GPS Time synchronisation

Question

I'm parsing NMEA GPS data from a device which sends timestamps without milliseconds. As far as I heard, these devices will use a specific trigger point on when they send the sentence with the .000 timestamp - afaik the $ in the GGA sentence.

So I'm parsing the GGA sentence, and take the timestamp when the $ is received (I compensate for any further characters being read in the same operation using the serial port baudrate).

From this information I calculate the offset for correcting the system time, but when I compare the time set to some NTP servers, I will get a constant difference of 250ms - when I correct this manually, I'm within a deviation of 20ms, which is ok for my application.

But of course I'm not sure where this offset comes from, and if it is somehow specific to the GPS mouse I'm using or my system. Am I using the wrong $ character, or does someone know how exactly this should be handled? I know this question is very fuzzy, but any hints on what could cause this offset would be very helpful!

Here is some sample data from my device, with the $ character I will take as the time offset marked:

$GPGSA,A,3,17,12,22,18,09,30,14,,,,,,2.1,1.5,1.6*31
$GPRMC,003538.000,A,5046.8555,N,00606.2913,E,0.00,22.37,160209,,,A*58
-> $ <- GPGGA,003539.000,5046.8549,N,00606.2922,E,1,07,1.5,249.9,M,47.6,M,,0000*5C
$GPGSA,A,3,17,12,22,18,09,30,14,,,,,,2.1,1.5,1.6*31
$GPGSV,3,1,10,09,77,107,17,12,63,243,30,05,51,249,16,14,26,315,20*7E
$GPGSV,3,2,10,30,24,246,25,17,23,045,22,15,15,170,16,22,14,274,24*7E
$GPGSV,3,3,10,04,08,092,22,18,07,243,22*74
$GPRMC,003539.000,A,5046.8549,N,00606.2922,E,0.00,22.37,160209,,,A*56
-> $ <- GPGGA,003540.000,5046.8536,N,00606.2935,E,1,07,1.5,249.0,M,47.6,M,,0000*55
$GPGSA,A,3,17,12,22,18,09,30,14,,,,,,2.1,1.5,1.6*31
$GPRMC,003540.000,A,5046.8536,N,00606.2935,E,0.00,22.37,160209,,,A*56
-> $ <- GPGGA,003541.000,5046.8521,N,00606.2948,E,1,07,1.5,247.8,M,47.6,M,,0000*5E

Answer 1

You have to take into account things that are going on in GPS device:

• receive satellite signal and calculates position, velocity and time.
• prepare NMEA message and put it into serial port buffer
• transmit message

GPS devices have relatively slow CPUs (compared to modern computers), so this latency you are observing is result of processing that device must do between generation of position and moment it begin transmitting data.

Here is one analysis of latency in consumer grade GPS receivers from 2005. There you can find measurement of latency for specific NMEA sentences.