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Differences between GPS and WWV time
From: Dan Allen
Date: 2001 Jun 22, 11:23 AM
From: Dan Allen
Date: 2001 Jun 22, 11:23 AM
I can only speak to Garmin GPS receivers, of which I have had several. Garmin GPSes have a very crummy built-in clock. If you put one on a shelf for a month or two, you can easily be off by many minutes. I have often wondered if this was the reason it seems that such receivers have a hard time acquiring satellites the first time after a long period of unuse... but I digress. As soon as the GPS receiver locks on to several satellites and has a 2D or 3D fix, then I have seen the time jump to the correct time. Well, almost. Internally there is no question that the GPS receivers have the correct time once they have a fix, since the entire system is based on the notion of absolutely correct time in order to fix your position. The apparent problem lies in the software that displays time. So sometimes on Garmin GPS 40, GPS 12XL, GPS III, GPS 230, and GPS eTrex units the displayed time can be off by 1-2 seconds from what WWV is broadcasting. I have checked this on several occasions. Another empirical result: these delays appear to be minimized once the GPS receivers have fully caught up with several tasks: 1) locking on to all visible satellites (this takes many minutes after boot-up) 2) downloading the latest almanac from the satellites (this takes several minutes) 3) displaying the position on a map Once these tasks are all done then the displayed time is usually within a half a second of WWV, perhaps even better. My gripe is that there is no indication of how accurate the time display is at any moment. I've just figured this all out by comparing my GPS to WWV many times. Another digression: a good way of determining correct time is NTP, the Network Time Protocol used to sync computer's clocks automatically with various atomic clocks around the country. Mac OS 9.1, Mac OS X, and Windows 2000 all support this, as well as many varieties of Unix, Linux, and FreeBSD. NTP can be setup automatically to do this every day or even every few minutes, and it is quite good. If you are interested more in NTP, go to http://www.eecis.udel.edu/~mills/ntp/ Dan -----Original Message----- From Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Robert Owens Sent: Friday, June 22, 2001 9:36 AM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: Sextant accuracy with short distance to horizon Has anyone noticed a difference between GPS and WWV time. I hear some units display a different time depending on software. Bob Owens TUGLY 26' Nordic Tug