Mir16609@aol.com wrote: > GPS units are set to the proper time at manufacture but do not adjust for the > leap seconds that are added every 2-3 years. The last leap second was added > on January 1st, 1999. This is not quite right. As Don says, the GPS system does have its own time base consisting of week numbers and seconds into week which does not follow the leap seconds added in UTC so it becomes progressively further from UTC over the years. However, the signal transmitted by the satellites also includes the current offset between GPS time and UTC. Most GPS receivers make the adjustment and display UTC. The Garmin GPS-100 produces a position and time fix once every two seconds. Therefore, if the UTC times start as odd numbers they tend to stay this way normally but as the leap second is added to UTC its offset from GPS time is updated and the receiver corrects its output changing to even numbered seconds. This takes a few seconds to happen as this recording of the NMEA output from a GPS-100 I made over midnight 1995-12-31 to 1996-01-01 when a leap second was inserted shows: > $GPRMC,235955,A,5137.55,N,00047.49,W,001.6,020.2,311295,,*06 > $GPRMC,235957,A,5137.55,N,00047.49,W,001.6,020.2,311295,,*04 > $GPRMC,235959,A,5137.56,N,00047.48,W,001.6,019.7,311295,,*07 > $GPRMC,000001,A,5137.56,N,00047.48,W,001.5,021.4,010196,,*0E > $GPRMC,000003,A,5137.56,N,00047.48,W,001.6,024.1,010196,,*0F > $GPRMC,000005,A,5137.56,N,00047.48,W,001.7,026.7,010196,,*0C > $GPRMC,000007,A,5137.56,N,00047.48,W,001.6,025.8,010196,,*03 > $GPRMC,000009,A,5137.56,N,00047.48,W,001.7,027.5,010196,,*03 > $GPRMC,000012,A,5137.56,N,00047.48,W,001.8,028.1,010196,,*0D > $GPRMC,000014,A,5137.56,N,00047.48,W,001.7,027.4,010196,,*0E > $GPRMC,000016,A,5137.56,N,00047.48,W,001.6,028.8,010196,,*0E > $GPRMC,000018,A,5137.56,N,00047.48,W,001.5,028.9,010196,,*02 > $GPRMC,000020,A,5137.56,N,00047.48,W,001.4,029.4,010196,,*04 I've just shown the GPRMC sentences for compactness. The time is the first field after "GPRMC", of course. Ed Davies ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Wed Feb 14 2001 - 09:35:26 PST