[aprssig] Mammoth Cave APRS Test (quick look).

Robert Bruninga bruninga at usna.edu
Mon Mar 4 09:36:23 CST 2013


APRS Cave Test:

Despite some initial hours of frustration, once the bugs were all found,
we successfully established a continuous network of 13 digipeaters the 1
mile length of the Mammoth Cave Cleveland Ave.  (Compare that to the 2000
mile path we get on our Golden Packet tests in July!).   We demonstrated
the ability to provide real-time texting and position reporting from
anywhere along that length to anywhere else along the chain including
topside.

It will take time to publish the detail results, but here is what I think
we learned now:

1) VHF range average hop length was 388' with a max of 530
2) UHF range average hop length was 439' with a max of 680
3) UHF does better.  Even better than the 13% statistics improvement
suggests.
4) Range with a 90 degree passage bend is not drastically different than
straight
5) If waveguide effect exists, it is insignificant.
6) Higher power (50W) made little difference to 5W or even less (.5W FRS).
7) Typical cave passages were between 30 to 50' wide and 10 to 20' tall
(big cave)
8) More testing needed in smaller passages (though UHF should be good down
to 2' wide).

Once everything was configured properly, extending the communications
system down the cave was as easy as walking until signal was lost, backing
up 20 to 30' and setting a walkie-talkie(relay) on a rock.  Then
proceeding.

Overall conclusion, APRS Radio brings a new range multiplier dimension to
in-cave communication.  Although the radios are expensive ($500) and need
a licensed ham radio operator present, the frequency band used is
identical to the $20 FRS radios available everywhere, making any caver a
potential contributor to gathering more data on ranges in a much more
varied environment than the subways of Mammoth.

Although cavers have used the short range FRS radios in some cases, the
short range  (a few hundred feet) has not made them a routine part of
caving.  However, now that APRS has demonstrated how these short ranges
can be chained together in series up to 14 hops or so, this offers some
significant opportunities for in-cave communication.  Since anyone can
test these individual links with just a $20 FRS radio, we would invite any
cavers to document any FRS radio experience on future caving expeditions
to gather more data on ranges in a greater variety of passages and to
document those results.  A paltry 300' radio range in the past has not
been impressive, but when they can be linked up to 14 times in series, we
are beginning to see some real potential.  Especially when some of those
300' distances may take an hour or more to crawl, communicating at the
speed of light sure looks handy.

Results and future info will be maintained as developed on our web page:

http://aprs.org/cave-link.html

Bob Bruninga, WB4APR
US Naval Academy
410-293-6417


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