The American Meteor Society Radiometeor Project AMS Bulletin No. 203 January, 1977 Revised: December, 1996 copyright: 1997, The American Meteor Society Compiled and Revised by: James Richardson AMS Radiometeor Project Coordinator Dr. David D. Meisel AMS Executive Director ------------------------------------------------------------------- Table of Contents 1.0 INTRODUCTION 1 2.0 PERSONNEL REQUIREMENTS 3 3.0 BASIC PROCEDURE 4 4.0 PHASE I: PRELIMINARY SURVEYS 4 4.1 Receiver Site Survey and Selection 4 4.1.1 Radio Background Noise 4 4.1.2 Other Considerations 6 4.2 Band Survey and Frequency/Transmitter Selection 7 4.2.1 Transmitter Requirements 7 4.2.2 Radio Frequency BAND SURVEY 8 4.2.3 HF Band (Frequencies Below 30 MHz) 8 4.2.4 Frequencies Between 30 MHz and 50 MHz 9 4.2.5 The 6-Meter Amateur Radio Band (50-54 Mhz) 10 4.2.6 The Low VHF Television Band (55-88 Mhz) 11 4.2.7 FM Commercial Band (88-108 MHz) 13 4.2.8 Aircraft Band (108 MHz - 140 MHz) 15 4.2.9 Frequencies Above 140 MHz 15 5.0 PHASE II: SYSTEM ESTABLISHMENT 16 5.1 Establishment of Receiver System 16 5.1.1 Basic Radiometeor Receiver System 16 5.1.2 Antennas and Mounting 17 5.1.3 Transmission Lines and Connectors 21 5.1.4 Filters and Traps (Optional) 21 5.1.5 Pre-amplifier or "booster" 22 5.1.6 Frequency Converter (optional) 22 5.1.7 The Receiver 23 5.1.8 Noise Cancellation Receiver 24 5.2 Establishment of Data Collection System 24 5.2.1 Receiver/Computer Interface 24 5.2.2 Audio Detection of Meteor Events 25 5.2.3 Automated Meteor Event Detection 27 6.0 PHASE III: SYSTEM TESTING AND OPERATION 30 6.1 System Grooming, Testing, and Calibration 30 6.1.1 Meteor Event Verification Test 30 6.1.2 System Calibration Test 31 6.2 System Full-Time Operation and Maintenance 32 7.0 FINAL COMMENTS 33 APPENDIX I: Forward-Scatter Fundamentals 34 A1.1 Introduction to Meteor Radio Scatter 34 A1.2 The Meteor Scatter Signal 35 A1.3 Meteor Scatter Geometry 35 A1.4 Sporadic Flux Variations 37 A1.5 Meteor Velocity Considerations 37 A1.6 Forward-scatter by Aircraft 38 A1.7 Other Propagation Modes 39 APPENDIX II: Link Bearings/Offsets for Meteor Showers 42 APPENDIX III: VHF Frequency Lists and Information 43 APPENDIX IV: Amateur Radiometeor References 44 A4.1 AMS Bulletins and Publications 44 A4.2 Amateur Radiometeor Efforts 44 A4.3 Meteors and Propagation (Advanced) 45 A4.4 Symposium Results (Advanced) 45 APPENDIX V: General Radio Astronomy References 46 A5.1 Radio Astronomy 46 A5.2 Radio Broadcast Guidebooks 46 A5.3 Radio Equipment 47 ------------------------------------------------------------------- AMS Radiometeor Project Bulletin 203 1.0 INTRODUCTION For nearly three-fourths of a century, Dr. C. P. Olivier, the founder of the American Meteor Society (AMS), collected data from amateur astronomers on the hourly rates of meteors seen by single visual observers. This information formed the basis of four catalogs (Olivier, 1960, 1965, 1974a, 1974b) giving the average visual meteor rates seen for each hour of the night during the year. Three of the catalogs were for the northern hemisphere and one for the southern hemisphere. The northern hemisphere catalogs were average rates over the years 1901-1958, 1959-1963, and 1964-1972 (Meisel, 1987). In addition to these visual observations, the AMS Kansas Meteor Group also pioneered efforts in establishing an amateur operated forward-scatter radio system for the automatic counting of meteor events (Houston, 1958). With the advent of highly sophisticated photographic and radio scatter detection methods during the 1950's and 1960's, United States professional interest in supporting amateur based data collection waned considerably, along with funding for such efforts. In the decade following the publication of the last Olivier catalog, no serious attempt could be made to update these compilations. During these years, dedicated AMS visual observers continued to make routine observations, educate the public, and investigate alternative ways of contributing to professional meteor science. Beginning in the early 1980's, however, there were some developments which indicated that an effort to modernize and update Olivier's work might be possible, utilizing AMS amateurs. First, it was becoming increasingly expensive for professional meteor observatories to maintain high-power radio transmissions on the continuous basis needed for a world-wide, routine patrol of the meteor flux. Second, amateur radio operators and amateur astronomers around the world were able to acquire receiving equipment with a sensitivity comparable to that in the hands of professional radio astronomers of one or two decades before. Finally, these same amateurs were in possession of one or more microcomputing systems which were capable of tabulating, storing, and processing large amounts of meteor monitoring data continuously and unattended. In 1977, the AMS established the AMS Radio Scatter Program (now the AMS Radiometeor Project) in an effort to germinate this potential within the amateur community (Meisel, 1977). During the decade of the 1980's, this program carried out experiments involving the establishment of meteor radio scatter receiving stations by groups of amateur astronomers, as well as preliminary work in using microcomputers for data collection. Notable successes included the work of William Black (1983) of Florida; Michael Owen (1986) of New York; and Meteor Group Hawaii, led by Michael Morrow and George Pokarney (Meisel, 1987). The most promising results were the experiments performed by Kenneth Pillon (1984), a Canadian amateur, who successfully demonstrated that a TRS-80 personal computer could be used to detect and make graphic printouts of meteor events. The long term goal of this program was, and is now, to establish and maintain a network of amateur operated radiometeor stations, each collecting data on the meteor flux on a continuous operating basis. Building upon lessons learned from these previous attempts, the first full-time prototype station became operational in 1993, utilizing an Apple IIe platform for data collection (Richardson, 1993). Since that time, a second operational station has been added to the project, with additional stations under development. In the upcoming years, AMS efforts will continue to go toward expanding this network, as well as to increase the sophistication of individual stations. The purpose of this bulletin is to aid those persons who wish to establish a forward-scatter station for inclusion into the AMS Radiometeor Network. A recommended procedure will be outlined for such an effort, along with suggestions and technical information. In addition to this bulletin, other materials and assistance can be provided by the AMS staff to those persons attempting this endeavor. A Coordinator for the AMS Radiometeor Project is assigned to assist developing stations and to direct network activities. It is important to note that the AMS Radiometeor Project is designed to augment, not to replace the active AMS Visual Program. In addition to the independent research conducted with the visual database, the visual observer is still the standard upon which most other meteoric rate research methods are based. The AMS will continue to use visual meteor observers to calibrate the radiometeor network back to the set of visual hourly rate observations that were tabulated in the past. 2.0 PERSONNEL REQUIREMENTS Unlike visual meteor observing, where observational skills are stressed and no complex equipment is required, radiometeor work (if it is to be more than intellectual curiosity) requires a large degree of careful experimentation and technical design to find the best combination of frequency, equipment, and transmitter location for a given receiver site. In order to achieve success in the AMS Radiometeor Project, it is essential that each participant develop, through reading and consultation, the maximum technical expertise possible. Unless there is a considerable degree of personal electronic/radio expertise, the AMS does not encourage single-handed attempts at participation in this project. Instead, novices are encouraged to contact nearby amateur radio groups, individuals with amateur or professional electronics expertise, or amateur optical and radio astronomy groups in their geographic area for assistance. If such contacts are not fruitful, appropriate references may be possible through the AMS Radiometeor Project Coordinator. Group participation in this project is encouraged, which allows participants to pool their various talents. Affiliate group memberships are a long AMS tradition, especially in the area of radio scatter. It is also highly recommended that individuals contemplating participation in this project be familiar with personal computer (PC) use and basic maintenance. Participants must be able to install and remove peripheral devices and circuit cards; install, remove, and operate software; create, copy, and maintain a disk data library; as well as performing normal computer operation and maintenance functions. While not a requirement, it is also very helpful for participant have access to electronic mail via the Internet for routine communications. 3.0 BASIC PROCEDURE The constraints outlined below are stringent but are necessary if results of permanent scientific value are desired. There are three basic phases, comprising six total steps, involved in the establishment of a successful Radiometeor Station: * Phase I: Preliminary Surveys (1) receiver site survey and selection (2) band survey and frequency/Transmitter selection * Phase II: System Establishment (1) establishment of receiver system (2) establishment of computer automated data collection system * Phase III: System Testing and Operation (1) system grooming, testing, and calibration (2) full-time system operation and maintenance In the following sections, each of these phases will be discussed in detail. Section 4 discusses Phase I, Section 5 discusses Phase II, and section 6 discusses Phase III 4.0 PHASE I: PRELIMINARY SURVEYS This phase is made up of two basic surveys: a Site Survey and a Radio Frequency Band Survey. By the end of this phase, final selections will have been made for both of these aspects. 4.1 Receiver Site Survey and Selection The most obvious site requirements for this project is sufficient outdoor space for an antenna system, and sufficient indoor space for the receiver and computer systems. The receiver and computer systems will also require adequate electrical power, ventilation, and environmental control. Beyond these standard housekeeping requirements, the most important characteristic of the receiver site is its radio background noise level. 4.1.1 Radio Background Noise Any radio astronomy experiment requires a site as free from man-made (artificial) radio noise as possible. This is analogous to the situation in optical astronomy where street lighting makes it difficult to observe deep-sky objects from urban areas. Sources of radio noise pollution include: A. Nearby Commercial and Private Transmitters Even when not generated in the desired band, interference from other radio sources can reduce radiometeor detection. Complicating this situation is the fact that the antenna is not the only place where interference can enter the system. Highly sensitive radio equipment, although shielded, may still be susceptible to strong interference entering into various parts of the receiver or converter circuits. This is called the image frequency sensitivity of the receiver system. Nearby radio and television transmitters, Ham and CB transmissions, mobile communications systems, and other radio transmissions can all be sources of potential interference. If this type of interference cannot be avoided, additional shielding or filtering against unwanted radio transmissions may be necessary for successful system operation. B. Computer Generated Interference This source is of particular concern for this project since the system will utilize a computer for data collection. All computers generate radio frequency interference (RFI), and the following steps will help to minimize this effect upon the system: (1) ensure that all computer components are properly grounded, and all covers and cases are installed. (2) maintain as much distance as practical between the computer system and receiver system, both the antenna and receiver hardware. Computer generated RFI usually follows the inverse square law, such that doubling the distance between the computer and the receiving equipment will reduce the RFI by a factor of four. (3) Avoid using a computer with board clock frequencies at or near the desired receiver frequency, or on some harmonic of the desired frequency, i.e., 1/2x, 2x, or at some other multiple of the receiver frequency. (4) All receiver components should be solidly grounded to an outside grounding rod. This should be a solid metal rod driven several feet into the ground, and is often available from local hardware stores. This will also help to reduce noise from other sources, as well as provide additional lightning protection for the system. C. Ignition and Electrical Noise All electrical devices which produce sparks or discharges generate copious amounts of radio noise. Included are automobile ignitions, neon and fluorescent lamps, electric fences, large electrical motors, electrostatic dust filters, and a variety of household electrical devices. Such electrostatic noise sources should be considered during initial site selection, as well as identified and reduced during the band survey and receiver establishment steps. D. Atmospheric Noise The most serious atmospheric noise comes from lightning or precipitation noise. Local lightning can be strong even on the FM band. In addition, a direct hit on the system by lightning can be disastrous. It is therefore stressed that all antenna systems must be properly grounded and protected from the possibility of electrical discharges including lightning. Natural electrical disturbances are most drastic in the summer and in warm, moist climates. Note: The automated data collection system currently employed is designed to cancel out and ignore electrostatic noise spikes generated by intermittent sources, preventing such noise spikes from being detected as meteoric events. This is done through the use of a second radio receiver designed to detect such noise sources specifically. It is recommended, nonetheless, that participants make every effort to minimize all sources of artificial noise since even canceled noise spikes will interfere with the detection of true meteoric events. 