goesrecv randomly loses signal -- usually requires restart of goesrecv

[Aleksey Smolenchuk]

Hello!

Over the past week or so, I've been running into an issue in which goesrecv would lose signal after a fairly decent lock-on in terms of frequency offset and gain (from what I can understand). When it happens, the vit error rate goes up, the frequency offset goes back to 0, and the gain adjustment starts jumping all over the place.

I've recorded some data of such event: https://goes.lxe.co/dashboard/snapshot/5AMn57zneNoefAWdDceR0jbbrGhLHO4f?orgId=1

This one was a bit strange: it didn't lock onto the signal for a while... I had to take the dongle out, wait a bit, try restarting goesrecv a few times, and it re-locked onto the signal after a few minutes. 

I didn't see any outages at https://www.ospo.noaa.gov/Operations/messages.html

Have you observed something similar?
Do you think this might something to do with my RTL-SDR dongle?
Is there some condition that would cause goesrecv move away from the frequency offset? Is there a way to "hardcode" the offset? 

I'd also like to monitor the signal through something like gqnx before it reaches goesrecv, so I'm thinking of switching the source from rtl-sdr directly to nanomesg. Has anyone tried something like this?

Thanks,

Aleksey

[Pieter Noordhuis]

Hi Aleksey,

Sounds annoying. What model of RTL-SDR are you using? I have had success with the Airspy Mini and RTL-SDR with R820T2 tuner (from NooElec). I have tested their model with E4000 tuner chip (they call it the SMArTee XTR) and saw some problems with thermals. While the E4000 is known for its wider tuning range it also has some notorious problems with its onboard L-band LNA heating up and causing problems.

Any time when thermals are an issue it might take a couple of minutes for the SDR to become usable again. I have seen this with every model I tried when the whole receiver was boxed up in a non ventilated enclosure with not even an internal fan. On a hot day, usually after mid-day, I would lose signal and see some of the same issues that you describe; no lock and a couple of restarts before it works again. It is possible that the chip gets into a particular internal state when its thermal protection trips (if they have it at all) and it requires a reset before it can work again, although this is speculation. I have since fixed this and it is no longer a problem. Do you keep your setup in an enclosure of some kind?

The frequency offset is needed to correct for the oscillator in the SDR. Typically these have stability of about 1 PPM or lower, meaning that at 1.7 GHz you may tune +/- 1.7 KHz the real signal. In practice I've seen the corrected frequency go up to +/- 10 KHz. This is not a fixed number and drifts around all the time due to temperature fluctuations. The frequency correction is directly derived from the signal and can't be fixed. If it starts to drift all over the place this is an indication the signal has disappeared.

Instead of using a nanomsg source you could also use a nanomsg subscriber to the sample publisher of the source or the AGC stage (after the gain is applied).

Cheers,

Pieter

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