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Re: Time references (UT vs. GPS) Whoa, there!



Sounds like a good question for Garmin.. I have the GPS-12 myself. Works
nicely with a program called Fugawi.

----- Original Message ----- 
From: "Bill Jones" <wejones@megalink.net>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, July 09, 2003 12:26 PM
Subject: Re: [amsat-bb] Time references (UT vs. GPS) Whoa, there!


>
> I've owned 5 or 6 GPSs over the years , and some of these GPSs have
displayed
> time in GPS time, and some of these have displayed time in UTC.  I'm
currently
> using about a 5 year old Garmin GPS12XL, so when this topic came up, I
decided
> to check my GPS to see what it was displaying.  After replacing the
batteries,
> which had died, and letting it attain a fix, I checked the time vs one of
the
> HF time standards, and sure enough, it was displaying GPS time, ie 13
seconds
> ahead of UTC.  I checked the time both on the display and on the NMEA
output
> from the serial port.
>    I thought this was interesting since the earlier poster also using a
GPS12
> said that his GPS displayed correct UTC.  After finding the manual, and
going
> through the options, I could find no way of selecting between UTC and GPS
> time, only a setting for converting to local time, which doesn't change
the
> seconds in the time displayed.
>    After 45 minutes of studying, I checked my GPS again, and NOW, it is
> correct relative to UTC.  ????
>    My guess is that before the GPS can display UTC correctly, it needs to
> receive a message from the satellites telling it the current number of
leap
> seconds, and when I initially got my GPS running, although it had a fix,
> perhaps it hadn't had time to collect a complete almanac of data including
> leap seconds???  However after leaving it running for 45 minutes, it
> apparently had all the data it needed to display UTC correctly???
>
>     My question now, however, is how could I have determined whether my
GPS
> was displaying UTC or GPS time.  I've looked through all the NMEA
sentences
> that my GPS12 sends out, and none of them seem to indicate what time
standard
> it is displaying.  I recorded the NMEA output when it was offset 13 sec,
and
> later when it was correct re UTC, and I can't see anything obvious in the
NMEA
> sentences that would have told me that the time was GPS time when I first
> turned it on.
>    My GPS displays $GPGLL, $PGRMZ, $PGRMM, $GPBOD, $GPRTE, $GPRMC, $GPRMB,
> $GPGGA, $GPGSA, $GPGSV,  and $PGRME sentences.  Can someone tell me if my
> guess re incomplete almanac was responsible for the observed behavior, and
> perhaps suggest a way that I could have determined whether the unit was
> displaying UTC or GPS time from the above NMEA sentences?
>    BTW, the NMEA sentences were indicating valid data.  Any suggestions?
>
>
>
>
>
>
>
> Bill Jones  N3JLQ  Sweden Maine   wejones@megalink.net
>
>  Main home page   http://www.megalink.net/~wejones
>  WWII/B-17 page   http://www.megalink.net/~wejones/wwii.html
>
>
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur satellite program!
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>


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