Bob Bruninga, WB4APR, led a group of radio amateurs earlier this month to Mammoth Cave -- the world’s longest known cave system -- at Mammoth Cave National Park in Kentucky to test how the Automatic Packet Reporting System (APRS) can be used as a means to extend radio communications underground. “When used underground, VHF and UHF radios can only work within a few hundred feet of each other and only when in the line of sight of another radio,” Bruninga explained. “This makes routine use of these radios of little value when underground; however, when APRS radios act as packet digipeaters, these few hundreds of feet can be extended by an order of magnitude.” Bruninga, a senior research engineer at the US Naval Academy, developed APRS as a global, on-air protocol for supporting handheld position reporting and text messaging via VHF radio.

Bruninga’s team used 14 APRS radios to establish a network that provided real-time position and text message communications along a route nearly a mile long in the cave. “Cavers carried a cave map that had a latitude and longitude grid so that they could know their exact coordinates when manually entering their position into their handheld transceivers,” Bruninga said. “Texting via APRS provided communications end-to-end, and even included e-mail into the topside global APRS system.” The team set up the APRS network in what Bruninga called “large subway-sized cave passages” that were 30-50 feet wide and 10-20 feet tall.

Bruninga said that extending the communications system along the cave was easy: “We would walk until we lost the signal, then back up 20-30 feet and set a digipeating APRS handheld transceiver on a rock and then keep going, repeating the process. This proved to be far more convenient than current methods, such as dragging along a World War II-era twisted pair phone line and Army field phones, or using HF and LF systems with their large antennas.”

After going over the data from the cave test, Bruninga found two interesting facts: Even with the average link between each radio at 450 feet, UHF outperformed VHF by about 13 percent. In addition, power did not make much of a difference, with the APRS-compatible handheld transceivers performing as well as several portable 10 W mobile radios. “One of the advantages of using UHF for this APRS network was that individual links in other caves can just as easily be pre-tested by cavers without an Amateur Radio license by using an inexpensive Family Radio Service (FRS) radio,” Bruninga noted. “This way, cavers can plan and individually test the topology of an APRS network before actually gathering the APRS equipment and setting up the actual expedition.”

Bruninga said that APRS radios bring “a new range multiplier dimension to in-cave communication. In the past, a paltry few-hundred foot VHF/UHF radio range has not been impressive to cavers, but now with APRS, they can be linked up to 14 times in series, demonstrating some real potential for ham radio caving support. This is especially true when some of those few-hundred foot distances may take an hour or more to crawl, whereas APRS can get the message through at the speed of light.”

Bruninga welcomes cavers who have VHF/UHF-range data experience with other types, sizes, shapes and humidity of cave passages to contact him via e-mail.