Keeping track of the boats and missiles pays the bills. Everything else is just a happy accident of zero marginal use cost.
The GPS system cost ~$12 billion to make, and needs ~$750 million a year in maintenance (mainly launching new stats as they have a service life of 15 years). This means it costs each US taxpayer $5/year to run. Quite a few road/subway projects cost more than the GPS system.
Navstar-GPS works fine though, so there must be some other reason.
Possibilities: military use, jobs program, pride, etc.
Doing war stuff with GPS does not generate any revenue. It costs American citizens the same amount of dollars regardless of whether it is used for war or not.
It’s not national, and it’s not security.
At this point I just assume you are trolling.
The cheapest Iridium phone service plan is $50/month for just 10 minutes of talk time. Would you pay $50/month for 10 minutes of GPS navigation a month?