OPS.txt 6.0 APRS OPERATIONS NOTES Version 6.0: See the alt-IGNORE command description under the SPECIAL EVENT section below. It allows special events to IGNORE all other APRS traffic on frequency to keep their screens clear for the special event stations only. This OPERATIONS file may help you to understand the finer points of operating an APRS net. It covers the two categories of operations. ROUTINE and SPECIAL EVENT. Also read the section on OBJECTS since the information there applies to both cases. Since APRS was designed to facilitate real- time tactical communications, operating APRS on a routine basis is sometimes about as exciting as 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 in a crisis! APRS has a very powerful BULLETIN feature permitting stations to post multiple-line BULLETINS to all stations. This is very importnat to "get the word out" to all stations in a net, either routine, or special event. Also, do NOT think that you need GPS for tracking special events. With version 5.7, it is so easy to update your vehicle's position just by moving the cursor and hitting the INPUT MyPOS command (I M)., that the only stations that need GPS are the ones that are lost! BULLETINS: To send a bulletin to all stations, simply use the normal SEND command, but instead of a callsign, Send the message to BLN# where # is the line number of your multiple line Bulletin. APRS stations will grab these messages addressed to BLN# and sort them onto the BULLETIN page. Like any other message, these BULLETIN lines will be transmitted on the decaying time period and will eventaully settle out at once every 15 minutes. This way, new stations joining the net will quickly pick up the BULLETINS. Since lines are sorted onto the BULLETIN page, a new BLNx line will overwrite the BLNx on line at all stations so that corrections can be made at any time. If your bulletin is time sensitive, be sure to include the TIME in the text, since BULLETINS are not time-stamped on receipt. When your BULLETIN is no longer needed, simply ERASE the outgoing BLN# message lines. This will stop your transmitting of the BULLETIN lines, but they will remain on everyone's screen that heard them. To erase them on everyone's screen, replace them with somthing new, or send a BLN# with a few blank spaces to overwrite the old lines. With this new bulletin feature, it should be much easier to keep all stations posted on local APRS events... 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 OFTEN ILLEGAL TO LINK from VHF to HF. The illegality comes about if the GATEWAY is unattended. With current hardware, a gateway sysop cannot selectively restrict the VHF input, so it is incumbent on users to respect this deficiency and NOT put the sysop of the gateway at risk. 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 100% BLOCK ALL HF OPERATIONS NATIONWIDE! The capability is there for linking special events on VHF out for the entertainment of all HF listeners, but DO NOT ABUSE IT, OR WE WILL LOOSE IT! See HF.txt for suggested frequencies. 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 still being unnecessarily broadcast to all stations in the net. For this reason, APRS operators should consider using TNC TALK mode (connected) to do intense one-on-one keyboard QSO's. Especially if a direct connect without using APRS digipeaters is possible! See MARATHON.txt for lessons learned at the Marine Corps Marathon (1993) and the many imporvements to APRS that resulted! 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 giving a 50% chance of a packet getting through, 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 fourth transmission! 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 communicating 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. 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 be transmitted for 6 minutes and then 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. Watching this effect of lost ACK responses on HF this summer between the 25 Naval Academy boats running APRS on HF, I made a significant change in APRS503a. Now, when APRS recognizes a duplicate message, it sends out the usual ACK, but stores a copy for transmisssion in the blind 30 seconds later. The reason for 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 were lost and it is time to double up. This simple change to the ACKING process has the potential of doubling throughput on a poor channel! SHORT MESSAGES: As with any packet, especially on HF, the shorter the packet the better the chance of getting through. Since a packet often has about 25 characters of overhead, however, there is not much sense of 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 (under poor conditions), keep 'em short. A neat 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 "stby". THen I type my lengthy several line message and walk away. This way, only the very short "stby" line is transmitted (often for hours on HF) until the band opens, and then once the station ACKs that line, my remaining lines are transmitted. One last suggestion on HF and other poor paths that will make messages more sensible to the receiving station is to indicate to the receiver of a multiple line message that there are more lines to follow. I use the > character to indicate that there is more to follow. Without this, and by sending short lines, often it is not clear to the receiver that more is comming. Often he tries to respond to my half sentences and then we spend twice as much time clarifying what in the world I am trying to say! Another way to do it if you know how many lines you will be sending about the same subject is to indicate what line number it is out of the total. For example, in a four line message, end the each line with >1/4, >2/4, >3/4, .4/4. ROUTINE OPERATIONS: The APRS default digipeater path of RELAY is ok for a few users starting up an APRS net, but you will soon need to focus on a few good stations to serve as WIDE area digipeaters. 