I would be curious on and interested in guidance from a GNSS expert on what the risk profile is when you're using jam resistant frequencies with a multi GNSS constellation chipset (GPS, Galileo, BeiDou), WAAS (GNSS corrections received via satellite), and when close enough to airports that support it, LAAS (GNSS corrections broadcast from airports). Follow up questions would be what countermeasures are available to bolster GNSS reliability during critical phases of flight (takeoff and landing), as well as how cellular tower networks could be used as distributed sensor fabrics for constantly monitoring for and triangulate GNSS jamming using SDR (or perhaps start with a handful of GNSS reference monitoring stations at major airports for this monitoring).
Because GPS and other GNSS have only a very weak signal strength there are no jam resistant frequencies, you can jam them all very efficiently, or you can jam them very efficiently in selective way (jam BeiDou, but not GPS).
GPS was jammed even as early as Iraq war.
"Coalition troops also got a glimpse of GPS’s greatest weakness during the Gulf War. Iraqi forces installed jammers, for example, on top of landmarks such as Saddam Hussein’s palaces to prevent them from being hit, Mastalir says. This helped the military realize early on that it would have to further develop its laser-guided munitions and other weapons that acquire targets when GPS is unavailable"
A lot of work is being done to make GPS more jam resistant, and - just as important - more spoof resistant. The latest batch of satellites has a whole raft of countermeasures to help a receiver to identify whether the received signal is genuine or whether it was spoofed. Receivers too can do a lot to detect spoofing by just observing the signal (for instance: unrealistic power levels on reception, impossible response to vehicle movement and others besides), but quite a few of those are unrealistic or too expensive for regular (non military) applications. Jam resistant frequencies do not exist as you already mentioned, from a physics perspective. But GP might mean this to refer to the new M-Code signal, and these are sent out on different frequency offsets from the main carrier, so effectively different frequencies.
L5 is considered to be more jam resistant, but it is still in the process of being rolled out and most receivers do not have that capability. It will take the better part of a decade to be rolled out completely but the space side should be done by 2027 so if you need it you can get it but you'll need a completely new receiver.
For obvious reasons hard information about all this is tricky to come by, you can read what is available from non-classified sources though and you can glean quite a bit from there.
> L5-band signals are 30x harder to jam and interfere with compared to L1, and they offer superior performance in difficult-to-navigate areas such as urban canyons and tree-covered regions.
If Russia’s proof of concept moves to active use, those satellites (Kosmos 2546 and any others) will need to be removed from service with anti satellite technology (either munition or X-37B equivilent). Clearly Russia is attempting to shield an overt military capability behind the satellite’s early warning launch detection primary mission to dissuade attempts to disable it. Remove the orbital capability, deny them access to replace it.
Ongoing Ukrainian strikes against Rassvet satellite production and launch sites contribute to further impairment of Russia's ability to get any new or replacement satellites to orbit.
I think Starshield (Starlink for the military) is going to use lasers as the means of communication (and location). Those are highly directionaly, and likely not possible to jam. Though I'm not sure how resistant are they to atmospheric interference, and the sats need to know where your receivers are, which I can't imagine the military is too happy about.
https://news.ycombinator.com/item?id=49186101
https://news.ycombinator.com/item?id=49184484
GPSJam: Daily maps of possible GPS interference - https://news.ycombinator.com/item?id=37868106 - October 2023 (89 comments)
GPSJam: Daily Maps of GPS Interference - https://news.ycombinator.com/item?id=32245346 - July 2022 (82 comments)
https://en.wikipedia.org/wiki/Transmitter_hunting | https://en.wikipedia.org/wiki/Pseudo-range_multilateration | https://www.arrl.org/direction-finding
https://geodesy.noaa.gov/CORS/ | https://arcg.is/18fWq8