Observation 1910077: NO-84 (40654)

Regarding Observation 1910077

interesting waterfall with something other than NO-84 in it (marked as “good” because of the evidence of a couple of packets in the 150 to 200 second range).

However the stronger signal seems to be moving opposite the doppler correction, indicating a non terrestrial (or at least, non-fixed) signal. Anybody recognize that one?


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You can easily recognize it from waterfall and from audio, it is a NOAA. I had to run Gpredict to find out which one was it and the culprit is NOAA 15. It’s very interesting that it is received so well almost 8.2MHz away from its central frequency.

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Huh, I never even thought to listen to it! Even if I had, I would’ve been curious as to why I could hear it so well here. I’ve never seen this type of image on any of my stations - in fact the only time I see this is on my shack RTL-SDR when I turn on my pre-amps (usually when I’m using the rtl as Rx for linears). But I’m not running pre-amps on the satnogs stations.


Your gain is wound up to full. At that kind of level it’s quite possible you could be getting IMD in your receiver. All it takes is a strong local signal at some opportune frequency and very odd things can start happening.

You could test this out by using rtl_tcp, or SoapySDRServer (as described here: https://wiki.satnogs.org/Omnidirectional_Station_How_To#Setting_the_gain ) to watch the output of your station live and find a suitable gain value.

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Yep, totally understandable, I guess. I may experiment cranking the gain back a few notches, but in my early experimentation I found best performance at the highest gain setting. This is the first image I’ve seen like this in ~25k observations (or at least that I’ve noticed) - so it may not be worth trying to fix it.