OPS.TXT APRS OPERATIONS NOTES Document version: 8.6.0 Previous was 8.3.4 of 7 Mar 99 Document dated: 1 Dec 2003 Author(s): Bob Bruninga, WB4APR OPS.TXT Operating an APRS net, both for routine and special events. Rules for digipeaters and gateways, objects, messages. Also using ZIP-LAN to share PCs to one TNC. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ FOR a general APRS overview see APRS.txt. FOR MOBILE Operations, see MOBILE.txt FOR HF Operations, see HF.txt FOR an APRS command summary see HELP.txt FOR multi-PC operations on a piece of ZIP cord, see ZIP-LAN.txt FOR Accessing the worldwide APRServe system, see APRStel.txt OVERVIEW: This OPERATIONS file may help you to understand the finer points of operating an APRS net both ROUTINE and SPECIAL EVENT. Since APRS was designed to facilitate real-time tactical communications, operating APRS on a routine basis is sometimes like watching the grass grow! The reason for building a routine APRS net is primarily for operator training and familiarity. If your operators are not familiar with APRS in a benign environment, then they will not be able to use it under stress! Although APRS is desiged for local real-time operations under stress, it does extend worldwide. The worldwide IGate system and the APRS-IS (Internet System) lets you see any of the thousands of other stations you can contact live just on 144.39. Normally you only see other local stations. But if you keyboard with anyone and your packets are linked through the internet, then the local IGate will send you a position report of the distant station. Again, the worldwide aspect of APRS is only there to give you something to practice with. This is not the design objective. SImilarly, GPS is a powerful tool linked to APRS. But, do not think that you need GPS for tracking special events. It is so easy to update your vehicle's position just by moving the cursor on your map and hitting the INPUT-MY command, that the only stations that need GPS are the ones that are lost! DIGIPEATER RULES: The advantages of APRS are many, but there is a price. Since APRS uses a fixed digipeater path sometimes different for different stations depending on geographic location, there is a duplication of on the air packets. This assures that all stations in the net are maintained up to date, but also proves to be less efficient during intense operator-to- operator QSO's where this point-to-point traffic is also being unnecessarily broadcast to all stations in the net. In such cases ALWAYS CHOOSE THE MINIMUM PATH TO THAT ONE STATION. You will be amazed at the improvement in throughput! Watch the DIGI page, and if you can work him direct, DO SO! WIDE-RELAYS AND MOBILES: The greatest advantage of APRS is in the use of generic alias callsigns for digipeaters so that mobiles do not have to change their paths as they move from area to area. Since WIDES are widely separated and are made for WIDE area and LONGHAUL comms, many mobiles (who have half the range due to mobile flutter) cannot hit them reliably. For this reason, the favorite mobile path is RELAY,WIDE,WIDE so that the mobile can be injected into the long haul WIDE hops from any other APRS station (RELAY). To make RELAY universal, the WIDE digipeaters also have this alias too. See DIGIs.txt. FIXED stations should NOT use RELAY generic paths except under the unusual circumstances. ROUTINE OPERATIONS: The APRS default path of RELAY is ok for a few users starting up an APRS net, but they should soon focus on putting up a WIDE digipeater. The reason for this is obvious. As soon as you get 3 or more local stations on APRS, any station living equi-distant (RF wise) from any two other stations will ALWAYS hear a collision of EVERY packet digipeated by both of those stations. That is why, once your network begins to grow, users need to designate paths by specific calls and digipeaters. It is important to keep generic WIDEs well separated to minimize duplicate repeats. ALTERNATE PATHS: In APRSdos prior to version 860, you could store up to 12 different, frequently used DIGI paths using the OPS-DIGI command for instant recall to tailor your DIGI path for the exact calls and path for each QSO. Proper use of this capability can significantly improve APRS effeciency and reduce the use of long generic paths! But under the new N-N Paradigm of 2003, long paths of any kind are just not desired anymore so this flexibility is no longer worth the significant code it used. The following paths are reasonable under the circumstances shown: RELAY,WIDE - Default. Good starting place to see whats out there WIDE,WIDE - Goes out 2 hops in all directions with minimal foldback WIDE,WIDE2-2 - Gets out 3 hops in all directions and identifies the callsign of the original digi where the packet entered the system. The following paths are NOT considered good practice and should not normally be used... and if you do, some Do-gooder will fuss at you... xxxx,RELAY - Never put RELAY anywhere other than the first hop! You may key up every station in hundreds of miles! WIDE,WIDE,WIDE - Gets out 3 hops in all directions but also results in as many as 27 duplicate packets in old networks that have not upgraded to KPC-3 or PacComm TNCs. WIDE,GATE - QRMs the HF net which you cannot hear! WIDE,GATE,GATE,WIDE - QRMs every APRS net in the country! TRACE DIGIS: These digis substitute their callsign in place of the generic call when they digipeate a packet. Thus, the path a packet took can be traced. Also, this prevents duplication, since