So did some poking around; apparently, there are two timestamps in GPS messages, and the more precise one is packaged in a custom hash function and called the "

So did some poking around; apparently, there are two timestamps in GPS messages, and the more precise one is packaged in a custom hash function and called the "ranging code". The timestep (called a "chip") is a bit under a microsecond. But yes, the receiver does have to measure small time differences between received signals; but that just requires a local clock with high resolution, not one with low drift or low absolute error. For example, if your local clock is a second off, but still measures time intervals to pretty high precision (say, 0.01% - losing a second every three years) that's probably not going to mess up your location fix very much. Especially when the problem is overconstrained as it usually is intentionally. Compare the theoretical minimum requirement of three visible satellites to make a measurement with the 4-12 satellites that the system guarantees will be in sight at any time.