4.1.2 Other Considerations Good radio astronomy observing sites are usually in rural valleys with surrounding hills shielding lines-of-sight to urban areas. For meteor scatter, where relatively narrow received bandwidths can be used, the requirements for transmitting station shielding are perhaps not as stringent as for detection of distant celestial objects. Nevertheless, remote geographic sites are still considered best for meteor scatter work. This is because the radio background noise level is lowest in such areas, and impulsive noise interference can still cause severe problems for radiometeor detection, particularly when automated meteor detection is employed. All efforts should be made to detect and plan for such intermittent radio noise sources during the initial band surveys. The most convenient site, of course, is the observer's home or at least in an adjacent building. If this is in a remote suburban or rural location, the prospects are good that useful meteor scatter work can be done, but tests must be performed to verify the suitability of a given site and geographic location. Alternatively, those living in an urban area may be able to arrange to have their equipment set up at a rural location, perhaps at the residence of a friend. It should be remembered that most electronic equipment (particularly of the microchip and transistor variety) is designed for operation at approximately room temperature. For proper operation, equipment should never be allowed to be subjected to the rigors of the elements. Even if the receiving equipment will take temperature extremes, computer systems are particularly sensitive to adverse environments. Finally, power lines and even non-electrified fences can pick up and amplify unwanted interference. Locations without fences, with buried power lines, and remote from power transformers and high voltage lines will generally be the quietest. Whenever possible, suspended power lines and pole mounted transformers should be situated behind the desired direction of signal receipt. In particular, avoid placing such objects within the beam of yagi antennas. Power poles may also be sources of intermittent RFI due to degraded insulators and poor grounding systems, especially in cold humid conditions. Local power companies may be persuaded to assist in some circumstances. 4.2 Band Survey and Frequency/Transmitter Selection Following site selection, the next step is to perform a band survey of the desired frequency range to help determine which frequency and potential transmitters can be utilized for meteor scatter. 4.2.1 Transmitter Requirements The main transmitter requirements for suitable meteor scatter observations include (1) Continuous, 24 hour transmissions (modulated or unmodulated). (2) Commercial transmitters should be located less than 1500 km (about 950 miles) away, but be sufficiently distanced to reduce the effects of atmospheric scatter. Atmospheric scattering contribution should be at most 2 times the receiver background noise level (3 dB S/N). It is recommended that commercial transmitters (usually 50 kW to 100 kW) be at least 300 km (about 200 miles) distance from the receiver. (3) To prevent the receipt of transmitter groundwave or line-of-sight transmission, low powered non-commercial transmitters utilized for this project should be distanced sufficiently below the radio horizon, or situated behind a natural barrier, such as high hills or mountains. Groundwave reception contribution should be at most 2 times the receiver background noise level (3 dB S/N). As a general rule, low powered transmitters (preferably not less than 50 watts) should be at least 80 km (50 miles) away from the receiver. (4) The transmitted frequency must be above that propagated by the ionosphere, but low enough to be efficiently reflected by a meteor trail. This will be discussed in further detail in the following sections. (5) For short range forward-scatter systems, the transmitting antenna must have significant power at high angles of elevation. Conversely, the receiving antenna pattern must be able to receive high angle radiation. The further the distance to the transmitter, the lower will be the angle of propagation for most meteor scatter signals. 4.2.2 Radio Frequency BAND SURVEY The following sections discuss each major radio band in relation to its potential for meteor scatter work. Note that this discussion applies to North America only and would not be accurate for other areas. The frequency bands to be considered are divided as follows: (1) HF band (less than 30 MHz) (2) frequencies between 30 and 50 MHz (3) 6-meter amateur radio band (50 to 54 MHz) (4) low VHF television band (55 to 88 MHz) (5) FM commercial band (88 to 108 MHz) (6) aircraft band (108 to 140 MHz) (7) upper VHF band (greater than 140 MHz) 4.2.3 HF Band (Frequencies Below 30 MHz) Meteor scatter theory shows that the lower the radio frequency, the more effective meteor radio scatter becomes. At the same time, the lower the frequency, the more prevalent other forms of radio wave propagation also become. Therefore, for a system in which meteor scatter is to be the primary propagation mode, the transmission frequency must be kept above that value which is likely to be propagated by other means. This frequency below which normal ionospheric propagation, or "skip," is likely to occur is called the critical frequency. Meteor scatter systems generally function best close to, but still above, the critical frequency. In considering the HF band, the lowest useable frequency for meteor scatter work depends on the time of day, season of the year and phase in the solar cycle. During daylight hours, propagation of signals vertically via the ionosphere can occur as high as 20-22 MHz at sunspot minimum, and 30-35 MHz during sunspot maximum. Consequently, These high critical frequencies render the HF band generally unsuitable for serious radio meteor work. It is important to know that even when operating above the critical frequency, other forms of propagation can still interfere with meteor scatter systems. High aircraft can cause reflections in short-range systems. Temperature inversions in the troposphere can create ducting effects. A solar related phenomena called D-layer scatter can occur during daylight hours, especially in the early afternoon. Aurora activity in the north, and "Spread F" activity near the equator can also cause unwanted propagation. For most meteor scatter systems, however, the most common form of unwanted propagation is "Sporadic E" activity. It is one of the ironies of nature that the influx of meteors during meteor showers enhances considerably the probability of Sporadic E ionization. An intense meteor shower has been shown to leave residual ionization, especially light metals, at the 100-120 kilometer level (E layer). Given the right upper atmospheric circum-stances (high winds and shearing), this ionization can often prohibit direct meteor observations by reflecting HF, VHF, and occasionally UHF radio waves back so strongly that meteor scatter signals are swamped. Sporadic E activity during the year roughly follows meteor shower activity: it usually begins to occur around April; peaks about June-August; dwindles through October-November; with a minor peak often seen in December and is usually absent again by January. Sporadic E tends to be most prevalent in the mid latitudes from about 20 degrees North to about 50 degrees North (a rough thumb-rule only). While not scientifically useful, the HF band does provide many persons with their first experiences in listening to radiometeors. For casual radiometeor observing, quiet frequencies in the Shortwave and amateur radio HF bands can successfully be used to listen to the rapidly descending "whistles" of meteor head echoes. A long wire or dipole antenna is all that is required, with the receiver set to monitor CW or SSB signals. 4.2.4 Frequencies Between 30 MHz and 50 MHz While this band (30-50 Mhz) is the most favorable for a 24 hour meteor scatter survey, the majority of the best frequencies in use in North America are for point-to-point intermittent communication. Continuous beacons are rare and of very low power (such as paging transmitters), rendering this band unsuitable for this project. It is noteworthy, nevertheless, that most Meteor Burst Communications systems operate in the 40 to 50 Mhz band. While the great majority of these systems operate on an intermittent basis, such as in the early morning hours when the meteor flux is highest, a few do operate continuously. These generally use transmitter powers ranging from 500 watts to 2000 watts. it is highly unlikely, unfortunately, that such transmitters can be utilized for this project. Most of these transmissions are used for secure communications systems, and transmitter frequencies and locations are classified by the U.S. Department of Defense. 4.2.5 The 6-Meter Amateur Radio Band (50-54 Mhz) The 6-meter amateur radio band possesses great potential for successful meteor scatter work through the availability of high-quality radio equipment via the commercial amateur radio market. At the same time, this frequency range also presents a disadvantage. The primary difficulty of attempting to utilize this band is the scarce nature of continuously operating beacon transmitters of sufficient power for serious radiometeor data collection. Most amateur radio beacons operate on an intermittent basis only, and at low powers, typically 10 to 100 watts. To be useful for this project, a beacon transmitter must broadcast a continuous AM or FM signal, preferably omnidirectional, at a power level of at least 50 watts. The transmitter to receiver distance should be between 50 and 150 miles (80 to 240 km), with some form of natural barrier between the two locations. Although AMS experiments have successfully detected meteor events using nearby transmitter powers as low as 20 watts, such systems are close to the limit of amateur detectibility (Meisel, 1982). From these guidelines, it is obvious that suitable 6-meter transmitters will generally not be available to most project participants. Lists of operating 6-meter beacons, including frequency, location, power level, propagation directions, and owner, can be found in the back of the amateur radio Repeater Handbook, published by the American Radio Relay League (ARRL) each year. It is also highly recommended that all participants wishing to utilize this band for radiometeor work obtain at least a Technician class amateur radio license from the FCC in order to become completely familiar with amateur radio equipment and procedures. Such a license is not required for receiving systems only, but non-licensed participants are not permitted, by law, to make even casual or experimental transmissions. Further information and help can be obtained from local Amateur radio clubs in the participant's area. The other great potential offered by this band is the ability of licensed participants to set up and maintain their own transmitter station for this project. This will require the setup and maintenance of two stations at some distance from one another, as outlined above. Because of the logistics and expense involved, it is recommended that such an endeavor be attempted by group participants only. A great many amateur radio operators utilize meteor burst communications for point to point short-lived communications. "Ham" activity using this mode is especially prevalent during major meteor showers. While the majority of hams are not interested in continuous scientific data collection, it is from amateur radio operators that the AMS learned the techniques for utilizing low VHF television transmissions as discussed in the following section (Owen, 1986). The serious hams utilize meteor events detected by a television frequency receiving system to indicate when "openings" exist on their amateur radio band. 4.2.6 The Low VHF Television Band (55-88 Mhz) Perhaps the most promising band for use in this project is the low VHF television band, comprising broadcast channels 2 through 6. In the past, these broadcasts were determined to be unsuitable for radiometeor work due to the fact that most stations usually signed off and ceased transmissions shortly after midnight, resuming transmission again in the morning. In recent years, however, competition from cable networks and more durable transmission equipment have allowed most broadcast stations to take up 24 hour schedules, making them ideal for this work. Table 1 lists the actual frequencies used by these channels. In addition to the primary picture signal, each TV channel also carries a subcarrier for sound which is set at 4.5 MHz higher than the picture carrier frequency. Also, a color subcarrier is located 3.58 MHz higher than the picture-carrier frequency. Since