APRS - Putting ham radio on the map
Automatic Packet Reporting System (APRS) is a digital communications system that uses packet radio technology to transmit information and text messages over amateur radio. It is the brainchild of Bob Bruninga, WB4APR, a senior research engineer at the United States Naval Academy and first came into use during the late 1980s. The acronym is said to have been derived from the inventor's callsign.
APRS is often incorrectly said to stand for Automatic Position Reporting System. This is an understandable error, as GPS-derived position reports are the most common type of information reported using the system. However, APRS 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. Of course, to do all this you need an adequate local APRS infrastructure and people willing to keep the real-time information up to date. So the extent to which APRS meets these objectives varies from place to place.
APRS is becoming steadily more popular due to the falling cost of GPS hardware and the advent of commercial transceivers from Kenwood and Yaesu with built-in APRS support. The purpose of this article is to explain the background to the system, how it works (in general terms), what it may be used for and how to get started in it. It is not intended to be a comprehensive review of the hardware and software available - products and programs mentioned are just examples - nor does it go into great technical detail about APRS itself or how to set up a system. If your appetite is whetted then there is plenty of information available elsewhere that you can find with a bit of help from Google.
How APRS 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. On the VHF 2m band - the most commonly used band for APRS - APRS uses 144.800MHz in Europe and 144.390MHz in the USA. 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 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.
All of the computer based APRS client programs connect to APRS-IS in order to display information that has been collected by the network. These programs can also, optionally, be configured to function as internet gateways. With the widespread availability of 24/7 home broadband connections, most home based APRS stations that are connected to a radio transceiver operate as internet gateways and the internet now plays a major role in integrating the worldwide APRS network.
Uses of APRS
Thanks to the APRS network it is possible for a ham sat in his shack in Germany to follow the progress of a Summits On The Air activator as he hikes up a mountain in England. He can even see (if the activator has put this information into his beacon) the frequency he will be using. They could even exchange text messages if they wanted to. Using APRS it is theoretically possible for someone using a handheld transceiver in Sydney, Australia to send a text message to another ham buddy in New York, USA.
Another facility of APRS-IS is the ability to store objects. Objects are created using the client software and typically contain a geographical position of something such as a repeater or a clubhouse, together with free-text information about it such as the repeater frequency and access tone. Objects are displayed on a map by APRS client software running on a computer. They are also transmitted on RF periodically by IGates in the area so they may be received by mobiles within range. In this way, APRS can provide visitors to an area with information about what repeaters to use or where the hamfest is being held and the frequency of the talk-in station.
APRS in use
If your interest has been piqued by that you have read so far, you don't need to install any software in order to get more of a feel for what APRS can do. There are several websites that display information from APRS-IS in real time, the best being aprs.fi.
The map shown above is embedded from aprs.fi. It shows a view of Prague in the Czech Republic. As it is a live map, what you will see will depend on when you are looking at it, but during the daytime there are usually at least a couple of mobiles driving around (shown by a red car symbol) whose positions are shown on the map. You can also see digipeaters and gateways, shown by a green star. The red dots on a vehicle's track are the waypoints that were transmitted by the radios in the vehicles. If you point at the dots with your mouse, a line will be drawn connecting the waypoint with the gateway that received it and sent the report to APRS-IS.
The symbol WX in a blue circle is a weather station. Click on these and you will see the latest weather report. Some of the weather stations are operated by hams, and may use RF to send their data to the system. Others are not. They use APRS but report over a separate network and are never transmitted on RF. When viewing coastal locations at aprs.fi you may also see the positions of ships, which are also reported via their own network.
Other symbols you may see on the map are generated by amateur APRS users using their client software. You might see a repeater, shown by a blue antenna tower symbol. Some amateur - perhaps a repeater group member - has taken the initiative to create that object.
A question mark in a blue circle may be used for a variety of different information. In the Prague area it appears to be used to mark the position of severe traffic holdups, and whilst writing this article I was tracking an APRS-equipped vehicle making its way through back streets to avoid just such an obstacle!
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.
There are several APRS software packages that can be run on your home computer to display activity which, when connected to a radio, will also provide a digipeater and/or gateway facility to other APRS users in your area. For many years the most popular program for Windows PCs was one called UI-View. However, that program has not been developed for many years as its author G4IDE is now SK. It is time for something new.
The principal actively developed and supported APRS software for Windows is now APRSISCE by KJ4ERJ. This software is free can be obtained from the Yahoo group that is also its support forum. APRSISCE is also available in a version that runs on later versions of Windows CE, the Pocket PC operating system, and Windows Mobile smartphones.
An alternative APRS tracking software for Windows is APRSpoint. This is a commercial program which is actually a plug-in for Microsoft MapPoint. It uses locally stored maps so it can function without an internet connection, but it doesn't support KISS TNCs nor can it function as an internet gateway or digipeater. Another tracking application for desktop Windows and Windows Mobile is APRS-Go.
