Feature suggestion - auto locking to a stable beacon

[Rob PE1CHL]

I am planning to use Gqrx as receive part of a station to work over the narrowband transponder of the amateur radio
payload on the upcoming Es'hail-2 satellite.
For now I have tested with a standard PLL LNB and an SDRplay RSP1a and Gqrx.  (RTL-SDR dongle works as well)
I receive the beacons transmitted by TV satellites for engineering purposes.

As expected, the LNB local oscillator is a little too unstable (crystal outdoors), which of course can be fixed
by using a GPS-locked oscillator instead of the freerunning crystal.   That requires mod of the LNB, and maybe an
extra cable.

Now Peter DB2OS posted an interesting idea on the AMSAT-DL forum: it would be possible to take the stable frequency
of one of the two beacons and "lock" the SDR software (in this case Gqrx) to that, so that the drift of the LNB local
oscillator is compensated by parameter changes in the SDR software so that the actual signal being tuned to always
remains on the same (indicated) frequency.  With the 500 kHz bandwidth of the transponder it should be no problem
to keep the two beacons inside the visible spectrum at a samplerate of 1MSPS or slightly lower.
(drift of the LO between hot day and colder night is of the order of 50 kHz)

How about that?
Of course the beacons will not be plain carriers, but rather they will be BPSK modulated.

[microp11]

Hi, I am using a similar technique to "track the frequency" of a BPSK carrier inside an SDRSharp plugin I wrote as an open source Inmarsat-C decoder. Instead of altering the CF in the given signal band (in my case 4KHz) I am adjusting the SDR frequency to keep my CF at 2000Hz. I am running it continuously, day and night and I find it works in almost all the cases, if fails only when parasite tones are coming and overlapping the band: https://bitbucket.org/scytalec/sdrsharp.scytalec/src/master/

Cheers!

[Rob PE1CHL]

Ok please understand that in this case I do not want to tune the receiver to track the carrier, but rather want to
be able to tune independently but keep the "calibration" of the receiver locked to the carrier.
So e.g. you have a passband of 700-701 MHz, at 700.1 there is a carrier that you lock to, and you tune and listen to 700.2
MHz.  Whenever the apparent location of the carrier at 700.1 changes within the passband, the location of the 700.2
signal is changed too, so tuning to 700.2 still means you are listening 100 kHz above the 700.1 carrier.
(due to transient effects when changing hardware freq it would be best to keep the hardware freq the same during
this operation so the tracking is performed only with the offset)

[Alexandru Csete]

Hi Rob,

It is a good idea and common practice to do this; however, at the
moment I don't anticipate any further development of gqrx as I am
working on other SDR projects.

Do you know of any specification and sample signals for this beacon?

Alex
OZ9AEC

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[Rob PE1CHL]

Hi Alexandru,

I think it will be a 400bps BPSK signal as "de facto standard" for Oscar 10,13,40.
However it can be changed as it is not really sent from the satellite but just relayed via the bent pipe from a transmitter on the ground.

The satellite is planned to be launched tonight, let's first see how that develops.