# nav_hdg

##

Last updated Fri Nov 19 12:58:23 1999

Generates heading correction from navigation files

## 1. USAGE

nav_hdg control_file

## 2. DESCRIPTION

This utility scans the NMEA navigation files and an ADCP CODAS database to generate an ASCII output file with the following columns:

1. ADCP profile time in decimal days
2. Database heading - Navigation mean heading
3. Database heading
4. Navigation mean heading
5. Standard deviation of navigation heading
6. Number of points used to calculate the navigation heading

This file is for input in parameter ANGLE_FILE for the rotate utility which corrects heading values stored in a CODAS database. This utility was written for a dataset collected during a cruise where GPS heading from a Seapath 200 (HDT string) was logged at a high sampling rate separately together with all other navigation NMEA messages, while heading logged together with ADCP data was from a gyro compass.

## 3. PARAMETERS

The control file must define the following parameters:

dbname: name

  Name of database (with path) containing heading data to be corrected.

nav_list: file

  Name of file (with path) containing list of all navigation files to parse sequentially for heading data. Comments following the C-syntax are allowed in this file.

out_file: file

  Name of output file, with path.

first_time YYYY/MM/DD hh:mm:ss

  This parameter is required because some NMEA messages containing time information, such as the GGA message, do not provide the full date. The given year is taken as the year base for conversion of time into decimal days. Furthermore, it is assumed that database profile times are given for the end of profiles. Hence, full first time information is required to perform heading statistics for the first ensemble. as

db_hdg_from: keyword

  Heading information can be stored in different variables within a CODAS database. This parameter specifies which variable to read for heading corrections. The following keywords are implemented:

  mean_heading

    Read heading data from element mn_heading in the ANCILLARY_1 structure.

  last_heading

    Read heading data from element last_heading in the ANCILLARY_2 structure.

## 4. OPTIONS

In addition to the parameters listed above, the following options are available:

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.

nav_messages:

  Select which navigation messages to parse. One should mention at least one message containing time and another one with heading information. All other messages are not necessary. Unselected messages will be considered as bad navigation lines. Possible keywords are:

  TRANSECT

  ZDA

  GGA

  HDT

  PRDID

  VTG

  end

    This keyword is mandatory and closes the selected list of NMEA messages to parse.

  In addition, the keyword ALL will select all the keywords appearing in the above list. To deselect some specific parameters, prefix the undesired keyword with character !.

  WARNING:

  > Avoid specifying more than one message containing time information, because these do sometime differ slightly. In the case of ZDA and GGA messages, if the latter has a slightly earlier time, the whole date will be set to the next day.

skip_nav_lines: n

  Number of navigation lines to skip when a bad input navigation line is encountered. This is mainly for situations where the navigation device hangs up and needs a reset: time and position information is usually wrong during a couple of seconds (typically 30 to 60 seconds). Hence, the number of line to skip in such a situation will depend on the navigation sampling frequency.

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 file specified in parameter out_file.

## 7. AUTHOR

Pierre Jaccard, Geophysical Institute, University of Bergen, 1999

Back to the top documentation Index of GFI Extensions