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 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, you need to designate your path by specific callsign and designate certain high stations as permanent digipeaters. If you put up a few good wide area digipeaters with the generic ALIAS of WIDE, the coverage of the network can be extended significantly. It is important to keep generic WIDEs well separated (40 miles or more over smooth terrain) to minimize duplicate repeats (or you end up with the same collison problem but on a larger scale). Most users should be able to hit at least one of these WIDEs. Just like with the RELAY's, however, users should use the specific digipeater call instead of the generic WIDE in routine cases to minimize collisions. In version 5.7 APRS permits the user to store up to 13 different, frequently used DIGI paths for instant recall. THis way, the operator can tailor his 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 the WIDE,WIDE generic path! 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. 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. For maximum effectiveness, every station should use the INPUT-PWR command to enter his transmitter power, antenna height, and antenna gain. Also APRS permanent digipeaters should include this info in their position beacons. See DIGIPTR.txt for the exact formats. 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. Paths of WIDE,WIDE,WIDE should be avoided for routine operations because it folds back on itself. The same area can be covered by using WIDE,WIDE,W3XYZ where the unique call of the third digipeater is specifically specified. If you think about it, stations at the end of an area can specify a pretty long string of digipeaters since the path is linear. Stations in the middle can only specify a symetrical double hop with WIDE,WIDE before they have to begin favoring one direction or another with unique calls. CAUTIONS ABOUT APRS MESSAGES: Remember that the general condemnation of multiple digipeater hops in the packet community applies only to connected protocols. This is because the probability of success goes down drastically because all ACKS must be successfully returned or all packets are repeated. This is generally NOT a problem with most APRS operations since only UI frames are used, and there are no acks. HOWEVER, APRS one-line MSGS are ACKED, and the inefficiency of digipeaters 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. (As noted above, in version 503a, APRS doubles up on ACKS on a poor channel and helps this situation somewhat.) But, in general, NEVER expect an APRS MSG to be successful beyond 2 digi's except if everyone else is DEAD. 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 BLUE-BOXED prompt, never linger in a blue-box prompt. The SEND command is a BLUE-BOXED prompt and should not be left un-completed! OBJECTS: As noted previously, anyone may place an object on the map and all other stations will see it. In their systems, on their P-list, the object's position report will be marked with the last three letters of the station that is currently uplinking that position to the net. A neat feature of APRS is that any station that has more current information on the location of that object can update its position by hooking, moving the cursor, and then hitting the insert key. Now this new station begins uplinking the new posit, and all stations, will update their P-list entry for that object INCLUDING THE ORIGINAL UPLINK STATION! The new position overwrites the old one so that the original station will now no longer uplink it. This came in handy during hurricane tracking. Who ever had information on the latest NWS EMILY position, uplinked it and everyone then always saw 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 original station stops sending the object position, it will remain there forever. Once the object or station has not been heard from for 2 hours, it will fade to gray so that you know it is an old contact. In version 4.01 a feature was added so that you can suppress the callsigns of old contacts. Just press the J command, and select LATEST instead of selecting any specific object type. The result will be to redraw the map showing ALL symbols, but only the calls of the recent ACITVE stations less than 2 hours old. Another feature added recently is the KILL function. This permits the uplinker of an OBJECT to 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 4 minutes to be sure everyone has received the KILL indicator; then he can delete it from his list. SPECIAL EVENT OPERATIONS Version 6.0: In preparation for the MC Marathon 1994, I have added a very important feature. The alt-IGNORE command sets up an APRS station to send all packets via the UNPROTO address of SPCL... vice APRS... It also sets up that station to IGNORE all other packets. 