The times recorded in the navigation file are in UTC. Keywords and the information they provide are as follows. Not all keywords or elements may appear in all files.
FTP: The first record located at the top of the header used to identify the file format.
VER: HYPACK version number.
INF: General survey information filled in by the data technician.
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 -69 indicates UTM, zone 19.
DTM: Datum transformation record.
GEO: Geoid Model File. Blank if not present. GEO <geo_file> <h_corr> where 'h_corr' is orthometric height correction in meters.
HVU: Horizontal and Vertical Units (meters)
TND: Survey time and date in UTC.
OFF 0: device offsets for device 0. Format of the value is OFF dn n1 n2 n3 n4 n5 n6 n7 where dn=device number; n1=starboard, port offsets where positive is starboard; n2=forward, aft offset where positive is forward; n3=height (antenna) or depth (transducer draft) offset - always positive; n4=yaw rotation angle where positive for clockwise rotation; n5=roll rotation angle where port side up is positive; n6=pitch rotation angle where bow up is positive; n7=device latency in seconds. No offset for device 0 during this data collection.
PRD: Private Device Data
DEV 0: Indicates that the lines with device designation 0 are lines of DGPS data.
DEV 1: Indicates that the lines with device designation 1 are bathymetry values.
USR: User information in the format u r l k, where u=user name, r=reseller, l=license type, k=key number
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 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 19, 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 1=GPS fix and 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 in the format ddmmmm.mmmm. To convert to ddmm.mmmmm divide by 100; long=raw longitude in the format ddmmmm.mmmm. to convert to ddmm.mmmmm divide by 100; alt=antenna altitude above ellipsoid (meters); utc=GPS time in the format HHMM.
EC1: Echo Sounding (single frequency) in the format "EC1 dn t rd" where dn=device number; t=time tag (seconds past midnight); rd=raw depth.
GYR: Gyro Data (heading) in the format "GYR dn t h" where dn=device number; t=time tag (seconds past midnight); h=shipheading angle.
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, three different messages were sent from the DGPS: $GPVTG, $SDDPT and $GPGGA. These will be defined later.
FIX: Event marked manually by the user in the format "FIX dn t event number where dn=device number; t=time tag (seconds past midnight); event number=event number (1,2,3...) automatic increments. The device number is automatically supplying a value of 99.
The NMEA strings present in the HYPACK file are the $GPVTG, $SDDPT, and $GPGGA. These are defined as follows.
$GPVTG is track made good and ground speed in the format "$GPVTG, t, T, m, M, n, N, k, K refcheck" where t=true course made good over ground, degrees; T indicates the previous number refers to true course; m=magnetic course made good over ground, degrees; M indicates the previous number refers to magnetic course; n=ground speed; N indicates the previous number refers to Knots, k=ground speed; K indicates the previous number refers to Kilometers per hour, refcheck= checksum at the end of the string with the first character an *.
$SDDPT is water depth relative to the transducer in meters in the format "$SDDPT, data_meters, offset_meters, * checksum. If the offset is positive, it's the distance from transducer to water line. If negative, it's the distance from the transducer to keel.
$GPGGA is GPS fix data in the format "$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.