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APRS introduction

APRS is an abbreviation for Automatic Packet Reporting System, and it is a graphical method of broadcasting positioning information in "real time" from packet radio-equipped stations. It was originally designed by Bob Bruninga WB4APR in the early 90's, but it has seen explosive growth in the last few years due to excellent software such as UI-View and Kenwood's APRS enabled rigs

People often ask why get involved with using APRS, which I find rather bemusing. I often wonder why we are also not asking each other to justify why we rag-chew while mobile or play with getting signals around the world. At least with APRS you can see other people exist, when calling frequencies are often silent all evening. Whether you get hooked or not will be for your own reasons, all I ask is that you give it a try for a few days and see for yourself. It does not even need to cost anything, presuming that you already have a receiver or transceiver on 2m and a PC.

Low res screen grab of UI-View CLICK for higer resolution

A screen grab from my home station
Click on the image to see it at higher resolution (170kb).

When APRS equipped stations send latitude and longitude information, their locations are instantly shown on a map on your PC screen. As well as the actual location, the course, speed and altitude of the station can also be included.

Even the US Space Shuttle has used APRS, with stations down here on the ground literally "watching" it, or at least its position, on their PC screen as it goes overhead. Many stations have also used the digipeater on board PCSAT, a purpose built satellite, to relay their APRS positions over a wide area. PCSAT was developed by students from the US Naval Academy and it was built entirely from standard amateur equipment for a total cost of $30,000 including the launch.

Photo of PCSAT courtesy of WB4APR

PCSAT and the "team"

UK emergency groups such as Raynet are a "natural" for APRS and it has been used extensively in Berkshire for support of the user services, such as St. John Ambulance.

I find that APRS provides an excellent indication of changes in radio propagation on VHF and over time you can build up a accurate picture of coverage from your home station.

What do I need?
Probably what you already have! If you're already active on packet radio, say with a transceiver and a packet TNC (Terminal Node Controller), all you need is to run the appropriate APRS software on your PC. If you don't have a TNC, you can use the soundcard on your PC with a simple audio cable to link to your rig. To make the soundcard work as a TNC you will need to use some free software called AGWPE. Have a look at the LINKS page for a step by step guide on downloading and using AGWPE.

There's software readily available for DOS, Windows, Apple and Palm PDAs. Take a look at if you'd like to download the latest versions of APRS and try them out. All the software I've detailed is either shareware, freeware, or public domain, so you can try it out without first putting your hand in your pocket. I would strongly recommend UI-View written by Roger Barker G4IDE. A link to a mirror of Roger's original site is on my LINKS page.  Sadly Roger died of cancer on the 8 September 2004, but I am sure that such an advanced and user friendly software application will continue to be the best in the world for many years to come.  Add-on utilities continue to be written and enhanced, plus the "family of UI-View users" is continuing to support it via the Yahoo eGroup.  

To take a look at what's on the air right now, point your browser at (Use page up or down to zoom and alt-click to centre the map)

Connecting up
To get started on APRS yourself, apart from the software you'll need a packet radio station. The "traditional" way is to use a TNC plugged between your PC and transceiver. This converts the RS-232 (also know as V24) from your PC into small data "packets" and transmits them as audio from your transceiver. On receive, the reverse happens and the TNC decodes the packets into RS-232 strings which are sent to your PC for display. Here you'll just need a suitable TNC, an RS-232 cable, and audio / PTT cables to your transceiver - the TNC's instruction book usually gives plenty of information on connection pins etc.

Running the software (See my other APRS page for how to set up UI-View)
Let's assume you're set up on packet with your PC, TNC and transceiver. Tune your transceiver to 144.800MHz on FM and you may already hear APRS "beacons" being sent. This is the coordinated frequency across much of Europe for APRS type transmissions.  Please check your local band plan for frequency details if you are operating outside of the UK.  Station icons will then begin appearing on your screen fairly quickly, and you'll typically see stations with different symbols displayed. Most home-based stations will be shown as a small "house" icon, and digipeaters will show up as a star. APRS weather stations will appear as blue dots with white wind barbs or a "WX" indication on the icon. Cars, lorries and motorbikes will also pop up within a short while.

Weather reporting
APRS can also be used for weather reporting, as well as just for location information. Click on one of the weather stations and you'll see what the conditions are at that station's location. This is because APRS position reports can also include the wind speed, direction and other important weather conditions.  If you look at the screen grabs on this page you will see examples of weather stations.