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. To keep this from preventing other NON-participating stations from seeing the packets, I have also added SPCL to the normal list of packets that APRS DOES monitor. This means that APRS stations with version 6.0 (or who put OTHER on) WILL still receive all SPCL event posits on their screens, and they will be automatically marked with the # for special display. This means a NON- participating station can still see just the SPCL stations by pressing the # key, instead of the SPACE bar. SPECIAL EVENTS: Let me use the Cycle Across Maryland (CAM) bike tour as an example of a special event which took a lot of daily APRS coordination. We had two of three relief vehicles configured with GPS packet transponders. These were assembled in cake pan enclosures for duct-taping to the roof of any vehicle. The uside down cake pans are reasonably aerodynamic and support both the GPS antenna and a 19 inch 2 meter whip. A single power cable extended down the windshield and was clipped directly to the vehicle battery. The package could be moved to another vehicle in about five minutes. The cake pan included only a walkie talkie transmitter at about the one watt level. Other packet mobiles running APRS did not even need their GPS units since with the map on their laptop, they can just use the INPUT-MyPOS command at any time to update their positions to the network. In this manner, my normal APRS/GPS combo can be split into TWO separate tracked vehicles for an event. THe GPS/TNC combo as a stand-alone tracker, and my laptop and another TNC only for showing people where I am. Since we only have two WIDE area APRS digipeaters in the state, and the CAM tour never went near them, we were dependent on home stations all across the state to serve as digipeaters for the event. The GPS packages were set to digipeat via the WIDE,WIDE path. By setting the alias of all home stations along the route to be WIDE, the vehicles were never beyond range of at least one WIDE station. Since the outgoing GPS packets were set up for WIDE,WIDE, the second digipeat was always picked up by one of the existing permanent WIDE digipeaters so that stations throughout the state could see the position of the one watt GPS units! We were looking for home stations about every 10 miles. Of course, as soon as a station was passed and was no longer in direct contact with the GPS units, it was IMPORTANT to remove the WIDE alias to minimize duplicative repeats. For this seven day event, home stations were organized on a nightly basis. Assigned stations would be WIDE for a whole day so that operators did not have to be home during working hours. As an added technique, we also set up both GPS units with the alias of WIDE so that they would also help digipreat each other along the trail. The disdavantage of this technique was evident as both vehicles returned to the evenings command post (also WIDE) and you had three WIDE digipeaters in 100 yards of each other! It was noisy within local simplex range of that site, but stations all over the state still saw the packets via the permanent WIDE digipeaters. Eighty percent of the home stations used as WIDE digipeaters had never even heard of APRS. They simply heard about the need for home packet stations and only had to change their ALIAS (and frequency) as directed by local announcements posted on all area BBS's. The event was an exciting success! Occasionaly there were not enough HAM voice operators per day to have HAMS in all of the relief vehicles. When ever a shortage occurred, the HAMS were removed from the GPS vehicles and assigned elsewhere. The location of the GPS vehicles were always known by net control via the APRS system so the need for a HAM rider was not necessary and in fact, only took up valuable space. Whenever voice communications were needed with the GPS relief vehicle, a mobile HAM was directed to the location indicated on the APRS screen. SYMBOLS: During the 94 MS Bikethon in Northern Virginia, KD4SJJ noticed that with four GPS mobiles and several fixed stations along the route, that there were so many callsigns that you could not see the map. He suggested making several numbered symbols so that all stations could be distinguished even with CALLSIGNS off! This was such a good idea, that not only did we make lettered balls (A-J) for the mobiles, but we also made square boxes 0-9 for the fixed stations! With these tactical symbols, and callsigns off, the map display is improved by an order of magnitude. Also added in 5.03a were an additional 7 different mobile symbols for various vehicle types! 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. That way they minimize QRM on channel. While the transmit function is disabled, a one-time transmission can be forced each time the X key is pressed. The X key enables one cycle of APRS transmission which may contain up to four packets containing your Beacon, Position, Objects, or Messages. You can send only your MESSAGES by simply hitting the T (TRAFFIC) key once. LOAD SHARING: Since any station can take over reporting of any objects, one approach is to let only one station hook 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! MARINE CORPS MARATHON: See the MARATHON.txt for details and lessons learned using APRS at the Marine Corps Marathon on 24 October in Washington DC.