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Automatic Position Reporting System (APRS) is an amateur radio based automatic position reporting system for tracking and digital communications. It was developed by Bob Bruninga (WB4APR) at the United States Naval Academy. In its simplest implementation, APRS is used to transmit real-time reports of the exact location of a person or object via a data signal sent over radio frequencies. In addition to real-time position reporting capabilities using the Global Positioning System, APRS is also capable of transmitting a wide variety of data, including weather reports, short text messages, radio direction finding bearings, telemetry data, and storm forecasts. Once transmitted, these reports can be combined with a computer and mapping software to show the transmitted data superimposed with great precision upon a map display.

At the sender’s end, data from various sensors (GPS, temperature, etc) is collected and encoded into the APRS format then transmitted to another station. At the receiving end, the APRS data is then decoded and displayed on a map or it is retransmitted on to the APRS radio network or sent to the Internet.

In its most widely used form, APRS is transported over the AX.25 protocol using 1200 baud Bell 202 audio frequency-shift keying. Amateur radio operators use frequencies located within the amateur 2-meter band – usually 144.390 MHz in North America, 145.175 MHz in Australia, 144.575 MHz in New Zealand, 144.930 MHz in Argentina, 145.570 in Brazil and 144.800 MHz throughout Europe and Southern Africa. An extensive digital repeater, or “Digipeater” network provides transport for APRS packets on these frequencies. Internet gateway stations (I-Gates) connect the on-air APRS network to the APRS Internet System (APRS-IS), which serves as a worldwide, high-bandwidth backbone for APRS data. Stations can tap into this stream directly, and a number of databases connected to the APRS-IS allow web-based access to the data as well as more advanced data mining capabilities. A number of low-earth orbiting satellites and the International Space Station are also capable of relaying APRS data.


What you need to receive APRS

This section describes one way of setting up a Windows PC to receive and decode APRS data. Be aware that there are quite a few applications that will do the job.  An in depth step by step guide is beyond the scope of this simple article so only a brief overview is presented. If you are wanting an in depth step-by-step guide, please take a look in the internet. Now you need:

  • PC running Windows 98, NT, 2000, ME or XP with a built in sound card & connected to the Internet
  • A transceiver or receiver tuned to the same frequency as the APRS data being sent
  • An audio cable to connect your transceiver or receiver to your PC sound card input

Then download and install what is known as a “Sound Card Packet Engine”. I use AGWPE – a program written by George Rossopoulos, SV2AGW. It can encode and decode packet tones using your computer’s sound card. It is particularly valuable since it can act as a “host” program for several good packet programs that do not have sound card packet capabilities of their own like Tumonz, AGWTracker, Etc. Download AGWPE from http://www.sv2agw.com

Once you have installed the software, you need to make up an audio cable from your PC audio port to the transceiver or receiver.

Next you want to configure AGWPE to recognize the transceiver or receiver connected to your computer – the sound card radio port.

Now you should download and install AGWTracker. This is the program that will display APRS packet data from the Sound Card Radio Port you set up. Among other things, it allows you to receive and send APRS data to or from the radio port or Internet. You can monitor the decoded APRS packets, view a list of stations received, send and receive APRS messages and view station location (globally) with the stunning on-line VirtualEarth or GoogleMaps. You can even load in your own maps! Download AGWTracker from http://www.agwtracker.com


How to transmit APRS

Like receiving APRS, there are various ways to send APRS. Whilst you could no doubt use a computer running a program like AGWPE, most people want a smaller solution. Open Tracker and TinyTrack are small modules that you can build up to directly connect to a transceiver’s microphone and speaker ports. These devices are known as TNC’s, APRS encoders, etc.

Next decide what it is you want to do with APRS so you can determine what you need:

  • To plot a moving object on a map (car, boat, bike, person, etc), you’ll need a transceiver, APRS Encoder and a GPS receiver
  • To report a fixed position and some telemetry data (temperature, voltage, switch position, etc) you’ll just need a transceiver and APRS Encoder
  • To set up a weather station you’ll need a transceiver, APRS encoder and a compatible weather instrument

In all cases, I would suggest you read the Open Tracker user guide. These are very handy references for the respective products.


APRS frequencies used in Southern Africa

The APRS frequencies and repeaters that are in use around Southern Africa.  However, if you are simply looking for get your APRS signal from point to point, there are a few frequencies recommended for this:

  • 2m      144.800 MHz
  • 17m       18.108 MHz *
  • 30m       10.144 MHz *
  • 30m       10.149 MHz *
  • 30m       10.151 MHz *
  • 40m         7.036 MHz *

It also pays to refer to the latest call books for the most accurate information.

* See ‘APRS on the HF bands‘


APRS on the HF bands

Transmitting APRS on the HF bands is not necessarily as simple as it may seem. Because of the reduced available bandwidth, data is sent at 300bps as opposed to VHF APRS which is 1200bps. Additionally, simply quoting a single dial frequency is meaningless unless you also know the audio tone frequencies being used.  If you load the standard OpenTracker HF firmware into the OpenTracker, the tones will be 2110 & 2310.

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