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[questions] Re: GPS+PPS vs NTP server, why a huge offset ?



On 17/06/2022 20:16, Terje Mathisen wrote:
The key idea is of course that in order to know where a GPS is located with better than 3 m precision, the unit by implication also knows what time it is, to within 10 ns of UTC(USNO). The only problem is to be able to convey that info to a connected NTP server.

It only needs the time relative to the visible constellation, as the ultimate position accuracy depends on time differences not absolute time. Absolute time errors affect position at about 7.5mm/µs.

<https://www.gps.gov/systems/gps/performance/accuracy/> quotes ≤30 nanoseconds, 95% of the time. That probably includes uplink allowances. Also, I'm not sure if UTC(xxxx) exists in real time. UTC proper only exists some weeks after the event. There might have to be an allowance for the difference between the instantaneous estimate of UTC(USNO) and the final value.
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