PPM drift for new SatNOGS station

Hi all!

I’ve recently installed a SatNOGS station Guatemala (first station in the country!) and am currently in the Testing phase. I am yet to improve the performance of my antenna, but have been able to receive faint signals from some birds.

I am noticing something slightly odd, that the frequency correction seems to be not exactly on point. Things I have checked:

  • Checked and double checked for correct Lat, Lon and Altitude configuration
  • SATNOGS_PPM_ERROR is set to 40 for my Nooelec NESDR Mini 2. I have used this value before for good performance.

Only thing I can think might be the problem, is that the PPM seems to vary depending on how long the RTL-SDR has been sampling. Say, if I just started sampling, PPM is about 36, and as time passes (and the SDR gets hotter) it stabilizes around 40. Could this be an issue?

Example observations:



P.S. Here’s a screenshot of the waterfall for GOMX-1, note how it drifts to the left:

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This sounds like it should be your issue. Checking at the waterfalls the curve you see is typical to change of PPM and not a wrong location (since wrong location would produce a diverging and then converging signal)

This is awesome! Welcome to our community!

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Thank you for your response!

I will look into it. Any good tools to determine PPM? I had measured it by hand with a HackRF transmitting with a very accurate TCXO, but that is no longer available to me.

Is the NooElec a TCXO version? If not, the crystal inside will drift with temperature over the course of an observation, and it will be difficult to correct for it.

I would strongly suggest using a SDR with a TCXO, as usuall you will only need to set the PPM once, and the temperature compensation bit of the crystal takes care of the rest…

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No! It’s not. Ah, this makes so much sense. Hence why I had seen PPM would vary from 36 to 40 as I left the SDR running. Would adding a script prior to the observation that ‘pre heated’ the SDR work? Or just too much of a workaround?

I think trying to ‘pre-heat’ the SDR is a bit futile TBH. Better off just getting a SDR with a TCXO. They aren’t expensive!

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