the picture carrier frequency contains most of the broadcast power, it is of primary concern for radiometeor work, although the sub-carriers may be investigated. Table 1: The Low-VHF Television Band chan Picture color Sound 2 55.25 58.83 59.75 3 61.25 64.83 65.75 4 67.25 70.83 71.75 5 77.25 80.83 81.75 6 83.25 86.83 87.75 The one factor which makes these signal broadcasts most useful for radiometeor work is the assignment of offset frequencies by the FCC to stations on these channels. Stations will be assigned to broadcast at the central frequency (called 0 offset), 10 kHz above the central frequency (called + offset), or 10 kHz below the central frequency (called - offset). Thus, instead of all channel 2 stations broadcasting at 55.25 MHz only, the frequency assignments are evenly divided between 55.24, 55.25, and 55.26 MHz. The subcarrier frequencies are also affected. The reason for this scheme is to permit the Automatic Frequency Control (AFC) circuit in television receivers to "seek" the station with the highest power output when a receiver is located midway between stations on the same channel. Rather than seeing a garbled, mixed signal, the viewer is able to watch the stronger of the two stations. The FCC attempts to evenly distribute offset frequency assignments throughout its geographic area to facilitate this feature. A radiometeor receiver will generally have a bandwidth of 10 kHz or less, and will be set up to monitor the carrier wave signal only, utilizing CW or SSB mode. This will allow the participant to choose the offset frequency for a selected channel which best optimizes meteor scatter. As a general rule, this will be the offset frequency with the most distant stations. For example, the prototype station at Poplar Springs, FL, utilizes the channel 2 broadcast. Using the - offset, the closest channel 2 station is located near Montgomery, AL, at 200 km (125 miles) distance, and is too close for meteor work. At the 0 offset is an Atlanta, GA, station which is about 300 km (190 miles) away. This might be useful for some monitoring, except that the atmospheric scatter signal is occasionally high enough to swamp out meteor events. On the + offset, the nearest station is Mississippi State, MS, and is 382 km (238 miles) away, making this the best frequency choice for monitoring. it should be noted that the Mississippi station is still avoided with the Yagi antenna to minimize atmospheric scatter effect from this station. Instead, most meteor events are reflections from stations in South Carolina, Tennessee, and Maryland. For survey purposes, the only equipment required is a good quality television receiver connected to an outside mounted, steerable (manual or electric), log periodic VHF antenna. Use of a store bought pre-amplifier will also aid in this survey, but is optional if the television receiver is of high quality. The participant should look for a "clear" or open channel, relatively free from reception in all directions. If poor reception is received in one or two directions, the channel can still be used, as previously discussed. If more than one channel is open, then the lower of the two should be selected. Open channels adjacent to very strong local stations should be avoided, even at the cost of selecting a higher channel. Once a potential channel has been selected, the participant should consult one of the several published listings of North American television stations to determine what offset and stations have the best potential for meteor scatter work. The effective radiated power (ERP) and operating schedule of potential stations should also be determined. Stations which do not operate continuously should be avoided. The reason that a channel with a local station on one of the offsets cannot be used is because it will generally bleed over onto ALL of the offsets, rendering that channel unusable for the project. Very strong local stations can also render adjacent channels unusable as well. It may, however, be possible to purchase a notch filter, or "trap," to reduce the signal from the unwanted adjacent channel station to a manageable level. Experimentation will be needed to achieve the best possible results. It should be noted that in order to utilize this band for radiometeor work, a standard television receiver cannot be used. The bandwidths of such receivers are too broad to allow selection of specific offset frequencies, and such receivers do not allow monitoring of the selected carrier wave only. It will, therefore, be necessary to obtain a higher quality receiver with the desired selectivity and modulation modes for establishment of the radiometeor system. 4.2.7 FM Commercial Band (88-108 MHz) While these frequencies are not as efficiently scattered as are lower ones, the FM band contains a proliferation of potential transmitters for use in this project. But this wealth of stations leads to considerable band congestion, and finding a suitably open frequency slot may be very difficult, especially in the more populated areas of the country. Channels in the FM commercial band are assigned in 200 kHz increments from 88 to 108 MHz, using the odd numbers, such that stations will be located at the x.1, x.3, x.5, x.7, and x.9 fractions of the MHz frequency. For Radiometeor work, the selected frequency should be as low in the band as possible. The participant should look for three "clear" channels in a row, with the middle clear frequency used to monitor radiometeor reflections. Channels adjacent to local FM stations are not suitable due to "bleed over" from the nearby occupied channel. With the high density of FM stations on the band, it can generally be assumed that any clear channel will have multiple stations available for meteor scatter, in a variety of directions. A couple of things should still be kept in mind, however. The desired distance to useful transmitters is to again be less than 1500 km (about 950 miles) but more than about 300 km (about 200 miles). The operating schedules for target transmitters should also be considered. For example, most of the transmitters in the lower end of this band are NPR stations, which often sign off late at night just as meteor activity is reaching its height. Searches for suitable FM stations are much easier if an "FM Atlas" is used. Another advantage of this band is the great availability and variety of relatively inexpensive equipment which can be obtained. Survey equipment for this band should consist of an outside mounted, steerable (manually or electric) log-periodic VHF antenna; a store bought FM pre-amplifier, usually about +20 dB; and a good quality FM receiver with digital display. Using this equipment alone, meteor bursts from overdense meteor trails can be monitored. For purposes of the survey, it is preferable to listen to the demodulated intelligence (music, voice, etc.) from the signal in order to aid in identifying the distant stations. For actual receiver system establishment, it is preferred that participants utilize a high quality receiver capable of operating in the CW or SSB mode on the desired frequency. Alternatively, it may be possible to utilize a high quality, digitally controlled standard FM receiver if the AGC or Power Level meter circuit voltages are tapped for input to the data collection computer. Again, experimentation will need to be done to determine the best equipment setup. The FM band also has the advantage that it may be possible to select several suitable stations for meteor scatter in various directions, and optimize the radiant-to-transmitter aspect for a particular major meteor shower. As a general thumb-rule, shower detection by a forward-scatter system is optimized when the shower radiant passes through an azimuth which is perpendicular to the transmitter-receiver baseline azimuth. As an example, the Geminid shower radiant, in the northern hemisphere will, roughly speaking, rise in the east, pass directly overhead, and set in the west. For optimal detection of this shower, the transmitter-receiver baseline for a system should be perpendicular to this, or have a roughly north-south azimuth. In addition, radiant altitude also plays an important role. Generally speaking, forward-scatter systems function best at radiant altitudes of 30 to 60 degrees, with peak detection at about 45 degrees altitude. Above or below this radiant altitude range, shower detection drops off rapidly. Thus, the Geminid shower radiant mentioned above would generate two distinct system shower maxima as it moved across the sky, rather than a single peak. Optimum directions for transmitter-receiver baseline azimuth are list in Appendix II for the major meteor showers. It is important to remember that because of the high sensitivity of forward-scatter systems to radiant directivity (both in altitude and azimuth), meteor showers detected in this way will create detected shower maxima which do not correspond to the actual shower maximum. For showers which occur over a period of several days, the radio shower observer will notice a system shower maximum occurring at roughly the same time(s) each day, with peak detected activity occurring within the same 24 hour period as the actual shower maximum. This gives a single forward-scatter system an accuracy of only +/- 12 hours with regard to detecting true shower maximums. To detect true shower maximums using radiometeor data, data from several systems must be analyzed simultaneously. 4.2.8 Aircraft Band (108 MHz - 140 MHz) Although there are beacons in this range, they are used for air navigation and therefore of ultra-low power. Information on these can generally be obtained at FAA flight information centers at major airports. Use of these transmitters is also discussed in the supplement to AMS Bulletin No. 203 (Meisel, 1982). During the 1980's, the AMS successfully utilized low-powered aeronautical beacons used for Instrument Landing Systems (ILS), transmitting at 75 MHz (Black, 1983). Since that time, unfortunately, the FAA has lowered the power output of such beacons below the threshold level for meteor detection, making these transmitters no longer useful. It is highly recommended that participants wishing to investigate the use of aeronautical radio transmissions as a possible meteor scatter source first gain a thorough knowledge of their function and current operating characteristics. Also in this band are the so-called "stationary' satellite beacons. Whether it is possible to detect meteor scatter from such transmissions remains to be demonstrated using amateur obtained equipment. 4.2.9 Frequencies Above 140 MHz Above 140 MHz meteor scatter is much less efficient, so the use of high VHF and UHF is not recommended unless absolutely nothing else is available. Television channels 7 to 13 probably offer the best potential for useful transmitters, with the 2-meter Amateur Radio band (144-148 Mhz) as a secondary option. 5.0 PHASE II: SYSTEM ESTABLISHMENT Once the receiver site and the desired frequency/transmitter have been selected, the participant can then begin to actually establish the radiometeor station. 5.1 Establishment of Receiver System Selection of receiving equipment is probably the most bewildering task facing the prospective radio meteor observer. There are several options and before making a big investment, it is always wise to experiment with used or borrowed equipment. With a good, stable shortwave communications receiver available, the construction or purchase of a frequency converter may be the easiest route once the frequency choice has been made. Also, some commercially available FM tuners and aircraft monitors may have sufficient stability and sensitivity to be satisfactory for routine meteor scatter work. Local amateur radio operators are good sources of information on the availability of both new and used radio equipment in the participants local area. This will include sources for antennas, antenna masts and towers, transmission lines, connectors, amplifiers, receivers, and other equipment. Ham radio operators also frequently hold "swap shops" in which used equipment of a wide variety is traded and sold. These operators are also good sources of information about catalog dealers in mail order radio parts and equipment. Participants in this project will be well served to make contact with this valuable resource. 5.1.1 Basic Radiometeor Receiver System The equipment making up a meteor scatter system includes: (1) Antenna and Mounting (2) Transmission Lines and Connectors (3) Filters and Traps (optional) (4) Pre-amplifier (or Booster) (5) Frequency Converter (optional) (6) Receiver (7) Noise Cancellation Receiver The following sections will give some guidelines for equipment selection and setup. It should be emphasized, however, that success in this area will largely depend upon the ingenuity of the individual participant. The only portion of the radiometeor systems currently being used by the AMS which is standardized is the Computer Automated Data Collection System. It is the responsibility of the participant to assemble a receiving system capable of successfully receiving meteor events. 