@Konrad_Roeder - The SDR GPS receiver, without particular attention to timing, has been done here: https://www.rtl-sdr.com/rtl-sdr-tutorial-gps-decoding-plotting/
I had this running a few weeks ago and it works reasonably well. Turning it into a GPS-disciplined clock could be interesting but I’m not sure where to start. Would you replace the rtlsdr’s xtal with a 28.8MHz OCXO and control the oven until the signal frequencies look right? Then you’d have the task of deriving a stable lab clock out of that weird frequency, and frankly that seems like more trouble than it’s worth. (Or get a 10MHz OCXO and synthesize the 28.8 from it? But still, awkward every which way.)
Inmarsat traffic looks a lot more interesting, and it’s in-band for the existing L-band antenna and LNA equipment, so I’m completely on-board with this plan. (It would’ve been neat for skylark to decode that and include it in the regular interface, but I suspect there wasn’t enough CPU to do both at once.)
What other stuff is out there on L-band? I feel like the moving-pictures-talking-heads people have really good lists of free-to-air channels and programming, but we data nerds haven’t assembled a good listing of free-to-air data services.