APRS stands for Automatic Packet Reporting System. In short, APRS is simply low bandwidth packet data over amateur ( or ham ) radio.
What's it good for?
The most common use of APRS is advertising your location over radio. A "beacon" can be set to automatically send over the air, at fixed intervals. Typical intervals would be every 5 or 10 minuets. Other APRS users can receive these beacons on their radios, or anyone with an internet connection, can use sites such aprs.fi to see APRS stations displayed on a map ( and more ).
The national APRS frequency is 144. 390MHz. If you tune any 2 meter radio to this frequency you will, eventually, hear what sounds like an old dial-up modem. Those are APRS packets.
Aside from position / location information, here are some other aspects of APRS that make it a powerful tool.
-Digipeaters / APRS repeaters;
Unlike voice repeaters, APRS digipeaters are all on the same frequency* and no offset or tone is needed to get your signal through. If your APRS beacon transmits on 144.390MHz and a digipeater receives it, it will be stored and forwarded.
Also, unlike (most) voice repeaters, a digipeater will transmit to and receive from another digipeater, so where a voice repeater will give you a single signal “hop”, a digpeaters will get you two signal “hops”.
The above applies to a typical position beacon as well as APRS messages and status fields described below.
*In some cases digipeaters can be set up on different frequencies and tone can be required.
-Beacon Status Field (under utilized in my opinion);
Just about any small bit of information can be entered into an APRS beacon status field, but the most useful information I have seen in this field is the frequency, offset and tone information that you are monitoring for voice. Some radios can be set to change this information automatically as your primary voice frequency changes. And some radios will recognize this information and provide a single button press option (QSY) to tune your radio to the frequency, offset and tone advertised by someone elses beacon.
This has proved useful on a number of occasions. I have been 100+ miles away from another wheeler I was head out to meet, knew nothing about the voice repeaters in their area, but saw their APRS beacon, with frequency status and was able to tune in and communicate over the voice repeater they had tuned up.
-Text Messaging and Bulletins;
If the status info in someones APRS beacon doesn't get you communicating over a voice repeater, try an APRS text message. Because an APRS text message can jump through a couple digipeaters, it has a better chance of getting where you want it. A typical APRS message is sent to a specific call sign, but “Bulletins” can be sent to all APRS radios in signal range by sending to call sign “ BLN#” (# = number 1, 2 ect. if you plan to send many).
-Call Sign Alerting (if your set-up is capable);
If your radio is capable, you can set it up to alert you (with a unique tone) when a specific call sign beacon is received.
This will typically alert you only once for each call sign, or every time the status field of their beacon changes. This works as a good alert of frequency change by another radio in the field. When someone bumps their radio off frequency (and has their status set to follow their voice frequency), I get an alert and I can use voice alert (next bullet) to tell them their radio is off frequency.
-Voice Alert as a secondary / temporary simplex frequency;
Voice alert can get you back some limited voice use on the APRS channel of your radio. This uses a 100Hz receive and transmit tone to mute most APRS beacons, but you will still hear other voice alter beacons in simplex range. While this might sound annoying, it actually works as an alert that someone with voice alert capabilities is near. To make use of this, you must keep your APRS channel volume up.
Once set up, a quick voice communication on the APRS frequency will get through to any other voice alert users (or anyone listening to 144.390MHz) and a voice alter user can get a quick voice communication through to you.
While transmitting voice on the APRS channel, you will be blocking all packet data, so you should keep it short. Typical voice communications using this method would be to refer someone to another simplex frequency or, as mentioned above, to let someone know they bumped thier primary frequency off.