The Automatic Packet Reporting System (APRS) is a real-time tactical data reporting system developed and maintained by amateur radio operators to exchange digital information. The APRS system is accessible over the air with a radio and modem (often referred to as a terminal node controller, or TNC), or the internet, which also acts as a high-speed backbone for the APRS.
Data is transmitted between two radios using a two-tone Audio Frequency-Shift Keying (AFSK) modulation. In the 2-meter (VHF) band, data transmissions are typically 1200 baud (with 1 symbol = 1 bit), and use the Bell 202 frequency tones of 1200 Hz for low, 2200 Hz for high. In the HF band, transmissions are typically 300 baud and use the Bell 103 tones.
Data Link Layer
The APRS data link layer is called AX.25
The AX.25 protocol specifies a basic routing system. When an APRS packet is received by a station, the station will typically rebroadcast it. In this way, a packet can travel from the transmitter to the destination by passing through a series of repeaters.
The APRS packet has a feature similar to TCP's timeout. A maximum number of hops can (and should) be specified in the packet to prevent it from traveling indefinitely.
The packet may also be given a specific routing sequence - a receiving station will only rebroadcast the packet if it is listed in the routing sequence. For mobile transmitters, a more general WIDE routing will be rebroadcast by all repeaters.
A packet will travel through the network until it encounters a radio with a connection to the APRS internet backbone.
The modem will need to be combined with a microcontroller that can generate the AX.25 frames to form a complete TNC. The TNC will need to connect to a radio for broadcast.
Pre-built DPS-based Modems
$40 from Argent Data, but all code, schematics, and PCB layouts appear to be open source. We could build one for about $10.
Custom DSP Modem
Texas Instruments has a document with source code that explains how to use an MSP430 as a V.23 (very similar to Bell 202) modem.
An AVR-based DSP modem has been built by someone, somewhere.
MX614 is a Bell 202-compatible modem IC with limited availability. A microcontroller could be used to generate AX.25 frames, then pass them to the MX614.
Using a pre-built modem like the OpenTracker has several advantages:
- Little development time required
- Small and light
- Open-source, so we can modify it and build our own
However, the pre-assembled device has a fairly hefty price tag of $40. Building our own based on their schematics and source seems like a great idea. However, I think we should link up with the local ARC guys and get some recommendations from them. Alex W 21:15, 1 April 2010 (UTC)