5.1.2 Antennas and Mounting A good antenna system is the keystone of a successful radiometeor receiver system, rivaled in importance only by the receiver itself. Participants should take great care to construct and maintain the highest quality antenna system possible. A. The Antenna Although for survey purposes a broadband antenna such as a log-periodic VHF antenna is satisfactory, once a station choice has been made, a high gain antenna cut specifically for the chosen frequency should be used. This type of antenna can be either bought commercially, or the participant can custom build their own. The most common antenna used for radiometeor work is the Yagi antenna, usually built for a gain of about 10 dB S/N, and a beam width of about 30 degrees. Formulas and matching techniques can be scaled from data found in many radio amateur handbooks. For Radiometeor purposes, a Yagi antenna of 4-5 elements is sufficient, although antennas of 3 or 6 elements have also been used successfully. Higher numbers of elements are not required because the extremely narrow beam width of such antennas will counteract any gain advantage. For short range forward-scatter systems, such as those utilizing close, low powered transmitters, an omnidirectional receiving antenna may be desired. An example of this would be the crossed dipole, or turnstile antenna. The disadvantage of omnidirectional sky coverage, however, is the loss in antenna gain. Dipole antenna's generally increase S/N by a factor of only 2 dB. Another option is a 3-element Yagi antenna set up for very high angle propagation. Participants will have to experiment to find the most suitable antenna for their particular system. One disadvantage of the use of frequencies in the 30 to 70 MHz range is the large physical size required for the antenna. If this becomes a limiting factor for the participant, a choice of frequency in the 70 to 140 MHz band helps to reduce this antenna size problem. B. Antenna Mounting Properly mounting a radiometeor antenna will depend largely upon transmitted signal polarization, distance to the transmitter(s), and ground quality. These factors are interrelated and can be easily confusing. (1) Polarization Transmitted signals in the Ham 6-meter, low TV, and FM bands (the major ones considered) are usually linearly polarized. Linearly polarized signals are divided into two types, horizontal or vertical, based upon the physical alignment of the transmitting (and receiving) antenna elements. FM Commercial band signals usually have vertical polarization for reception by automobile antennas. Low TV band signals usually have horizontal polarization for receipt by pole mounted broad-band antennas. Six-meter amateur radio transmissions can be either vertically or horizontally polarized depending upon the transmitter. Participants should match the polarization of their receiving antenna to the transmitted signal polarization, if known. Having said this, however, another factor needs to be mentioned which somewhat negates this advice. Whenever linearly polarized radio waves enter the magnetized ionosphere, they become circularly polarized. if these signals are reflected back toward the ground, the signal usually regains its linear polarization as it leaves the ionosphere. This is especially true if the signal path is east-west, cutting across the earth's magnetic lines of force. Conversely, a signal on a north-south path, parallel to the earth's magnetic lines of force, has a probability of never regaining its linear polarization and may reach the surface with circular polarization. This causes a resulting loss of signal integrity. A meteor trail reflection can add slightly to this circular polarization effect. As a result, a linearly polarized radiometeor antenna, such as a Yagi, will only see at full strength, those events which have effectively regained their initial polarization. While this effect is generally only noticeable in the statistical analysis of data from the system, it should be kept in mind by the participant, especially when employing north-south signal paths. (2) Antenna Height and Beam Tilt Angle Antenna height is usually measured in wavelengths above the ground, while antenna tilt angle refers to the angle above the horizon to which the antenna axis is pointed. In determining desired antenna height and tilt angle, the primary factors to be considered are distance to transmitter and desired angle of propagation. For midpoint reflections, the following approximate propagation angles apply: 1500 km (4 deg), 1250 km (6 deg), 1000 km (8 deg), 750 km (12 deg), 600 km (15 deg), 500 km (18 deg), 250 km (30 deg), and 125 km (40 deg). A more detailed list of propagation (altitude) angles is shown in Appendix II. Thus, for a radiometeor system, it is desirable to match the antenna beam altitude as closely as possible to these propagation angles. This is achieved very differently for horizontally and vertically polarized antennas. It is most important to remember that antenna beam altitude, the sky angle from which most signals will be received, and the antenna tilt angle, the angle at which the antenna axis is physically mounted, are often INDEPENDENT of each other, due to the effect of ground reflection. If a good ground is present, the desired beam altitude for a horizontally polarized (HP) Yagi can be achieved by varying the antenna height only, while leaving the antenna itself at a 0 degree tilt angle. For a near 0 degree beam altitude, the HP yagi should be mounted at least 2 wavelengths above the ground. This would be 38 feet for a 6-meter antenna. For a desired beam altitude of 45 degrees, the HP Yagi should be 1 wavelength above the ground. This is 19 feet for a 6-meter antenna. For a near vertical beam altitude, the HP Yagi should be placed 1/2 wavelength above the ground. For a 6-meter antenna, this would be only 9.5 feet. Higher frequencies will require lower antenna heights than low frequencies. Above 2 wavelengths height, the HP yagi loses this effect from ground reflection, and tilt angles of the desired amount will need to be applied to the antenna if beam altitudes of greater than 0 are desired. It should also be noted that the typical 5 element Yagi has a 30 degree beam width even in the vertical plane, such that a beam altitude of 45 degrees will yield coverage from about 30 to 60 degrees altitude. This also implies that a 0 degrees beam altitude is sufficient for transmitter distances of 600 km and greater. In an area with a good ground, the vertically polarized (VP) yagi will show very little effect from antenna height. The antenna can be mounted at whatever height is convenient to the participant. Any desired beam altitude, consequently, will need to be created by placing the antenna at the desired tilt angle. (3) Ground quality A final factor to be considered is the quality of the natural ground in the receiver site area. Wet or moist areas with high soil conductivity will have very good ground quality, and antennas will behave as described above. Dry, sandy areas with poor soil conductivity will tend to reverse the effects of ground reflection on the antenna beam, although installing grounding rods on the antenna and receiver will restore some ground potential. In the case of the HP Yagi, a poor ground will tend to cause the beam altitude to more closely match the actual antenna tilt angle, reducing beam elevation by ground reflection. In the case of the VP yagi, a poor ground will cause the beam altitude to be higher than the antenna tilt angle, causing beam elevation by ground reflection to occur. The antenna height will again become a factor affecting beam elevation of the VP Yagi. Under such conditions, mounting the antenna at least 2 wavelengths above the ground will prevent these ground reflection effects. C. Antenna Azimuth In considering antenna azimuth, transmitter bearing and distance are the primary factors. The link distance is important because most meteors which cause forward scatter are not located over the transmitter-receiver baseline, but instead occur primarily in two broad regions, called the "hot spots." These "hot spots" are roughly located about 50 to 150 km to either side of the transmitter-receiver baseline midpoint. For transmitter distances of greater than 750 km, the antenna can generally be pointed directly at the bearing of the transmitter. In these long distance forward-scatter links, activity from both "hot spot" regions will already lie within the beam-width of the antenna at the transmitter bearing. At transmitter distances of less than 750 km, the more active "hot spot" is generally selected to lie within the beam, requiring that some offset angle be applied to the receiving antenna to optimize reflections. Based on a desired radiant elevation of 45 deg, approximate values for offset angles are: 1500 km (10 deg), 1250 km (11 deg), 1000 km (13 deg), 750 km (15 deg), 600 km (18 deg), 500 km (21 deg), 250 km (37 deg), and 125 km (56 deg). A more detailed list of antenna offset angles is listed in Apppendix II. As with beam altitude, the shorter the distance to the transmitter, the more pronounced is the required offset angle. At transmitter distances of less than 100 km, the system more closely resembles the back-scatter situation, with one primary hot spot located to the north of both transmitter and receiver, at very high propagation angles. In this situation, the receiving antenna can be pointed north regardless of transmitter bearing. Once the initial system testing has been completed, a final antenna direction will be chosen for the site, and the antenna will be fixed at that selected azimuth. Therefore, rotors and other antenna steering devices will not be required. D. Other considerations It should be noted that vertically polarized antennas are much more susceptible to detecting electrostatic noise spikes than horizontally polarized antennas. This may be a factor to consider in warm, moist climates where thunderstorms are frequent. All antennas should be mounted on a sturdy, well reinforced support system, able to withstand winds from the most severe storms which pass through the site area. If a mast is used, it should be properly supported by guy wires. Large antennas, such as those for 6-meters or TV channel 2 may require a small tower for proper support. Ensure that the antenna system is well grounded to maximize protection from lightning. 5.1.3 Transmission Lines and Connectors For use with this system, it is recommended that the highest quality transmission lines and connectors be used, to minimize signal losses and noise. Also, the shortest practical runs of cable should be used between antenna and receiver equipment. For 50 ohm systems it is recommended that RG-8 (or equivalent) cable be used rather than the smaller RG-58 cable, due to the larger cable's better signal loss characteristics. At 50 MHz, RG-58 has a signal loss of about 3.2 dB/100 ft while RG-8 has a signal loss of about 1.5 dB/100 ft. It is important to remember that the weakest link in any receiver system is the cable connectors. A poorly made or unprotected connector can cause severe signal loss and noise introduction into the system. Connectors used should be of the best possible construction, fit snugly and correctly, and be water-proofed and protected if employed outside. 5.1.4 Filters and Traps (Optional) In order to reduce the background noise level to a suitable degree for meteor scatter work, it may be necessary to employ some filtering in the line coming from the antenna before it reaches the receiving equipment. Whenever possible, filters should be installed in the line prior to the signal reaching the pre-amplifier, although this may not be possible if a mast mounted pre-amplifier is used. The number of filters should be limited to the minimum amount necessary for good system operation since each filter will create some small loss of the desired signal. Common filters which may be necessary include: A. High-pass filter This type of filter will generally pass only frequencies greater than 30 MHz, and will help to greatly reduce transmissions from HF Ham and CB radios, which can cause severe interference. This filter will be essentially required for those receiver systems which use a frequency converter to step down a desired frequency to within the range of a standard shortwave receiver (less than 30 MHz). B. Low-pass filter, This type of filter will pass only frequencies below a certain cutoff frequency, and is usually designed to prevent receipt of unwanted commercial transmissions, such as those in the TV or FM band. obviously, the desired frequency must be below those discriminated against by the filter. C. Notch Filters or "Traps" These are filters which are designed to discriminate against unwanted transmissions of a particular nature, such as a very close commercial transmitter. Examples of this include a notch filter for the entire FM commercial band, or a trap for a particular television channel to prevent interference with an adjacent open channel. D. Band-pass Filter The band-pass filter is the opposite of the notch filter in that only a narrow band of frequencies is passed, with all other frequencies attenuated. In today's congested radio spectrum, this type of filter is becoming increasingly necessary for low level signal work. A single band-pass filter designed for the desired frequency range should greatly improve the Signal to noise (S/N) ratio for most radiometeor systems, unless the background noise is already extremely low. A band-pass filter is also desired in situations where multiple filters of the previously mentioned types had to be employed, and will replace all of them. Although this filter type is the most elegant, it is also the most expensive, and must be custom made for a particular frequency and band. One potential source for Custom filters is: Digital Communications, Inc. (DCI) P.O. Box 293 White City, SK, Canada F0G5B0 phone: (306) 781-4451 Toll free: (800) 563-5351 E-mail: dci@dci.ca 5.1.5 Pre-amplifier or "booster" Detection of faint meteors often requires some preliminary signal amplification, particularly for frequencies above 88 MHz. For survey purposes, a commercial "mast-mounted' FM or VHF broadband transistor amplifier may suffice. If a suitable frequency has been selected, a high gain pre-amplifier can be purchased or constructed. Typical gains for pre-amplifiers are +20 to +30 dB. It is important to remember that pre-amplifiers are usually designed for a wide band-pass range, and will, therefore, amplify both the desired signal and the background noise. Such amplifiers cannot be used to improve the signal to noise (S/N) ratio. In some cases, a pre-amplifier can be combined with a frequency converter design, perhaps also including quartz-crystal frequency control. One potential source for high quality, low noise pre-amplifiers is: Advanced Receiver Research, Inc. (ARR) P.O. Box 1242 Burlington, CT 06013 phone: (860) 485-0310 5.1.6 Frequency Converter (optional) If a reasonably high quality receiver for the ordinary shortwave (HF) band is available, a frequency converter (oscillator plus mixer) may be used to step the incoming frequency down to the range of the selected receiver. This may be the lowest cost solution to setting up an operational meteor scatter system. it is recommended that such converters be purchased through some of the numerous radio supply sources. Alternatively, enterprising participants can attempt to construct their own. This has been successfully done for the TV channel 2 frequency, as well as for the 75 MHz ILS beacons used in past experiments. A high degree of electronic expertise is required for this route. 