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Subject: Differences between GPS and WWV time
From: Dan Allen (danallen@XXX.XXX)
Date: Fri Jun 22 2001 - 13:23:19 EDT
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@XXX.XXX]On Behalf Of Robert Owens
Sent: Friday, June 22, 2001 9:36 AM
To: NAVIGATION-L@XXX.XXX
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
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