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.