The June 2017 issue of the Astronomical League
Reflector had an interesting article about someone's participation in the Radio Astronomy Observing Program. What caught my eye in Dr. Alex Vrenios' description of his efforts was his attempt at detecting meteors using forward scattering.
"Seeing" meteors using distant FM station signals scattered off ionized trails of meteors is nothing new. I've never given it a serious try because the FM spectrum in the Twin Cities is almost saturated. It's difficult to find a frequency that isn't taken by a local station. Dr. Vernios solves this problem by using National Weather Service weather radio stations. NWS stations operate at seven different frequencies, and their coverage is designed to not interfere with other stations. It's usually the case where only one station comes in clearly. Tune to a station frequency that doesn't serve your location and you'll probably get only static, which is perfect.
The downside of using NWS frequencies is that they are not particularly good at scattering from meteors. Compared to an optimal frequencies like TV channel 2 (55.25MHz, more about that in another post) they reflect only about 1/25th as well. That's a whopping 14dB drop in potential received signal. NWS stations also tend to be very low power and run only 1.0 or 0.3 kW. (That's compared to a typical 50 or 100 kW for TV stations.)
That said, the approach seemed innovative and worth a try. Also in its favor is the extremely low cost of the needed components. Here's what is needed:
- A crossed dipole antenna that's built to receive NWS frequencies.
- An RTL-SDR USB dongle. This is a simple device that turns your computer into a digital tuner. It has excellent frequency resolution and sensitivity.
- SDRSharp software for contolling the dongle and presenting the results. It's free!
- Chronolapse freeware for performing timed screen captures of the SDRsharp display.
The antenna is easy to make if you don't plan on requiring it to be weatherproof. The dongle is available from Amazon, and the needed software is downloadable. The entire project can be assembled in one or two days, but finding the optimal settings for SDRSharp may take longer. I'll have more on that later, but for the moment let me show you what the results look like.
To start, here is a map of the surrounding NWS stations of interest with their frequencies. My location is indicated by the red circle.
Next, here is a typical FFT power spectrum and display of power levels shown in a "waterfall" diagram.
You will want to click this to see it full scale. The top portion of the diagram shows instantaneous power as a function of frequency; the seven NWS frequencies are marked. During daytime three stations can be heard: The one serving my area, loud and clear; A 300W station in Norwood/Young America, poor; a 1000W station at Clearwater, very poor. A signal is evident at 162.4MHz, but no voice is discernible. That leaves three frequencies (162.4, 162.45, and 162.525MHz), none of which has any signal evident. However, as you can see, there has been a short burst of signal at 162.525MHz.
Another example:
This shows an approximately seven second burst at an otherwise silent frequency.
Note that in both cases only one frequency showed a burst, suggesting that the cause was something local to the transmitting station or the path between it and the receiver.
Bursts like these appear to happen frequently during the daytime. However, there are also longer and weaker bursts that show more complex structure. Are those cause by meteors or some other phenomenon?
Sadly, it appears most of the bursts are being caused by reflections from local aircraft. After using the system for a while it became obvious that the bursts were coinciding with air traffic departing from the nearby airport. Recording the waterfall chart all night showed that there were no bursts overnight until a little after 5 A.M., when departures starting taking place.
That's circumstantial evidence. But there is more: I zoomed the frequency scale to see what was happening in the way of Doppler shifts:
Most of the bursts looked just like this, with a significant Doppler shift that diminishes over time as an aircraft passes. The magnitude of the shift indicates a speed of about 200 mph, which is consistent with a departing, climbing commercial jet. A meteor would not show this kind of signature.
So the NWS-based system won't work for me unless I can reject the aircraft signals. This is a problematic task. There's a chance a system like this would work if it wasn't operating a few miles from an airport, but I'll have to leave that to others to discover.
For now I'm moving on to trying out the method utilizing Canadian TV stations that broadcast video carriers at more optimal frequencies. More on that next time.
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One more piece of the mosaic imaged (but not yet processed). There's an outside chance I'll get it half finished this year. 7 down, 9 to go:
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Oh, and I'm pretty much done lambasting Trump for a while. It's not that he's matured or gotten any smarter or wiser--he's still a spoiled child with the mind of a dirty old man. It's just that there's apparently no changing him. The Republicans will use him to their advantage as long as possible, then either ignore or discard him based on their own limited self interest.
The best way to deal with him and toadies like Jason Lewis is to educate yourself on the issues, support candidates who run against them, and to VOTE.
