Answer by Jason Yeaman:
Planes and ships navigated fairly well before there was a GPS constellation.
I don't want to sound like a Navstar constellation doesn't do a Great job as far as providing a force multiplier for guided munitions. But let's just say we didn't have navstar gps. Guided bombs would be less accurate. More of the OPFOR civilians would die.
Let's say we didn't have the DMSP. The OPFOR would have to bleed on rainy days when the weatherman predicted a sunny day.
Let's say we didn't have SBIRS. We have a reduced capability of early warning missile detection, so shooting them down would take a few seconds longer.
MILSATCOM. Reduced capability to communicate with units from s C4I perspective. Resulting in units operating from a tactical perspective in real time, rather than a having a strategic posture.
If we didn't have Magnum or Trumpet, we would have a reduced capability of intercepting RF and voice based communications overseas, making gathering foreign intelligence more of a challenge.
Effectively, more people would need to die before we would win wars.