[amsat-bb] PSAT spinning fast?
aflowers at frontiernet.net
Mon Jul 20 01:56:34 UTC 2015
I started looking at this because the fades on the uplink signal can cause a character synchronization problem with PSK31. Because of the variable character length, you can't write a synchronous decoder that locks onto the character period to help correct framing errors. I was interested in the "rotation rate" (or so I thought) because the fade period might inform how to do any FEC or other error correction scheme.
I've only looked at the recording that that W0DHB made and posted to the list. The PSK signals in the passband show a relatively short fade period and independent uplink signals are not always in phase. In fact, it looks like the period of the fade appears to slowly drift independently--between 1.8 to about 3 seconds in this recording. I'll take another look and see if I can find a really good section of the recording where you can see the independent drift in the fading frequency between KO6TZ and W0DHB.
It's unlikely that the satellite is speeding up and slowing down in its spin rate, and if it were satellite rotation, the period of the fade should be the same for all stations that are transmitting simultaneously to the satellite (neglecting other effects like nulls from the antenna pattern on the satellite, which may be significant).
*If* I'm right, and the frequency of the fading is independent for each uplink station, I would suggest that you are hearing the effect of Faraday rotation. The drift in the frequency of the fading for the different uplink signals would be a result of the differing electron density between the independent stations and the satellite. As the satellite moves, the amount of rotation changes depending on the "thickness" of the ionoosohere, thus causing the indepedendent fading.
I haven't done a hardcore signal analysis to extract the exact fading rates, but it sounds like a great student project. It looks like there is even a good research opportunity to dig into the analysis of the HF beacons of the very first satellites and see how they compare to this.
Just an alternative hypothesis before you start worrying too much about the sensors...
On Jul 19, 2015, at 3:29 PM, Robert Bruninga <bruninga at usna.edu> wrote:
> On the last pass 1500 EDT or so, I was hearing a fade almost every 3
> seconds in the PSK31 audio downlink. That would be 20 RPM.
> So I upped the Sun Vector telemetry sample rate to the highest to get now
> 24 samples per minute. But this plot shows only about 6 RPM. But if the
> audio is right, then we are sampling below the Nyquist rate and so the plot
> can be aliasing…
> So the question is. What do “you” hear as the audio spin rate in the PSK31
> downlink on 435.350?
> Bob, Wb4APR
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