Automatic Position Reporting System
Many of todays Ham Radios have GPS Built in. D-STAR and Fusion rigs have the ability to report APRS Data.
Long-time packeteer Bob Bruninga, WB4APR, developed the Automatic Packet Reporting System (APRS), which allows packet radio to track real-time events. It deviates markedly from the usual message- and text-transfer activity. Instead, APRS concentrates on the graphic display of station and object locations and movements.
For example, if you know the latitude and longitude of your station, you can add this information to the beacon transmissions sent by your packet TNC. Any monitoring station that's equipped with APRS software will translate the data and display your location on a computer-generated map.
Taking this idea a step further, if you own a portable Global Positioning System (GPS) receiver, you have precise position information at your fingertips. Connect the GPS receiver to your TNC and you can transmit your location informationeven as you're moving!
When any person in an APRS network determines where you're located, he can move his cursor and mark your position on his map screen. This action is then transmitted to all screens in the network, so everyone gains, at a glance, the combined knowledge of all network participants. In other words, everyone knows where you are. The map screen retains this information for future reference. This means that moving objects can be dead reckoned to their current locations with one keystroke--based on their previous positions.
With a small GPS receiver, a TNC and a hand-held transceiver stuffed in a cigar box, almost any object can be tracked by packet stations running APRS software. You can place these boxes on bicycles for a marathon event and, of course, in automobiles. This system is an excellent tool for triangulating the location of a hidden transmitter or jammer.
If you have a packet station (TNC, transceiver, and computer or terminal), you have all the hardware you need to start monitoring APRS activity in your area. All that remains is to obtain an appropriate version of the APRS software. There are currently three versions, for Apple Macintosh, DOS, and Windows. By the time you read this, there might even be an X Windows version to run on Sun and Linux workstations.
Any of the Web pages listed in this article will direct you to the latest version of the software. The software is offered as shareware, which means you may freely download the software and try it. The Macintosh and Windows programs are full working versions, only the ability to save settings has been removed. If you decide to continue to use it, you should send the authors the requested registration fee. This entitles you to future updates and support. The software comes with installation instructions, but if you need help, there is the http://email@example.com/ mailing list, where you will typically get an answer within hours, if not from the authors, by thousands of "Elmers" ready, willing, and able to help you.
Where to listen is the next piece of information you need. There are presently two main frequencies dedicated to APRS work: 10.151 MHz LSB on HF (the mark/space frequencies fall within the 30-meter ham band) and 144.39 throughout North America and Canada. Monitoring the HF band will show activity across the US, because of the propagation characteristics of 30 meters. The VHF band provides a picture of activity predominately on your home turf. However, as we shall see, there are exceptions to this rule!
Although I've indicated that there are only a few APRS frequencies, the situation is rather "fluid" and varies around the country. If you have Internet access, you'll find Web pages for the major metropolitan areas that provide excellent frequency coordination tools. There is APRS activity on HF bands from 40 to 10 meters, and also on 6 meters.
With standard packet station equipment and software, tune to one of the frequencies indicated and watch the activity. It won't be too long before you'll want to put yourself on the map, so to speak! So let's see what it takes to transmit APRS packets.
Taking It on the Road
If you intend to go mobile with APRS, you must have a GPS receiver because your position is constantly changing. The good news is that GPS receiver prices have dropped drastically. With a little careful shopping you'll find a useable GPS unit for under $200. Just make sure it has a NEMA-compatible data port.
Connecting the GPS to the TNC can be a bit tricky, but your TNC manual should offer a step-by-step description. Most GPS devices provide 4800-baud RS-232-compatible signals in a standard format that GPS-friendly TNCs can easily digest.
For mobile use, we are still typically concerned with three parameters: BTEXT, BEACON, and UNPROTO. If the TNC is configured properly, it will take the information provided by the GPS (latitude, longitude, time) and automatically use that as the beacon text. The beacon rate should be set to one-minute intervals, since we are now moving and need to update our position more frequently. Lastly, the UNPROTO path must be set. In our fixed station example, the UNPROTO path was set to APRS VIA WIDE2-2. For mobile applications it is common practice to insert the digipeater WIDE1-1 so that our final path is:
APRS VIA WIDE1-1,WIDE2-1
The APRS Network
After spending a lot of time looking at moving dots, I started to examine the packet frames to see exactly what was being transmitted. I had been in packet radio a few years, but seeing frames with a digipeater path of RELAY, WIDE, WIDE, or WIDE, WIDE, GATE, and virtually every other combination imaginable, I was lost! In this section I'll give you some examples of digipeating paths and explain why they are used. Let's pause for a quick AX.25 packet refresher course first.
