Smaller NME Files With GPSSLOG.CFG

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When you use the recording function (% key) without instructions from you concerning the resulting NME file content, GPSS records every data rolling in from your GPS. These are known as "NMEA sentences," and record much more than your location, velocity and altitude.

NMEA Sentences

File line begins with:	What they contain:	Useful to GPSS*?
$GPGGA	GPS position fix data	NO
$GPGLL	Your geographic position in lat/lon	YES
$GPGSA	Location and identification of satellites	NO
$GPGSV	Identification of satellites in view	NO
$GPRMV	Recommended minimum data (lat/lon, speed, direction, time, and so on)	YES
$GPVTG	Course over ground and ground speed	YES
$P....	Proprietary information (e.g., type of chip in receiver)	NO

* Useful for recording a trip, that is. Uses are of other NMEA sentences are likely within GPSS, in functions beyond the trip recording.

Other sentences your GPS unit might send (but not of interest for trip recording on GPSS):

$GPAAM - Waypoint Arrival Alarm

$GPALM - Almanac data

$GPAPA - Auto Pilot A sentence

$GPAPB - Auto Pilot B sentence

$GPBOD - Bearing Origin to Destination

$GPBWC - Bearing using Great Circle route

$GPDTM - Datum being used

$GPMSK - send control for a beacon receiver

$GPMSS - Beacon receiver status information.

$GPRMA - recommended Loran data

$GPRMB - recommended navigation data for GPS

$GPRTE - route message

$GPVTG - Vector track an Speed over the Ground

$GPWCV - Waypoint closure velocity (Velocity Made Good)

$GPWPL - Waypoint information

$GPXTC - cross track error

$GPXTE - measured cross track error

$GPZTG - Zulu (UTC) time and time to go (to destination) ZDA - Date and Time

Filtering out sentences you don't need
(GPSSLOG.CFG file)

You can instruct GPSS to ignore all but sentences of interest to it by creating a GPSSLOG.CFG file.

Step-by-step:

1. Record a short NME file by pressing % (SHIFT + 5) while you, your GPS and GPSS are in your vehicle. A trip of a few minutes is enough, as long as you cover some ground.

2. Open the NME file in Notepad or other text editor.

3. Note which NMEA sentences are being recorded versus what GPSS finds useful. My unit, an extremely low-cost Rand McNally GPS, sends 4 different sentences to GPSS; only the lines beginning with $GPRMC on my unit have any value for recording trips.

4. In Notepad, create GPSSLOG.CFG

5. In the first line of this file, list the sentences that GPSS finds valuable (in my case, the file contains only "$GPRMC" without the quotation marks.) If you have more than one, just run them together: $GPRMC$GPVTG.

6. Save the file in your GPSS directory.

Your unit might be sending to GPSS any or all of the sentences in the table above. Pare back to only those of use to GPSS for recording.

Result: smaller NME files
The result is a dramatically smaller NME file. For example, one AAAAA000.NME file of 1.37Mb is only 281Kb once all the extraneous lines were removed.

A second result is dramatically faster plotting when you hit CTRL + A and click the PLOT button (but note that playback, as opposed to plotting, will still mimic the original length of time).


By Dave Gehman
© Copyright 2004, Robin Lovelock
Send changes, suggestions to Dave Gehman