The Digital Domain of Amateur Radio

Automatic Position Reporting System (APRS)

APRS was developed by Bob Bruninga, WB4APR, as a way to track the position of an object or person using a GPS receiver and packet radio. In addition to being a cool way to see where you are in relation to other hams, APRS has a number of uses for the kinds of public service that we provide.

APRS works by sending unconnected information (UI) packets over the air. These packets can contain a lat/lon position, weather information, a text message to a specific person, or a bulletin message or announcement. UI packets are used because there is no real data connection between any 2 stations - they are simply broadcast packets to be picked up by other stations using APRS.

To help extend the range of an APRS station, hams set up digital repeaters, or digipeaters, to receive and retransmit APRS packets. The two most common digipeaters are RELAY and WIDE. RELAY digi's are mainly used for RELAYing a packet to a WIDE area digi, so a local operator has the ability to reach a large area. WIDE digi's are usually located in high positions and can retransmit over a much larger area than RELAY's can.

Latitude/Longitude Position
The primary purpose of APRS is to obtain the location of object or person. To do this, the latitude and longitude of that object or person has to be given. For most fixed stations  - a home station or digipeater - this position is entered into the APRS software once, and retransmitted at a set interval. For mobile stations, a GPS receiver is used so that as the station moves, its position is updated. This kind of information can be helpful when

Weather Information
A ham that is interested in weather might have a simple weather station set up. With APRS, some basic observations - temperature, wind speed/direction, and rainfall over the past 24 hrs - can be reported over APRS. This can help provide amateurs with an idea of what the weather might be like in an area. It can also be helpful during Skywarn to be able to provide these kinds of weather observations to the Nat'l Weather Service for their records.

Text Messaging
One of the great features of APRS is its ability to send text messages to other APRS users. This, of course, has great advantages because of the ability to use digipeaters to communicate with hams that are potentially outside of your direct communication range. However, if you add in the power of the internet, now you can use APRS to communicate with someone elsewhere in the world! This is accomplished through the use of I-Gates, or internet gateways. On one end, the I-Gate takes the received message and sends it out through one of several central servers for APRS data. On the other end, an I-Gate sees the message for someone in its area and sends it out for the intended recipient to receive.

Bulletins/Announcements
The last feature of APRS is the Bulletins feature. This can be used to send a message to a group of people, or to all. The bulletin could be for an upcoming hamfest, a special event, or a warning of impending severe weather. 

Where to get more information
Keith and Mark Sproul have developed three versions of APRS software for Windows, Macintosh, and an X-Windows version for Unix/Linux users. Their software can be downloaded from their web site at http://aprs.rutgers.edu. More information on APRS and how it is used can be found on both Bob Bruninga's web site and the Tucson Amateur Packet Radio web site.