these digi's will not digipeat their own packets twice. WIDEn-n DIGIS: These are APRS unique digipeters that can digipeat based only on the number of hops indicated in the digipeater SSID digit. For example, a WIDE3-3 packet will be digipeated out 3 hops in all directions with little or no duplication... See DIGIS.TXT ALTERNATE PATHS: If you live in the middle of a network going both directions, then you should consider saving one or more paths using the OPS-DIGI-SAVE command. Save one as WIDE,NORTH1,NORTH2 and save the other as WIDE,SOUTH1,SOUTH2. Then use the OPS-DIGI-ALT command to specify them as alternate paths some percent of the time. These percentages subtract from your normal path and beacon rate, so understand that your main path will be used less often. This capability was removed from APRSdos in APRS860. All users must understand that they are responsible for setting their outgoing VIA path so that their packets hit the intended area of interest. Unlike normal CONNECTED protocols which automatically return ACKS via the reverse path of incomming packets, APRS is an unconnected broadcast protocol only and each station's packets will only go via the outgoing path set up by that station. If your station receives a duplicate APRS MSG packet more than twice, it gives you a beep and an alert that your ACK's are probably not being heard by the other station and that you should check your outgoing VIA path to make sure it gets to the sender. APRS has a very useful feature for determining the best path between stations. The Power-Height-Gain reporting capability lets APRS plot range contours around all stations that have included the P-H-G data in their position reports (using the INPUT-PWR command). Of course, Height here is height above AVERAG TERRAIN, not above sealevel or tower height! If you do not understand the difference, ASK! See DIGIs.txt for the format details. Those stations between WIDE area digipeaters only need to use the single hop of WIDE and their packets will go in both directions. Stations that can only hit one WIDE area station may set the path of WIDE,WIDE without any conflicts. Areas with TRACE or WIDEn-n digipeaters can use just about any path without fear of duplication. But in general it is always useful to begin your path with your most reliable digipeater so that even without a position report, stations can see approximately where you are by the first digi in your path. CAUTIONS ABOUT APRS MESSAGES: Remember that multiple digipeater hops has long been condemned in the packet community because the probability of success for CONNECTED packets goes down drastically because all ACKS must be successfully returned or packets get re-tried over and over. This is generally NOT a problem with APRS operations which use UI frames without acks. HOWEVER, APRS one-line MSGS are ACKED, and the inefficiency of multi-hops DOES APPLY! If you do a lot of one-line messages between operators, you will experience the same hopeless probabilities of success as with conventional packet. But, in general, NEVER expect an APRS MSG to be successful beyond 2 digi's except if everyone else is off the air! Operator messages are a secondary function of APRS, and should not be used as a primary means of passing traffic! One further caution, since APRS suspends all packet processing while waiting for the operator with a BOXED prompt, never linger in a prompt. The SEND command is a BOXED prompt and should not be left un-completed! ACKS THAT DONT MAKE IT: Just like connected packet, the chance of a message packet getting through is usually the same as the chance that the ACK will get back. If the radio path is only 50%, that means that the receiver will probably get the message by the second transmission, but that the sender may not get an ACK until after his 4th! This is because the sender had to send 4 packets to get two through and the receiver then ACKed twice in order to get one through. You see this effect frequently on APRS, when you are talking with a station over a long poor path. You will notice that the person at the other end has already responded to your message even before you get an ack from your outgoing message. BUT your next line will never go out UNTIL it gets that ACK. The reason that you will probably get his response message before your ack, is because his response message is being repeated over and over in the usual APRS decayed algorithm, but his ACK is ONLY transmitted once each time he gets a dupe of your message line to him. What this means is that whenever it is obvious that the other station has responded to your message line, you should ERASE it so that APRS will move on to the next line. Sometimes if you know that the other station is probably hearing the digi better than the digi is hearing him, and you are not getting ACKS, then simply send him messages in the blind. Let each line will be transmitted for 6 minutes and then you can erase it. APRS will then move on to the next line. Remember that APRS will have transmitted 6 times in the first 6 minutes, but that it will then be over 3 minutes, then 6 and then 12 minutes for further transmissions. To improve on this effect of lost ACK responses, APRSdos recognizes a duplicate message, and not only sends out the usual ACK, but stores a copy for transmisssion in the blind 30 seconds later. The 30 second delay is to avoid cluttering up the frequency if the path is good, since the sending station will have sent the message at least twice in the