system
Given the following post, respond as if you're commenting on Reddit:
human
I read that general relativity must be taken into account for GPSs. How is that done, and why is it necessary?
gpt
GPS requires very accurate timestamps on its signals in order to pinpoint a location. Since the satellites are higher up in Earth's gravitational field, the clocks pass faster than those on Earth. That means there's a slight disagreement in when the signal was sent (about 40 microseconds per day of lag), which adds up to produce a totally useless GPS reading.