gps

1. USAGE

gps control_file

2. DESCRIPTION

This utility extracts GPS position and time from non-NMEA navigation files. It was motivated for data collected in the Azores during the AGMASCO project in November 1997. During this cruise, differential GPS was not available, however the ship was equipped with an Azhtech receiver logging GPS fixes every 10 seconds. A similar receiver was located in Faial. These two GPS data sets were then compared and processed in order to create better quality fixes. Its purpose is to create position files for input to refabs and refabsbt.

Currently, only the above mentioned fix file can be read. It contains the following columns:

1. Time in decimal days
2. Unused
3. Unused
4. Unused
5. Latitude in decimal degrees
6. Longitude in decimal degrees
7. Unused

Position fixes are interpolated to the ADCP ensembles times. A good fix is determined as follows: the time difference between two consecutive fixes is less that a certain threshold value (see parameter tolerance) and the ADCP ensemble time must be within these two fixes.

3. PARAMETERS

The control file must define the following parameters:

dbname: name

Name of database (with path) containing profiles to be fixed.

year_base: YYYY

Base year for converting time into decimal days. This value must be compatible with the decimal days listed in the fix file.

gps_list: file

Name of file (with path) containing list of gps fix files.

output: file

Name of output file.

tolerance= integer

Maximal allowed time gap, in second, allowed between an ensemble and two fixes. Because fixes might not cover the whole cruise, this parameter gives the possibility to cancel position fixing within these holes.

4. OPTIONS

The following options are implemented:

OPTIONS:

This key word can be used to improve the clarity of the control file and specify the beginning of options. It has no effect.

only_good_fixes

Use only good fixes to correct only ADCP positions. Positions without fixes are left as is.

longitude_180

Specify this for converting longitude values between -180 and +180 degree before using them.

end

This keyword is mandatory and terminates the list of selected options.

5. TIME RANGES

Finally, the control file is terminated by a list of time ranges. See time_rng(5) for the syntax of these.

6. OUTPUT FILES

The generated output file contains the following columns:

1. Ensemble time in decimal days
2. Longitude fix in decimal degrees
3. Latitude fix in decimal degrees
4. Fix status: -1 for a bad fix and +1 for a good fix
5. Time in second from previous fix to ADCP ensemble time
6. Time in seconds from ADCP ensemble to current GPS fix

The last columns are meant for diagnostic purpose and eventually adjustment of parameter tolerance.

7. AUTHOR

Pierre Jaccard, Geophysical Institute, University of Bergen, 1999

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