Radio Detection Basics
There are two primary methods being used by amateurs to detect meteors via forward-scatter technique; the FM method and the AM/CW method.
Prior to the 1960's most of the radio meteor research was conducted at universities, government and military sites. As it is now, such institutions were limited by their current funding. This meant meteor observation were often spotty and they were not usually continuous over many days. They utilized radar and back-scatter techniques to detect meteors.
In the 1960's amateur radio observers listened to a vacant commercial FM radio stations which have their channels in the 88-108 MHz range. As FM radio became more popular it quickly became harder and harder to find a vacant channel to listen for meteors. Even if a vacant was clear locally an observer might be plagued with the local stations' 'spilling over' which interferes with hearing meteors echoes. Since FM, frequency modulation, there is no easy way to see the Doppler signature of a meteor.
To avoid these limitations, crowding being the biggest problem, observers started using the video carriers of television stations. The video carriers are continuous wave (CW) and narrow band in nature. In North America each of the lower TV channels had one of three possible offsets; minus, zero, and a plus offset. What this means in practice is if channel 3 is an 'empty' channel locally, then a listener could listen for the video carrier at 61.250 MHz (Zero offset), or on either side of it at 61.260 MHz (+ offset) or at 61.240 MHZ (- offset). This provided an additional means of reducing possible interference. TV stations are also more spatially isolated than FM stations are so there is again, less chance of interference.
Compared to FM, using a CW signal also gives the observer a means of observing the Doppler signature of each echo by means of FFT (Fast Fourier Transform) routines. This enables studies on Epsilon type echoes, head echoes and other echo phenomenon. Using the Doppler of head echoes the height of a meteor can also be determined by amateurs.
Changes are in the wind
North American radio observers as well as European observers are facing a crisis. The video carrier method is on the verge of disappearing as the two continents switch from analogue TV signals to digital signals. The United States have already made the change and Canada is due in 2011. Many European stations have switched already while others linger on with analogue.
We will discuss alternatives signal sources to TV video carrier below.
Most people wonder how it is possible to hear a meteor. The answer is when a meteor enters the upper atmosphere it begins pushing atoms aside as it penetrates the ionosphere. These high speed collisions leads to high temperature heating of the meteor. When the energy becomes sufficient the meteor begins to glow at visible light wavelengths. Not only does the leading front of the meteor glow it also creates a plasma trail behind it. We call this ablation. Mass is being converted into energy and light. The ionized plasma rapidly looses it's energy and the electrons recombine so most meteors are a brief flash in the sky; the common shooting star we all knew as kids. Most of the visible phase of a meteor ablation occurs between 110 km and 60 km above earth's surface.
The reason amateurs listen to TV video carriers or FM stations is because the stations provide the source of the RF, radio frequency, power that illuminates (reflects off) the meteor's plasma trail. Commercial TV stations run 100,000 Watts (100 kW). That is a lot of power! While the stations want their signal to reach their customers' TV sets in reality much of the signal is radiated out above the horizon and vertically into the sky itself. Usually these signals are lost to the sky as they penetrate the ionosphere without being reflected and continue out into space. If a meteor produces an ionized reflective trail then the VHF (TV and FM) signals can be reflected off the plasma and back down to earth. When the geometry is right radio observers receivers hear a brief "ping"; a musical sounding note of the signal reflecting off the meteor's trail.
For forward-scatter work the transmitter is located well below the receiving station's horizon. Usually we strive to have a transmitter between 600 to 1200 km away from the receiving site. See below for the geometry of forward scatter signals.
Diagram from Richardson and Knuteh (1998).
As mentioned, back-scatter is used by the professionals. In this case the receiving station is not below the horizon from the transmitter, rather, it is co-located with the transmitter. The power is borrowed as in forward-scatter it is produced by the transmitter at the site. The signal is sent from the stations transmitter outwards and the signal is reflected back to the receiver at the same location. Radar is a prime example of back-scatter.
More to follow on video carrier method... For now please see ABMO Radio page to see examples of a working TV video carrier set-up.
Hopefully, one of our members will discuss using the FM method and it will be placed here. If you are interested in the FM I highly recommend going to Ilkka Yrjöllä's web site.
Even if you're not interested in FM detection his discussion on forward scatter is the best I've seen as is his discussion on CCD, light intensifiers and other meteor subjects.
Software for automatic counting section follows:
- Spectrum Lab
- Colorgramme Lab V 2.3
- Roll your own Colorgramme
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