Do I need a GPS receiver?
APRS can also be used with manually entered positions while you're operating from home or a few fixed positions, for example while helping out on a marathon.  The Kenwood TH-D7E handheld transceiver, or the Kenwood TM-D700 and TM-D710 mobile rig, have a built-in TNC and an APRS-compatible operating mode.  With these rigs you can enter your position from the front panel as you move around.  However the "easiest" way is of course to plug in a GPS receiver.  A GPS connected to your TNC will automatically update your positions as you move.  

APRS while you are on the move
Your APRS software will usually have asked you to enter your station details in when you first started it, and it uses this in your packet station's beacon text. Another method typically used by mobile and portable stations, who don't need a PC screen themselves to keep track of others, but do want others to keep track of them, is to connect a GPS receiver directly into the TNC. The TNC uses a special "GPS" mode in its firmware. I use a TinyTrak unit in my car, connecting the GPS to a single channel 20 year old Yaesu handheld rig. The TinyTrak units are available as PCB kits or pre-built, from this site

The TNC (or TinyTrak) then periodically automatically beacons APRS position reports, at intervals you've set in your TNC, from the virtually continuous NMEA data stream sent by the GPS to the TNC. Such stations are called 'trackers' and many stations around the world have also added such a system to their car. You could of course add one to yours, your partner could then see where your are at any time and get your meal ready when you arrive home from work! One station in the USA even watched his stolen car being driven across town on his APRS map. The police couldn't believe it when he told them his car was still on the move and that he was tracking it!  I watched my car being "test driven" by a garage mechanic at 80 mph in a 30 mph limit, past a school.  I complained when I collected the car at the end of the day and hence saved the 20 charge for the test.

Any GPS receiver capable of outputting its data in standard NMEA-0183 format can be connected, and the position information sent by the GPS to the computer will automatically place you on the map.

Messaging, position intervals and digipeating
As well as position reports i.e. "one-way" data, APRS also supports two-way messages. You're alerted when there is an incoming message for you, and it is stored for you until you delete it. It's also possible to send short one or two line bulletins in the same way, which everyone sees.

Each new message line is transmitted immediately, then a few seconds later. After every transmission, the period is increased, until an ACKnowledgement message is received.

To get your APRS packets that bit further, many stations have set up specific digipeaters covering given areas. Individual stations can of course also be used as digipeaters and in the United Kingdom they would set their software 'Alias' to
'WIDE1-1'. This way you can set your UNPROTO address to 'APRS via WIDE1-1' for your UI packets to be automatically digipeated. Don't worry about this jargon, or knowing what it means, just have a look at the how to set up UI-View page on this site and copy what I have done. This will get you started and you can tune your own preferences in the future.  In many countries where there is a higher density of stations, for example the USA, there has been a move away from using RELAY, to WIDE1-1.  For consistency across national boundaries many are trying to standardise on one set of recommendations, but you will find conflicting guidelines.  It is always worth finding out how the majority of local users have set up their stations and digipeaters.

Diagram showing how digipeaters work

Wide-area digipeaters are normally set to digipeat stations that have "WIDEn-N" as their UNPROTO path (where n and N are a number between 1 and a maximum of 7 e.g. WIDE3-3).   In the UK many stations in the past used and alternative to WIDEn-N, called TRACEn-N  e.g. TRACE3-3.  TRACEn-N was used so that receiving stations can see which digipeaters the beacon has travelled via. However many digipeater owners have now modified their software so that WIDEn-N also traceable.  Each digipeater will add its own callsign and decrement the number by one e.g. 

12:59:05T G4IQI>APU25J,WIDE3-3,RFONLY <UI C Len=60>:
=5125.11N\00046.93W-Mac using TM-D700, Datum=OSGB36 {UIV32}

12:59:08R G4IQI>APU25J,MB7UJ*,WIDE3-2,RFONLY <UI C Len=60>:
=5125.11N\00046.93W-Mac using TM-D700, Datum=OSGB36 {UIV32}

12:59:10R G4IQI>APU25J,MB7UJ*,G1GQJ*,WIDE3-1,RFONLY <UI C Len=60>:
=5125.11N\00046.93W-Mac using TM-D700, Datum=OSGB36 {UIV32}

This tells me that my home station transmitted a beacon at 12:59 and 5 seconds, it was retransmitted by my local wide area digi MB7UJ, then G1GQJ heard MB7UJ's retransmission.


