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Re: [gpsd-users] Determining when the time has been corrected

From: David J Taylor
Subject: Re: [gpsd-users] Determining when the time has been corrected
Date: Tue, 16 Oct 2018 08:26:45 +0100

My uBlox NEO-6M device always corrects its timestamp after boot. After a warm start, for example, it is approximately 3 seconds ahead of UTC but will start issuing the correct timestamp in about 1-7 minutes. I'd like to get a 'confidence' value that indicates whether the timestamp is accurate or will be corrected sometime in the future (there is often more than one correction).

I've looked at the values 'fix.ept' (estimated time uncertainty) and 'dop.tdop' (time dilution of precision), but these remain stable before and after a correction at 0.005 and ~1.26 respectively, and I'm receiving PPS signals minutes before the timestamp is corrected.

I'm not really looking for a Chrony/NTPD solution--I don't want to slew the system clock or resort to statistical techniques to determine precision. I'd like to know if there is any way to determine if the GPS timestamp is accurate as quickly as possible in an isolated, non-networked environment. Possible, or a pipe dream?



Having a 3-second offset sounds strange to me. However, if you can look at the (GPS-time - UTC) offset value, and determine that it's 18 seconds or greater, I think that would be a good check. Unless that offset value is being held across reboots!

You did know that the GPS timescale is offset from UTC, I suppose, and that the GPS device will only output the correct time once it knows the offset. How individual devices handle this across reboots will be different, of course.

There are those more expert than I here, but I recall something like that the shared memory value isn't set until the time is considered correct. Correction appreciated.

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