Using APRS to Support a Marathon

This file provides important notes about Field operation of APRS. The first part shows how even ONE station at NET control can provide a valuable service even with NO ONE ELSE on APRS! After that portion is some other lessons learned from the Marine Corps Marathon in 93 and 94.


Today we just finished another one-man APRS marathon (* see footnote). All it takes is ONE good APRS VGA Display Screen setting in front of the Voice NET operators. As long as they see the current position of the LEAD and TAIL, and less importantly the FEMALE and sometimes PACK at all times, They are just tickled pink. No one even notices that there is NO TNC, no Radio, no GPS, and NO other APRS operators anywhere else along the entire coure. The race officials are happy, the Voice OPS are happy and everyone sees these important ICONS moving along the map.

Heres how. Set the APRS operator off to the side with his laptop. He operates entirely from his own LCD screen, but his laptop is driving the full size VGA display in front of the two primary Voice Net operators. The APRS operator places the LEAD on the map with the correct course and a speed of 9 knots. The object magically moves along, and after the event proceeds for about 15 minutes, the TAIL is pretty well known and it too can be placed on the map with an initial speed of 6 knots.

ALready the Voice Net operators can see the progress of the marathon. As the APRS operator overhears reports on the passage of the LEAD, or TAIL, the he simply uses the hook-INSert capability to update the positions of the main objects whenever the dead-reckoned objects stray from the course or are ahead or behind the predicted positions.

The race proceeds, and the Voice OPS, and race officials and everyone that swings by the APRS display sees "High-Tech" at work. The question of where everything is, is always there at a GLANCE. After a while, the Voice ops are using the screen to alert check points to the arrivial of the LEAD. The Police liason comes and looks at the screen to see when he can open or close roads, and when he can release personnel. Everyone operates from that ONE screen, prominately displayed for all to see. The APRS operator still just sits in the corner with his laptop and LISTENS. Occassionally he may make brief announcemnts to the radio OPS, that the LEAD should be arrriving at.... or the TAIL should be passing.... etc.. but his job is to keep the display accurate.

The POINT here is that sometimes we trip over our own high tech ambitions by trying to do TOO much and end up with nothing working. We didnt decide to do this marathon until 2 days before. ALthough 5 trackers actually showed up, this marathon is along a railroad bike trail where cars have no access, and even then CARS and TRACKERS are usually RARELY exactly where the LEAD and TAIL are, but seem to start and stop and wander around. On the otherhand the Dead-reckoned position of that LEAD object just keeps on trucking along at 9 knots, just like the runner that it represents. In my experience, the DR'ed object for the LEAD of a marathon will always work better than any other solution other than a helemet mounted tracker on the guy himself (which just will not happen). Clearly, this marathon was a low profile event with only 200 runners and 20 voice ops, so our only need was to keep NET control and RACE officials informed of the progress of the event. Therefore, we didnt even use a single one of the 5 trackers. In fact all they did was add QRM until we told everyone to turn them off.


  1. The most important thing at net control is for the APRS station to NOT transmit (QRM to voice ops). Set CONTROLS-XMT off. Use O-C-T to set your TNC DIGI OFF and then set MYA NONE. (we dont know why but even with DIGI off, a local station using via RELAY was still causing our APRS station to TRANSMIT every now and then... SO we finally changed the MYAlias so that our station could not be used as a digipeater by anyone..

    (Actually, there were a few other APRS stations in the field monitoring so the APRS operator simply used XMT-OBJ every now and then, either during lulls in activity OR when Netcontrol was TALKING! You wont cause QRM if you transmit simultaneously with net control).

  2. If your event route is circuitous, and you do not have a good mind's eye for the 360 degrees of the compass, you may want to have some penciled in COURSE directions along major legs of the route. Also be familiar with the fact that the LEAD is moving .3 miles in 2 mins, and the tail is moving .3 miles in about 4 minutes. THink or make notes as to what these distances are on the 2, 1 and 0.5 mile map scales.. Depending on your map detail, at a one minute reporting rate, I found the 1 mile scale to be the most useful, providing a useful coverage for about a 10 minute period.

  3. APRS DR's by the minute. But it only updates the screen when it XMTS or when the screen is re-drawn. WIth CONTROLS-XMT off, then you need to hit the space bar occassionally or set alt-S-OTHER-REDRAW to 60 minute or so automatically.

  4. WHenever the OBJECT approaches a turn, hook his little anchor circle and move the cursor to his current position in the turn and hit INSert to give him a new COURSE.

  5. Even on a straight-away, when you have time and he has moved a few minutes, hook his circle, and move the cursor to his current DR'd symbol location and INSert. Two reasons for this. FIrst, Remember that APRS is using a decaying timing period, so your updates to everyone on the net (if you were transmitting) are getting less and less frequent and since they only plot new positions when YOU transmit (unless they also have redraw on), you need to do this to keep their screens looking more alive. ALso, on your own screen it clears up the long DR lines and makes for a nicer display. These two steps 4, and 5, are where a good APRS talent really shines. Mess this up, and you will quickly lose the bubble and look real bad... This means that this APRS operator must have NO OTHER RESPONSIBILITIES.

  6. On our Bike Trial where there are also lots of bikers, walkers, and runners not involved in the marathon, NETcontrol *frequently* gets conflicting voice reports of "first runner" FIrst female, tail runner and stragler sometimes off by as much as a few miles. Here is where the methodical DR'ing of APRS really helps. The APRS ICON moving along at 9 knots never waivers in its progress. If we get a voice report that is more than a few hundred yards off from the DR'd report, NET control has learned to treat it with suspicion. In almost all cases, the APRS DR'd posit is right, and the observers at the check point were confused... and seeing the wrong runner..

