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Text File | 1993-05-04 | 14.3 KB | 248 lines

AUTOMATIC PACKET REPORTING SYSTEM (APRS) APRS is the result of my experience over the last 13 years with trying to use packet radio for real-time communications or public service events. Packet radio has great potential but so far has been best used for passing large volumes of message traffic from point to point or into the national distribution system. It has been difficult to apply packet to real time events where information has a very short life time. Typically, several steps are involved in preparing and passing message traffic including decisions about routing and connectivity. APRS avoids the complexity and limitations of trying to maintain a connected network. It permits any number of stations to participate and exchanges data just like voice users would on a single voice net. Any station that has information to contribute simply transmits it, and all stations receive it and log it. Secondly, APRS recognizes that one of the greatest real-time needs at any special event or emergency is the tracking of key assets. Where is the Event Leader? Where are the emergency vehicles? Where is the head of the parade? Where are the VIP's? Where is the mobile ATV camera? Where are the mobiles? Included on the distribution disk are several README text files on specific applications of APRS such as for weather nets, direction finding, plotting satellite contacts, and so forth. APRS accomplishes the real-time display of operational traffic via UI frame broadcasts and map displays. There are three major display subsystems and a number of other minor displays 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 is still on line within the last few minutes. POSITIONS - This display maintains a separate list of the positions of each station. Each position report can also contain a brief comment. These lines retain the time of FIRST receiving a given position report and give an indication of the latency in the network over unreliable paths such as HF. MAPS - Maps to any scale from .5 miles up to 2000 miles can be displayed. Stations are instantly displayed when they transmit a properly formatted position beacon. Stations with a reported course and speed can be dead-reckoned to their present position. You can center the map anywhere. 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 a one line message to any other station. On receipt, the messages are acknowledged and displayed on the bottom of the receiving stations screen until the operator hits the K key to kill them. ALL TRAFFIC LOG - This display is a time sequenced log of every new beacon or one line message sent. Beacons are logged the first time they are received. This is in contrast to the LATEST display which shows the most recent time of receipt of a beacon text. WHEN-HEARD - Since UI frames are redundantly transmitted, 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 the system automatically tracks mobile packet stations interfaced to GPS or LORAN-C navigation, the graphic capability of the maps works perfectly well with manual tracking. Any station in the network can place an object on his map including himself 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, he just updates his position on his map and that movement is transmitted to all other stations. To track other event assetts, only one packet operator needs to monitor voice traffic to hear where things are. As he maintains the positions and movements of all assets on his screen, all other displays running APRS software display the same displays. USING DUMB TERMINALS IN AN APRS NETWORK: The simplicity and usefulness of this geographic capability cannot be over stressed. Stations running APRS simply move the cursor to where they think they are on the screen and their LAT/LONG coordinates are automatically transmitted to all other stations. Even the simplest of portable packet stations with dumb terminals can report their positions if a pre-printed map is made available to all net participants which has a LAT/LONG grid reference. The portable station just looks at the map and enters his LAT/LONG 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. Eventually, it is hoped that all stations, no matter how they are using their TNC, will include their LAT/LONG in their Beacon Text so that their location is immediately available. See the file README.DUM for details on APRS formats and help in using dumb terminals in an APRS network. 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, and that is the display on a map of the location of those stations. If all stations were encouraged to simply insert their LAT/LONG as the first 19 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. All it would take to implement this capability is a single AMSAT news bulletin to ask all stations to insert their LAT/LONG in their beacon text. No changes onboard the shuttle or MIR would be required. See README.SAT for further details. FOX HUNTING OR DIRECTION FINDING: APRS is an excellent tool for triangulating the location of a hidden transmitter, balloon, or interfering signal. The X command (cross fiX)has been added to permit displaying the intersection of bearing lines from a number of reporting stations. To use APRS in this manner, each station having a bearing report on the direction of the target, simply enters that bearing in the COURSE field for his own station. His station is now not only reporting his station location, but also a line of bearing. All stations running APRS can simply hit the X key to display the intersection of these bearing lines. Further, if a DF vehicle has a GPS or LORAN-C device on board, he can be tracked and directed right to the location of the target. PROTOCOL - Since the objective of APRS is the rapid dissemmination 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 a longer and longer repition rate. Each new beacon is transmitted