MB7UO in our APRS digipeater which runs from our cabin site on 144.800 MHz.
What is ARPS?
APRS (Automatic Packet Reprorting System) is a digital communications system that uses packet radio technology to transmit information and text messages over amateur radio. It is really a tactical digital communications system, which is to say that it can provide information about anything of amateur radio interest particularly for mobiles visiting an area – local repeaters and their frequencies, club meetings, hamfests, the local weather and the existence of traffic hold-ups. It also supports two-way text messaging.
How It Works
Unlike packet radio, where stations connect to each other in order to exchange information, APRS operates in an unconnected fashion. All stations use a single frequency. HF is also used, with many HF stations acting as gateways between HF and VHF creating the potential for a worldwide radio-based network.
Stations transmit their position reports, beacons, telemetry, messages and so on using unnumbered AX.25 frames for any other stations within range to hear. Other stations that receive the frames may retransmit them after inserting their callsign into the data. Stations that do this like our own MB7UO are called digipeaters. They are generally well-sited stations with a good coverage area that help to extend the range of low power mobile users.
Much of the information sent by APRS stations – beacons, position reports, objects, telemetry and bulletin messages – is not addressed to anyone in particular. Other information such as text messages are addressed to a specific station, which transmits an acknowledgement when it receives it. To try to guarantee reception, an APRS station will retransmit a message several times with an increasing delay between each attempt until an acknowledgement is received from the recipient station. However after several failed attempts retransmission will stop. Unlike with cellular SMS APRS has no mechanism for storing an unacknowledged message until the recipient comes on the air and is able to receive it.
When a digipeater retransmits a packet it adds its call to show it has done so. This also shows how many times the packet has been retransmitted. When a packet is originated it contains a path, which specifies what stations or type of stations can retransmit it and how many times it may be digipeated. When wisely used, the path prevents congestion of the radio channel, but it also limits the distance that messages can reliably be sent by radio unless both sender and recipient are close to an HF gateway station. However, APRS is not intended to be a purely radio-based system.
APRS also makes use of the internet. APRS Internet Service (APRS-IS) is a worldwide internet-connected network of servers that are also connected to individual APRS stations called internet gateways (IGates). APRS-IS servers collect information received from APRS stations via IGates, filter out duplicates (where packets have been received by more than one gateway station) and distribute them to the other servers in the network. So an APRS user anywhere in the world can see APRS activity anywhere else in the world, not just their local traffic.
There are many ways to get involved with APRS. You need a ham radio license, because even if you are only using computer software connected to the internet and not to a radio, it is still capable of generating messages that could be relayed over RF. What else you need depends on what exactly you want to do.
To find out more, here are some websites that you might like to look at:
APRS is an interesting and fun aspect of the amateur radio hobby that provides features and facilities you couldn’t get by any other method.
- It provides a way for you, your family and friends to track your position.
- It provides a way for you to track the position of other things using low powered radio trackers over amateur radio.
- It provides a way to present radio-related information like the location and frequency of local repeaters, the location of a club house, hamfest or field day site, in a way that is useful to visiting hams.
- It provides a free ham to ham text messaging service.
- It provides a way to send and receive alerts, warnings and other ham radio or safety related bulletins.
- Using the APRS to email gateway you can even send short emails from your radio, though the return path isn’t supported.
APRS is an easy and inexpensive aspect of the hobby to get into, requiring in many cases no more than a free software download and your existing radio.