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(meteorobs) Radio Meteor Obs
In light of the recent thread about radio meteors, I wanted to
add my bit with the FAQs.
Your comments or questions are appreciated, and will help as a
motivation to work on an update :-).
Yours,
Chris
--
Meteors by radio: the FAQs
METEOR SCATTER - the FAQs Rev. 2, Sep 3, 1995.
0. About these FAQs
These FAQs are the result of your various questions and remarks to the
monthly RMOB (Radio Meteor Observation Bulletin), and discussions in the
Radio Astronomy Forum of Compuserve.
Constantly amateur astronomers, radio amateurs and schools looking for
new projects, get involved in observing meteors by radio methods, and
need the same basic information.
0.1. Revision / posting frequency
As promised in Rev. 1, the references have been updated.
Several persons have been observing meteors by radio for the first at the
occasion during the Perseids 1995. Accordingly, the practical sections how
to build a meteor station and to recognize meteor reflections have been
expanded.
Rev. 3 is due in November 1995.
0.2. Distribution
These FAQs are posted on:
sci.astro.amateur
rec.radio.amateur.space
uk.radio.amateur
Compuserve's Astronomy Forum, Section 11, Radio Astronomy.
Please let me know if you feel that other newsgroup are interesting too or
more appropriate.
These FAQs are also e-mailed to the RMOB subscribers (see further).
0.3. Acknowledgement
Many thanks to Tom Ashcraft (72632.1427@compuservedot com) and other
participants to Compuserve's Astronomy Forum for the stimulating
discussions. Jeroen Van Wassenhove (100101.734@compuservedot com) compiled
the larger part of the references, and made other useful comments.
1. What is meteor scatter (MS) ?
Meteor scatter is a form of electromagnetic wave propagation. The ionized
trail of meteors (typically appearing between 100 km and 80 km high) acts
temporarily as a reflector for radio waves.
A meteor does NOT generate detectable radio emission itself!
In the case of forward scatter, the transmitter and receiver are at
different locations. Backscatter can be seen as a special case, whereby
the sender and receiver coincide (typically the case of radar).
Meteor scatter can be used day and night, allowing daylight meteors streams
to be studied.
Meteor scatter can also be used for secure communication.
Historically, meteor streams were conclusively detected by means of radar at
the end of World War II.
2. How to observe MS ?
Tune to an "empty" frequency in the VHF band, and wait until a suitable
oriented meteor trail reflects a distant broadcasting station. You hear a
more or less short (a fraction of a second to tens of seconds) fragment of
the remote broadcast. Occasionally, more than one station (together or one
after the other) can be heard. Routinely, stations of 300 km to 1000 km away
can be observed. The problem however in densely populated areas can be to
find a non-locally used frequency.
2.1. Which frequencies to use ?
The lower the frequency, the better... but at too low frequencies, the
radio- waves bend and follow the curvature of the Earth.
A lower practical limit is 40 MHz. More commonly, the VHF is suitable.
VHF Band I ranges from 41 to 68 MHz, and is used for TV.
The FM band (88 MHz - 108 MHz) is also used frequently. Try as low as
possible in the band, and avoid free radio's in the upper band (104-108 MHz).
Not to forget are beacons at e.g. 50 MHz or 75 MHz.
Radio hams listen also to MS in the 2 m (144 MHz) band. The reflections are
shorter and weaker than at the lower frequencies. Radio amateurs worldwide
have standard procedures for using meteor scatter. They listen or transmit
during well defined periods to well defined areas. For more information,
please contact Bob Bruninga, WB4APR @ KA3RFE.MD
(bruninga@greatlakes.nadn.navy.mil).
There is little use in trying MS at still higher frequencies.
2.2. Which equipment to use ?
A regular FM receiver is sufficient. An all-band receiver is of course
better in terms of sensitivity and features such as bandwidth selection.
Connect a horizontally polarized Yagi antenna with 4 to 6 elements (not
too directive) to your receiver. The antenna can be pointed horizontally,
or at a certain angle (the latter avoids direct reception from nearby
stations). Keep the antenna feed cable short to minimize losses, or add
an antenna amplifier.
With a good setup, it should be easy to obtain 10 to 20 meteor reflections
per hour when no stream is active (the sporadic background). During
streams, this number can get as high as several hundreds.