It is possible to use WIDEn-N to allow your beacon to go via up to 7 digipeaters. Some digipeaters will not add its callsign to the path. The following example shows a mobile station using an UNPROTO of WIDE1-1,WIDE3-3. It was first digipeated by MB7UJ. However it is not possible to identify all the three WIDE digipeaters.

13:01:01R G3XYZ-9>UP4RVR,MB7UJ*,WIDE3-3 <UI R Len=15>:

13:01:02R G3XYZ-9>UP4RVR,MB7UJ*,MB7UG*,WIDE3-2 <UI R Len=15>:

13:01:05R G3XYZ-9>UP4RVR,MB7UJ*,MB7UG*,WIDE3-1 <UI R Len=15>:

If you compare the example above, you will also notice that the second line is also very different.  The beacon from my home location shows the latitude and longitude in an uncompressed format that can be easily read.  The beacon from G4XYZ's vehicle is in a compressed format known as MIC-E.  With MIC-E transmissions the latitude, longitude, speed direction etc. are contained in both the second line and in UP4RVR (this is called the destination address field).  UI-View can also compress its beacons as can be seen in the following examples from my home station.  It is worth turning on Compressed beacon as it reduces the amount of data and therefore increases the capacity of the network.  It should be noted that this is not the same as MIC-E compression:

Compressed beacon by UI-View:
11:27:39T G4IQI>APU25J,WIDE3-3 <UI C Len=54>:
=\4Ne/N<!u- BMac using TM-D700, Datum=OSGB36 {UIV32}

Uncompressed beacon by UI-View:
11:28:40T G4IQI>APU25J,WIDE3-3 <UI C Len=60>:
=5125.11N\00046.93W-Mac using TM-D700, Datum=OSGB36 {UIV32}

The destination address field in the above two example beacons is used to identify the software type and version e.g. APU25J is for 32 bit UI-View version 1.99.  It is also possible to use another encoding format which uses raw GPS information. 

Raw GPS beacon:


Internet linking
At the beginning of this page, I gave an Internet web site with "live" APRS data, and it's true to say that the Internet has given APRS a tremendous boost.  It is now possible to connect in and have access to APRS in other countries, including trackers, messaging, and weather data.

Within the current UK licence it is permitted for any station to port beacons from radio onto the internet servers, however to transmit information from the internet onto radio requires a Notice of Variation (NoV). It is very easy to overload the radio frequency with the vast amount of beacons on the internet, so this should not even be considered without a lot of experience of local conditions and without the agreement from other local stations. The distance that these internet ported transmissions should always be very limited and not normally sent via the digipeaters.

The best Windows APRS software in the world - UI-View
Its three main features are:

  • It will plot the locations of stations that it hears transmitting beacons, so you can see who is on the air around you.
  • It will show weather reports from any stations sending out weather beacons.
  • It has a casual messaging facility, which is designed to encourage local packet 'chat'. 

Low res screen grab of UI-View CLICK for higer resolution

UI-View screenshot. Click on the image to see it at higher resolution (96kb).

The program can be used with virtually any type of packet hardware, or with AGWPE. This means you can use packet modems, or your sound card running AGWPE software. UI-View's name is derived from the fact that it does everything via UI (Unnumbered Information) frames, e.g. beacons.

What's the difference between UI-View and APRS?
UI-View also has a few specific features which can be used with other UI-View equipped stations. You can "ping" another station, this sends an ASCII character which the other system recognises as a special message and simply replies with a packet "ACK". When your system receives this, it'll show you how many times, with a maximum of five, that the ping was sent. You can also use this "ping" facility via digipeaters if you wish, making it useful for trying different routes. A UI-View "query" can send a message to another station as a different ASCII code, the other system then automatically sends back its station information. A modification of this is a "DX query", which again sends a different ASCII code to which the other system replies with information of the best DX in its current station list, ignoring stations heard via digipeaters. 

More information?
If all this has whetted your appetite to have a go on APRS, take a look at the sites on my link page.

With that, have fun, and enjoy getting going with APRS 


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This information was partially based on an article written by G4HCL with additional sections and updates by G4IQI.