  7. Notice that even with the only APRS station at Net Control having XMT off, he could still plot all movements of TRACKERS without any QRM to the station. In this case, or especially in the DR'd case, APRS locals monitoring the freuqncy may not even see that a MARATHON is in progress or that APRS is providing the PRIMARY displays for the entire event. The first year we did this event, we had 8 APRS laptop checkpoints, 3 trackers on Bicycles, but the only place that anyone was really using and seeing APRS was net control. Thats why over the years, we have devolved to this one-man APRS net for simplicity. It provides 95% of the functionality with only 5% of the effort.

  8. Understand Dead reckoning. As the race proceeds, drop the TAIL's speed to 5 and then eventually 4 knots. If you notice that your OBJECTS are running a little ahead or behind, especially the TAIL, then modify their speed. But the lead will always be 9 knots. At that speed, the lead is doing about 300 yards a minute. Even so, a 1 MPH error will still be within 100 yards even after 3 minutes! The Tail is much less predictable, so keep an eye on it closely.

  9. Since the LEAD is done after 2.5 hrs, the rest of the day focuses on the TAIL as each check point is itching to close down, and is always asking where the TAIL is. Here is where the APRS operator can have a voice radio too, and can answer those questions direct without having to bother primary NET CONTROL.

I am certainly not meaning to discourage having an APRS display at EVERY checkpoint, but recognize that if you do, the most important problem that you will have is QRM. THE BEST WAY TO DO ALL OF THIS, is to have RECEIVE only displays at NETCONTROL and all checkpoints. Then have the APRS operator 100 Yards away (or 5 miles away at home in his Shack). He LISTENS to all radio channels, and simply updates HIS display which in-turn updaes everyone elses. But he is TRANSMITTING from a distant location and causing NO QRM. ALso it is useful to have a separate APRS voice coordination net.

The disadvantage of this is that you still need a GOOD APRS DISPLAY operator at the NETCONTROL, making sure that the big display is zoomed into to the focus of interest at any instant. The VOICE ops do not have time to move cursors and ZOOM. That is the APRS operators job. In that vein, use the MAP-SAVE to store up to 4 zoomed in maps in the HOT keys 3, 5, 7 and 9. I usually keep map 9 saved on the LEAD, and map 3 saved on the TAIL. Then I can save other focus areas in 5 and 7. In fact, the APRS operator can judge from hearing each voice communication what is the best map display to put infront of the operators. In fact, I was getting darn good! As soon as I heard the callsign of an incomming transmission, I would hit the appropriate hot-map so that the Voice operators just always had the "visual" on that guy's area as soon as he bagan speaking. WOW.. (this also had the benefit of forcing a map-redraw so that it was always current too). As the race proceeded, I was frequently re-saving new views in these hot-keys... to always present the best focus of interest.

Do NOT think that you can just put up an un-attended display. It will look cute, but it will either be zoomed out too far so that you can see everything (all clutterd up) or zoomed in on the wrong area at the wrong time. PLAN ON HAVING A GOOD APRS OPERATOR DOING NOTHING BUT MANAGING the DISPLAY for NETCONTROL.

Again, on race day, if you want to track STRATEGIC vehicles, water wagons food trains, VIPs, downed runners, pickup vehicles , etc.. even without GPS, just have an APRS operator (Anywhere, even at HOME) listening to voice freqs and UPDATING these objects. This can really spread out the work load. If you have more APRS volunters, assign each one a different net to "listen too" and to keep HIS objects current. This lets all APRS stations "on-site" operate in RECEIVE only for 0 QRM, but still show where everything is... on ALL displays..


1995: Had fewer GPS trackers and relied almost entirely on the DReckoning in APRSDR.EXE to move the LEAD, PACK and TAIL objects along the marathon route without operator intervention except when needed to correct for long term drift between the runner and the Dead Reckoned object. The LEAD runner goes about 9 Knots and the Tail goes about 4 Knots. Also, these tracks DO follow the exact course, (this is in contrast to actual GPS vehicles which often cannot follow everywhere that a runner can go. We also DR'ed the lead Handicapped, Woman, Special-Olympics and PACK. See DR.HTM.

MASTER-SLAVE. This year we also operated three 486 color laptops and two larger VGA displays all connected to the single APRS TNC. Only one laptop was operated by the APRS operator as MASTER, and all other laptops were placed in SLAVE mode in front of the other voice net controllers, so they could independently zoom in to areas of their immediate interest. If we had had more VGA monitors, each laptop could have also driven an extra large display. The small size of the laptops fit unobtrusively at the operating positions. See OPS.HTM.


14,000 runners, LOTS of hams, and our second year with APRS! In 93 we put GPS on the LEAD, LEAD Handicapped, & TAIL chase cars. It was great, but predictible. This year we let APRS dead-reckon the predictible movements of the chase cars and built 11 Trackers for the ambulances. Lessons: Of 11 units, 2 never quite got finished, one just couldn't be attached in the rain, one leaked, flooded and died, the tinyest (running on AA cells) lasted 6 hours. It rained from 5 AM until 1400. Now for the good news:

CONCLUSIONS: Next year, we will probably go back to tracking the high- profile chase vehicles and HAM mobiles that are always moving, rather than ambulances.


REPLAY MARATHON.hst to see how it went. To make sense out of it all, try playing back only one mobile at a time, and turning Callsigns off. WB4APR-9 was the lead Handicapped vehicle (started 15 minutes early, W3ADO-9 was the lead runner, and MOBILE-9 was the Tail. Statistically, we did very well. W3ADO-9 was turned on at 0827 but did not move until 0902. It was removed from the vehicle at about 1127. Transmitting at once a minute, there should have been 145 posits transmitted. We counted about 115 in the file. (many could have been filtered out by APRS as duplicates). The result computes to almost an 80% success rate!

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