When early TH-D7 came out (it was 1999), it was a great success because of their innovative concept of features packed inside a radio. A proof of that is the large number of units sold and that this model, together with the TM-D700 mobile, is still in production. What suddenly made these radios different from the others was the immediate APRS capability simply connecting them to a GPS unit. At that time, all GPS units were compliant to the NMEA 183.2, but soon the new NMEA 183.3 started to be adopted by all GPS manufacturers: to support the increased accuracy, 4 or more decimal digits appeared in the position data, while Kenwood radios were designed to read fixed-lenght data with 3 decimal digits. After that many TH-D7 owners discovered that newest GPS units did not work with their radio, Kenwood installed a new firmware in the CPU to support the NMEA 183.3, fixing some other bugs and adding new functionalities. This made happy the new buyers, while to the owners of older s/n was given the possibility to send the radio at their expenses to authorized service labs for an upgrade. Useless to say, we preferred to find our own solution for our little HTs...

The TH-D7 uses two NMEA sentences out of the many sent by the GPS unit every second: the $GPGGA and the $GPRMC. As said in the BACKGROUND section, early firmware version did not recognize the commas separating the various fields inside these sentences, using instead a fixed-length method that could lead to wrong data interpretation. Let's look more in detail to the following examples of NMEA sentences as sent, respectively, by a non-compatible GPS and a compatible one:

The 4 decimal digits in the Lat and Long fields instead of the 3 expected by the TH-D7 are the source of the incompatibility, so why not to snip the extra digit before it reaches the TH-D7? Our "potator" (ital-english term for "snipper")  project is based on this idea, simple but needing a minimum of hardware able to:

• receive the NMEA strings

 

• identify the $GPGGA and $GPRMC sentences

 

• recognize the length of the Lat/Long fields inside those sentences

 

• remove the least significant decimal digits exceeding the 3 expected by the radio

 

• recalculate the checksum

 

• reformat both sentences and send them to the radio

The schematic is the following:

(click on picture to open in new frame)

The code has been written by Andrea, and correct operation has been verified with the GPS units that formerly didn't work with our radios: Garmin E-TREX, Garmin E-MAP, Garmin 65, Magellan 315. There should be no reasons why other NMEA183.3 units will not work with our project. The .hex code to allow you to program the micro by yourself can be found in the downloads section.

The circuit can be put in a small plastic box and powered by a 9V battery, still keeping the portability of the system GPS+potator+THD7.

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The original project was not intended to be put inside the radio, but to stay externally and process the data flowing from the GPS to the TH-D7. Nevertheless, having a couple of SMD versions of the microcontroller and a lot of courage, Johnny has managed to definitively implant the circuit inside the radios and forget about incompatible GPS units.

The implant is a risky operation, and even if we have been successful in doing it in the two radios we own, it is not recommended to do the same with yours.