Due to the daily variation in meteor activity, more reflections are
received during the morning hours (5h to 6h local time) than in the
evening (18h). This can serve as a proof that you are actually observing
meteors!
2.3. Always possible to observe ?
Unfortunately, other forms of propagation interfere with MS.
The worst one is sporadic-E or Es, consisting of conduction clouds in the
high atmosphere that make permanent reception of remote stations possible
during minutes to tens of minutes. The sporadic-E season is from May to
July in large parts of the northern hemisphere. In some regions however,
sporadic-E is unknown!
There are also tropospheric influences. A temperature inversion can also
cause reception over wider ranges than normal.
Thunderstorms cause very sharp peaks. On FM, most of these (amplitude
modulated) spikes are suppressed. However, DO NOT connect your antenna to
your receiver during thunderstorms or when you are absent for longer time.
Lightning strikes have ruined several radio shacks.
2.4. One step further: recording meteor reflections
Meteor reflections can easily be recognized by listening. But a better
measure of the shape of the reflection can be obtained in accessing the
signal strength and recording it, either on a pen recorder, either feeding
it into a computer via an analog/digital (A/D) convertor. Special care has
to be taken to shield well the computer, power supplies, add filters etc.,
or the computer signal will be more or less noticed in the receiver.
Optocouplers can be used to separate the grounds. However, some
sensitivity and linearity will be lost in using them.
Alternatively, the game port input of a PC can be used too. The sampling
rate is lower (say 20 samples per second), but this is often good
enough.
Meteor reflections can broadly be divided in two classes - underdense and
overdense - with different reflection profiles. These profiles can be
related to the physical characteristics of the meteors.
3. Who else is observing ?
During recent years, amateurs in following countries have been recording
meteor reflections: Austria, Belgium, Denmark, Germany, Finland, Japan,
the Netherlands, Portugal, the UK, the USA.
Some of them observe only during the periods of the great streams, others
run continuous automated stations.
Only a few radars are still used for monitoring streams. One is run by
O. Belkovitch (oleg@astro.kazan.su), at Kazan University.
K. Suzuki, Aichi, Japan operates the MU radar wave of Kyoto University
(46.5 MHz, 1 MW) at Toyokawa Meteor Observatory in Aichi, Japan.
4. How to relate radio- to visual observations ?
Hearing and seeing a meteor is a unique experience. There is a fairly
simple relation between the visual magnitude of a meteor and the duration
of the reflection. At 70 MHz, a zero magnitude meteor gives typically
rise to a 1 second reflection. (At higher frequencies, the duration is
shorter).
5. How to correct MS counts to standard conditions ?
The number of meteor reflections that will be observed from a certain
stream is not a simple function of radiant height, as is the case with
visual counts (cfr. the zenithal hourly rate).
Conversely, deriving the true number of meteors from the observed number
of meteors is not a trivial matter, certainly not when more than one
transmitter is involved. Certainly, a stream does not necessarily peak when
the observed number of meteors is highest!
The various parameters, such as antenna characteristics, position of
transmitter and receiver, radiant position and other stream characteristics,
power of the transmitter etc. can be fed into a simulation program, such as
FORWARD (by the undersigned). Although the general activity pattern can be
reconstructed, one has to be careful.
For visual meteors, the observed number is highest when the radiant is in
the zenith (all other remaining the same). This is not so for the number
of meteor reflections: the maximum number is observed for a radiant
elevation of 45 to 50 degr. When the radiant is too high (say more than
80 degr), very few reflections are received!
6. Where to find meteor stream data ?
The International Meteor Organization (IMO), specializing in visual meteor
observations, issues a yearly meteor stream calendar, which can be found
on several places on the Net and on Compuserve. Contact otherwise Juergen
Rendtel (jrendtel@aipdot de) or Peter Brown (peter@canlon.physics.uwodot ca).
Gary Kronk's home page [http://wums.wustldot edu/~kronk/index.html]
gives also a good overview of the various streams.
7. Where to find observational data and reduction software ?
Ilkka Yrjola, OH51Y (ILKKA.YRJOLA@915000.KY.KYMMENE.MEMONET.mailnet.fi)
makes his data available on ftp server FTP.FUNET.FI in directory
pub/ham/vhf-work/mssoft42.zip. The software for viewing is available
too, as well as stream data.
