An Introduction to
Automatic Packet/Position Reporting System Reporting System (APRS)
Bob Bruninga, WB4APR
APRS is a registered trademark of APRS Software and Bob Bruninga, WB4APR.
What is APRS?
APRS is a real-time tactical digital communicatons protocol for exchanging information between a large number of stations covering a large (local) area. As a multi-user data network, it is quite different from conventional packet radio.
APRS is different from regular packet in four ways. First by the integration of maps and other data displays to organize and display data, second, by using a one-to-many protocol to update everyone in real time, third, by using generic digipeating so that prior knowledge of the network is not required. Since 1997, a worldwide transparent internet backbone, linking everyone worldwide has been implemented.
APRS turns packet radio into a real-time tactical communications and display system for emergencies and public service applications (and global communications). Normal packet radio has only shown usefulness in passing bulk message traffic (Email) from point to point. It has been difficult to apply conventional packet to real time events where information has a very short life time and needs to get to everyone.
Although the recent interfaces to the Internet make APRS a global communications system for live real-time traffic, this is not the primary objective. But like all of our other radios, how we use APRS in an emergency or special event is what drives the design of the APRS protocol. Although APRS is used 99% of the time over great distances, and benign conditions, the protocol is designed to be optimized for short distance real-time crisis operations.
APRS provides universal connectivity to all stations by avoiding the complexity and limitations of a connected network. It permits any number of stations to exchange data just like voice users would on a voice net. Any station that has information to contribute simply sends it, and all stations receive it and log it.
Secondly, APRS recognizes that one of the greatest real-time needs at any special event or emergency is the tracking of key assets. Where is the Event Leader? Where are the emergency vehicles? Whats the Weather at various points in the County? To answer these questions, APRS is a full featured automatic vehicle location and status reporting system too. It can be used over any 2-way radio system including HAM, CB, Marine Band, and Cellular Phone.
Now there is even a nation- wide live APRS tracking network on the Internet! Click here to view the AMSAT Live Nationwide APRS Traffic page.
Note - the Live Nationwide APRS Traffic page requires the Java 2 Platform, Standard Edition (J2SE) Runtime Library. If you do not have the Java Runtime Library installed, you can download it free of charge from Sun Microsystems.
APRS is on 144.390 MHz throughout the North American Continent. Other countries often use other frequencies. Check locally.
APRS and APRS software packages provide a number of features. A generalized list is provided here.
This display maintains a list of the latest UI frame received from each station. In effect, this is a multi-station one-line broadcast status system. On a DX cluster freq, this display accumulates a list of all users and what was their latest command to the cluster.
APRS software generally maintain a separate list of positions of each station often including a brief comment. They can also contain direction finding (DF) and or weather info. Maps can be displayed and APRS stations will be plotted. Stations reporting a course and speed are dead-reckoned to their present position. Overlay databases of the locations of all APRS DIGIpeaters, National WX Service sites and even HAM stores are and you can zoom in to any point on the globe!
In addition to the one-line STATUS packets, two-way messages with acknowledgment are supported. Incoming messages can alert the user on arrival and the recipient can reply using the same process.
APRS has the capability to send important multi-line BULLETINS addressed to everyone. These bulletins can include virtually anything but by convention these are generally used for high priority traffic that could not be sent by MESSAGES.
Most APRS software displays statistics such as the total number of packets per station per hour. These statistics may also show the connectivity of the network over varying paths, such as HF, or to see when stations enter and leave the net.
Although APRS automatically tracks mobile GPS/LORAN equipped stations, it also tracks perfectly well with manual reports or GridSquares. APRS will use a GridSquare in brackets at the beginning of any packet. Additionally, any station can place an object on his map including himself and within seconds that object appears on all other station displays. In the example of a parade, as each checkpoint with packet comes on line, its position is instantly displayed to all in the net. Whenever a station moves, he just updates his position on his map and that movement is transmitted to all other stations. To track other event assets, only one packet operator needs to monitor voice traffic to hear where things are.
Using APRS on All Digital Channels
You can use APRS posit packets on all frequencies as a general purpose network topology display on ANY packet frequency!
