How precise is the time from GPS?

clockDuring one of my recent passages, while checking a few LOP of stars by comparing them to the position shown by the GPS receiver, my eye fell upon the time displayed on the screen of the GPS and found it very different (14 seconds fast), from that of my radio-controlled wristwatch, the same that I had just used to label the heights taken with the sextant.
I was so puzzled that I rushed down the companionway to compare the wristwatch time with another radio-controlled clock installed on the chart table, and found them perfectly sinchronised, so I checked out the GPS receiver that I keep in my grab-bag , I switched it on and discovered that this latter was fifteen seconds fast. Still in disbelief, I turned on the HF radio and tuned one of the time and frequency stations listening in silence to its tics until the minute signal. Also these signals were in agreement with radio-controlled clocks.
At this point, after completing my astronomical calculations with the times taken by my watch, and realizing that the accuracy of the astronomical point was good enough, I began to suspect that, the GPS recivers were not that reliable, in particular given that four seconds correspond to one mile.
The calculations performed again the following day using the GPS time, have confirmed this hypothesis, putting the celestial fix far from both the GPS and previous celestial fixes.
Returning from the cruise, I started to search the web for an explanation of such a gap since it appeared unbelievable the possibility for such an error on GPS receivers. I had always read that the time onboard each satellite is given by four or five cesium and rubidium atomic clocks of extreme accuracy and the precision of the position is based on such clocks.
After some correspondance with the U.S. government and some manufacturers of GPS receivers, the whole matter became clearer. Here are in breaf the conclusions:

The time used internally by the GPS receiver to calculate the position has an extreme precision, which amounts to about forty nanoseconds.

The clocks of the GPS system have been aligned together in 1980 and is kept in synch through the TAI (International Atomic Time). Periodically, the UTC time is updated by removing a second (leap second) to adjust the atomic time with the mean solar time based on Earth’s rotation over time compared with a grid of reference fixed in space. Since then, the gap accumulated between UTC and TAI is several seconds, for the sake of precision they were eleven in 1996.

The data on this gap is transmitted to GPS receivers only once every 12.5 minutes, and in case of bad reception of satellites, could not be properly captured at the first occurrence.

When this information is processed by the GPS receiver, the time shown on the display is correct, although this time processing and display can show time with a certain delay which is typically between one and two seconds, this delay affects also the NMEA signal transmitted on serial link from the GPS receiver.

Moral: Do not blindly trust the time shown by the GPS receiver to make your calculations for celestial navigation, but if you can, check it via a radio clock or one of the standard stations for time and frequency!

Note: Here I have tried to trivialize the matter for not introducing large explanations and masses of numerical data, those who want to know more about the network may seek additional information here and here.

Personally, I still love to label the time on my celestial calculations as GMT like my celestial navigation guru David Burch. GMT, for the most meticulous, will always be in between UTC and UT1, while maintaining a high accuracy for our purposes.

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