first 30 seconds. After the third transmission, it is clear that the ACKs are getting lost and it is time to double up. This algorithm has the potential of doubling throughput on a poor channel! REPLY ACK: In the year 2000 time frame, we figured out how to embed ACKS in reply messages so that for only 2 more bytes in a message in a dialog between 2 users, we can carry the ACK back for free. This vastly improves the reliability of ACKS in a dialog. APRSdos and APS+SA are the only ones that implemented it immediately, but this, combined with APRSdos decaying retry algorithm can result in an ORDER OF MAGNITUDE faster message dialog than other client software that does not use these two fundamental communications techniques. SHORT MESSAGES: As with any packet, especially on HF, the shorter the packet the better the chance of getting through. With 25 characters of overhead, however, there is not much sense in making the message part much shorter than a half line (40 characters). The chance of a 40 character line getting clobbered compared to a 75 character line is 65%. On HF keep 'em short. A trick that I frequently use whenever I know that a station is not currently on the air, or the path is not currently good, is to send the first message line with only the word "test" followed by additional lines with the body of the message. This way, only the very short "test" line is transmitted (often for hours on HF) until the band opens, and then once the station ACKs that line, the remaining lines are transmitted. BULLETINS: To send a bulletin to all stations, simply SEND a message to BLN# where # is a line number from 1 to 9. Like any other message, these BULLETIN lines will be transmitted on the decaying time period and will soon fade out of the system. If you want the bulletin to remain at about a 15 minute rate, then instead of using numerals in the BLN# mesage, use a LETTER. This way, new stations joining the net will quickly pick up the BULLETINS. Since lines are sorted onto the receiver's BULLETIN page, a new BLNx line will overwrite any previous BLNx at all stations making changes and corrections easy. If your bulletin is time sensitive, be sure to include the TIME in the text, since BULLETINS are not time- stamped. When your BULLETIN is no longer needed, simply ERASE your outgoing BLN#. This will stop your transmission of the BULLETIN lines. Receiving stations can erase all old bulletins by using the ALT-E command. GATEWAY RULES: I have interjected this paragraph because of the large number of APRS HF to VHF gateways now in operation. First, it is very important that users understand that GATEWAYS ARE ONLY INTENDED TO LINK HF ACTIVITY INTO LOCAL VHF NETS. IT IS INNEFFICIENT, DISCOURTEOUS, AND MAYBE ILLEGAL TO LINK from VHF to HF. Linking HF operations into every local VHF APRS net in the country is not a problem, because the slow 300 baud data rate could never saturate ANY 1200 baud local net. HOWEVER, linking just ONE active VHF net ANYWHERE in the country out onto HF WOULD CERTAINLY BLOCK ALL HF OPERATIONS NATIONWIDE! The capability is there for linking special events or cross country travelers on VHF out for the entertainment of all HF listeners, but DO NOT ABUSE IT, OR WE WILL LOOSE IT! See HF.txt. GATE SUMMARY: On HF, use the path of GATE,WIDE and everyone in the country within one WIDE hop of a GATE will likely see you. *Never* use GATE,WIDE,WIDE because your packets will now go 2 hops on VHF and be seen MULTIPLE times from multiple gates! No one can tell where the GATE is and it is just a BIG MESS. Believe me! Second, never routinely go through a GATE on VHF. USING THE OPS-COMM-TNC dumb terminal mode. This mode works OK for using your TNC normally, but it doesnt have any file transfer capabilities. You might try to find a small .EXE comm program that you can FILES-SHELL out of APRS, do your COMM thing, and then EXIT back into APRS.... YAPP and PACCTERM both work, but be careful of how these programs change your TNC parameters... OBJECTS: As noted previously, anyone may place an object on the map and all other stations will see it. On their P-list, the object will be marked with the last three letters of the originating station. Any other station that has more current information on that object can also update its position by SELECTing, moving the cursor, and then hitting the insert key. His station will begin uplinking the new posit, and all stations, will update their P-list entry for that object INCLUDING THE ORIGINAL UPLINK STATION! Since the new position overwrites the old one, the original originating station will now no longer uplink it. This comes in handy during hurricane tracking. Who ever has information on the latest HURICANE posit uplinks it and everyone then always sees the latest storm track without anyone in the net being dependent on any one station for updates! Once objects are transmitted on to other station map screens, they will remain there until that operator deletes them, even if the originator stops transmitting them. It will, however, fade to dark gray after 2 hours to show it as an old report. You can use the CONTROLS-FADE command to bring them back to bright colors, or use the J command to see JUST-the-LATEST symbols. The KILL function permits the originator of an OBJECT KILL it from all displays on the net. His station will continue to uplink the object, but tagged with a special KILL flag to suppress its display on all screens. It remains in everyone's P-lists, though, so they can refer