Most notable world powers are developing anti-satellite weapons and I could see a scenario where a significant number of the GPS satellites get knocked out. I'm wondering if there are ground based GPS transmitters or if there is any public knowledge on how to solve this problem?
Basically every modern piece of US autonomous equipment runs on GPS.
Yes, ground-based GPS transmitters have been a thing since GPS was introduced. Search on "pseudolite" or "GPS pseudolite" ("pseudolite" = "pseudo satellite").
GPS doesn't depend on a transmitter being in orbit -- that just makes each transmitter work over a longer distance. If you put the guts of a GPS satellite in a known location then it'll function as a normal GPS satellite for the area that it can cover.
They are -- or at least were -- a thing, to increase GPS reliability around airports and other spots that depended on GPS coverage. Once the GPS constellation was complete there was less of a need for them, but there's no reason it can't be done.
I'm just gonna point out the fact that it only is called a satellite because it orbits a moon, planet or a star. So if your looking for an alternative to a gps system you should search by it with different terminology. Communications by radiofrequency(in sight) and undersea cables are already in place, which are alternatives.
Look into latest developments on eLoran now under close review as a reliable augmentation to the GPS system. In contrast to GPS, eLoran is very high power and can penetrate buildings, tunnels and much less susceptible to jamming and spoofing.
Related to the near term urgency of finding an alternative to GPS, see Section 1601 of the 2021 National Defense Authorization Act where the US Senate Arms Services Committee details the urgency to find an alternative to GPS as a backup system.
https://www.congress.gov/congressional-report/116th-congress/senate-report/236
Relevant excerpt below:
The committee recommends a provision that would require the Secretary of Defense to: (1) Prioritize mission elements, platforms, and weapon systems that require position, navigation, and timing (PNT) and are critical to operations; (2) Mature, test, and produce sufficient quantities of equipment that can generate or process available resilient alternative PNT signals to equip the prioritized force elements; and (3) Integrate and deploy such equipment into the prioritized operational systems, platforms, and weapons. In addition, the provision would require the Secretary, not later than January 1, 2021, to submit a plan to the congressional defense committees for achieving these goals, including budget requirements, and to begin implementation in fiscal year 2021.
The committee further directs the Secretary of Defense to coordinate with the National Security Council, the Department of Homeland Security, the Department of Transportation, and other appropriate departments and agencies to encourage civilian and commercial adoption of Department of Defense- developed technologies and capabilities for resilient alternative PNT to back up the Global Positioning System.
