Electronic – gps performance for synchronization


I can't understand why GPS reciever clock is very good (stratum 0). I know it's very accurate and pure within itself; but when it passes through a long distance wireless channel, then it has low power and much noise and jitter, so how it can be better than other clocks?

Best Answer

A GPS receiver creates a local replica of something called "GPS system time", which is a virtual timebase created from all of the clocks on the satellites and ground stations. This replica is integral to the process of coming up with a navigation solution, which is based on measuring the signal delay from each satellite to an accuracy on the order of nanoseconds.

The algorithm that keeps this replica synchronized guarantees that there is no long-term drift with respect to GPS system time. Furthermore, it is specifically designed to deal with the errors introduced by the radio channels, so there is minimal jitter as well.

It is relatively easy to provide outputs based on this replica timebase, typically in the form of 1 pps or 10 MHz logic-level signals. Usually, the biggest source of jitter in these outputs is due to the fact that the replica timebase is asynchronous with respect to the receiver's own physical clock.

Therefore, the peak-to-peak jitter of these outputs is usually equal to the local clock period1, the short-term stability is equal to the stability of the local oscillator2 and the long-term stability is equal to GPS system time, which is based on atomic clocks. A PLL can easily filter out the jitter if the application requires it.

1 Many receivers have 10.23-MHz local clocks, which is why you frequently see a specification of ±50 ns on the 1 pps output.

2 Quartz oscillators generally have very good short-term stability. In fact, the best laboratory-grade quartz oscillators have better short-term stability specs than cesium time standards.