Timing receivers have many different tasks. Let's look at them and the possible accuracy of their solutions.
Precise time
Precise time on the GPS (or GALILEO) scale can be determined by a single-frequency receiver (e.g., NEO-M8T or LEA-M8F) with a CEP50 of about 16-20 ns. This is due to ionospheric and tropospheric delays of GNSS signals. Of course, the receiver must be configured for the length of the antenna cable and consider the delays in the antenna and receiver. However, it should be understood that each satellite system has slightly different time. There are four main time references, like satellite systems. These references differ from each other by about 10 ns, and satellite systems differ from their references by another 5-10 ns.
A dual-frequency receiver (e.g., ZED-F9T or F10T) independently measures and eliminates ionospheric delay, taking advantage of the fact that the delay depends on the frequency. Therefore, its accuracy is higher, approximately CEP50 at 5 ns.
Precise time on the UTC scale. Each satellite system has a way to convert to UTC. But the accuracy of the least significant digit is usually 1 ns. This is because the coefficients are embedded in the satellite's board, at best, twice a day, and at worst, once a day. This results in an additional loss of about 5 ns.
Synchronization.
RTK (e.g., ZED-F9T) allows synchronizing two identical receivers with an accuracy of 100 ps (picoseconds). Naturally, this assumes identical antennas and accounting for cable length. However, fractional noise of 4 ns due to insufficient clock frequency of the generator interferes. In the end, the standard deviation of 2.5 ns is specified in the documentation. The disadvantage of this method is the maximum distance of hundreds of kilometers.
PPP (more precisely, PointPerfect, e.g., this technology is available in ZED-F9T) enables global synchronization with the same accuracy as RTK. More precisely, PointPerfect provides accuracy within 200 ps, but the overall accuracy is determined by fractional noise, so the result is 2.5 ns. The advantage is that there are no distance limitations.
Precise frequency.
Here, everything is determined by fractional noise, i.e., jitter. So, ±4 ns is quite achievable for ZED-F9P, while ±11 ns is possible for NEO-N8T.
Additional capabilities.
Special timing receivers, like as LEA-M8F. First and foremost, this is a receiver that can adjust the frequency of its generator according to the time of the received satellite systems. Other abilities stem from this.
Reliable precise time. For example, in case of a hurricane that destroys the antenna, or failures in satellite systems that occur once every ten years, or enemies with spoofers diverting precise time from your clocks (there have been such cases in high-frequency trading). By the way, most spoofers can only spoof GPS, so relying on GALILEO and BEIDOU is a good strategy.
Reliable frequency - for the same reason.
External generator control. This is useful if its stability is even better, for example, atomic clocks.
Frequency measurement.
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