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Re: 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   


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