APRS uses the AX.25 protocol, a wireless point-to-point protocol based on the international standard X.25 protocol. Enhancements to the X.25 protocol were necessary to accommodate the unique requirements of wireless communication and the Amateur Radio environment in particular. X.25 is a connection-oriented protocol. This means that it assumes that two stations will connect to each other and handshake before any information is passed between them. However, this would mean that Amateur Radio operators could never call CQ. Remember: By definition a CQ is a one-way broadcast to an unknown destination. For this reason, Ul (unnumbered information) frames are provided in AX.25. This is important because all APRS information uses UI frames, so that a single packet can be heard by everyone. When an APRS station broadcasts its location, it has no idea if it is being heard, nor does it expect a response (acknowledgment) that the packet has been received. This means that the packet is operating on an unreliable channel. Because it is only a matter of a few minutes before the information is repeated again, a lost packet for APRS is not as critical as a lost packet in normal packet communication.
Since most APRS activity is on 2 meters, transmission is limited to the line of sight. Add the limitation of mobile communication where the power transmitted is low (typically a few watts) and the antenna system is limited (a whip), it would appear that a mobile APRS station has little or no chance of being heard outside a radius of a few miles. This obstacle is overcome with a little help from your friends. Other local, fixed APRS stations with more power and better antenna systems serve as strategically placed digipeaters. It is through the use of these digipeaters that a mobile can broadcast its location to a large metropolitan area-and beyond.
Let's look at a standard packet station example:
C KK5SU VIA WA3ZFE, KC5PVL
In this example, we are sending a packet to KK5SU through stations WA3ZFE and KC5PVL. This is known as digipeating and many packet users do it when the desired station cannot be contacted directly. WA3ZFE receives the packet first and rebroadcasts it to KC5PVL, who repeats it again, sending it to its final destination, KK5SU. This is old hat to the veteran packet user, but its importance cannot be overemphasized in APRS work. Digipeating is a powerful tool and forms the building blocks for APRS networks. It allows the APRS users to extend their range far beyond line of sight.
For standard packet station communication, we know the call signs of the digipeating stations (WA3ZFE and KC5PVL, in the previous example). The same cannot be said for APRS. So how do you digipeat through a station when you don't know its call sign? No problem. All you need to know are the standard aliasnames. Any station that is set up to respond to an alias is capable of handling your packets automatically, even if you don't know its call sign!
Commonly used aliases are: WIDEn-N on VHF and GATE on HF. Each alias denotes a very different type of station. A WIDE1-1 station is one with a limited range (a few miles). A WIDEn-N station is usually a dedicated station with wide local or regional coverage. A HF GATE station has a very wide coverage area (500 miles or more). Most, if not all, GATE stations are really gateways from 30 meters to 2 meters. If you're monitoring your local 2-meter APRS network and suddenly see a symbol showing a station 500 miles away, chances are it is a packet relayed through a GATE. (Depending on your station setup, it might also be a packet that reached you directly via meteor-scatter. Yes, they're doing APRS meteor scatter, too!)
We are now prepared to return to the topic of the PHG (power, height, gain) parameter. As every Amateur Radio operator knows, the line-of-sight range of a station is primarily a function of PHG. Therefore, if we look at the coverage circles of local APRS stations, we are in a good position to develop digipeating paths that are customized to our particular location. Figure 1 shows coverage circles for southern California. If we examine these displays and we determine that we cannot hit a WIDEn-N digipeater, then we would be justified in adding RELAY as the first hop of our digipeating path. in a similar way, if we know that we can hit two WIDEn-N digipeaters, we may wish to use a digipeater path that specifies the call sign of the digipeater rather than using the generic WIDEn-N alias. In this way, we bring up only one WIDEn-N station and reduce channel congestion in the process.
APRS is an interesting and fun aspect of the amateur radio hobby that provides features and facilities you couldn't get by any other method.
- It provides a way for you, your family and friends to track your position.
- It provides a way for you to track the position of other things using low powered radio trackers over amateur radio.
- It provides a way to present radio-related information like the location and frequency of local repeaters, the location of a club house, hamfest or field day site, in a way that is useful to visiting hams.
- It provides a free ham to ham text messaging service.
- It provides a way to send and receive alerts, warnings and other ham radio or safety related bulletins.
- Using the APRS to email gateway you can even send short emails from your radio, though the return path isn't supported.
APRS is an easy and inexpensive aspect of the hobby to get into, requiring in many cases no more than a free software download and your existing radio.
I hope that this article has whetted your appetite to try APRS for yourself. If you want more information then here are some links to good resources that explain specific aspects of APRS in some detail.
In its most widely used form, APRS is transported over the AX.25 protocol using 1200 bit/s Bell 202 AFSK onfrequencies located within the 2 meter amateur band.
Sample APRS VHF frequencies
An extensive digital repeater, or "digipeater," network provides transport for APRS packets on these frequencies. Internet gateway stations (IGates) connect the on-air APRS network to the APRS Internet System (APRS-IS), which serves as a worldwide, high-bandwidth backbone for APRS data. Stations can tap into this stream directly, and a number of databases connected to the APRS-IS allow web-based access to the data as well as more advanced data-mining capabilities. A number of low-earth orbiting satellites, including the International Space Station, are capable of relaying APRS data.
Many hams are using APRS imbedded rigs in Emergency situations, special events, public service events ARES/RACES nets and deployments. Station location is an important part of these events to track and deplot resources where they are needed. These radios can also transmit Photos and text to relay messages while communicating.
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