The GNSS jamming in Ukraine is mostly from Ukraine themselves, to defend against Russian drones and guided bombs.
Just as Iran jams GNSS, and Venezuela jammed GNSS ahead of the attack. Didn't really help though.
https://arstechnica.com/gadgets/2026/05/nasa-satellites-can-...
Probably because Wester jammers are used from ground and drones have antennas on the top side. For comparison, Russian satellite can jam the signal from above and on a large area. Russian technology is superior.
If Russian GNSS receivers wasn't affected they wouldn't need fiber optic drones, Ukraine is jamming both the small and large drones. Russia have both their own Kometa system to try to filter out the jamming signals, and plenty of Chinese tech as well.
Russian technology is very dependent on both western and Chinese tech, yet they couldn't even defend their own oil refineries in St Petersburg or make any relevant progress along the front in years.
To be fair, USA also could not defend their allies and their bases, and didn't move the frontline in Iran as well. And if Iran buys a submarine capable to launch missiles from a friendly country the situation might get worse.