5.1.7 The Receiver For initial testing and detection experiments, it is probably wise to use borrowed or used radio equipment. After the initial feasibility tests, it can then be decided if the sometimes considerable investment in higher quality equipment is warranted. Unlike ordinary radio astronomy, where extremely wide band receivers are usually required, meteor scatter studies can often be done with relatively narrow band equipment (generally less than 10 kHz). Although there are numerous types of portable monitor radios now available for various parts of the 30-150 MHz band, it is doubtful if any have the required sensitivity and selectivity needed for meteor scatter work. Certainly, the equipment designed and recommended for amateur radio astronomy detection of the sun and Milky Way will have ample sensitivity for meteor work. Although microchip and transistorized receivers are more elegant than tube types, the older equipment is easier for the novice to modify and repair. Parts for such equipment, however, are becoming increasingly difficult to obtain. By far the best receiver for this project is one of the high-quality multi-band receivers currently available on the commercial market. Cost does become a considerable factor, as these receivers can be quite expensive. The great advantage of these receivers is the relative ease with which different frequencies can be investigated, while a frequency converter, for example, can only be designed for one frequency. Combined with the pre-amplifier, the receiver should have a sensitivity of between -130 dBm and -100 dBm (0.071 to 2.2 microvolt)* This places the receiver sensitivity within the range of the natural background noise level. This will allow the receiver (and computer system) to detect faint meteor events which may be less than 5 dB above the background noise. Additionally, the receiver and computer will be adjusted to achieve as wide a measurable signal range as possible (preferably greater than 20 dB) from "low peg" to "high peg." The preferred receiver should contain a beat frequency oscillator (BFO), which will be used to monitor the carrier wave signal from the transmitter, in either continuous wave (CW) or single side-band (SSB) mode. A receiver which allows the user to disable the automatic gain control (AGC), automatic volume control (AVC), and squelch from the radio front panel will prevent the participant from possibly having to disable these features from within the receiver itself. * Calculated from dBm = 20*log10 V -90 -10*log10 R (V = microvolts, R = ohms) where R is the input impedance, assumed here to be 50 ohms. 5.1.8 Noise Cancellation Receiver The receiver used by the system to detect and cancel out electrostatic noise spikes does not have to meet any of the stringent requirements of the meteor scatter receiver. A fair quality shortwave receiver with an indoor antenna is all that will be required. The best frequency settings for detecting noise is in the low HF band, about 2.5 MHz to 10 MHz, with AM detection employed. A standard AM receiver (0.56 to 1.6 MHz) is not recommended because of the common nighttime reception of AM stations across this band. Some AM receivers, however, may permit tuning beyond 1.6 MHz, making their use as a noise receiver potentially feasable. The participant should place the Noise receiver on as clear a frequency as possible to prevent the reception of broadcasts, while easily detecting lightning and other electrostatic noise spikes. 5.2 Establishment of Data Collection System The actual establishment and use of the Personal computer system used for automated data collection in an AMS radiometeor system is discussed in detail in the AMS Meteor Burst Software Users Manual, (Richardson, 1996b). It will be helpful, nevertheless, to provide a brief discussion on the computer interface, audio detection of meteors, and automated detection of meteors. 5.2.1 Receiver/Computer Interface The computer system currently being used employs a low impedance optical coupling device to provide a signal from the receiver to the Game I/O port on the computer. There are two places from which the receiver signal can be obtained. The easiest signal source to use is the audio signal output from the receiver, provided that the carrier wave signal is being monitored, and not the modulated intelligence. To use this output, the AGC, AVC, and Squelch must all be disabled on the receiver. This ensures that the audio signal level of the receiver is directly proportional to the received power level of the meteor event. The most convenient source for this signal is an earphone or speaker jack on the receiver. Unfortunately, utilizing this jack will normally disable the receiver speaker at the same time. It is desired that the participant devise some manner in which the audio signal output from the receiver can be both sent to the computer, and monitored by ear at the same tine. Some receivers provide separate audio output jacks at the back of the receiver for this purpose. The second method in which the receiver output can be obtained is by leaving the AGC of the receiver enabled, and tapping either the AGC voltage or Power Level Meter voltage to be sent to the interface device. Most modern AGC circuits have extremely fast response times to incoming signals, and can be used to directly measure the incoming signal power level. At the present time, the disadvantage of using the second method is the low-impedance of the optocoupler. This causes the interface device to load down the circuit being monitored, and affect the voltage being measured. Power level meter circuits are generally more durable to such loading effects, but extreme care must be taken not to damage the receiver circuitry. Experimentation will be required to utilize this interface method and it may be necessary to build a special operational amplifier (OP Amp) to match the device correctly. In the future, the AMS will develop and use a high-impedance A/D converter as the computer interface device, in which case the AGC voltage will become the desired parameter to monitor. Using this input device, the voltage can be monitored without affecting the receiver operation. Receivers will require a slight modification to tap this voltage and deliver it to a jack on the back of the receiver. The modification can be performed by a local electronic shop if the participants do not wish to perform it themselves. 5.2.2 Audio Detection of Meteor Events Just as the human eye is a superior device for image evaluation, the human ear is a superior device for aural discrimination particularly where a large dynamic range is required. Learning to recognize and classify radiometeor events by ear is a desired skill for participants in this project. The descriptions below apply to a receiver which has been set up to monitor the carrier wave signal, via the use of a BFO. It is recommended that participants audibly learn to listen for meteor events during the morning hours, when the incoming meteor flux is the highest. With no meteor events present the listener should hear normal background static, with one or more continuous audible tones faintly superimposed on this background. These tones are the atmospheric scatter signals from each of the transmitters closest to the receiver. The BFO should be adjusted for the most comfortable tone pitches. It will be noted that when more than one transmitter is present, the tones will usually be on different pitches. This is because the tolerance for transmitters assigned to a particular frequency, while quite narrow, will still permit deviations of up to a few hundred hertz, which is easily detected as different pitches by the human ear. Above this background noise the listener will be able to hear occasional tonal "pings," similar to the sound made by striking a tuning fork. These are the underdense meteor trails, and generally last less than 1 second. The term underdense refers to the fact that the meteor trail electron line density is below a critical factor, usually taken to be 10^14 electrons per meter. Below this density, the electrons in the meteor trail scatter the radio wave independently, creating a received signal which rises very sharply when the meteor trail forms, but then rapidly decays exponentially as the trail diffuses. Most meteors forming underdense trails will be just at, or below, the magnitude level seen by the naked eye. Less frequent than the underdense trail "pings" will be the louder "bongs" of the overdense trails. These trails are above the critical electron density, and reflect the radio wave as a unit. generally, this causes a relatively slower signal strength rise (compared to the underdense trail) up to some sustained peak level which usually last for only a few seconds but can stretch on to several minutes. Following this, the signal level will decay back to the noise level in the same gradual fashion. Overdense trails are generally caused by meteors which can be easily seen with the naked eye. The third type of event which can be heard is the oscillating overdense trail. As soon as it is formed, a meteor trail undergoes twisting and scattering by upper atmospheric winds. An overdense trail may be broken apart to form separate "glints," each capable of reflecting radio waves. These moving patches will create an oscillating diffraction pattern at the receiver, with the received signal strength fluctuating in a pulsating pattern during the duration of the event. Because Sporadic E and D-layer scatter can also create such oscillating events, distinguishing true meteor trails of this type can be difficult. During times of light sporadic E and D-layer scatter, the listener will often hear "waves" of such oscillating events in the receiver background noise. Very infrequently, the rapidly descending tonal pitch "whistle" or "whoop" of a meteor head echo can be heard. This is caused by the compressed and rapidly expanding ionization around the meteor head itself reflecting doppler shifted radio waves as the meteor descends through the atmosphere. Occasionally, the meteor head echo will be accompanied by a loud underdense "ping" or overdense "bong" as the meteor head reaches the first fresnel zone and causes a specular reflection as well as a meteor head reflection. The first fresnel zone is, roughly speaking, the primary point at which the meteor trail meets the requirements of forward-scatter geometry for a particular system. Those participants desiring further explanations should consult the lists in the appendices. Due to frequency effects, it should be noted that receiver systems which operate in the FM band (88-108 MHz) or higher will generally only be able to "see" overdense trails, with very few, if any, underdense trails detectable. Additionally, due to propagation angle effects, longer range systems will have a higher probability of detecting fainter events than short range systems. This is because the increase in signal attenuation due to increased distance is counteracted by a sharper decrease in signal attenuation at the reflection point. That is, signal loss at the meteor trail is less at lower propagation angles (lower trail reflection incident angles). At some point beyond about 1000 km distance between transmitter and receiver (depending upon the system), the attenuation due to distance again becomes the limiting factor. 