The US military has many use cases for GPS far beyond weapon guidance and navigation. Many of these use cases cannot be served by INS and are both high value and done primarily in peace time i.e. the loss of the GPS constellation in a war won't affect the military utility of having the GPS constellation generally. Spoofing these use cases is still a giant nuisance.

There are a couple things that people forget about US military INS:

US military INS actually operates as a giant swarm of INS computers with other sensor inputs that compare notes -- wisdom of crowds -- to correct accuracy. If one INS computer drifts too much, the other INS computers will notice and correct it. Even without the swarm, many larger military systems will have multiple INS computers distributed throughout the platform that can compare notes.

The US military has been at the forefront of inventing exotic INS technology since the 1960s, and the details of their INS capabilities are a closely guarded secret. It is used ubiquitously in almost every combat system they produce. What is known is that even in the 1990s the accuracy of INS significantly exceeded design requirements, and there are current military research programs that suggest they know how to build INS that exceeds current GPS accuracy over long temporal baselines.

Given that they already have a GPS constellation that they use for a wide variety of other cases, GPS-corrected INS guidance is a very cheap way to squeeze some additional precision out of weapons that were already pretty precise in the first place. It allows them to build a dirt cheap and reliable guidance package that is also highly precise in environments where GPS spoofing is not a credible problem. Remember, these particular guidance packages tend to be modular and swappable: use the cheap stuff that leans on GPS more for low tech enemies, use more sophisticated and expensive INS for the high tech enemies.

Ummm...no.

Certain systems do indeed operate as you say, with INS as the primary guidance/navigation mechanism. Submarines for instance, cannot rely on a GPS fix. Strategic missiles as well.

But ground forces do indeed use GPS systems. INS systems are very costly to build and maintain, go GPS is a go-to for missions and requirements where it's a good fit. You simply cannot outfit every ground-pounder with an INS system and then keep it properly operational in their task envelope.

Also, parts of the GPS system are quite jam-resistant. Not entirely, but when it was first developed, GPS was pretty secure for it's time, using a cryptographically derived spreading code for the carrier.

Here is an example of a system in use today:

https://en.m.wikipedia.org/wiki/Defense_Advanced_GPS_Receive...

Some of you might be interested in an article I wrote on the subject:

https://sofrep.com/46818/gps-spoofing-how-iran-tricked-us-pa...

You are right. The military relies on INS and GPS. Most systems such as the navigation in an F/A-18 are INS with GPS to maintain accuracy like you described. I thought JDAM was entirely GPS, but apparently it is only an INS supplement to.[1] Cruise missiles use both plus terrain mapping.[2] Military could probably handle total GPS denial, but it would be ugly, especially since it may mean no SATCOM either. I ponder what WW3 looks like and total space denial seems like a strong possibility. Spoofing is probably worse than denial since it isn't just no information but bad information that could throw off the INS.

[1] https://en.wikipedia.org/wiki/Joint_Direct_Attack_Munition [2] https://en.wikipedia.org/wiki/Tomahawk_(missile)