3 ms·

We don't need super accurate clocks in the GPS receivers; instead, the GPS satellites carry atomic clocks with them, and include timestamps in their broadcasts.

We don't need super accurate clocks in the GPS receivers; instead, the GPS satellites carry atomic clocks with them, and include timestamps in their broadcasts. So you get two signals at the same time, compare the time the satellite said it sent each signal, and you've got the difference in travel time.

baddox 9y ago
Oh, interesting. What’s the time granularity transmitted by the satellites? I assumed that the satellites broadcasted a course stream of times and the receiver had to finely measure its local time difference between receiving the same time from multiple satellites.
- baddox 9y ago
  My assumption, for example: Receiver receives time “noon” from GPS satellite 1 at receiver-local time T=0. Receiver receives time “noon” from GPS satellite 2 at receiver-local time T=2 nanoseconds. Receiver receives a couple more “noon”s from other satellites, then uses its receiver-local time differences to trilateral its location.
- azernik 9y ago
  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.

baddox 9y ago

Oh, interesting. What’s the time granularity transmitted by the satellites? I assumed that the satellites broadcasted a course stream of times and the receiver had to finely measure its local time difference between receiving the same time from multiple satellites.

baddox 9y ago
My assumption, for example: Receiver receives time “noon” from GPS satellite 1 at receiver-local time T=0. Receiver receives time “noon” from GPS satellite 2 at receiver-local time T=2 nanoseconds. Receiver receives a couple more “noon”s from other satellites, then uses its receiver-local time differences to trilateral its location.
azernik 9y ago
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.