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Amateur Position Reporting System
Amateur Position Reporting System (APRS) is a system of transmitting and receiving reports of position, direction and speed of travel, weather, and other data using software that displays the data on a computer screen as plots on a map. APRS was developed by Bob Bruninga,an amateur radio operator who is also a faculty member at the US Naval Academy, Annapolis, MD. An APRS system has several components:
How Does It Work?
Back in the early days of radio communications -- even today, in some areas of the world -- messages, "traffic" in radio jargon, were passed using Morse code or, later, voice. Because of the vagaries of radio transmissions, messages were often not received completely. Static crashes, fading, interference from other stations would all cause the receiver to miss parts of the transmitted message. For this reason, message transmission became standardized. When a station had a message to transmit, first would be transmitted a preamble with such information as the transmitting station, the destination of the message, and the group count, that is, the number of words in the message. If the receiving station missed some of the message, the operator on the receiving end would reply with requests for "fill" -- asking the transmitting station to fill in the blanks.
Packet radio, developed by a group of amateur radio operators in Tucson, Arizona, is a modern, computer-driven version of old style message transmission. Suppose you want to transmit a message containing a long status report, listing various types of equipment, the mileage or hours on each piece of equipment, service data, and current condition. You would type the message into your computer which would then send the message to the Terminal Node Controller, a fancy modem that runs packet firmware. The packet firmware arranges your message into "packets." Each packet is a specific length and has inserted into it "check bits," formatted bits that are standardized in the packet protocol. The TNC then sends this formatted data to your transmitter where it is transmitted.
Because the receiving station is running a TNC with the same protocol, the receiver knows that each packet should be a certain length and that "check bits" are to be inserted at certain points. If a static crash, a fade, or some other type of interference causes the TNC not to receive what it is expecting to receive, the receiving TNC automatically tells the transmitting TNC what it received and what it did not receive. The transmitting TNC then re-transmits the missed packets.
All this happens at the speed of light. The result is perfect data transmission. All the operator sees is a clean message showing up on the computer screen. The TNCs are doing all the work of filling in the missed packets.
A Simple APRS Setup
The simplest form of APRS setup consists of a computer, a TNC, a transmitter and receiver capable of operating on the amateur radio bands, and APRS software. To make it work:
Here is what it looks like. This is a screen shot from my computer running WinAPRS, a windows version of APRS. I am located in Bristol, Tennessee. What you see here are call signs of stations that are transmitting their locations. Note that there are different symbols. These symbols indicate a station that is transmitting weather, a repeater station, a station located in a vehicle, or other types of stations. The yellow lines are state lines; red and blue lines are highways.
Click here to see more shots of my APRS screen.
And here are more APRS screen shots.
A Better Look
For another view, go to this link. This is an APRS station in Florida running APRS data with a Java applet that allows real-time tracking to be shown over the Internet.
There is another APRS application known as a stand-alone tracker. This application requires a GPS receiver, a TNC, and a transmitter-receiver. The GPS receiver receives data from the GPS satellites, thus, it knows its location and, if moving, the speed and direction of travel. That data is fed to a TNC which packages it into packets and sends the data to the radio, which transmits location, speed, and direction of travel.
I have in my old VW Beetle a Garmin GPS-40 GPS receiver with an external antenna lying in the back window, where it has a clear shot at the sky. The GPS-40 sends data to the TNC. The TNC is set to transmit my location every two minutes. The TNC controls an old handheld transceiver, the output of which (1/2 watt) goes to an amplifier with 25 watts output. Thus, every two minutes, my tracker transmits my location. This data shows up on APRS screens as a little red car with my call sign.
Here is an article with photos describing my APRS stand-alone tracker.
UPDATE -- As of August 2007, we are living in an apartment in Knoxville, TN -- after losing everything in Hurricane Katrina and a number of other personal challenges. I have an APRS station running in the apartment -- follow this link for the update.
