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.
ONE-MAN APRS MARATHON
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.
DETAILS AND LESSONS LEARNED
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 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).
- 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
- 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.
- 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.
- 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.
- 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..
- 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.
- 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.
- 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.
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..
MARINE CORPS MARATHON 1993,4 & 5
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.
APRS LESSONS LEARNED @ MC Marathon, 1994!
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:
- Completely "sealed" GPS/TNC/Radio boxes should have drain holes!
- Maritime GPS units withstand immersion in water. TNC's don't
- You can't duct-tape GPS trackers to vehicles in the pouring rain
- New Marine Corps Tents (made by lowest bidder) leaked everywhere!
- 14,000 runners, vehicles, etc + RAIN = MUD
- Mag-mount GPS trackers wont stick to aluminum HUM-VEES.
- Tracking ambulances, which are parked for 99% of the event is BORING!
- Ambulances with emergencies are under such close control by the
ambulance direction net control, that he knows EXACTLY where they are anyway.
- We got double milage out of most APRS mobiles. They put their GPS's in
stand alone trackers for the ambulances, but kept their LAPTOPS and used
the INPUT-MY command to manually report their position.
- The alt-SETUP-MODES-sPecial command let the entire event operate on
145.79 while ignoring ALL other non-participating stations. THis keeps
all APRS pages free of non-participants. Many stand-alone trackers
are XTAL controlled, so you MUST plan on using the normal APRS freq
for special events.
CONCLUSIONS: Next year, we will probably go back to tracking the high-
profile chase vehicles and HAM mobiles that are always moving, rather than
SUMMARY OF 1993 MARATHON!
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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