Greetings
Ladies and gentlemen, we must not forget the effects of jamming on GPS. Because of the very low signal strength(transmitter is 10k miles away) GPS is dead easy to jam with amateur -made jammers--THIS MEANS SOME FORM OF BACK-UP
In fact it is a legacy from the cold war, to achieve the greatest precision the US located most of the GPS ephemeris over Moscow(In fact when we study cases of best GPS reception we call it the "Moscow case") So the Russians being very upset about the gps constellation invented hand held jammers, the story priced these jammers 150 US$ in the streets of Moscow.