Explanation of APRS by its Author

AUTOMATIC PACKET REPORTING SYSTEM (APRS) APRS is a shareware program that takes advantage of the availability of inexpensive GPS receivers to display the locations of moving stations on your PC. By connecting your PC computer to any radio network via an inexpensive ($130) radio data modem, then all stations can see the movements of all other stations. APRS permits any number of stations to exchange data just like voice users but without the complexity of maintaining separate point-to-point links. The primary APRS display screen is the MAP display. To help mobile units find each other, there are radio nets standardizing on several radio channels:

145.79 MHz FM HAM radio VHF nets

10.151 MHz LSB Nationwide HAM tracking frequency
27.295 MHz CB Channel 29

APRS recognizes that one of the greatest real-time needs at any special event or emergency is the tracking of key assets:

Included on the distribution disk are several README text files on applications of APRS such as for weather nets, direction finding, plotting satellite contacts, and tracking mobiles. In addition to the map display from any range from .5 to 4000 miles, there are several other data screens:

STATION TRACKING. Although APRS automatically tracks mobile stations interfaced to GPS or LORAN navigation, it also easily tracks manual reports. 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 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, any operator can simply maintain the positions and movements of all assets on his screen, all other displays running APRS software display the same displays. There is also a Tracking command on the P display that will cause APRS to keep the map display always centered on a selected object.

SPACE APPLICATIONS: Some HAM radio satellites may not only transmit their current location, but can also be used for relaying station position reports between ground stations over large areas. The value of the map displays are in their ability to help students visualize the three dimensional geometry of an overhead pass.

FOX HUNTING OR DIRECTION FINDING: APRS is an excellent tool for plotting the location of a hidden transmitter, balloon, or interfering signal. APRS will display the intersection of bearing lines from a number of reporting stations and also overlapping signal strength contours if only signal strengths are reported. Finally, APRS includes the Fade-Circle Search and Rescue technique which can be used by a mobile with only an OMNI antenna to locate a hidden transmitter.

WEATHER STATION REPORTING: APRS position reports can also include the wind speed and direction, as well as other important weather conditions. APRS supports a serial interface option to the ULTIMETER-II home weather station. With this interface, your station includes WX conditions in your position report for display at all other stations in the network and you appear as a bright blue circle, with a line indicating wind speed and direction.

FREQUENCY COORDINATION: As more and more digital devices on amateur radio include APRS position information in their routine BEACONS, APRS makes an excellent tool for displaying the topology of radio networks as an aid to frequency coordination.

PROTOCOL - In order to efficiently use the radio channel, APRS assumes that old information is less important than new information. Therefore, all packets are redundantly transmitted but at a longer and longer repetition rate. Each new packet is transmitted immediately, then 10 seconds later. After every transmission, the period is doubled. After ten minutes only six packets have been transmitted, and they stabilize to once every 10 minutes beyond that.

COMMANDS: The keyboard is always active and responds to over a hundred different commands. Aside from selecting the major displays shown above, and manipulating the map display, there are several other sub-menu commands and the message commands:

DEMONSTRATION FILE: To see how the APRS system works on our frequency, use FILES-LOAD to load the file called FREQ579.BK. This file contains all the local stations on 145.79 MHz in our area. To see the tracking of the GPS equipped Army/Navy game football run, load the file named FBALL.BK and replay the file named FBALL.HST and select to see only FBALL, or CHASE1. To see the Marine Corps marathon event, load MARATHON.BK and replay the MARTHON.HST file. See Details in README.1st.

REPLAY: The positions of any moving station can be replayed either from memory or from a file. Tracks are kept in on-line memory until 150 have been saved, and then are saved to a HISTORY file. During REPLAY, use the Calls command to toggle on and off the display of call signs, and use the HOME and page keys to center and zoom the map display if the mobile station moves off the screen. During replay, use these commands:

FINDING A COPY OF APRS: APRS can be found on this site or on most other Amateur radio bulletin board systems. All you need is the APRSxx.zip file and the regional map file for your area, either E, C, W, or SEmapsXX.zip. You may also find even more local maps for your specific state, such as GA, CA, OH, TX, MI, IL, IA, and MOmapsxx.zip. After creating an APRS directory, be sure to use PKUNZIP -d. The -d option assures that the complete APRS directory structure is re-constructed as you unzip the files.

Copyright © 1996 Bob Bruninga

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