Entity_and_Attribute_Overview:
The times recorded in the navigation file are in UTC. The (National Marine Electronics Association) NMEA strings $GPGGA, $GPVTG, and $GPZDA are stored in these navigation files. These are defined 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,173356.00,4204.848996,N,07036.929067,W,4,09,01.1,00003.278,M,-028.888,M,01,0000*56
UTC Time = 173356
Latitude = 4204.848996 N
Longitude = 7036.929067 W
Fix Quality = 4
Number of satellites = 9
HDOP = 01.1 relative accuracy of horizontal position
Altitude = 3.278 meters above mean sea level
Height of geoid above WGS84 ellipsoid = -28.888 meters
Time since last update = 01
Checksum = *57.
$GPVTG is track made good and ground speed in the format "$GPCTG, 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,340.39,T,356.37,M,005.50,N,010.18,K,D*27
True course made good: 340.39
Magnetic course made good: 356.37
Ground speed (knots) = 5.5
Ground speed (kilometers) = 10.18
Mode=Differential
$GPZDA is UTC Date/Time and Local Time Zone Offset in the format "$GPZDA,hhmmss.ss,xx,xx,xxxx,xx,xx" where hhmmss.ss=UTC, xx=Day, 01 to 31, xx=month, 01 to 12, xxx=Year, xx=Local zone description, 00 to +/- 13 hours, xx=Local zone minutes description (same sign as hours), checksum.
Example: $GPZDA,173357.00,28,04,2007,,*69
UTC= 173357.00
UTC day = 28
UTC month = 04
UTM year = 2007
Local zone hours = null
Local zone minutes = null
Checksum = *69
$PASHR,SAT is GPS satellite tracking status information. This is an Ashtech proprietary format; no additional explanation could be found.
Entity_and_Attribute_Detail_Citation:
