Cross Country Wireless APRS TNC Digi Tracker project history

Back in October 2008 at the UK RAYNET National Convention at Liverpool I was part of a small group (Jack G8HIK, Ross G6GVI and myself Chris, G4HYG) giving a presentation on the latest developments in APRS (Automatic Position Reporting System.

APRS is an international network of amateur radio stations providing real-time GPS based vehicle or event position reporting, weather reporting and simple message handling for educational, experimental or emergency use. APRS uses a common radio frequency of 144.800 MHz throughout Europe. In addition to the radio network internet gateways provide a two-way port to the internet so that stations local to the gateway can be seen worldwide on APRS viewing programs such as Xastir or UI-View.

As always at events such as the Convention the feedback from those attending the presentation was invaluable. Many RAYNET members have simple APRS trackers but would like to upgrade to the next level where they could use APRS with a computer using Xastir or UI-View or operate an in-fill digipeater from home or from the car. At present the only hardware option is to buy an old packet radio TNC (rarely available these days) or buy a new product like the Tracker 2. The Tracker 2 is a fine product (I have one myself) but it's price in the UK is now approaching £100. The feedback from RAYNET members was that a cheaper unit with basic TNC, digipeater and tracker facilities would be welcomed.

After the Convention I researched the possibility of producing an APRS TNC with digipeater and tracker facilities. I came across an article "An Inexpensive Terminal Node Controller for Packet Radio" by Bob Ball WB8WGA in the ARRL magazine QEX where he describes a simple TNC using a PIC microprocessor IC based on previous work by Mike Berg N0QBH on modem-less receiver design using an external comparator. I made contact with Bob and Mike and gained their permission to further develop the original TNC concept for amateur radio use.

I'm including the following features in the TNC:

Use D type connectors for radio and GPS ports for power and audio inputs (compatible with Kantronics TNC, Tiny Trak and Open Tracker)...Done
Use DIL switches for power on/off, GPS/PC, PTT on/off, PTT for HT, 5V feeds to pins 4 and 9 on the GPS port to power GPS receivers...Done
Additional low pass and high pass filtering on the audio input to improve performance on noisy VHF signals and 300 bd HF signals...Done
Additional LEDs displaying activity on the serial inputs and outputs...Done
Add a FSK output for 300 bd HF transmitters in addition to audio tone output

In the firmware I plan to add the following features:

Add additional digipeater options (superdigi)
Add an FSK output on pin 3 of the PIC16F88 for 300 bd HF transmitters
Add a 300 bd speed option for HF use
Add a status packet every five position packets when using GPS NMEA input...Done
Make changes to allow use with UI-View and Xastir and 4800 bd NMEA GPS receivers...Done

Latest news of the project

22 December 2008
One prototype built undergoing tests.
The PCB design has been finalised and the first prototype batch of boards have been ordered and are due for delivery after Christmas.
Firmware development based on version 2 supplied by Bob Ball WB8WGA is continuing.

3 January 2009
In the firmware I've removed the temperature sensor routines from Bob Ball's original version 2 firmware to make space for more APRS routines. The first major firmware change is to output a status packet every fifth packet during GPS operation. It uses the BTEXT data so the status packet has to include an initial ">" character.
For example: BTEXT >Chris, Bolton RAYNET

The $GPRMC NMEA data string is now used as default. It's relatively long at 60 bytes but does include a datestamp, timestamp, latitude and longitude to four decimal places rather than two, speed, true course bearing and magnetic variation. It's also a known uncoded format so is acceptable for amateur radio use in many countries that do not allow coded messages.
In APRS symbols can be included in the destination packet as well as the more commonly used information packet and SSID.
For example: GPSC86,WIDE1-1,WIDE2-2 where GPSC86 is the symbol for a van.