Automatic Position Reporting System

Updated 01/23/2003 01:09 AM

Introduction to APRS

For those not familiar with the Automatic Position Reporting System (APRS), the most important thing to understand is that APRS is not just a communications tool like Packet or RTTY, but rather a whole set of tools - a toolbox if you will - all designed to work together. All of these tools designed to facilitate the management of any team effort across a wide area.

What's inside APRS

Let's take a peek inside this toolbox and see what we find:

Geographic Information Systems: APRS provides for the display of data on a wide variety of maps. Virtually any map which may be displayed on a computer may have APRS data displayed over it. The display may be in two dimensions (position on the earth's surface) or three dimensions (including altitude). Some of the items typically displayed on a map include: storms, weather observations (e.g. temperature, wind speed. etc) airplanes, moving vehicles, runners, bicycles, shelters, ham stations, homes, businesses, fires, accidents, etc.

APRS Software: This is the tool which ties all of the parts together and sorts incoming data into meaningful tables. The APRS software may be run on virtually any computer which supports one of the following operating systems: MS DOS (ver 2.0 and higher), MS Windows (ver 3.0 and higher), MS Windows CE, Macintosh, LINUX, and Java. APRS programs have also been written for a variety of postage-stamp processors, and at least two Kenwood and one Alinco receivers. This software provides for the handling of all of the following types of data: position reports, status reports, objects, weather reports, storm data, shelter data, telemetry, messages traffic, bulletins, warnings, and beacons.

Global Positioning Satellite (GPS) Systems: The Global Positioning Satellite System allows users to pinpoint their exact location on the surface of the earth (or above it). Typically, a small antenna, about the size of a computer mouse can provide digital data containing the latitude, longitude (and sometimes altitude) of the device. When this data is input to the APRS system, all users can see the exact location of that device on a map as it moves. For example, a mobile ham unit (car) using APRS could have its location constantly reported to an EOC (Emergency Operations Center). APRS can also work with LORAN systems used in the maritime environment.

UI Radio Packets Packet Radio is a system invented by hams to transmit digital data over radio circuits designed for voice. Data to be transmitted is split into small units called packets and then converted to audio tones (often via FSK) and transmitted as an audio signal. On the receiving end, the tones are converted back to digital data and assembled into the original stream of data. UI refers to Un-numbered Information, which is a specific type of packet which does not require acknowledgement (the data equivalent of a voice broadcast). Radio frequencies most commonly used by APRS include 144.39 Mhz, 7.085, 10.151, and 18.107 Mhz LSB. However, APRS packets may be transmitted over almost any voice channel, including over police radios, amateur repeater systems, business band, marine band, Citizen's Band, cell phones and even land-lines.

APRS Digipeater Network In the 2-meter band, APRS stations may have as little as 1 watt of power and yet have their packets reach stations as far as 400 miles away. This is accomplished by a network of dedicated digital repeaters (digipeaters) which relay signals over long distances. To avoid network saturation, packets are normally repeated only three times, hence the 400 mile limit.

Trak-Net For mobile users in remote areas not served by digipeaters, packets may be repeated via orbiting satellites. The mobile station requires a 10 watt output and a 5/8-wave antenna to uplink a signal on 144.90 Mhz.

APRS-NET In addition to staying in touch by radio, APRS can take advantage of the Internet to transmit data to stations out of radio range, or to stations not equipped with radio capabilities. By design, APRS data is only transmitted about 400 miles by radio. However, the Internet allows users all over the world to exchange data when desirable. Virtually any computer with APRS software and a modem may receive data from the APRS-Net. (ISP not required!)

ZIP-LAN For computers not already connected to a network, a very inexpensive serial data network may set up to provide all users access to APRS data. This would be helpful in a police headquarters or EOC where there may be computers that normally work independently, but all need access to APRS information during an emergency.

Weather Stations Inexpensive weather monitoring equipment may be connected to an APRS station to provide a continuous stream of weather observations from the station's location. Parameters such as temperature, wind speed (gust and sustained) and direction, barometric pressure, rainfall and relative humidity are often available on a real-time basis from locations all around a major city. This data can be of enormous value to weather forecasters, race organizers, and hazardous-materials- incident teams.

Aircraft Communications and Reporting System - ACARS - Commercial aircraft use this packet system to stay in touch in the 129 - 131 Mhz band. If your TNC is programmed to recognize these packets, they may also be reported by APRS, giving you position information on all participating aircraft within 200 miles of your location.

If all of this sounds a bit overwhelming to you, remember that our telephone system in the US is pretty complicated, too, with central offices, satellite links, switching centers, microwave circuits, cell sites and much more. BUT it does not take a rocket scientist to make phone call! Just as a six-year old can effectively use a telephone, a teenager could easily send and receive messages over APRS.

As you can see, there's a whole lot to APRS - more than you could learn in one sitting. The fundamental concept behind APRS is the old saying, "A picture is worth a thousand words." The same concept applies to learning APRS: you could do an awful lot of reading about APRS and still not have a good idea of how it works. Probably the best way to learn APRS is to obtain a shareware copy of APRS software for your computer, load it, and experiment with it. Note that you don't need any radio or packet equipment to get started. If you can connect to the Internet, you can see the whole world on APRS!

Photo of typical APRS tracker = http://web.usna.navy.mil/~bruninga/tracker.jpg

 For a sample screen shot of what the output data might look like, see http://aprs.rutgers.edu/images/NJ.gif

 APRS shareware program download site = http://www.cave.org/aprs/   (only those of us on Willit will need the software, but I mention the link for people who may be interested)

 Volunteer APRS helpers, state by state = http://www.kcaprs.org/elmers.htm.

 Master page of APRS links = http://web.usna.navy.mil/~bruninga/aprs.html

 
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