NavList:

Three countries have the capability to destroy satellites, to wit: the USA, the USSR and China.  This is a proven, demonstrated capability, not just desire or wishful thinking.  Should war between the US, China or the USSR breakout, you can be assured that much vaunted weapons systems, based upon GPS, will tempt GPS satellite elimination.  No satellites = no input to GPS receivers and therefore rendering weapons impotent and/or inaccurate.  I would also wager the US would be very tempted to destroy competitive positioning satellites for similar reasons. Denial of accurate positioning.

In consideration of the short list of GPS satellites (30 for the US), it would not be a monumental task for any capable opponent to eliminate all of them.  Ditto the US attempting to eliminate other satellite systems, small numbers.  The small numbers and the level of contribution makes these satellites extreme high value targets.  It would be foolish to ignore them.  All military conflicts from the date of GPS inception have involved opponents which cannot destroy satellites.  That is not a guarantee of future invulnerability.

Short of a major conflict between the mentioned principles, I can think of no scenario in which a systemic failure of the satellite system occurs.  Even if several of the 30 satellites become simultaneously degraded or broken, the level of system redundancy insures continued positioning availability.

Thanks for chiming in Jeremy.  Your backup plan matches Yacht Ruby Rose's backup plan.  Handheld, battery operated GPS receivers.  I was hoping you would chime in, as you represent the commercial mariner and would obviously be very diligent in your approach.  I take it that your interest in CN is therefore a hobbyist interest, but certainly, I could be wrong in that regard.

The backup to a GPS is another GPS.

Brad