5.2.3 Automated Meteor Event Detection A. Original Goals The following is an excerpt from the original (Meisel, 1977) version of this bulletin, stating the goals for automated detection: "In the AMS meteor survey program, we desire to obtain not only raw meteor counts, but meteor magnitude distributions to correlate with present and past visual and telescopic (optical) observations. Because of diffraction effects from a meteor trail, one meteor can produce several successive signal maxima. It is not easy to produce an automatic device which will (a) discriminate against background interference, (b) adjust for possible doppler shifting of received signal due to meteor motion (head echo), (c) count meteor events according to the received signal strength while ignoring diffraction effect peaks from the same meteor, and (d) allow reliable measurements to be made over a wide range of signal strengths." (p. 11) Producing an automated system which meets these requirements has been an ongoing project, with efforts continuing in this area. The following is a brief description of these efforts. B. Non-computerized automated Event detection. Early amateur radiometeor systems used a strip chart recorder to record receiver signal strength level over a period of time, without regard to detecting individual meteor events. This method, however, generated copious rolls of paper recorded events, all requiring data reduction by hand if they were to be of scientific value. Because of this, such systems could only be left in operation for short periods of time, such as during major meteor showers. The first automated data collection system designed by amateurs for continuous, long-term data collection was operated by the AMS Kansas Meteor Group in the late 1950's (Houston, 1958). This system employed an automatic event counter, which would add one event to its counter whenever the signal level crossed a predetermined threshold. Using this system, the group monitored the sporadic meteor flux over a period of several months. This threshold crossover detection method forms the basis for most forward-scatter automated detection systems used today, although other methods are available. C. Computer Automated Event Detection The computer automated detection method currently being used by the AMS system employs a floating, selective threshold crossover detection method (Richardson, 1996a). In short, the computer software monitors the output from the receiver, looking for the sudden, rapid signal rise indicative of a meteor event. The software contains the following parameters: (1) All detected events must rise greater than a preselected threshold value above the receiver background noise level. If, for whatever reason, the receiver background level floats above the low "peg" for the system, the event threshold will float along with it. The threshold value is chosen by the user when the software is started, and is selected to optimize a particular system's performance. (2) Long duration events (greater than 2 minutes) are discarded by the system. Meteor events of this duration are very infrequent, while other propagation modes, such as high aircraft (for short range systems), sporadic E, and D-layer scatter can all generate similar long duration events. (3) Oscillating events with periods of less than 3 seconds are generally detected only once. Because of the high variability present in oscillating events, further development will be necessary to perfect this feature. (4) Radio frequency interference (RFI), including lightning events, are ignored by the system through the use of a second receiver to detect such events. When the computer detects an event occurring on both receivers simultaneously, the event is ignored. (5) Further event discrimination occurs during the statistical analysis of the collected data at the University of New York. All questionable events are discarded. D. Meteor Event Data There are currently two types of data being collected by amateur operated data collection systems; full signature recording and parameterized data collection. The first type of system stores the full digitized signature for each meteor event detected, similar to a strip chart recorder. Because this method requires large amounts of disk space, followed by non-statistical analysis steps by the researcher, the AMS project opted for a different approach. Instead of recording full signatures, the AMS software stores only specific parameters about each detected event. This approach requires much less disk storage space, and permits direct computer statistical analysis of the collected data. Currently, three parameters are collected for each event: event occurrence time (epoch), signal amplitude, and event duration. More parameters are planned to be collected by future software versions, but it has been found that these few parameters allow a surprising amount of information to be inferred from them. 6.0 PHASE III: SYSTEM TESTING AND OPERATION Once the radiometeor system has been successfully established it is ready to undergo final grooming and testing, followed by full-time system operation. 6.1 System Grooming, Testing, and Calibration Following establishment of the radiometeor system efforts should focus on grooming the system to achieve the lowest possible background noise level; the best system sensitivity and signal level range; and continuous, reliable equipment operation. Two basic system tests will be performed when the system is ready, a meteor event verification test, and a system calibration test. 6.1.1 Meteor Event Verification Test Several methods will be used to verify that the system is properly receiving and detecting meteoric radio events: A. Meteor Signature Audio Analysis Audio recordings of receiver events should be sent to the University of New York for doppler analysis. The frequency doppler shift of meteoric signals will be extremely high, indicative of meteors entering the atmosphere at tens of km per second. With the rare exception of reentering space debris, these doppler signatures cannot be artificially produced. B. Meteor Signature Power Analysis: Signal Strength vs. Time signatures for numerous events should be observed and printed. These are to be compared with the expected signatures for underdense, overdense, and oscillating overdense trails described in the professional texts. The signatures should show the proper rise and decay characteristics. C. Diurnal Curve Verification Histograms from the data should clearly demonstrate a diurnal curve with a minimum in the evening hours, and a maximum in the morning hours. This diurnal variation will differ from system to system. D. Annual Curve Verification Histograms of the long term data should show an annual curve with a maximum around July-September, and a minimum around February-March. 6.1.2 System Calibration Test When all system grooming has been completed and the participant is ready for "hands off" continuous data collection, a system calibration test will be performed. The purpose of this test is to calibrate the signal levels recorded by the computer to the actual dBm or microvolt levels detected at the antenna. To perform this test, the participant will need to gain access to a VHF signal generator capable of outputting a signal in at least 2 dB increments, and values ranging from about -130 dBm to -50 dBm. This is a general guideline only, and some deviation from this is acceptable. The signal generator (SG) is connected via coaxial cable to the radiometeor system in the signal path prior to the filters and pre-amplifier, and as close to the antenna as possible. If a long test cable is used, the signal loss of the test cable will need to be determined and taken into account. An alternative method is to use a length of test cable equivalent to the cable coming from the antenna, taking the antenna cable out of the line and placing the test cable from the SG in its place. With either method, values obtained from the test should be the dBm values present at the antenna output point. The basic procedure for performing the test is to document three calibration "runs," recording computer numeric values for various signal generator output levels. Each run is performed from the software "low peg" value to "high peg" value and back again, recording computer levels along the way in as small a dB increment as the SG will allow. A software program is included which allows the determination of numeric signal values rather than graphical ones. if the computer "low peg" value for the system is above zero, this value must be subtracted from all data. The result will be a table containing six columns of data for various SG dBm values, ranging from a 0 signal strength value to the "high peg" value. A second portion of the calibration test is the system response time check. This test checks the system response to a sudden signal amplitude step change from low to high and back again. To prepare for this test, obtain a stopwatch, and set up the signal generator such that a sudden step change can be made from some signal level below the system's low peg, to some signal level above the system's high peg. To perform the test, time how long it takes for the system to change from low peg to high peg when the step change is introduced by the SG. Next, time the response to a sudden step change back to the low value. Make several runs to find an average time for each. If the system response is less than can be timed, record < 0.2 seconds as the response time. In conjunction with the calibration test, the participant should record all of the parameters for the station, including: 1. Antenna type, height, bearing, and tilt angle. 2. Cable type and length between antenna and receiving equipent. 3. Filter specifications. 4. Preamplifiers specifications. 5. Frequency converter specifications, if used. 6. Receiver specifications, including sensitivity and bandwidth. 7. Receiver front panel settings. 8. Optocoupler settings. 9. Computer software settings. Once a satisfactory calibration test is achieved, no further adjustments can be made to the system since doing so will invalidate the calibration. If adjustments are required at a later date, then additional calibration tests will need to be performed. Along with the initial calibration for the receiving station, the participant should also obtain detailed information about each transmitter station being utilized. If this cannot be obtained through published material, the Engineering Department for the station itself should be contacted. Information obtained should include: 1. exact transmitter location and ground elevation 2. tower and antenna height 3. antenna type and directional coverage 4. Transmitter power level 5. Signal polarization and modulation 6. Exact frequency and tolerance maintained. 7. Transmitter operating schedule. 6.2 System Full-Time Operation and Maintenance It is desired that the system be maintained in continuous operation to the fullest extent possible. Realistically, however, occasional outages will occur, and equipment maintenance and upkeep will be required. Brief power flickers will cause the computer system to halt, requiring restart of the software. Storms can damage outside antenna equipment, requiring repair. Electronic failures can occur in both the receiver or computer equipment, and mechanical failures can strike cooling fans and disk drive systems. In addition, the system can be expected to also gradually degrade in performance over time. Beyond emergency maintenance the participant will need to use his or her own judgement in determining what regular maintenance items will help keep the system at its best long-term performance. Suggestions for regular maintenance include: (1) Annual calibration test of the system to check system performance and make adjustments, if necessary. (2) Annual disassembly and cleaning of all outside cable connectors, including those at the antenna. Cables should also be inspected for signs of brittleness and aging, replacing if necessary. (3) Along with cleaning the antenna cable connectors, The antenna should be inspected at close range and, if necessary, it should be taken down, cleaned of corrosion and repaired. (4 All outside grounding system connections and hardware should be taken apart and cleaned annually. Other regular maintenance should be done at the discretion of the participant. It should be remembered that if maintenance is done to any components which would affect the calibration of the system, then a new calibration test should be performed. It is highly recommended that participants keep a written log book, documenting details about their system. Log entries should record, but not be limited to, system progress; problems and setbacks; equipment changes; antenna, receiver, and computer settings; and calibration test results. Such a record will aid greatly in tracking the establishment and operation of the station, as well as providing a quick reference if questions should come up. Participants are also encouraged to write short articles about their experiences for inclusion in Meteor News. This will not only help give much deserved recognition to our participants, but also keep the general AMS membership appraised of the status of the Radiometeor Network. Once a system has been declared operational by the project coordinator, participants should send copies of the data collected by the station to AMS headquarters on a quarterly basis. Under special circumstances, such as unusual shower activity, participants may be requested to copy and send in their data at a more frequent interval. Participants should keep and maintain their original data diskettes as a data archive, and diskettes should not be erased or re-used. 7.0 FINAL COMMENTS As can be seen from the preceding sections, participation in the AMS Radiometeor Project requires considerable time, effort, and commitment on the part of our participants. Major funding for each station is provided by the participant, with the AMS providing limited help in the acquisition of equipment, especially for the computer system. Participants should enter this project with a sincere desire to contribute to professional scientific research on meteors and their parent objects - comets and minor planets. The information and procedures given above are provisional and subject to revision as experience is gained. Comments and suggestions are solicited by the AMS staff, but enthusiasm for either extremely high or extremely low tech solutions must always be tempered with a dose of reality. Casual radio meteor detections, like casual visual