Not long ago there were at least 5 satellties that permitted APRS digipeating and 7 that could be received by your APRS equipped ground station. Unfortunately, as of January 2013, this is no longer true. There are no reliable APRS satellites left. The ISS digipeater is the most reliable but is frequently turned off, and PCSAT-1 goes days without digipeating a single packet. APRS is ideal for the short but congested satellite pass. APRS only requires one successful packet for everyone to see each successful station compared to the typical *CONNECTED* SAREX mode which requires 5 successful packets. Thus APRS reduces channel loading, while capitalizing on the most fascinating aspect of the amateur radio hobby, and that is the display on a map of the location of those stations.
Fox Hunting and Direction Finding
: APRS is an excellent tool for plotting the location of a hidden transmitter, balloon, or interfering signal. APRS software packages generally have several powerful DF tools:
- Plots the positions and Bearing lines of all participating stations whether mobile or fixed
- Plots the overlaping signal strength contrours for OMNI-DF reports. This techinque even plots big BLACK circles for NULL reports so that you see all the areas where the FOX is NOT! This OMNI technique is very powerful and locate a jammer to a neighborhood with NO beams or special equipment.
- Fade-Circle Search and Rescue technique for single station signal strength location using only a OMNI antenna
- Optional automatic DF interface to Doppler DF units for automatic plotting of DF bearings. For more DF info, see the DF.txt file.
Weather Station Reporting
APRS position reports can also include the wind speed and direction, as well as other important weather conditions.APRS software packages a serial interface option to many home weather stations to do this automatically. All weather stations show up as a blue circle, with a line indicating wind speed and direction. Finally, APRS users can generally set WX alarms and be alerted when WX conditions exceed those values. APRS Weather data is also fed into the National Weather Service's Radio Amateur Weather System (RAWS). You can view an example of observations for the San Francisco Bay Area RAWS system online.
APRS an ideal tool for the DX cluster user. With the proper software not only does the user get to see all DX spots on the map, but by operating in the monitor only mode, he has reduced the overall packet load on the DX cluster. This is a benefit to everyone on the channel. Usually the APRS monitoring station will see the SPOT as soon as the first station gets it, rather than later on down the list.
Internet and IGATES
APRS Users can access the worldwide APRServe system through stations that send received packets to it. The APRServe system can route packets though the internet to another IGATE station in a different geographic location. Using the normal 144.39 channel, you can send and receive message traffic to ANYONE.
The beauty of APRServe is that everyone connected can all FEED their locally heard packets in to the APRServe system and everyone everywhere can see them...
: Every packet asset on every frequency should include a position or at least gridsquare in all routine BEACONS. This alows APRS to be used to monitor network topography on any frequency. Thus, APRS makes an excellent tool for frequency coordination. In fact, javAPRS is now used by TAPR to maintain the entire digital data base for North America.
Although APRS redundantly transmits data, a fundamental precept is that old data is less important than new data. All APRS packets are repeated at an ever decreasing rate. Each new packet is transmitted immediately, then 20 seconds later. After every transmission, the period is doubled. After 20 minutes, only six packets have been transmitted. From then on the rate remains at 10 minutes times the number of digipeater hops you are using. This allows the rate to be every 10 minutes for a local event or every 30 minutes for the typical home station running WIDE3-3.
The following software packages will help you implement an APRS station. All are free except where otherwise indicated:
Includes Satellite tracking and Kenwood radio tuning
APRSdata serves the position, and frequencies of all satellites in view to all mobiles in the area.
UI-View is an APRS application for Windows that supports a wide number of TNCs and AGWPE host mode. It has a number of valuable plugins including an ISS and PCSat tracker and telemetry decoder. (€15, approximately $19).
WinAPRS provides real time station positions for Windows ($60).
Similart to WinAPRS but for Classic MacOS.
APRSPoint is an implementation of the latest APRS standard with high quality map support. ($169).
A Java based APRS client that is the same as the applet on the AMSAT Live Nationwide APRS Traffic page.
A Windows based APRS program with many features, including enhanced messaging, announcement support via ANSRVR, and QRU for object query support.
APRS Equipped Radios
Kenwood currently provides the TH-D7AG handheld and TM-D700A mobile transceivers that are APRS ready.
Trackers are generally small devices that interface to a GPS, format GPS positional data for APRS and control radios to transmit the information. Here is a list of trackers:
TinyTrak3 is a GPS position encoder transmits APRS packets with location at an adjustable rate. ($36 Kit, $60 Assembled)
- TAPR PIC-E
PIC-E is a PIC based encoder that can be programmed to encode GPS data for APRS use ($65)
APRS Related Web Sites
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