APRS is the result of 13 years of experience with trying to use packet radio for real-time communications for public service events. Packet radio has rarely been used effectively in real-time events, where information has a very short life time. To solve this problem, APRS avoids the complexity and limitations of trying to maintain a connected network. It uses Ul frames to permit any number of stations to participate and exchange data, just like voice users would on a single voice net. Stations that have information to contribute simply transmit it, and all stations monitor and collect all data on frequency. Secondly, APRS recognizes that one of the greatest real-time needs at any special event or emergency is the knowledge of where all stations and other key assets are located. APRS accomplishes the real-time display of operational traffic via a split screen and map displays. The major display subsystems and a number of other minor displays are as follows:

Latest Beacons

This display maintains a list of the latest UI frame received from each station. In effect, this is a multi-station, one-line broadcast message system. Since the lines contain the latest time of receipt, this display shows if a station was on line within the last few minutes.

Position

This display maintains a separate list of the position of each station. Each position report can also contain a brief comment, weather report, DF bearing, or other important information.

Maps

Maps to any scale from 0.5 miles up to the whole world can be displayed. Stations are instantly displayed when they transmit a properly formatted position beacon. Stations with a reported course and speed are automatically dead-reckoned to their present position. You can center the map anywhere in the world.

Messages

In addition to the beacon text, which is used to broadcast information to all other stations on the net, there is an operator-to-operator message capability. Any station can send multiple one-line messages to any other station. On receipt, the messages are acknowledged and displayed on the bottom of the receiving station's screen until the operator kills them.

All Traffic Log

This display is a time sequenced log of every new beacon or message line sent. Beacons are logged only the first time they are received.

When Heard

This display maintains a count of the total number of transmissions from each station per hour. These statistics are ideal for displaying the connectivity of the network over varying paths, such as HF, or to see when stations enter and leave the net.

Station Tracking

Although APRS automatically tracks mobile packet stations interfaced to GPS or LORAN navigation, the graphic capability of the maps works perfectly well with manual tracking or with grid squares. Any station on HF or VHF that includes its grid square in brackets as the first text in the beacon text will be plotted by APRS. Additionally, any station can place an object on its map, including itself, and within seconds that object appears on all other station displays. In the example of a parade, as each checkpoint with packet comes on line, its position is instantly displayed to all in the net. Whenever a station moves, the operator just updates the station position on the map, and that movement is transmitted to all other stations. To track other event assets, only one packet operator needs to monitor voice traffic to hear where things are. As that operator maintains the positions and movements of all assets on his screen, all other displays running APRS software display the same positions.

Using Dumb Terminals in an APRS Network

APRS automatically computes positions by latitude and longitude for all stations, based on the position of the cursor on their map display. No GPS or special hardware is needed in most cases. Even the simplest of portable packet stations with dumb terminals can report their positions if a pre-printed map which has a latitude/longitude grid reference is made available to all net participants. The operator of a portable station just looks at the map and enters his latitude and longitude into his beacon text. Using the same map, he can plot with pins the location of all other stations as he sees their position reports go by.

Space Applications

APRS could be a solution to the effective use of orbiting terrestrial style packet radio digipeaters in space such as on the shuttle, MIR, AO-21 and ARSENE. The problem with space digipeaters is the saturation on the uplink channel which makes the use of a normal connected protocol impractical. For a connected contact, a total of five successive-and successful-packet transmissions are required. Not only does APRS reduce this to one packet, but it also capitalizes on the most fascinating aspect of the Amateur Radio hobby by displaying the location of those stations on a map. If all stations inserted their latitude and longitude or grid square as the first characters of their beacon text, everyone within the satellite footprint would see the location of every successful uplink. Since the shuttle is a rapidly moving object, the locations of successful uplink stations will move progressively along the ground track. No changes onboard the shuttle or MlR would be required to implement this capability!

Weather Station Reporting

APRS also supports an optional weather station interface to the ULTIMETER-II home weather station. The wind speed, direction, temperature and rainfall are inserted into the station's periodic position report. The station shows up on all APRS maps as a large blue dot, with a white line showing the wind speed and direction. Several automatic APRS weather reporting stations, supported with additional manual reporting stations, can form a real-time reporting network in support of SKYWARN activities across your state.

Fox Hunting or Direction Finding

APRS is an excellent tool for direction finding (DF). The X command (cross fiX) has been added to permit displaying the intersection of bearing lines from a number of reporting stations. The DF stations can either be manually placed on the map, or they can automatically be plotted if the DF bearing is included in their BText position report. All stations running APRS can simply hit the X key to display the intersection of these bearing lines. As of APRS version 3.00, there is an optional Doppler DF interface for automatically plotting and transmitting instantaneous DF bearings.

Protocol

Since the object of APRS is the rapid dissemination of real-time information using packet UI frames, a fundamental precept is that old information is less important than new information. All beacons, position reports, messages and display graphics are redundantly transmitted, but at longer and longer repetition rates. Each new beacon is transmitted immediately, then again 20 seconds later. After every transmission, the period is doubled. After ten minutes only six packets have been transmitted. After an hour this results in only three more beacons; and only three more for the rest of the day! Using this redundant UI broadcast protocol, APRS is actually much more efficient than if a fully connected link had to be maintained between all stations!

Conclusion

APRS is the newest frontier in amateur packet radio and it parallels the explosive growth in the GPS technology. Although there are now-and will be- some amazing software products in the consumer marketplace, none have been written from the ground up to support amateur packet radio. Even without GPS, APRS is a fresh approach to using packet at real-time events.