meteor sightings, while satisfying for the individual are only of limited value to our long-range goals. All contributions by participants will be acknowledged in our annual reports, applicable scientific papers, and appropriate AMS publications. Emphasis will be concentrated on those efforts which conform to AMS recommendations. Good Luck! Compiled and Revised by: James Richardson Dr. D.D. Meisel AMS Radiometeor Project Coordinator AMS Executive Director Route 2, Box 118 Department of Physics and Astronomy Graceville, FL 32440 Geneseo, NY 14454 E mail: RICHARDSON@DIGITALEXP.COM MEISEL@UNO.CC.GENESEO.EDU APPENDIX I: Forward-Scatter Fundamentals The following sections are provided to give the reader a very basic introduction to meteor radio scatter principles. The primary references used are McKinley (1961) and Davies (1965). Readers are encouraged to investigate further on their own. A list of further references is provided in the appendices which follow. A1.1 Introduction to Meteor Radio Scatter Whenever a meteor passes through the upper atmosphere it creates a column (actually an elongated paraboloid) of ionized air behind it, usually less than 1 meter in diameter and tens of kilometers (km) long. This is called a meteor trail. Occurring at an atmospheric height of about 85 to 105 km (50 to 65 miles), the free electrons in this ionized trail are capable of reflecting radio waves from transmitters below on the Earth's surface. This type of reflection behaves very similar to light reflecting from a mirrored surface, and is called a specular reflection. Meteor trail reflections are brief, however. As the trail rapidly diffuses into the surrounding air, it quickly looses its ability to reflect radio waves, causing most reflections to last less than 1 second. Occasionally, a large meteor may create a trail capable of reflecting radio waves for up to several minutes. Meteor radio wave reflections are also called meteor echoes or events. if the radio waves from the transmitter below strike the meteor trail at a perpendicular, or right angle, then the reflected signal will be directed back towards the transmitter itself. This is called back-scatter, and a station set up to receive such signals is called a meteor radar. Back-scatter is one of the most common methods by which professional astronomers study meteors using radio waves. These reflections can be used to study the meteors which caused them because each meteor will generate a unique signal based upon its mass, velocity, angle and direction of entry into the atmosphere, and distance from the transmitter. If the radio wave from the transmitter strikes the meteor trail at some incident angle other than perpendicular, then the reflected signal will be projected to some point on the ground some distance away from the transmitter. This is called forward-scatter, and the area on the ground where the signal has been reflected is called a forward-scatter footprint. This footprint will only be a few km wide and several km long. Meteor trails can reflect radio signals over distances of up to 2000 km (1200 miles) between a transmitter and receiver. most forward-scatter systems, however, operate at a distance of about 300 km to 1500 km (175 to 925 miles). The forward-scatter of radio waves by meteor trails can serve two important purposes: First, the trails can be used to send brief encoded messages to distant receiver sites. This is called Meteor Burst Communications, and is frequently used as a backup means to satellite communications. In North America, the most widely known meteor burst communications system is the SNOTEL system, used by the U.S. Department of Agriculture to monitor rain and snowfall levels at remote stations throughout the Rocky Mountains. The second purpose for forward-scatter is the study of meteors, and is similar to that performed by the back-scatter systems. The advantage of this method is that transmitters broadcasting for purposes other than meteor radio scatter can be utilized, thus allowing the construction of a receiving station only by the researcher. The disadvantage of this research method is that the geometry is much more complex than in the back-scatter condition, making meteor parameters more difficult to determine. Through the use of commercial radio transmitters, amateur astronomers have also begun to successfully establish forward-scatter receiving stations of their own. While most of these amateur stations are for the purpose of enjoyment only, a handful of stations have been established for the purpose of collecting data usable by the professional astronomical community. A1.2 The Meteor Scatter Signal The various types of echoes or signals produced by meteors are discussed in Section 5.2.2. The listed references also discuss these signals in great detail. A1.3 Meteor Scatter Geometry Meteor scatter systems are very sensitive to the position of the meteor trail in the atmosphere as well as the trail aspect: its angle of entry and direction of travel. In order to cause a successful reflection, very specific geometry requirements must be met. In the back-scatter radar, the transmitter and receiver are located in very close geographic proximity to each other. In order to cause a radio reflection, the meteor trail must lie within a plane which is tangent to a sphere having the transmitter-receiver as its center. The path of the radio wave moving out from the transmitter, reflecting from the trail, and moving back to the receiver will mark the radius of this sphere. Meteor trails detected by back-scatter must meet the above geometry requirement, and also lie within the band of atmosphere most suitable for proper ionization, about 85 to 105 km height. The closest meteor trails detected in this manner will be about 90-100 km distance, directly overhead from the transmitter-receiver, and traveling in a plane horizontal to the ground. The most distant meteor trails detected will be about 400-500 km distance from the transmitter-receiver, located near the horizon, and traveling in a plane nearly vertical to the ground. In the forward-scatter system, the transmitter and receiver are located at some geographic distance from each other. In order to cause a radio reflection, the meteor trail must lie within a plane which is tangent to an ellipsoid having the transmitter and receiver as its focus points. The path of the radio wave moving out from the transmitter, reflecting from the trail, and moving back to the receiver will lie within a plane, called the plane of propagation, which is perpendicular to the tangent plane containing the meteor trail. This is an oblique reflection, with the radio wave being reflected from the trail at the same incident angle at which it struck the trail. The forward-scatter system is also sensitive to the aspect of the meteor trail within the tangent plane. A meteor trail which is perpendicular to the plane of propagation will generate a stronger, longer duration echo than a meteor trail which is parallel to the plane of propagation. Meteor echoes can occur from areas of the sky ranging from the area behind the transmitter to the area behind the receiver, and to either side of the great circle path between the two. The strait line connecting the transmitter to the receiver is called the baseline. Viewing this baseline in longitudinal view, the above geometry requirements dictate that reflections from trails occurring over the midpoint between transmitter and receiver lie within a tangent plane which is horizontal to the baseline, that is, having a very low elevation angle. Moving outward from the midpoint towards either transmitter or receiver causes the tangent plane elevation angle to increase until the tangent plane of the meteor trail must be nearly vertical to cause a reflection behind either transmitter or receiver. In the cross-sectional, or lateral view, the elevation angle of the tangent plane for a midpoint reflection must again be horizontal. Moving out to either side of the baseline midpoint again causes the required meteor trail tangent plane elevation angle to increase with distance from the midpoint. The result of this geometry in the actual meteor flux is that most meteor reflections in the forward scatter system will be generated from two broad "hot spot" regions, located about 50 to 150 km to either side (laterally) of the baseline midpoint for the system. This geometry also causes the system to be sensitive to meteors from radiants in particular areas of the sky. These radiant areas will be located perpendicular to the azimuth of the transmitter-receiver baseline, at about 30 to 60 degrees elevation. If the transmitter-receiver baseline runs north-south in azimuth, then the eastern radiant region will generate meteors which cause reflections in the western-most "hot spot." The radiant region in the west will generate meteors which cause reflections in the eastern-most "hot spot." A1.4 Sporadic Flux Variations In addition to the shower aspect sensitivity of the forward-scatter system, discussed in Section 4.2.7, The forward-scatter system is also affected by the uneven sporadic meteor radiant distribution as well. Due to the low inclination of most sporadic meteor orbits, the sporadic meteor radiant distribution is clustered near the ecliptic. In the northern hemisphere, the ecliptic is generally located in the southern portion of the sky, affecting hot spot illumination in forward-scatter links. The east-west link is the most affected, with the northern hot spot being far more active than the southern hot spot. For links with NW-SE or NE-SW baselines, this effect is not as pronounced, however, the more northern hot spot will still show more activity than the more southern hot spot. The system with a north-south baseline will show fairly equal activity from both hot spots on the long-term, but is the most greatly affected by the diurnal fluctuation in the sporadic rate. In the evening hours, when the link is on the trailing edge of the Earth, both hot spots are relatively quiet. By midnight, however, the western-most hot spot will become more active due to sporadic meteors originating from the Earth's apex direction. In the morning, both hot spots will be equally active and at the peak for the day as the apex point is highest in the sky. By noon, this activity has quieted, with the eastern-most hot spot now the more active one. When evening comes, the cycle begins again. A1.5 Meteor Velocity Considerations In order to cause a forward-scatter event, the meteor must generate a specific density of free electrons within the trail. At altitudes of greater than about 105 km, the electron density produced is too low and the diffusion rate too rapid to cause many detectable events. Below about 85 km, the density of the atmosphere becomes high enough to cause rapid attachment of the free electrons, also with a corresponding loss of meteor detectibility. The optimum altitude for forward-scatter detection is centered at about 95 km, where the ionization and diffusion/attachment effects are most equally balanced. Meteor velocity, mass, and angle of entry into the atmosphere all affect the ability of the meteor to cause a forward scatter reflection. Meteor velocity will affect the height of ionization and the path length, with faster meteors ionizing higher in the atmosphere and having longer path lengths. Meteor mass will affect the free electron line density of the trail and path length, with higher mass meteors producing the highest densities (strongest signals) and longest path lengths. Meteors with a high inclination will penetrate further into the atmosphere than those with low inclinations, but high inclination meteors will have shorter path lengths than those entering at low inclination. These parameters will have the following effect on the forward scatter system: Fast meteors have a tendency to ionize higher than the optimum altitude band, causing the forward-scatter system to discriminate against the lower magnitude, fast meteors. Only the brighter fast meteors will penetrate deep enough into the atmosphere to get below the "height ceiling" and cause reflections. On the other hand, slow meteors have a tendency to ionize below the optimum altitude band, causing the system to discriminate against the higher magnitude, slow meteors. Only the fainter slow meteors will ionize fully in the correct region to cause full reflections. Bright, slow meteors may begin their ionization in the proper region, but then pass under the "height floor," causing only a partial, low duration reflection. Along these same lines, very bright, slow fireballs which have the potential for dropping meteorites will generally also not cause noticeable reflections. These fireballs often do not produce visible light and ionization until they are below about 70 km, making them rather "stealth" to radio detection. Meteors of medium velocities tend to have the best forward scatter capability throughout their magnitude range. A1.6 Forward-scatter by Aircraft Aircraft travel at about 1/20th the altitude of meteors, and at about 1/125th the velocity. This gives them radically different reflective characteristics when compared to meteor trails. In order to cause a radio reflection an aircraft must have both the transmitter and receiver within its line of sight "radio horizon." The minimum altitude for a reflection is lowest at the transmitter-receiver baseline midpoint, with the minimum altitude about four times as high over either the transmitter or receiver. A few examples of the minimum midpoint altitudes are: (link distance, min. altitude) 1000 km, 20 km (65,000 ft) 750 km, 11 km (36,000 ft) 500 km, 4.9 km (16,000 ft) 250 km, 1.2 km (4000 