The author edits the monthly RMOB, which gives summaries of recent
observations from all over the world. Included are the setup characteristics.
Contact me for putting you on the distribution list.
8. References
8.1. Books
THE reference is still "Meteor Science and Engineering" by D.W.R. McKinley,
McGrawHill 1961. A fantastic book, giving the theory and practical results
of the heydays of the radio meteor science.
A recent, less astronomical and more communication oriented work is:
Meteor Burst Communications (Theory and Practice) Ed. Donald L. Schilling,
Wiley Series in Communications, 1993, ISBN 0-471-52212-0.
reference section.
A very concise, but technical, introduction to the subject is:
Schanker, Jacob Z., "Meteor Burst Communications", Artech House, Norwood,
MA, 1990 ISBN 0-89006-444-X
Another classical work is "Meteor Astronomy, Lovell, A.C.B., University
Press, Oxford, New York, 1954."
With their project write-up, "Automatic Registration of Meteors,
University of Ghent, 1991.", B. Callens and P. Vauterin won an
international science price. A thesis (in Dutch) built further onto this:
" De automatische waarneming van meteoren door middel van radioreflecties
op 66MHz., Van Bavinckhove, C., University of Ghent, 1994". It gives
interesting results obtained with a dual setup in order to eliminate
interference.
8.2. Periodicals
The regular astronomy magazines, such as Sky & Telescope, Astronomy,
or Astronomy Now run occasionally contributions about forward scatter
experiments.
WGN, the Journal of IMO, contains sometimes contributions and letters
about radio observations of meteors.
8.2.1. General
Black, W.H., Observing Meteors by Radio. Sky & Telescope, July 1983,
pp.61-62.
Pillon, K., Meteor astronomy at home (Computer Adventures).
Popular Science, May 1984, pp. 80.
Black, W.H., A visual/radio meteor observing interface. WGN 17-4, 1989.
Lynch, J.L., A Different Way to Observe the Perseids. Sky & Telescope,
August 1992, pp. 222-225.
Mason, J., Tuning in to meteor showers. Astronomy Now, February 1994.
8.2.2. Relation visual - radio meteor
Lindblad, B.A., The Relation between Visual Magnitudes of Meteors and
the Durations of Radar Echoes, Smithsonian Contributions to
Astrophysics, Vol. 7, pp.27-39.
Van Wassenhove, J., The Relation between Visual Magnitude and Echo
Duration. WGN 15-4, 1987, pp.116-118.
8.2.3. Reduction of data
Jones J., and Morton J.D., The determination of meteor stream radiants
from single station observations. Bulletin of the Astronomical
Institutes of Czechoslovakia, Vol.28, 1977, pp.267-272.
Jones J., Meteor radiant distribution using spherical harmonic analysis.
Bulletin of the Astronomical Institutes of Czechoslovakia, Vol.28, 1977,
pp.272.
Simek, M., Regression method for long-term meteor shower radar data analysis.
Bull. Astron. Inst. Czechosl. 36, 1985, pp.270-278.
Steyaert, C., FORWARD: A General Program for Calculating the Observability
Function. WGN 15-3, 1987, pp.90-93.
Steyaert, C., FORWARD: Extension for Multiple Stations. WGN 15-6, 1987,
pp.193-195.
Van Wassenhove, J., Forward Scatter Data and the Population Index. WGN 17-6,
1989, pp.265-266.
8.2.4. Technical literature
Black, W.H., An inexpensive A/D Converter for VIC 20 or C 64 Built form
Readily Available Parts. WGN 17-3, 1989, pp.96-97.
Greneker, G., Signal Processing techniques for separating meteor echoes
from those of aircraft. WGN 17-2, 1989, pp.43-48.
Van Doren, A., Data Acquisition Systems. Reston Publishing Company Inc., 1982.
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Chris Steyaert Tel : +32 (14) 31 51 04
V.V.S. Astronomical Association Fax : +32 (14) 22 13 73
Belgium E-mail : steyaert@vvs.innetdot be
Kruisven 66 Compuserve: 72650,3513
B 2400 Mol, Belgium 51 degr N, 5 degr E
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