The TNC allows the operator to designate how the signal is to be retransmitted through the use of the UNPROTOcol Via command. By inserting a callsign or other station identifier after the command UNPROTO V, the transmitted signal will be retransmitted by the station whose callsign appears after the V. You can designate several retransmitting stations, such as: UNPROTO V STATION1,STATION2,STATION3. "Stand-alone trackers" such as the one in my old VW are set up to be retransmitted as UNPROTO V WIDE,RELAY. Thus, any station whose callsign is WIDE or RELAY will receive the signal from the old VW and retransmit it.
Most APRS fixed stations set up with the station call sign and either RELAY or WIDE as an alias. Many TNCs allow the use of two aliases; my fixed station TNC is set up with W4HH as the primary call sign and WIDE and RELAY as the aliases.
With mobile stations set up to be relayed VIA WIDE,RELAY and with fixed stations identified as WIDE and RELAY, the relatively weak signal from a mobile station will be picked up by a fixed station and retransmitted. Mobile stations are generally low power and have only simple antennas. Fixed stations generally have more sensitive receivers, more powerful transmitters, and much better antennas. It is not unusual for me to sit at my screen and see tracking data on mobile APRS stations all over the country. A mobile station in Kansas will be received by a fixed station, retransmitted from WIDE, RELAY fixed station to another, until I see it on my map display here in NE Tennessee.
So, What Good Is It?
Commercial trucking companies use APRS systems. The next time you see an 18-wheeler up close, look at the top of the cab. You may see a device that looks like a big bowl. Inside that radome is a GPS receiver, a TNC, and a transmitter and receiver operating on a frequency used by satellites. The GPS receiver plots the truck's position, speed, and direction. That data is fed to the TNC which activates the transmitter. The signal goes to a satellite that retransmits the location, speed, and direction data to the trucking company headquarters. This way, the company can keep track of where its trucks are. The company can send a signal to the satellite that commands trucks to transmit (each truck has an identifier). By storing and replaying the GPS plots, the company can track a truck's travels for the past XX hours or days.
The Annapolis Amateur Radio Club uses APRS to assist in the Marine Corps Marathon in Washington, D.C. Stationed along the marathon route are reporting stations with stand-alone trackers. Aid stations, ambulances, and marathon officials have trackers in their vehicles. Each type of station transmits a different symbol. At marathon HQ, there is a computer that displays a map of the marathon route. Showing on that map are little red crosses designating aid stations, little red ambulances, etc., etc. When a report is received of a runner down or another problem, with just a glance at the screen, officials can locate and identify the closest aid and dispatch assistance.
Some luxury cars come with an emergency locator option. You have an emergency, you hit a button on the dash, and the car transmits a call for help. That is nothing more than an on-board APRS tracker that sends a signal to a satellite. The satellite retransmits the call for help to a central location from where help is dispatched.
APRS has great flexibility in that it can be programmed to transmit any sort of data and cause that data to be displayed in graphic fashion on a computer screen. How about: rain cloud symbols if it's raining; snowflakes for snow; and the like.
Want to Try APRS?
First you must have some sort of licensed radio operation. Most common APRS use is among amateur radio operators. If you are an amateur radio operator, when you purchase a TNC (I use the AEA PK-12), you get a shareware version of DOS APRS. Send Bob Bruninga $25.00 and he will register it for you. The difference is that the unregistered version has to be configured with callsign, location, etc., every time you use it.
One warning. I could not run DOS APRS under Windows 98. Ran okay under Win3.1 or Win95. But, there is something about how Win98 handles COM ports that required me to go to WinAPRS.
Here are some links:
Update: August 2007
Some of the information in this article is out of date. As you may have seen elsewhere on this website, we moved to the Mississippi Gulf Coast in January 2005 where we were wiped out by Hurricane Katrina. We moved back to Knoxville, TN, where between October 2005 and August 2007: my father died; mother was diagnosed with pancreatic cancer; son got married; mother died; daughter graduated from law school; and now -- August 2007 -- we are deciding where to settle down, finally.
Meanwhile, I decided to activate an APRS station in our Knoxville, TN, apartment. Click on this link to see the APRS station I am using as of August 2007.