[pps43]

Back of the envelope. The distance between Vnukovo airport and Kremlin is 30 km, speed of light is 300,000 km/s, time delay is 10^-4 seconds. Let's say it takes the moving car 100 seconds to get from outside the jamming area to inside. So receiver clock has to drift less than that. A year is pi * 10^7 seconds, so in a year receiver clock should drift by less than pi*10^(7-4-2), or approximately 30 seconds. My wristwatch can do better.

[jhayward]

Now please explain, in terms of how GPS receivers operate, how that information helps you defeat spoofing.

[pps43]

GPS receiver determines its position by measuring the distances to GPS satellites. Those distances are calculated from time delays. To get the delays, the receiver should know the exact time. Its own clock is not stable enough to do it, so our receiver has to determine exact time by monitoring one more satellite than is strictly necessary, and calculating the time from that.

Now if this "exact" time suddenly jumps (compared to internal clock), it probably means that the signal is not coming directly from the satellites, but relayed from Vnukovo.