back to it if needed. They must still manually DELete it from their P-list as needed. Once the originator has KILLED an object, he should let it remain on his P-list for at least 6 minutes to be sure everyone has received the KILL indicator; then he can delete it from his list. NEAT OPERATOR FEATURES: There are several menus and commands you can use to set up your operating envrionment. Here are some highlights: CONTROLS MENU: You can set filters and how you want packets displayed. MAPS: Turn on or off most map features plus overlay data files on the maps such as DIGIS, Radio Shacks, etc RADAR ALARMS: Use INPUT-MY-RADAR to set your"airspace". POSIT ALARMS: Set ALARMS on any mobile which will alarm if he moves. WX ALARMS: Set Weather alarms on temps, winds or rain... TRACK MODE: Lock on to one station and keep map centered SPECIAL MARKS: Mark any station on the P-LIST for SPECIAL highlighting Then you can see only those stations with JUST-S SPECIAL EVENT OPERATIONS: The alt-SETUP-MODES-SPECIAL command sets up an APRS station to send TO the UNPROTO address of SPCL... vice APRS... and to ignore all other packets NOT addressed to SPCL. This allows the event participants to keep their screens (P/L lists, etc) clear of unwanted other APRS stations on frequency, while tracking the event normally. All other stations watching the event will still receive all SPCL event posits on their screens, and they will be automatically marked with the # for special display using the JUST-SPECIAL command or SPACE bar. SPECIAL EVENTS: The Cycle Across Maryland (CAM) bike tour is a good example of a special event using APRS. We had two of three relief vehicles with GPS trackers. These were assembled in cake pan enclosures duct-taped to the roof with a small power cable extended down the windshield and clipped directly to the battery. These packages could be moved among vehicles in about five minutes. Some other packet mobiles ran APRS without GPS units by just using the INPUT-MyPOS command to update their positions. Since we only have two WIDE digipeaters in the state, we were dependent on many home RELAY stations all across the state to serve as digipeaters for the event. We simply asked many conventional packeteers to tune to 144.39 for the event and set their TNC aliases to RELAY for that day. We also set up both GPS units with the alias of RELAY so that they would also help digipreat each other along the trail. The disdavantage of this technique however, was evident as both vehicles returned to the evenings command post (also RELAY) and you had three RELAYS in 100 yards of each other! SYMBOLS: APRS now permits HUNDREDS of different mobile symbols by using the NUMERIC OVERLAY capability. This makes it easy to distinguish mobiles even with CALLSIGNS-OFF to reduce clutter! EMISSION CONTROL: If there are only a few APRS stations involved in an event but there are lots of APRS observers on frequency, then the observers can set their transmitter off using the CONTROLS-X command to minimize QRM on channel. They can still transmit under manual control by using the X key. LOAD SHARING: Since any station can take over reporting of any objects, one approach is to let only one station SELECT every symbol that comes in and then he becomes the reporting repsonsibility. The original station that uplinked the report in the first place will fall silent when it sees the report comming from the designated Net Control station. This way all positions are reported by only one station on frequency, although all other stations can still update the positions as needed. Remember that the last station to report the position of an object will be the one that continues to report it! APRSdos has a NET-CONTROL feature for OBJECTS that automates this process. MARINE CORPS MARATHON: See MARATHON.txt for the lessons learned using APRS at the Marine Corps Marathon for the last 3 years in Washington DC. ZIP_LAN MODE AND EMERGENCY OPERATIONS CENTERS: Dont overlook, that a handful of separate PC computers can ALL BE CONNECTED TO A SINGLE TNC AND RADIO! This fact can be used to create quite an impressive multi-station tactcal communications system that will rival some 911 consoles! No special LAN hardware is required other than a serial port and as much two conductor zip cord as you need. See ZIP-LAN.txt CAUTION: This ZIP-LAN capability is not backwards compatible to any software prior to APRS800, Mac/Win prior to 2.09 and APRSa4 ver 0408. With a ZIP-LAN, ALL consoles see the tactical picture, and these PC's are at the individual operator's disposal to zoom in, and hop from screen to screen to give them access to what ever info they need! Do not think that a big screen display is better. A single big screen is impressive, but actually useless. Only the person at the KEYBOARD of an APRS system can actually get useful info from APRS. In our county, you need to be below the 8 mile scale to get an idea of what is going on at a crisis, and while you are zoomed in there, others need to be focusing on other parts of the county, or different screens. You can wire every PC in the building using cheap 2 conductor speaker ZIP cord! You can carry hundreds of feet of this stuff in your briefcase with your portable laptop! This is a TREMENDOUS capability, since these days PC's are much more plentiful than TNC's and all available assets can be brought into the picture. Every SLAVE operator has his own INDEPENDENT access to all of the APRS info without bothering the APRS operator. See ZIP-LAN.TXT