ft) 125 km, 0.3 km (1000 ft) The shorter the distance between the transmitter and receiver, the more levels of air traffic that become available to cause reflections, over a broader geographic area. In experimenting with this effect, the AMS has found that aircraft reflections generally affect (to any significant degree) only those systems having a baseline of less than about 250 km (150 miles). This is not to say that aircraft will never cause reflections on systems of larger baselines; however, these reflections will constitute only a very small minority of the overall total events. Due to their slow speed, aircraft tend to have much longer reflection durations when compared to meteors. The aircraft will simulate an extremely slow "head" echo, with its associated oscillating diffraction pattern. The stronger the normal atmospheric scatter signal is at the receiver, the more pronounced these oscillations will be. This will create a symmetrical signal of a series of either accelerating or decelerating oscillations. The center of these oscillations is usually occupied by a large peak which last from about one to fifteen minutes. The higher the operating frequency, the more likely it is for an aircraft to cause a reflection. This is the opposite from meteor reflections. Thus, frequencies in the commercial FM band or higher have the highest degree of aircraft interference. A1.7 Other Propagation Modes Davies (1965) provides the most useful descriptions of the other forms of propagation which can affect the VHF range. The following list is provided as a very brief summary only. A. Atmospheric Scatter The term "atmospheric scatter" has been used throughout this bulletin to generically classify the various forms of scatter propagation. Atmospheric scatter can be roughly divided into three primary forms: (1) Tropospheric Scatter This is scattering via the lowest layer of the atmosphere, and is affected by weather conditions. Colder temperatures provide more effective scattering, and the passage of a cold front can be told from the rise in tropospheric scatter signal levels. This form is most prevalent on short range forward-scatter systems. Natural barriers between transmitter and receiver, such as mountain ranges can be used to reduce this propagation mode. (2) D-Layer Scatter In a solar related phenomena, the lowest, D-layer of the ionosphere can occasionally scatter VHF radio waves back strong enough to cause spurious propagation. D-layer scatter is caused by turbulence and wind shear in the level of the atmosphere just below the meteoric region, between 70 and 90 km. This effect generally occurs between 1200 and 1600 hrs local time, and will appear as long overdense and oscillating overdense events. Due to its solar dependency, D-layer scatter is worse in summer than in winter. (3) High-powered Ionospheric Scatter This scatter mode can occur when transmitter powers of greater than 25 kW are used, and should generally affect only those forward-scatter systems utilizing commercial transmitters in the low VHF Television and FM Commercial bands. Lower frequencies will be more affected than higher frequencies. This mode will not cause spurious events, but will instead provide a nearly continuous low-level signal from the transmitter, usually at or below the natural background noise level. This scatter source is meteoric in origin, and is created by the myriads of extremely faint magnitude meteors in the E region. B. Ionospheric Propagation (1) Sporadic E (Es) This propagation mode is discussed in Section 4.2.3. (2) Equatorial Spread f this propagation mode occurs primarily in equatorial regions, during times of high sunspot activity. Generally occurring below 20 degrees of latitude, peak activity occurs at night, and is highest around the time of the autumnal equinox. These reflections occur in the F2 region of the ionosphere, greater than 120 km height. (3) Radio Aurora In high northern latitudes, aurora activity takes the place of Sporadic E as the most common secondary source of propagation next to the desired meteor activity. The aurora activity is associated with solar flares and magnetic disturbances. It occurs at a height of 75 to 135 km, with a mean of 110 km. This activity can occur in daylight and is not directly associated with visible aurora. C. Other Atmospheric Effects (1) Temperature Inversion Ducting Occurring primarily in winter, a temperature inversion will not only trap a layer of cold air beneath a layer of warm air, but will also refract radio waves. If the inversion layer extends over a large geographic area, VHF radio waves can travel long distances along the duct formed between the ground and the inversion layer. This will usually cause the receiver to be swamped for several hours, primarily during the morning hours, but occasionally extending into the afternoon. (2) Polar Cap Absorption (PCA) While not a propagation mode, PCA can greatly affect the ability to receive meteor echoes. After a sudden ionospheric disturbance (SID) begins, absorption of radio waves by the lower atmosphere can become severe. The lower HF frequencies will be affected first, called an HF "blackout." At low latitudes, the meteor scatter frequencies will generally not be affected, and may even see an enhancement of meteor event signal levels. In upper latitudes, however, the absorption blackout can extend to the VHF frequencies, preventing all meteor scatter, especially during daylight hours. This effect can last for several days. APPENDIX II: Link Bearings/Offsets for Meteor Showers Suggested Station Location/Directions for Monitoring Tests for Northern Hemisphere Meteor Showers Using Radio Scatter and Directional Antennas DATES SHOWER MOST DESIRED STATION DIRECTIONS Jan. 1-4 Quadrantids N to NW or S to SE May 1-6 Eta Aqr NE to E or SW to W June 1-15 Arietids (Daytime) NE to E or SW to W July 26-31 d Aqr E to SE or SW to W Aug. 10-14 Perseids NW to E or SE to W Oct. 18-23 Orionids NE to SE or SW to NE Nov. 14-18 Leonids NE to E or SW to W Dec. 10-14 Geminids NE to E or SW to W Dec. 21-23 Ursids NE to E or SW to W Table of offsets to be added to the above* RANGE (km) TEST ALTITUDE TEST AZIMUTH OFFSETS 50 44 75 100 41 62 150 38 51 200 34 43 250 30 37 300 27 32 350 24 29 400 22 26 450 20 23 500 18 21 550 17 20 600 15 18 650 14 17 700 13 16 750 12 15 800 11 15 850 10 14 900 9 14 950 9 13 1000 8 13 1050 8 12 1100 7 12 1150 6 12 1200 6 11 1250 6 11 1300 5 11 1350 5 11 1400 4 11 1450 4 10 1500 4 10 2000 1 10 *If the meteor shower to be monitored has a radiant of southern declination (minus) the azimuth off-set should be to the north, if the radiant has north (plus) declination off-set should be to the south. APPENDIX III: VHF Frequency Lists and Information 1) FM Atlas and Station Directory Bruce F. Elving P. 0. Box 24 Adolph, Minnesota 55701 2) World Radio-TV Handbook Jens M. Frost, Editor P. 0. Box 88 2650 Hvidovre, Denmark 3) Worldwide TV-FM DX Association P. 0. Box 163 Deerfield, Illinois 60015 4) Handler Enterprises P. 0. Box 253 Deerfield, Illinois 60015 APPENDIX IV: Amateur Radiometeor References A4.1 AMS Bulletins and Publications Meisel, D.D., (1977, January). The AMS Radio Scatter program, AMS Bulletin 203. University of New York At Geneseo. Meisel, D.D., (1982). Radiowave Scatter Detection of Meteors Using VHF Aeronautical Beacons, Supplement to AMS Bulletin 203. University of New York at Geneseo. Meisel, D.D., (1987). New Approaches to Some Methodological Problems of Meteor Science. presented at the First GLOMET Symposium, August 1985,Dushanbe, Takijistan,pp.389 - 404 in Middle Atmosphere Program, Handbook 25, ed. R.G. Roper,published by ICSU SCOSTEP. Richardson, J.E., (1996, September). The Poplar Springs Radiometeor Station: A General Description. The American Meteor Society, State University of New York at Geneseo. Expanded and revised version of previous short article. Richardson, J.E., (1996, December). Apple IIe Meteor Burst Software Users Manual, Version 4.0. The American Meteor Society, University of New York at Geneseo. A4.2 Amateur Radiometeor Efforts Black, W.H., (1983, July). Observing Meteors by Radio, Sky & Telescope, pp.61-62. Fukuda, M., (1982). FM Radio Observation Report No. 2, Radio Meteor Res. No. 9, pp 78-101. Houston, W.S., (1958, July). The Amateur Scientist: Counting Meteors by Radio, Scientific American, pp 108-111. Describes work of W. S. Houston at Manhattan, Kansas, with the AMS Kansas Meteor Group. Houston, W.S., (1960, April). The Great Plains Observer, Vol. 3, No. 5. out-of-print. Jarrell, D.and Mike Morrow (1992,Summer,Radiometeor Detection: Hints for Changing From 75 MHz to 50 MHz, No.98, p. 5 Lynch, J.L., (1992, August). A Different Way to Observe the Perseids, Sky and Telescope, pp. 222-225. Mason, J., (1994, February). Tuning in to meteor showers, Astronomy Now. Mackenzie, R.A., (1974), Memoirs of British Meteor Soc No. 1. Report on Amateur Radio Meteor Work, 1969-1974. Owen, Michael. (1986, June). VHF Meteor Scatter, an Astronomical Perspective, QST, pp. 14-20. Pillon, K., (1984, May). Computer Adventures: Meteor astronomy at home, Popular Science, p. 80. Richardson, J.E., (1993, Fall). The Poplar Springs Meteor Patrol: A General Description, Meteor News, No. 102, p. 5 Riggs, James (1996), Implementing the Richardson Radiometer System, No. 111, p. 1 Suzuki, K., Nagafuji, N., & Kinoshita, M., (1976, May). Sky and Telescope, 51, P. 362. A4.3 Meteors and Propagation (Advanced) Al'pert, Y.L., (1973). Radiowave Propagation and the Ionosphere, 2nd Ed. Trans. Rodman Consultants Bureau. Davies, K., (1965). Ionospheric Radio Propagation. NBS Monograph 80. (Reprinted by Dover Pub., 180 Varick St.) Chapter 8 includes forward scatter. Lovell. A.C.B., (1954). Meteor Astronomy. Oxford: Clarendon Press. Excellent summary of both optical and radar work/theory up to 1952. McKinley, D.W.R., (1961). Meteor Science and Engineering. McGraw-Hill Book Co. Nearly complete bibliography to literature up to 1960. (Excellent - includes forward scatter.) Schanker, Jacob Z., (1990). Meteor Burst Communications. Norwood, MA: Artech House. Introduction to the engineering aspects of forward-scatter links. Schilling, D.L., (1993). Meteor Burst Communications (Theory and Practice) Wiley Series in Communications. A4.4 Symposium Results (Advanced) Kaiser, T.R., (ed) (1955). Meteors, Spec. Supplement to J. Atm. and Terrestrial Physics (Vol. 11). Kresik. L. & Millman, P.M. (1968). Physics and Dynamics of Meteors, I.A.U. Symposium No. 33, D. Reider Publishing Co. Dordrect. Holland. APPENDIX V: General Radio Astronomy References A5.1 Radio Astronomy Hey, J. S., (1973). The Evolution of Radio Astronomy. Neal Watson Academic Publications. Historical, short discussion of meteor scatter. Heiserman, D, (1975) Radio Astronomy for the Amateur. TAB Books, No. 714, Blue Ridge Summit, Pa. 17214. Contains good pages of optical astronomy review (some parts, not always error-free). One chapter on radio astronomy theory (with math). Remainder is devoted to specific construction projects. No mention of meteor studies. Heywood, John, (1964). Radio Astronomy and How to Build Your Own Telescope. Hyde, F.(1963). Radio Astronomy for Amateurs. Norton W.W., Lichtman, J. & Sickels, R.M. (1975). Amateur Radio Astronomers Notebook. order from Bob's Electronic Service, 817 N. Andrews Avenue, Ft. Lauderdale, Florida 33311. Good starter volume for transistorized equipment construction projects. Relatively expensive ($20 in 1976), but if all circuits are constructed properly, will save the amount in blind alleys. Only criticism is contents of Chapter 13 of first edition (which is devoted to interstellar communication) where a subject of professional radio astronomy is mentioned in too much detail. Lovell, B. and Clegg, J. A., (1952). Radio Astronomy. Dated but with good account of early techniques particularly of meteor echos (some math). Swenson, G.W., Jr. (1980) An Amateur Radio Telescope, Pachart Publishing House, Tucson. Swenson, G.W. Jr. and S.J. Franke, An RF Converter for Amateur Radio Astronomy, Sky and Telescope Vol.58,N0.5, 422 A5.2 Radio Broadcast Guidebooks Bennett, H. (1974). The Complete Shortwave Listener's Handbook. TAB Books No. 685, Blue Ridge Summit, Pa. 17214. Chapter on TV DX-ing, P. 190-200. - Mentions video carrier injection (VCI) to enhance audio reception of TV. Brier, H.S. & Orr, W.I., (1974). VHF Handbook. Radio Publications, Inc., Box 149, Wilton, Conn. 06897. Amateur radio with equipment, antennas and brief mention of meteor propagation on p. 67-69. Frost, J.F. (ed) (1971). How to Listen to the World, World Radio-TV Handbook. 2650 Hvidovre, Denmark. Article on FM Reception by Jack White, pp. 106-115 - Article on TV DX by Glenn Hauser, pp. 117-126 - Hauser's article introduces (VCI) and gives detailed instructions on split channel reception. Also mentions using paging stations on 35.22 MHz, 35.58 MHz, 43.22 MHz, and 43.58 MHz. MacKinnon, E.G. (1968). VHF Ham Radio Handbook. TAB Books, Blue Ridge Summit, Pa. 17214. Fairly elementary equipment (tube type only) described. No mention of meteors in propagation discussion A5.3 Radio Equipment Green, W. (1972). VHF Projects for the Amateur and Experimenter. TAB Books, No. 608, Blue Ridge Summit, Pa. 17214. VHF equipment instructions including pre-amplifiers and converters (tube and transistor types). Orr. W.I., (1974). Beam Antenna Handbook. Radio Publications, Inc., Box 149, Wilton, Conn. 06897. Orr, W.I., (1974). All About Cubical Quad Antennas. Radio Publications. Inc., Box 149. Wilton, Conn. 06897. Schultz. J. (1972). Understanding and Using Radio Communications Receivers. TAB Books, Blue Ridge Summit, Pa. 17224 General description of radio receivers and their operation.