GPS Settings On The Icom IC-91AD

For GPS operation a GPS receiver must be connected to the 91AD's data port. The GPS receiver must be set to send in the NMEA format.


Using the RS91 software select "Edit Set Mode(C)..." from the "View" menu button.

The following "Message" box will appear...
1. In the "Message" area enter the generated message text from the web site in the "GPS"  "Message" field.
2. The "Tx Message" should be set to "ON".

Below is a screen shot of the D-PRS Calculator from the web site
Enter your call sign at 1.
Choose a symbol at 2.
Enter a message at 3.
Your generated text is shown in blue.

As an example, I entered my information.

Note: The "LJ  " is the code for a jeep. The Symbol codes must be 4 digits long.  Please notice the gap in the generated message betweeb the "LJ" and the D-STAR". That's two blank spaces.

Next select "Digital Setting". The
"Digital Setting" box will appear:
3. Set the "GPS Mode" to "ON". When using an external program like D-Rats,  the GPS Mode must be set to "OFF".
4. Double click in the "GPS Control Mode box and select "RMC" and "CGA".
5. Set the "GPS Auto TX Timer" to "OFF" so the GPS data will only transmit when the PTT is pushed. Setting a time interval will cause the GPS data to beacon.


Why use both the RMC and CGA sentence formats?
Both of these sentences provide basic GPS position data.  The RMC sentence adds speed and direction. The CGA sentence adds altitude.

What is the difference between GPS and GPS-A modes? The GPS mode was the original D-Star GPS mode. The D-Star GPS mode just passes GPS NMEA data on one line with station ID & message lines added. This is not a standard APRS format and lacks strong error detection (just 8 bit XOR checksum). This is the only choice on the 91AD. The 91AD can not decode GPS-A position data sent from a newer radio like the IC-2820H. External software programs like D-Rats can decode the GPS-A data stream when connected to the 91AD.

Starting with the Icom 2820 the GPS-A mode was added. The GPS-A mode sends in the standard APRS TNC-2 format and adds CRC (16 bit Cyclic Redundancy Check) error detection and only uses one line per position report. The D-RPS generated message text is not needed with the GPS-A mode. GPS-A is a much better format but it's only available on the IC-2820H and the IC-92AD radios.
D-Star RadioGPS ModeGPS-A Mode

Why must the
D-PRS Calculator be used to generate the GPS message setting when using the D-Star GPS mode? Since this mode only sends the basic NMEA data, user call sign and message text, it lacks all the information needed for use on APRS.  For your location to appear on the APRS web sites, more information and proper formatting is required. The D-RPS calculator adds a station icon and error detection data. In the D-RPS generated text, four digits are added at the beginning of the text string for the station icon. Next is the message text. Then an "*" is added to mark the end of the text. At the end of the text string the checksum data is added for error detection.

Using D-Rats or DStar Comms: When using a computer for text messaging the "GPS Mode" must be set to "OFF". The text messaging software will control the GPS functions. D-Rats or DStar Comms will not work if the "GPS Mode" is left "ON".

GPS Unit
Here's a picture of the Garmin Foretrex 101 connected to an Icom IC-91AD.

Updated 11/11/2012