Moving Up in the World

A couple of weeks ago, in a telephone conversation, my older son mentioned that it would be interesting to see if we could measure the Doppler shift in a satellite radio beacon as it passed overhead. He suggested, if I was interested, that he could bring along a radio and we could try it when they came home to visit at Christmas. Of course I was interested!

After supper on Christmas evening, he found a candidate satellite and set up the radio. We fed the audio from the radio to the line input of my MacBook to make the recording and do the analysis. The long and the short of it was that we picked off a nice Doppler shift on two passes, one on the evening of the 25th and one on the evening of the 26th. The Raven Lite software was great for monitoring the signal, both visually and audibly but it had the shortcoming of limiting the recording time to only a minute. For the pass on the 26th, we split the signal and used his MacBook to record the full pass of several minutes into one file using Audacity while keeping Raven Lite running on mine. Since the frequency shift was greater than the radio’s passband, it had to be retuned a few times.

I picked several points off of each file and manually adjusted each point for the time and any retuning of the receiver as needed. Then I used my linear path model for the curve fit after replacing the speed of sound with the speed of light. The first pass data fit very nicely. I haven’t done the fit on the second pass yet. Once I do a little more analysis, I’ll put up a page on it.

My son reminded me that there was a docudrama on PBS’s Nova many years ago (December 1989) called “The School Boys Who Cracked the Soviet Secret” about a science class in a private boys school in England that did a similar analysis on the Sputnik satellite right after it was launched. It would be interesting to watch that again.

This is a very interesting subject that has lots of avenues to study in more detail. We’ll have to see what develops as time passes. For right now though, the basic process was pretty easy, gives me a lot of things to think about… and was very cool.

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