Tuesday, February 12, 2013

ISS in Graves radar recording

From time to time, while recording Graves' signal with Spectran for meteor head echoes, strange things appear on the screen. Some of them are just "birdies" from the neighborhood, or noises from frequency inhibitors ( 142 - 144.5 MHz is deliberately interfered in some areas of Spain ). But sometimes I detect some "traces" in the spectrogram from unknown origin... until now.

They are very weak, almost at my detection level, and they appear always as segments: The Graves' signature. I know from hams in central Europe airplanes and satellites can be detected using Graves' signal. I'm too far away to detect airplanes, but satellites? My antenna is only a triband collinear vertical with nearly 30m of coaxial cable. Not the best antenna for this kind of signals so I was skeptical.

Obviously the most probable satellite ( I mean the larger one ) is the ISS, so I started to look where was the ISS when I left the computer recording. I recorded during a ISS pass between me and Graves, almost perpendicular to my line to Graves. But I didn't see anything in the spectrograms. Later, that night I noticed a ISS pass along my line to Graves, and looking in the spectrograms just at the time the ISS was almost between Graves and me I found this:

It looked promising but some confirmation was needed, so I looked specifically for ISS passes, those passes where the ISS passed between me and Graves. The next pass the next night gave me this beautiful print:

The fast timebase used to measure the slope from a meteor head echo was too fast to catch the ISS, so the next night, I recorded again, but now with the slower timebase Spectran is capable of.

Murphy came with me to see the experiment with a irritating interference noise: It sounded like a humming free running oscillator moving up and down all the time. It was nearly 3 AM and I doubted between go to bed or continue.

But I was lucky. In fact I heard the tone by the speaker between the buzzing sound, obtaining this screenshot:

Can you see the four sloped traces?

There was no doubt. I can hear the Graves' signal reflected from the ISS with my vertical antenna. But surprisingly I can detect it only when the pass trajectory is like the green one in this map. In passes like the red one I can't detect it. I'm sure it is a question of geometry, and ISS shape and orientation along its orbit.

The most interesting part is I have found similar traces at times when the ISS was not even near Europe, like the first image in this post. There must be from other satellites. What other satellites can be up there so big in size? It's easy to look for future satellite passes but it is very difficult to look what satellite was between Graves and me at a specific time so I have no idea (yet).

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