immediately, then 20 seconds later. After every transmission, an additional 20 second increment is added to the beacon period. After an hour this results in one beacon in 6 minutes; after two hours, every 12 minutes, and so on. The 20 second nominal increment period is adjusted up or down automatically by each station depending on channel activity. This way a few stations at a special event can get as short as a 10 second cycle times; whereas a busy channel with dozens of stations transmitting could slow the increment up to a minute. A transmission can be forced at any time by hitting the T key. COMMANDS: In most cases the keyboard is always active. There is a mneumonic relationship between all functions and the appropriate key. For this reason, the PC function keys are avoided. The most useful commands are as follows: Space Key - Display map and all station locations H - Help - Two alternating pages of help L - Latest beacons - Displays the latest BText from each station P - Positions - Displays a list of all stations reporting positions C - Controls - Display a one line status of all control states W - When heard - Displays the number of beacons per hour per station S - Send - Send a directed message to a station Alt-C,L,T,R or W - Cities, Leaders, Tags, Roads or waterways on/off Ctrl-L Load file - Loads a previously saved file and screen location Ctrl-S Save file - Saves all positions and Beacons for later restart Ctrl-R Replay - Replay the recent movements of stations in memory or replay a track history file Ctrl-V Via digi - Used to change your VIA path for outgoing packets DEMONSTRATION FILE: To get an idea of how the APRS system works on an ordinary packet traffic frequency, use ctrl-L to load the file called FREQ5- 05.BK. This file contains all the local stations on 145.05 MHz in our area. To see the tracking of the GPS equiped Army/Navy game football run, load the file named FBALL.BK and replay the file named FBALL.HST and select to see only FBALL, or CHASE1. To see the marathon event we just completed, load MARTHON.BK and replay the MARTHON.HST file. See Details in README.1st. HOOKING STATIONS: The yellow circular cursor can be moved to select any station in the system using the arrow keys. On the MAP display move the cursor near any station symbol. Then hit the RETURN key to "hook" the station. Detail information on that station will be displayed in the upper left hand corner. Alternatively, use the gray + and - keys to step through each station one by one. Once hooked, several functions may be performed: 1. LIST BEACONS - hitting the L key will list all beacons from that station currently in memory. 2. POSITION CORRECTION - performed by moving the cursor to the desired new location and pressing the Insert key. You are then prompted to enter in a new course, speed, comments or time as needed. 3. DELETE - performed by hitting the Del key. Removes the station from the position file. The hook function also works on the LATEST and POSITION display lists by using the up/down arrow keys. If a position exists, you are given the opportunity to display the map screen with that station centered on the display. This is useful for finding a station which is far off the currently displayed map range. If a position does not exist, you are given the opportunity to create one. REPLAY: Since all beacons and position reports are retained, the ctrl-R key may be used to replay the positions of a moving station. The replay is performed either from active memory, or from historical files. Historical files are automatically saved periodically as shown below. The replay will always begin using the existing map and scale. Alternatively the map location may be set to automatic so that the map will be recentered whenever the mobile station moves off the existing screen. The range scale never changes. If there are several widely separated stations in the file, it is best to either avoid the auto-centering option or to select only one station at a time for playback so that the screen is not being constantly changed. During replay, the user has several single key commands available: F - Faster. Speeds up playback W - Warp speed. Skips every other point for still faster playback S - Slow. Slows down playback Q - Quit playback. Space - Pauses playback. Toggles on and off FILES: There are several files used by the system: MAPLIST .map - Contains a list of all map files to be used and also the default LAT/LONG, Range and GMT offset for your location USA .map - large scale map of the east coast xxxxxxxx.map - Maps of any area to any scale. At this writing there is a whole USA map and state maps for most of PA, MD, VA, & NC. TKxxxxxx.HST - Track history. The day and time that the file was saved is indicated by the x's. If the file contains an interesting trip or station movement for later replay, the file should be renamed using DOS to a more descriptive *.HST file BACKUP .BK - Automatic backup of system every time program is quit. This file is overwritten every time the program is quit. BKxxxxxx.BK - Backup file of the latest beacons and POSITS heard created whenever Ctrl-S save is commanded. Usually used to save a file backup of a special event or situation. BLxxxxxx.LOG - Log of all new beacons received in chronological order of first receipt. A good log of text used during an event. CHESSBOARD: To demonstrate the flexibility of APRS in reporting the movement of objects on screens in a net, I have drawn a chessboard map in the center of the Gulf of Mexico. Any two stations can play chess easily using APRS by placing pieces on the map using the alt-A key and updating their positions using the cursor and INSert keys! As a start, I have already loaded all the pieces and saved them in CHESS.BK. Unfortunately you need to enable each one for uplinking individually using the X key on the P screen. Is better to wait till you move a piece before enabling it for uplink so that channel loading is minimized.