system
Given the following post, respond as if you're commenting on Reddit:
human
GPS satellites work because they adjust for similar bending of spacetime to coordinate with systems on Earth. Why wouldn't they?

What possible errors could they make without this adjustment? I thought all they do is to point us direction or pinpoint our location. Why are they really concerned about time?
gpt
GPS satellites need to know the time. 

They don't know what direction you are in, and even if they did the error would be humongous. It would be very tricky to make a satellite system that could accurately pinpoint a transmitter in this way.

The way it works is GPS satellites continually transmit both the current time according to their onboard clocks and their current position, which is known very accurately. At a given time your receiver will simultaneously get this information from a handful of different satellites (I believe 4 at minimum should be necessary) . The time data won't be precisely the same since the different satellites are different distances from you.

If, for example, satellite A time is 1 second earlier than satellite B then you know that satellite A is 1 lightsecond farther away from you that satellite B. You use the data from several satellites to work out the relative distances to them from you and, since you know where each of them are, you can finally compute your location (and the time at your location actually).

The reason they need to adjust for relativity is that both the speed they are moving at (relative to the surface of the Earth) and the gravitational field they are in (relative to the surface of the Earth) effects how the onboard clocks tick relative to each other.

If the time on board each satellite does not precisely match each other then you will not be able to work out the time of flight of the signals and you cannot then work out the relative distances to the satellite. This would ruin the ability of GPS to function.