NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: GPS time display unreliable
From: Jared Sherman
Date: 2003 Sep 11, 18:19 -0400
From: Jared Sherman
Date: 2003 Sep 11, 18:19 -0400
Paul, those threads appear to deal with a bug or idiosynchracy in one model from one source (the Magellan Sportrak) and evn then there is some question about whether the problem just revolves around one firmware level or upgrade. The only solid information that I have found is that a properly functioning GPS does not get "all" information instantly. In my literally fevered mind I recall that there is a master string which is only broadcast something like once every 14 minutes that contains data the GPSes need in order to ensure full accuracy. Therefore, any GPS which has just been turned on is suspect--simply because it has not necessarily had time to read the full string unless it has been running for at least 15 minutes. Let's say 1/2 hour in order to ensure the string was read fully and clearly at least once out of two passes. IIRC that string contains the latest correction/update information and "how necessary" it is depends on what has changed since the last time the GPS was running, if anything. But in the case of my antique Garmin 12XL, I make a habit to check the displayed GPS time versus my chronometer time, which is usually set to the USNO master source via the internet the night or day before. They both hack--exactly--within the 1 second that I consider to just be "push the button" timing error. Given a "known good" GPS and the proper initialization time, I think the accuracy of GPS display time is going to be immaterial as compared to anything less than having an actual master time source with you. After all, it IS obtaining that time display from 16 atomic clocks.