Designing a Private APRS System

Most non-commercial users of APRS will use the existing network of digipeaters and Internet gateways. But if you're tracking a large number of stations with a fast update rate, or if you're using APRS for commercial purposes, you'll need to build your own system. Argent Data Systems is a hardware manufacturer and not an installer; while we can provide you with many of the pieces you'll need for a do-it-yourself AVL system, putting it all together is up to you. Fortunately it's not as difficult as it might sound at first, and building a working system is easily within the capabilities of the average land mobile radio shop. Below are some things to consider when designing your system.

Why APRS

For commercial users, the main attraction of APRS is the fact that it's an open standard. All APRS devices and software can interoperate, give or take a few of the more esoteric features. The average commercial AVL system, on the other hand, won't interoperate with equipment from another vendor. 'Base station' receivers can cost thousands of dollars. Technical details aren't disclosed, customization isn't possible or is prohibitively expensive, and customers are 'locked in' to a single vendor.

APRS, on the other hand, offers unmatched flexibility, openness, and low cost. It's particularly useful where technical ability is more plentiful than cash; APRS is used extensively by small, technically-savvy companies, and in developing nations where alternatives are too expensive or unreliable.

APRS Data On the Air

APRS is usually used with 1200 baud audio frequency shift keying. This means that APRS data can be carried by inexpensive voice-grade radios, but it also means that the tones are audible as a short burst typically lasting about half a second. This must be kept in mind if APRS is to be used on a shared channel with voice traffic.

Modes of Operation

You'll first have to decide how you want to use the radio channel. In its most common form, APRS is esentially a CSMA/CA protocol - a device wanting to transmit listens to make sure the channel is clear, and then transmits its packet. Each tracker transmits at either a fixed interval, or an interval that varies with the speed and turn rate of the tracked vehicle.

This system works well where a dedicated channel is available, and a relatively small number of trackers are active in any given area. It has the advantage of requiring no prior coordination, and it works well with digipeaters, but because in a typical system trackers are frequently unable to hear each other, and are unable to detect collisions if two trackers transmit at the same moment, channel utilization is limited. As a rule of thumb, about 20% of the full channel capacity is available in this mode. If each transmission occupies half a second of air time, and each tracker transmits once each minute, then adding more than 24 trackers to the channel will result in fewer packets being successfully transmitted.

A more efficient option is to use assigned time slots. By assigning each tracker a designated time offset for its transmissions, relative to the start of the hour (as indicated by the GPS receiver's highly accurate clock), channel utilization can approach 100%, with no collisions. Argent's products are unique in supporting half-second time slots, allowing up to 120 transmissions per minute. But because no standards exist for time slot implementation between vendors, one or two second spacing is generally required between trackers from different vendors

The primary disadvantage of using assigned time slots is that they require prior coordination, and adding trackers to the network can require reconfiguring all of the existing trackers if there are not enough open slots available. This scheme is also poorly suited for networks where trackers may move between different digipeater coverage areas, as time slots must be allocated network-wide. In some cases, however, the increased reliability of a time slot system can be more important than complete channel utilization.

The third major mode of APRS operation is mic encoder or 'mic-e' mode. In this mode, data bursts are sent at the end of voice transmissions. This is the easiest way to use APRS on an existing voice channel without causing excessive annoyance to users - the data burst is generally no more annoying than a courtesy tone or ANI burst. Setting up a radio for mic encoder operation can be more difficult, though, and may require building custom cable harnesses. Also, in mic encoder mode the trackers typically aren't making automatic transmissions, and no position updates will be provided when the user isn't talking. Polling of trackers is possible, but can further complicate radio wiring.

Radio Interfacing

APRS devices are typically connected to a radio's speaker output, microphone input, and push-to-talk input. Many commercial mobile radios provide an accessory connection that has these signals. Additionally, you may be able to configure an alternate PTT input to transmit with a different CTCSS tone, or on a different channel, allowing a voice radio to be shared with the APRS system without the users being constantly annoyed by data traffic.

The tracker and GPS receiver also need to be wired to a power source - usually that's the same power source the radio is connected to.

Digipeaters

A digipeater is a digital repeater. Unlike most voice repeaters, digipeaters are typically simplex devices - they receive and transmit on the same channel. A high-elevation digipeater can significantly extend coverage for the local area, and multiple digipeaters can be used to form larger networks. Adding a single digipeater to a channel effectively doubles the air time required for each packet, though, and the likelihood of a packet arriving at its destination decreases with each hop required. Networks should generally be designed to require not more than two or three digipeater hops.

Digipeater placement depends mostly on the required coverage area and the intended destination of the packets. For many private networks, the goal is to deliver all packets to a single, central location. For others, it may be more important for units in the same general area to be able to communicate with each other.

Because they are simplex devices, digipeaters can be very simple to deploy, and typically consist of nothing more than a radio, antenna, and TNC device like the Tracker2. More elaborate cross-band setups are possible where high channel utilization is important.

If the required coverage area is small (e.g., a racetrack) then no digipeaters may be needed at all.

Base Station

For many users, the primary objective is to have all tracked units displayed at one or more PCs. Assuming the base station can hear all of the tracked units either directly or via digipeaters, this is easily accomplished. A base station can be as simple as a PC running free sound card modem software (e.g., 'soundmodem' under Linux), a free mapping program like Xastir, and a scanner connected to the PC's audio input.

Of course, a hardware TNC like the Tracker2 can be used on the base station side, and this simplifies radio interfacing if there's any need for the base station to transmit data - whether for messaging, polling, or placing map objects.

It's also possible to link multiple receiving sites through the Internet or a private TCP/IP network. This involves running a hub program on the network, which aggregates the received traffic and makes it available to users on the network.

Mobile Terminals

Search and Rescue teams, storm spotter groups, and many other organizations need situational awareness and not just vehicle tracking - each vehicle needs to be able to see others nearby. In some cases installing a full-fledged mobile PC in each vehicle might be the best solution, but many users will find that a mapping GPS receiver is sufficient. Both the Tracker2 and OpenTracker+ can send received positions to a suitable GPS receiver to be plotted as waypoints. Garmin navigation systems that support the Fleet Management Interface (FMI) protocol can also be used with the Tracker2 to provide two-way text messaging.

What We Can Provide

In addition to the off-the-shelf solutions shown elsewhere on this site, Argent Data Systems can provide customized trackers for your specific application. Typical modifications include new board designs to accommodate a required physical form factor and radio interface, and software modifications to add new features to meet unusual requirements. Depending on the number of units to be purchased, we can often provide these customizations at little or no additional cost.