If you use Linux then APRSIS32 can be run on this platform using wine. Alternatively you can use Xastir, a native Linux APRS client developed using the X-Window manager which is the only other full APRS client (with APRS-IS support) under current development that I am aware of.
If you own an iPhone with GPS then there is a client called iBCNU. This is not a full featured APRS client like the ones mentioned above, but it allows your position to be tracked by other APRS clients, can be used to send and receive APRS text messages and relies on web access to aprs.fi to provide a map display. Android phone users have a very basic tracker application called APRSdroid.
APRS client software is all very well but it doesn't allow you to do much that you couldn't do using websites like aprs.fi or OpenAPRS. To start using APRS over radio you need additional hardware.
Even if your main interest in APRS is to use it mobile or on foot, most people start by setting up a home station internet gateway. This helps to provide the local APRS infrastructure that you will need to ensure that your position reports and other packets sent on the move are picked up and relayed to the internet.
For a home station you won't need a GPS, since your position is fixed. APRS client software allows you to enter your position manually so that it appears in the correct place on client maps.
You will need a way to decode APRS packets received over the radio so that they may be processed by your client software, and to encode any packets your station wishes to transmit. The traditional way to do this has been to use a packet radio TNC. This connects to your PC and client software using a serial interface or Bluetooth and to your radio using the mic and speaker connectors or a dedicated data interface.
TNCs are available secondhand on places like eBay, though they seem to fetch quite a high price. However you can also buy some quite reasonably priced new units designed specifically for APRS, such as the APRS TNC Digi Tracker from Cross Country Wireless, which is available with or without an internal GPS for mobile use.
Several amateur radio mobile transceivers including the Kenwood TM-D700 and TM-D710 and the Yaesu FTM-350R have built-in support for APRS. The Kenwood radios have a TNC that can be accessed directly from a computer making them much more useful - a TM-D710 is what I'm currently using with APRSISCE in my home station. The Kenwood TS-2000 shack-in-a-box base transceiver also has built-in TNC. Use of one of these transceivers simplifies the setting up of an APRS station.
Sound card software
An alternative to using a TNC is software that uses your sound card as a modem. This is the method I currently use for my HF station. It has the advantage that it uses the same computer to radio interface that I use for other digital modes like PSK31. As I already have such an interface it also means that there is no cost involved! AGW Packet Engine is a free program that was designed to allow packet radio software to interface with TNCs. But it also includes an option to use the computer sound card as a TNC which has sufficient functionality for APRS use. Other digital mode programs such as MixW, TrueTTY and MultiPSK also provide packet TNC functionality using a sound card but they are more complicated to set up and none of them work any better than AGWPE.
Linux has a similar software to AGWPE called Soundmodem. However it can be difficult to get working if you aren't a Linux expert.
APRS on the move
An APRS client program, sound card software, a simple interface and your 2m FM radio are all you need to set up your home based APRS IGate. But what to use whilst on the move?
If you are just interested in tracking the position of something - yourself, a balloon or whatever - then you can buy a ready to use APRS tracker such as the Byonics MicroTrak 300 consisting of a controller, modulator and transmitter. Some of these devices include the GPS, others require it to be provided externally but can often work with existing GPS navigation devices.
The inclusion of a transmitter makes these dedicated devices a bit expensive, so if you would prefer to use an existing transceiver you could use an APRS TNC and a GPS and hook it into the microphone and speaker sockets. The Cross Country Wireless APRS TNC Digi Tracker with built in GPS is ideal for adding tracking functionality to an existing 2m FM mobile transceiver. If you need something smaller then Argent Data's OpenTracker+ is available in an SMT version that could fit inside many radios.
Yaesu's VX-8 series and Kenwood's TH-D7A and the upcoming TH-D72 are hand held radios with integral GPS and APRS support capability. They can transmit position reports on the APRS 2m frequency at the same time as you monitor another frequency.
Position reports of other stations and objects received off-air are also displayed allowing you to see your distance and bearing from them. You can also send and receive APRS text messages using the alphanumeric keypad. These radios are one of the more expensive ways to carry APRS around with you but they are compact and tidy compared with the several small boxes linked with cables needed for a home-brew solution.
Finally, as mentioned earlier, you can use something like APRSISCE on a Windows Mobile cellphone with GPS. This lets you use the mobile data network instead of the amateur bands to get data to and from the APRS network, a solution that works well particularly in areas where amateur digipeaters and IGates are a bit thin on the ground. However you can also use the facilities of an amateur radio transceiver if both it and the Windows Mobile device support Bluetooth.
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.
I hope that this article has whetted your appetite to try APRS for yourself. If you want more information then here are some links to good resources that explain specific aspects of APRS in some detail.
- APRS Beginner Guide by K9DCI (15-page PDF)
- Sound Card Packet with AGW Packet Engine
- Using APRS on HF with PSK63
- aprs.fi User Guide