This question is often asked with regards to Raveon’s GPS tracking transponders. This tech blog describes how to determine your GPS transponder’s locations and addresses some additional questions.

The short answer

The NMEAOUT 1 command will enable output of NMEA 0183 compliant messages on the serial port. This will work on any GPS-enabled Raveon radio (M7 GX, ATLAS PL, etc). After exiting command mode, you’ll see the GPS transponder outputting messages similar to the following:

$GPGLL,3308.35228,N,11716.16936,W,213357.00,A,A*77
$GPGGA,213357.00,3308.35228,N,11716.16936,W,1,07,1.06,00134,M,-035,M,,*5F
$GPRMC,213357.00,A,3308.35228,N,11716.16936,W,000.8,277.0,040512,12.0,E,A*10
$GPGLL,3308.35218,N,11716.16947,W,213402.00,A,A*75
$GPGGA,213402.00,3308.35218,N,11716.16947,W,1,06,1.14,00135,M,-035,M,,*5E
$GPRMC,213402.00,A,3308.35218,N,11716.16947,W,000.4,254.1,040512,12.0,E,A*1E
$GPGLL,3308.35232,N,11716.16968,W,213407.00,A,A*75
$GPGGA,213407.00,3308.35232,N,11716.16968,W,1,06,1.21,00136,M,-035,M,,*5B
$GPRMC,213407.00,A,3308.35232,N,11716.16968,W,001.1,271.2,040512,12.0,E,A*1E

You probably noticed the GPS coordinates in the messages above. That’s Raveon in San Diego. In this example, there are typically 3 types of NMEA messages (RMC, GGA, and GLL) that the trasponder will periodicallly output. The NMEAMASK command in the Raveon GPS transponder can be used to enable/disable the various NMEA messages. The NMEARATE command can be used to configure how often these commands will be sent out the serial port.

GLL

The $GPGLL message is the simplest NMEA message to use (although they all contain position information). Its NMEA GLL format is:
$GPGLL,<Latitude>,<North/South>,<Longitude>,<East/West>,<UTC Seconds>,<Valid Data>,<Autonomous Mode>*<Checksum>
The possible values for <Valid Data> are “A” when the data is valid and “V” when it is invalid. <Autonomous Mode> is always set to “A” on Raveon radios.

GGA

$GPGGA,<Latitude>,<North/South>,<Longitude>,<East/West>,<GPS signal quality>,<Number of Satellites>,<Horizontal DOP>,<Number of Satellites>,<Altitude>,<Units>,<Geodial separation>,<Units>,<Age of diff GPS data>,<Reference ID>*<Checksum>
The GGA message is included because it contains the altitude information. The altitude field in Raveon products is always in meters.

RMC

$GPRMC,<Latitude>,<North/South>,<Longitude>,<East/West>,<Speed>,<Track>,<Date>,<UTC Date>,<Magnetic Variation;,<E/W>*<Checksum>
The RMC message contains the track/heading and ground speed information.

Why not $PRAVE?

$PRAVE is a special Raveon format that is intended for use with over-the-air position updates. It differs from a standard $GPGLL (or GGA or RMC) in that it also contains flags, addresses and signal strengths specific to Raveon GPS trackers. A number of the fields are not applicable for a message that wasn’t sent over-the-air.
Messages comming out of the Raveon GPS Transponder that begin with $PRAVE are due to the reception of an over-the-air message from another GPS transponder.

Parsing the $PRAVE message

The messages output by enabling the NMEAOUT switch are standard NMEA GPS messages. There are many open source and licensed parsing libraries available. The $PRAVE message format is a NMEA compliant “private message” (the $P indicates this) and most third-party parsers are able to parse it.

Raveon has VB, C and C++ example code that can parse NMEA messages, so contact Raveon tech suport if you would like copies of this code.

The GPS receiver in Raveon’s M7 and Atlas PL GPS transponder may be configured for different situations. By default it is configured for LAND operation.
Selecting the correct operating parameters has a significant impact on GPS receiver performance.  GPS receiver dynamics may be optimized for LAND, AIR, or SEA operation.
  
  LAND  = 1    Maximum speed the GPS will receive at is 233knots/268mph/430kmh.  Useful altitude from -2000 to +9000 meters.
  SEA = 2    Maximum speed the GPS will receive is not specified.  Useful altitude from -2000 to +9000 meters.
  AIR = 3   Maximum speed the GPS will receive at is 1000knots/1150mph/1800kmh.  Useful altitude from -2000 to +50,000 meters.
 
