APRS works to extend communications range underground.
So says the modes developer Bob Bruninga, WB4APR, who
reports on an experiment that took place on March 2nd . One
where he and several other hams tested the use of APRS as a
means to extend radio communications underground in Mammoth
Cave, Kentucky.
According to Bob Bruninga, WB4APR, typically, VHF and UHF
radio in underground caves are limited to only a few hundred
feet and strictly line-of-sight making their routine use of
little value. But with APRS radios acting as packet
digipeaters, these few hundreds of feet can be extended by
an order of magnitude.
Bruninga says that in the test a total of 14 APRS equipped
radios were used in the cave to establish a network almost a
mile long providing real-time position and text message
communications along the route. Cavers carried a map of the
cave marked with a Latitude and Longitude grid so they could
manually enter their position into their handheld APRS-
equipped transceivers. Texting via APRS provided
communications end to end.
Among the interesting findings were that UHF worked about
13% better than VHF withan average link distance of about
450 feet even in the large subway sized passages of Mammoth
Cave. Also power did not seem to matter much. The Kenwood
TH-D72 walkie-talkie performed as well as several portable
10 watt mobile radios housed in boxes.
Another advantage of using UHF for this APRS network was
that individual links in other caves can just as easily be
pre-tested by unlicensed cave explorers using inexpensive
FRS radios. This way, all cavers can plan and individually
test the topology of an APRS network before actually
gathering the required APRS equipment and setting up the
actual expedition.
