Not all files contain complete NMEA strings. These are the original files, no corrections have been made to these files. The times recorded in the navigation file are in UTC. The NMEA keywords and the information they provide are as follows:
$GPGGA is GPS fix data in the format: $GPGGA, t, lat, lath, long, longh, q, sat, h, a, M, alt, M, t2, refcheck, where t=time in UTC in the format hhmmss.ss; lat=latitude in the format ddmm.mmmmmm; lath= N or S indicating the latitude hemisphere; long=longitude in the format dddmm.mmmmmm; longh=E or W indicating the hemisphere; q=fix quality where 0=fix not available or invalid; 1=GPS fix; 2=Differential GPS fix; 3=GPS PPS Mode fix; 4=RTK fix; 5=RTK float; sat=number of satellites; h=Horizontal Dilution of Precision (HDOP); a=Antenna altitude above mean sea level (geoid); M= units of antenna altitude in meters; alt=height of geoid above WGS84 ellipsoid; M=units of geoidal height in meters; t2=time since last DGPS update; refcheck=DGPS reference station id and the checksum.
Example: $GPGGA,170003.00,4119.08134,N,07221.00464,W,2,09,1.2,5.5,M,-33.8,M,6.4,0008*70 UTC Time = 170003.00 Latitude Deg = 41 Latitude Min = 19.08134 N Longitude Deg = 072 Longitude Min = 21.00464 W Fix Quality = 2 (indicates Differential GPS) Number of satellites = 09 HDOP = 1.2 relative accuracy of horizontal position Altitude = 5.5 meters above mean sea level Height of geoid above WGS84 ellipsoid = -33.8 meters Time since last update = 6.4 Checksum = 0008*70.
$GPVTG is track made good and ground speed in the format: $GPVTG, true, T, mag, M, grsp1, N, grsp2, K, mode, where t=true course made good over ground in degrees, T= True, m=magnetic course made good over ground in degrees, M=magnetic, grsp1=ground speed, N=Knots, grsp2=ground speed, K=Kilometers per hour, mode=Mode indicator (A=Autonomous, D=Differential, E=Estimated, N=Data not valid).
Example: $GPVTG,70.98,T,,M,0.09,N,0.17,K,D*01 True course made good: 340.39 Magnetic course made good: 70.98 Ground speed (knots) = 0.09 Ground speed (kilometers) = 0.17 Mode=Differential.