APRS 

 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.

Getting Started:

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://aprssig@tapr.org/ 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.

The APRS protocol relies on Unnumbered Information (Ul) packet frames to transmit location information. If you have previously used packet radio, you have used UI frames when you called CQ or activated your beacon function. For APRS work, all that is required is changing your beacon text, beacon rate, and path. On most TNCs these parameters are BTEXT, BEACON, and UNPROTO, respectively. In practice, there may be a few other parameters needing initialization, but these three are particularly important.

 

 

 

APRS is becoming so popular, virtually all TNC manufacturers advertise APRS-compatible TNCs that provide additional functionality specifically designed for APRS work. These features include the ability to easily interface with an external GPS unit. These TNCs also have parameters and buffers dedicated to APRS use. In those cases, the standard parameters (BTEXT, BEACON, UNPROTO) remain the same and you specify APRS specific parameters.

For example, one manufacturer of an APRS-friendly TNC has a Location Text (LTEXT) parameter where you specify your position and therefore leave BTEXT unchanged. This assumes that the TNC is being used in a stand-alone configuration (no computer control). When a computer with APRS software is used to control the TNC, you specify the parameters with simple "fill in the blank" boxes and the computer system controls initialization. However, for simplicity and clarity, let's assume that you have a standard TNC and that it will be transmitting position reports independent of a computer.

Setting the beacon rate requires a little understanding of the APRS network. Technically, there is no reason you can't transmit a location as often as you like. In practice, however, fixed stations (eg, home stations), should not transmit more frequently than every 30 minutes. There is a good reason for this: APRS is anunconnected broadcast protocol. This means there is no acknowledgment between stations when a packet has been received. Therefore, if packets collide, there is no retransmission and the information is lost. This is significantly different than the normal AX.25 connected protocol that assures error-free transmission. So, to reduce collisions, the rate between transmissions should be extended to assure a high probability that the channel is available.

Since a fixed APRS station, by definition, is not moving, updating its position more often than every 30 minutes is superfluous and counterproductive, so every 30 to 60 minutes is a reasonable interval. Some versions of the APRS software set rates to 20 minutes and others to 30, but you can set the rate as desired. For our current fixed location application, set your beacon rate to 30 minutes or BEACON 180 on most TNCs (180=1800 seconds=30 minutes).

The beacon text is the second important parameter that needs to be initialized. Because we're only talking about a fixed APRS station, there is no need for a GPS device. All you need to know is your location. Borrow a GPS receiver just long enough to determine the latitude and longitude of your station. With that information, you can set up your BTEXT. For example, BTEXT for my fixed station is:

/120800z3300.28N/11702.39W-PHG2230/Rick in Poway, CA

If you don't know your latitude and longitude, or if you don't want to tell the world your exact location, the standard procedure is to use your six-character grid-square designator, which provides accuracy to within a few miles. For example, here is an alternate beacon text for my location:

/120800z[DMI31a]-PHG2230/Rick in Poway, CA

All of these strings may seem like gibberish, so let's examine them in a little more detail. The first field consisting of the single slash character (/) indicates you are a fixed station with no APRS messaging capability. The second field uses the format, DDHHMM to indicate the date and time. Normally this field contains the current time and date based on real-time GPS information, but since we don't have a connected GPS for this system (or a real-time clock embedded in the TNC), the accepted practice is to specify the date that you started your APRS. In our example, 120800z indicates that the system was started on the 12th day of the month at 0800 Zulu Time. The next two fields contain the latitude (3300.28N) and longitude (11702.39W), or a single field for the six-character grid square (DM131a). The next character is a hyphen (-), which indicates that you are a fixed station at your home. There are nearly 200 symbols (characters) available that will designate your station as being in anything from an ambulance to an airplane!

 

 The Power Height Gain (PHG) field contains four digits that represent the power of the transmitter, the height, gain and radiating pattern of the antenna. For our example, 2230 represents a transmitting power of 4 W, and the antenna is 40 feet high with 3 dB gain, with an omnidirectional radiating pattern. If this has you scratching your head in bewilderment, don't worry! The APRS documentation (see PROTOCOL.TXT in the APRS software bundle) explains it in detail. We'll come back to this in a moment.

After the PHG field you append a comment. This is typically your city and state, name and perhaps a short salutation ("Howdy!").

The only other major TNC parameter that must be set is UNPROTO (unprotocol). Like the PHG parameter, we will cover this in more detail when we discuss networks but, for the present, a good starting path for a fixed station is:

APRS VIA WIDE2-2

That's all there is to it. Slight variations may be required depending on your TNC. In most cases, however, it is as easy as setting BTEXT, BEACON, UNPROTO and tuning to 144.39 MHz. At this point you are able to see APRS activity and transmit your location to others. Now let's hit the road.

 

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.

Summary:

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.

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.

 

 

 

 

 

 

 

APRS Maps and Charts
Open APRS.net Live tracking Map APRS FI Maps Live Tracking Map
APRS Client Software KA2DDO APRS Client Software
D-Star on APRS Fusion on APRS

 

 

 

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Last updated December 2015 (C) W3PGA

 

 

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