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[aprssig] TM-D700/TH-D7A uses what NMEA sentences for APRS?

Stephen H. Smith wa8lmf2 at aol.com
Sat Dec 31 21:34:07 UTC 2005


preed at dnaco.net wrote:
> Does anybody know what NMEA sentences the Kenwood APRS radios use from
> the GPS for their various APRS transmissions?
>
>   
In the standalone "APRS" mode, all they require is $GPRMC which give the 
radio LAT, LONG and SPEED.   If the GPS is also providing  $GPGGA 
sentences, elevation will also be encoded into the Kenwoods' Mic-E 
format transmissions


The Kenwood TH-D7 and D700 are operated in the "PACKET" (not "APRS")  
mode when used with an external computer attached.  In this mode,  the 
internal firmware will echo data received at the dedicated GPS port out 
the main data port along with received packets.    This echoing 
operation can even convert baud rates -- GPS data arriving at the 
radio's GPS port at the NMEA-standard 4800 baud can come out the main 
port at 9600 baud.

The program running on the attached computer needs to know how to 
separate received off-air data from GPS data.    Most current APRS 
applications do this automatically when you select the same COM port 
number for the TNC and GPS. 


However, there is one significant limitation in this mode.  GPS 
receivers output several different "sentences" (strings of data) which 
contain various combinations of lat/long, time, heading and speed and 
altitude information.  These sentences are normally repeated,with 
updated data, once every second. The key fact is that the Kenwood GPS 
passthrough function can echo ONLY ONE of these sentences at a time.   
The most commonly used GPS data sentences, and the data they contain, are:


=== GPGLL   (LAT, LONG, Time)

=== GPRMC   (Time, LAT, LONG, SPEED, MagVar)

=== GPGGA   (Time, LAT, LONG, GPS Valid, Number of Sats Used, Hdop, 
ALTITUDE)

=== GPGSV   (Satellite Status)

=== GPZDA   (Time, Date)

==== GPVTG  (Compass Heading, Ground Course Velocity)



Assuming the GPS device actually outputs GPRMC, GPGLL, and GPGGA (in 
some GPS units each sentence has to be turned on or off in a setup menu) 
the results will vary depending on how the D7 or D700 is set. 


1)    When the US version D7A(G) or any D700 is used in  *standalone*  
'APRS" mode, the transmitted beacons (always sent in the 
highly-compressed Mic-E format) will contain altitude IF the GPS outputs 
the GPGGA string.    The Euro versions of the THD7 don't encode 
altitude, even if altitude IS present in the GPS data stream.  Note that 
you can shorten the Mic-E burst (useful for posit-after-voice operation) 
by turning off the GGA string in the GPS device, so the comment field 
doesn't get filled with altitude data.
 

2)    When the D7/D700 are used in "PACKET" mode with an external 
program, the internal firmware echos ONE (and only one) selected string 
of your choice received at the Kenwood GPS input out the main serial 
port connected to the PC.    The command " GPSTEXT " sent to the D7/D700 
internal TNC determines which one GPS string is captured and forwarded 
to the PC.  You can see and/or alter this command in the TNC 
initialization file THD7.CMD or D700.CMD) in UI-View.

If you initialize with " GPSTEXT $GPGGA ",  the APRS program on the PC 
will be able to "see" (and therefore transmit) LAT, LONG, TIME and 
ALTITUDE but not speed and heading.

If you initialize with " GPSTEXT $GPRMC ",  the APRS program on the PC 
will be able to see LAT, LONG, TIME and SPEED. but not altitude.

If you initialize with " GPSTEXT $GPGLL " the APRS program on the PC 
will be able to see LAT, LONG, and TIME only with no altitude or speed 
information.


3)    If you connect the GPS device directly to the PC through a second 
serial port (i.e. not using the Kenwood pass-through feature)  --AND-- 
your PC-based APRS software knows how to parse and extract data from 
multiple GPS sentences simultaneously  and then format this data into 
APRS format (UIview does this), then you can transmit  LAT, LONG, SPEED, 
and ALTITUDE at the same time.






Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://webs.lanset.com/wa8lmf/DigiPaths

Updated APRS Symbol Chart
  http://webs.lanset.com/wa8lmf/miscinfo/APRS_Symbol_Chart.pdf   


New/Updated "Rev G" APRS     http://webs.lanset.com/wa8lmf/aprs
Symbols Set for UI-View,
UIpoint and APRSplus:


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