Load GPS Based Data

Use the Coordinates > Differential GPS > Load GPS Based Data menu option (GPSLOAD GX) to load differential GPS locations from a Geosoft ASCII table into differential positioning channels of the database. The differential correction data must be contained in a Geosoft Table File that contains at least Date, Time, X, Y and Alt fields.

Load GPS Based Data dialog options

GPS table name	Differential GPS data table (date, time,x, y, alt) Script Parameter: GPSLOAD.GPS
Flight #	Flight number to which the table applies, or blank for all flights. If the flight is left blank, the "Date" and "Time" fields of the table are used to find the lookup values. If the flight is specified, only the "Time "field is used for the lookup. Script Parameter: GPSLOAD.Flight
GPS time reference channel	Time channel name. Script Parameter: GPSLOAD.GPStime
Time lookup resolution (seconds)	Time lookup resolution. Lookups will find values from the table within this resolution. If left blank or set to 0, the nominal sample interval of the time channel will be used. Script Parameter: GPSLOAD.Res
New differential longitude channel	Output GPS X channel name. The "X" field in the GPS table must be one of: "X", "Long" or "Longitude". Script Parameter: GPSLOAD.X
New differential latitude channel	Output GPS Y channel name. The "Y" field in the GPS table must be one of "Y", "Lat" or "Latitude" Script Parameter: GPSLOAD.Y
New differential altimeter channel	Output GPS altimeter channel name. The "Alt" field in the GPS table must be one of "Alt", "Z", "Elev" or "Elevation". Script Parameter: GPSLOAD.Alt

Application Notes

Because of intentional signal degradation in GPS, the absolute position provided by a single GPS receiver can be in error by as much as 90 metres. This error can be determined by also measuring the recorded location of a ground GPS receiver, which has a known and fixed location. The difference between the recorded location and true ground location is the error. Clearly, both the ground GPS and moving GPS must measure the GPS signals from the same satellites at the same time in order for the error to be accurate.

You will require software such as the Picodas C3Nav program to actually compile the differential corrections. C3Nav matches the ground GPS and moving GPS readings at the same time, and uses the data only from the common set of satellites that both are observing at that time. C3Nav produces a listing file that contains the GPS time (seconds from the start of the week), and the differentially corrected location of the moving GPS receiver. The Geosoft program C3N2TBL can be used to convert theC3Nav output file to a Geosoft table ready for processing by GPSLOAD.

C3N2TBL has the following usage:

C3N2TBL gps_file tbl_file date {-x= -y=} tset

Where:

gps_file	Input GPS data file from C3NAV (.OUT assumed).
tbl_file	Output Geosoft GPS table (.TBL assumed).
Date	date of time 0 in the input GPS file. It should be in YYYY/MM/DD format.
-x	GPS base X location.
-y	GPS base Y location.
tSet	The "TIME" field to hours of the day.The default is seconds on the week. This setting must match the time channel in the database.

Example of an output GPS table:

The GPSLOAD GX uses the line date and GPS time channel to find the appropriate GPS correction. GPS tables are normally compiled on a flight basis, and only lines in the specified flight (or group) will be processed.

The differentially corrected locations can, and often do, have gaps, primarily because of differences in the constellation of satellites visible to the ground GPS and the moving GPS. These gaps can be filled by assuming that the moving GPS data is sufficiently linear in it's noise characteristics during the time of the gap. The GPSGAP GX can be run to fill the gaps.