APRS (or "Automatic position reporting system") is in many ways completely different to standard packet operation. It is designed for real-time data flow only, with no store and forward arrangements, and needs no BBS stations. It is designed to be on one frequency, and provides its own infrastructure, with higher power stations helping lower power stations.

It is not just about GPS and positions. It is totally unconnected, existing within UI frames, and relying on redundancy of paths to get its beacon to the desired destination.

There are three components of APRS - Firstly the originating station, which sends out a beacon. Secondly, intermediate generic digipeaters, which can operate point-to-point, and thirdly the receiving stations.

Beacons can take many forms, but are designed to be a container for data (much like AX25). They have some pre-designed formats, but other formats allow application specific data to be sent transparently.

The main device required to run APRS is a device to generate the beacon. There are dedicated tracker / telemetry unit (eg Tiny Trakk or Mic-E / Pic-E), but also most recent tnc's will do the job. Even suitably equipped PC's, pocket, palm device or Linux systems will enable APRS packets to be sent and monitored.

An APRS beacon contains the sending callsign, the path, a symbol (if a positional beacon, eg car, truck, etc), and data. Receiving callsign is irrelevant.

Beacons can report a station's position (Lat/Long/Height), but also its speed, in the case of a car/truck, etc. This data can be input manually or obtained from GPS equipment.

Beacons can also be used for direction-finding. The station's kit outputs an APRS-compatible string, puts a signal into a computer, tnc and radio - then triangulation is possible.

APRS can put "objects" on maps. These are "owned" by any station which cares to advertise them. They can mark events/places eg rallies, and can also "move" to be tracked by monitoring stations.

APRS can be used to broadcast weather information: wind speed, direction, etc. Also used for Telemetry, broadcasting anything you like (alarms, environmental information, etc).

Although APRS is different to AX25, it is capable of sending messages - no large documents, but single line communication, generic bulletins, group bulletins. It has a system of acknowledgement packets as part of APRS protocol (but not AX25 RR/REJ frames).

The path for these messages need not be known. Instead APRS uses generic paths, and when a station responds, the path is now known, so the next beacon uses an explicit path. If that path fails, the system reverts to a generic path to find another route.

Queries and Status - these are APRS commands by which intelligent stations can be pinged and queried; some stations have databases of information which can be interrogated.

In order for APRS to cover a wide area digipeating is required. Local "relays" work as Level2 hop, helping low power stations, by filling in RF "black spots". Smart digipeaters have wider area coverage, with some being linked to the Internet (so-called "IGates") to give even further coverage.

Digipeaters have three generic aliases. "Relay" (the basic one), "Wide" and "Trace". Additionally APRS uses "smart digipeating (using "WideN-N" and "TraceN-N").

With APRS there is a real need to know the path the packet has taken. If an explicit path cannot be established, messages will continue to be sent as a general broadcast, producing considerable QRM. Ideally, when stations repeat they change the generic alias in the path to their own callsign.

G1HUL went on to illustrate how digipeating operates, showing examples of digipeated packets. WIDEn-n and TRACEn-n routines were shown, with successive stations transmitting diminishing Wide7-7, 7-6, (etc) packets.

All TNC's can do L2 digipeating, and alias substitution (except MFJ's). Later Kantronics, TNC-2 clones and Kenwood TM-D700's can perform smart digipeating.

At the receiving end, most computer platforms are catered for with suitable software. Additionally dedicated monitors are available (eg HamHud, D7 and D700). Typically these programmes display maps of an area, possibly linking into AutoRoute and other street atlases. These programmes show output from weather stations, organize message management, have movement alarms, and can track stations.

The source of most confusion and positional is defective mapping. Generating your own maps requires an understanding of how local mapping co-ordinate systems relate to the world wide Latitude and Longitude system.

APRS uses the GPS system reference WGS84 datum. But most maps are surveyed to their own country's datum (eg UK to OSGB36). Fortunately datum difference is at worst only +/- 250m, which not normally critical. But most errors occur due to poor map calibration. Ideally all stations would use the same datum when mapping.

On the RF side of APRS, the allocated frequencies are 144.800 FM (1200 baud) & 14.105 LSB (300 baud). Other frequencies exist, but unless you are setting up a closed system (eg for a specific event) there is no point in using any other than these two frequencies.

There is an increasing number of IGATEs. These stations take everything they hear on RF and put it onto the internet. Conversely, they listen to Internet APRS traffic and relay anything which is unknown locally. These stations can monitor and relay several thousand station beacons within a few hours, and it is interesting to monitor such things as earthquakes (noting the reported Richter scale reading) and track paths of satellites.

On the net, station location queries can be made at http://www.findu.com.

APRS can be used for more than just hobby purposes. It can serve real time communication and carry out important data gathering. Monitoring can be conducted of significant weather "events", earthquakes, volcanic eruptions, etc. APRS can be put to emergency use (RAYNET, vehicle tracking, search and rescue, etc). It can help with pathless messaging, can enable remote status monitoring and can further propagation studies.

However it has a great appeal to many amateurs simply as part of their radio hobby. Many amateurs are using APRS (see the maps of observed stations). Perhaps packet decline is not as big as we think - G1HUL showed how he had monitored over 570 stations from his qth over a 2 week period.

For those with Internet the following URL's were recommended for further study:

http://www.aprsuk.net/
http://www.ui-view.com/
http://welcome.to/uiview/
http://www.ew.usna.edu/~bruninga/aprs.html
http://www.findu.com/
http://groups.yahoo.com (for ui-view, aprsuk and aprs groups)
http://www.tapr.org (for US-based aprssig and aprsnews groups)

During the question session, he agreed that the guaranteeing of message delivery was rather hit and miss over long distances with several hops. If such guarantees were required (eg for RAYNET use) a dedicated network of digipeaters was desirable.

Jim also agreed that most APRS use was confined to weekday evening and weekends, with relatively few users operating 24/7.

He tackled the issue of congestion caused by usage of one frequency, stating that congestion was not particularly an issue, although it would be if there more reliable links in the UK. Apparently US stations often discuss congestion.

Jim invited contacts for advice on APRS, advising use of e-mail in the first instance. His details:

Jim Andrews, G1HUL
E-mail: jim@stuckinthemud.org
Tel: 01530 249218
 

Minutes by Mike Wager, G7RAZ @ GB7WIS.#24.GBR.EU