NMEA WPL Message

The waypoint location sentence (WPL) is used by GPS receivers and plotters in different ways, often to share waypoint data or to show waypoints on-screen.  Many plotters and GPS map displays can take in WPL messages, so a sending device can send a list of waypoints to a map.  RavTrack transponders may be configured to output the WPL sentence to a plotter, PC, or GPS map display.  The RavTrack transponder will send a new WPL message every time it receives a new GPS position from a tracked object.  The WPL sentence specifies the latitude, longitude, and ID of the object that reported its position.  The device connected to the RavTrack transponder that receives the WPL seentence will interpret the WPL sentance as the location of a waypoint, and place a named Icon at the location specified in the WPL sentence.  

The general format of the message is shownhere:

$GPWPL,1,2,3,4,5,6 * cks

When the RV-M7 GX is set to GPS 4 mode of operation, it will output this message every time it receives a position report from another RV-M7 GX transponder.  Within the WPL message, is the latitude, longitude, and “waypoint ID”. In the waypoint ID field, the RV-M7 GX puts the ID of the RV-M7 that transmitted its position. 

For example, if a RV-M7 GX receives a position report from ID 0003 located at 4917.16N , 12310.64W it sends out the following message.

$GPWPL,4917.16,N,12310.64,W,3*65

The lat/lon is sent using the dddmm.mm format, where ddd is the degrees, and mm.mmmm is the decimal minutes.  There is no sign to these numbers. 

GPS 4 mode with the $GPWPL message is the most common way of using the RV-M7 GX with a hand-held or mobile GPS.  Most any GPS with a serial-data input will accept the $GPWPL message, and put an icon on its screen, with a label showing the ID of the RV-M7 at that position. 

Some GPS display have an issue when a new position report is received. Most will move the waypoint to the new location, but some re-draw a new waypoint at the new position, and leave the old waypoint in place. 

To help make the waypoints easier to read, the RV-M7 GX may be programmed with an ID prefix, using the PREFIX vvvv command.  The default is a capitol letter V.  Set it to a dash “-“ to disable the prefix string.  If a prefix is programmed into the RV-M7, it will add the prefix characters whenever it outputs the ID in a $GPWPL message.   For example, if the prefix is set to the phrase “Car”, the RV-M7 GX will output the following message when it receives a position report from RV-M7 Transponder with ID 0003. 

$GPWPL,4917.16,N,12310.64,W,Car3*65