The accuracy depend on the receiver used. Inexpesive GPS models are available with a claimed PPS signal accuracy of 1 ms or better relative to the broadcast signal. However, in most cases the actual accuracy is limited by the precision of the timecode and the latencies of the serial interface and operating system.
If the Operating System supports the PPSAPI, RFC-2783, it will be used.
The various GPS sentences that this driver recognises look like this:
(others quietly ignored)
$GPRMC,POS_UTC,POS_STAT,LAT,LAT_REF,LON,LON_REF,SPD,HDG,DATE,MAG_VAR,MAG_REF*CC<cr><lf> $GPGLL,LAT,LAT_REF,LONG,LONG_REF,POS_UTC,POS_STAT*CC<cr><lf> $GPGGA,POS_UTC,LAT,LAT_REF,LONG,LONG_REF,FIX_MODE,SAT_USED,HDOP,ALT,ALT_UNIT,GEO,G_UNIT,D_AGE,D_REF*CC<cr><lf> POS_UTC - UTC of position. Hours, minutes and seconds [fraction (opt.)]. (hhmmss[.fff]) POS_STAT - Position status. (A = Data valid, V = Data invalid) LAT - Latitude (llll.ll) LAT_REF - Latitude direction. (N = North, S = South) LON - Longitude (yyyyy.yy) LON_REF - Longitude direction (E = East, W = West) SPD - Speed over ground. (knots) (x.x) HDG - Heading/track made good (degrees True) (x.x) DATE - Date (ddmmyy) MAG_VAR - Magnetic variation (degrees) (x.x) MAG_REF - Magnetic variation (E = East, W = West) FIX_MODE - Position Fix Mode ( 0 = Invalid, >0 = Valid) SAT_USED - Number Satellites used in solution HDOP - Horizontal Dilution of Precision ALT - Antenna Altitude ALT_UNIT - Altitude Units (Metres/Feet) GEO - Geoid/Elipsoid separation G_UNIT - Geoid units (M/F) D_AGE - Age of last DGPS Fix D_REF - Reference ID of DGPS station CC - Checksum (optional) <cr><lf> - Sentence terminator.Alternate GPS sentences (other than $GPRMC - the default) may be enabled by setting the relevent bits of 'mode' in the server configuration line
The driver will send a $PMOTG,RMC,0000*1D<cr><lf> message each time a $GPRMC string is needed. This is not needed on most GPS receivers because they automatically send the $GPRMC string every second and will only work on GPS receivers that understand the $PMOTG string. Others will just ignore it.
"$PGRMO,,2<cr><lf>"Now switch only $GPRMC on by sending it the following string.
"$PGRMO,GPRMC,1<cr><lf>"On some systems the PPS signal isn't switched on by default. It can be switched on by sending the following string.
Reference Clock Drivers