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(meteorobs) Radio Observation of Meteors (very long)
Hello all,
To help answer some recent questions on the topic, here is a long excerpt
(Section 4.2) from the American Meteor Society document describing the
Radiometeor Project, which discusses the selection of an appropriate
forward-scatter frequency/transmitter. I have the full document available
as an ASCII text file for those who are interested further. There is also
some additional information available at the AMS web site on the project,
including 5 *.wav files demonstrating the various types of meteor echoes
from low VHF television transmitters. See:
http://www.amsmeteors.org
As an FYI, I have also recently become the chair person on radiometeor
monitoring for the Society of Amateur Radio Astronomers (SARA), handling
requests for information from enquirers to that organzation. Thus, you get
me via that route also. But, there are also several other good sources of
good information on the web, many of which are linked from the AMS site.
As an example, try the innocuous little MS (meteor scatter) link on the AMS
Radiometeor Project page...
Best regards,
Jim
-------------------------------------------------------------
Excerpt from:
Richardson, J.E., and Meisel, D.D. (January, 1997). "The AMS Radiometeor
Project," AMS Bulletin No. 203 (Revised). The American Meteor Society,
Ltd.: Geneseo, New York.
-------------------------------------------------------------
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, digitally controlled standard
FM receiver with the AGC or Power Level meter circuit voltages
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.
-------------------------------------------------------------
James Richardson
Tallahassee, Florida
richardson@digitalexp.com
Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.amsmeteors.org
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