The times recorded in the navigation file are in UTC. Keywords and the information they provide are as follows:
INF: General survey information filled in by the data technician. In this case, the information recorded the first two days of the survey (jd249 and jd250 folders) was from a previous cruise. The correct information was entered for the final survey day (jd251 folder).
FIL: Raw format file.
ELL: Ellipsoid information. The name of the ellipsoid followed by the semi-major axis in meters and the flattening ration.
PRO: Project information record where TME indicates Transverse Mercator and the central meridian of -75 indicates UTM, zone 18.
TND: Survey time and date in UTC.
DEV 0: Indicates that the lines with device designation 0 are lines of Ashtech DGPS data.
OFF: device offsets (none used during this data collection)
EOH: end of header
The remaining elements have similar information in the first 3 columns. The first column will indicate the data type, the second column will indicate the device that recorded the information (0 for Ashtech) and the third column is the time tag (seconds past midnight) that is also sometimes referred to as the latency. The remaining information on each line is specific to the data type.
POS: Position of the ship - in this case the antenna location - in the format "POS dn t x y" where dn=device number; t=time tag (seconds past midnight); x=easting; y=northing. On this cruise these values are in UTM, Zone 18, WGS84.
QUA: Position quality information in the format "QUA dn t n m h sat mode" where dn=device number; t=time tag (seconds past midnight); n=number of values to follow; m = 10 minus HDOP (horizontal dilution of precision); h=HDOP; sat=number of satellites; mode=GPS mode (NMEA 0183 standard values) where 2=Differential GPS fix.
RAW: Position information in the format "RAW dn t n lat long alt utc" where dn=device number; t=time tag (seconds past midnight); n=number of values to follow; lat=raw latitude X 100; long=raw longitude X 100; alt=antenna altitude above ellipsoid (meters); utc=GPS time in the format HHMM.
MSG: Message string in the format "MSG dn t message" where dn=device number; t=time tag (seconds past midnight); message is the message sent from the device. On this cruise, two different messages were sent from the DGPS: $GPGGA and $GPGLL. These will be defined later.
FIX: not defined in the HYPACK manual
The NMEA strings present in the HYPACK file are the $GPGGA and $GPGLL. 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 longitude hemisphere; q=fix quality where 2 is a DGPS fix; 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.
$GPGLL is geographic position date in the format "$GPGLL, lat, lath, long, longh, t, vcheck" where 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 longitude hemisphere; t=time in UTC in the format hhmmss.ss; vcheck=data valid and checksum.