The default setting for the GPS receiver used in Raveon’s M7 series of GPS transponders and Atlas PL Transponders is LAND.
The default LAND operating parameters allow the receiver to perform well in most environments. Transponders with firmware version C12 or higher are able to view the GPS receiver’s configured dynamics, and change the dynamic mode between AIR, LAND, and SEA. Upon power up, the firmware reads the internal GPS receiver’s current configuration.

The user can optimize the internal GPS receiver in the transponder to a particular application.  If the receiver is then taken out of this environment, the specifically tuned receiver may not operate as well as a receiver with the default options.
The dynamics feature default setting is LAND mode, where the receiver assumes a moderate dynamic environment. In this case, the satellite search and re-acquisition routines are optimized for vehicle type environments. In SEA mode, the search and re-acquisition routines assume a low acceleration environment. In AIR mode, the search and reacquisition routines are optimized for high acceleration conditions. 

Reading the GPS Receiver Configuration

The GPS receiver configuration may be determined by using the GX command to show the overall configuration of the M7 or Atlas PL transponder. The “Dynamics” command will also return a string in the following format that indicates how the GPS receiver in the transponder is currently configured.   

MMM, EE.E, SS.S

 MMM: dynamic mode AIR, LAND, or SEA
 EE.E: Elevation Mask
 SS.S: Signal Mask  

Elevation Mask

This is the minimum elevation angle for satellites to be used in a solution output by the receiver. Satellites which are near the horizon are typically more difficult to track due to signal attenuation, and are also generally less accurate due to higher variability in the ionospheric and tropospheric corruption of the signal. When there are no obstructions, the receiver can generally track a satellite down to near the horizon. 

Signal Mask

This mask defines the minimum signal strength for a satellite used in a solution. There is some internal hysteresis on this threshold which allows brief excursions below the threshold if lock is maintained and the signal was previously above the mask.

Configuring the Dynamics

Use the DYNAMICS command to set or read the dynamics.  DYNAMICS with no parameter will return the configuration.  The following commands may be used to set the dynamics:

DYNAMICS 0    (factory default dynamic mode)
DYNAMICS 1    (LAND)
DYNAMICS 2    (SEA)
DYNAMICS 3    (AIR)

When issuing the DYNAMICS x command, give the GPS receiver a few seconds to execute it.  The M7’s firmware will also send a “flash save” command to the GPS receiver after the dynamics configuration is changed so that the change becomes permanent in the GPS receiver.  Upon power-up, the GPS receiver will use the new dyncamics setting. 

To verify the dynamics setting was saved, cycle power on the M7 or Atlas PL, enter the CONFIG mode, and enter the GX command to view the overall configuration of the device. The GPS receiver dynamics will be displayed.

Advanced GPS Receiver Configuration

To facilitate the advanced user, Raveon added a command “PASS” in the C12 firmware.  PASS will pass the parameter of the command to the internal GPS receiver in NMEA format. The GPS receiver used in the M7 transponder and ATLAS PL personal locator is a Trimble Copernicus II.  The technical manual for the Copernicus II contains details on how to configure it using NMEA type messages.  

For example, to set the GPS receiver to AIR mode, issue the following command while in the command mode.
    PASS $PTNLSCR,0.60,5.00,12.00,6.00,0.0000020,0,3,1

The command’s paramter is a NMEA formatted sentence without the * or the checksum.  The M7 or Atlas will append the * and the checksum to the sentence before sending it to the internal GPS receiver.  To set the dynamics, use the DYNAMICS x command, not the PASS command. The PASS command is provided to the advanced user who wished to reconfigure the receiver’s low-level configiruation paramters such as signal and elevation masks. 

If you change the GPS receiver’s configuration with the PASS command, do not forget to issue the save configuration command to the GPS reciever. The save command for the GPS receiver in the M7 and ATLAS PL is:
 PASS  $PTNLSRT,H,2,7,0

The M7 GPS transponders and the Atlas PL personal locators may be configured to output NMEA 0183 GPS messages from its internal GPS receiver.  For GPS tracking, these GPS transponders can receive GPS position reports from other radios, and they may also be configured to output their own GPS location via their serial port.

Following is a list of the NMEA messages that are available (as of revision C2 of the Firmware).

Refer to the product’s technical manual to see which NMEA messages are sent out in the various operating modes.  Once you set the “GPS Mode” of the radio using the GPS X command, you can change the NMEAMASK parameter to modify with of the NMEA sentences will come out the serial port.

For example, to have only the RMC sentence come out the serial port, use the following command”

NMEAMASK  256

To have the GGA and GLL come out the serial port use this command:

NMEAMASK 3

The NMEAMASK parameter is the sum of all of the decimal values of the individual bits corresponding to the NMEA messages.