The author isn't actually familiar with how GPS works. First, there's no timestamp transmitted by the satellite (aside from the nav message, which isn't used fo

The author isn't actually familiar with how GPS works. First, there's no timestamp transmitted by the satellite (aside from the nav message, which isn't used for this). The satellite time is estimated as an offset from receiver time. Secondly, relativity is compensated for by fixing the satellite clocks so you don't get that location drift. Thirdly, if you had a perfect clock and used a naive algorithm, yes you'd have problems with changing elevation or gravity. Fortunately, GPS accounts for both receiver clock offsets (whatever their cause), and drift in offsets over time. The receiver is also constantly adjusting its clock based on the satellite signal so that error never accumulates beyond acceptable limits.