Communications of the ACM
How Software Enabled the Thai Cave Rescue
Software-defined radios allowed the rescuers to communicate, when simple radio technology could not.
Credit: DW.com
One day in late June, 12 junior soccer team members (all boys) and their 25-year-old assistant coach set off on a day-trip of exploration in the Pong Cha cave network in Thailand. Unfortunately, when they had reached a point more than two miles from the cave entrance, flooding rains caused the water flowing into the cave to rise, cutting off their return route. The cave labyrinth was not entirely submerged, luckily, because of low and high points along the route, but the low points became impassible without scuba gear; the high points still permitted the free flow of air, allowing those trapped in the cave to breathe.
Authorities raced to get to those trapped in time to save them. The Thai government mobilized divers, along with trained rescue workers from around the world who converged on the scene. After two days, the situation was looking bleak, since divers' oxygen tanks were insufficient to reach the trapped soccer team, and there was no way for forward divers to communicate with rescue coordinators at the cave's entrance, or with other divers who were out of their line of sight.
A turning point came when the local Thai distributor for Maxtech Networks contacted the company's home office in Israel, proposing that their handheld software-defined radios (SDRs) could be used to establish a mesh network underground to enable the rescuers to communicate all along the hazardous route from the cave's entrance to the trapped children. Within 12 hours, Maxtech was flying an engineer equipped with 17 Maxtech SDR handsets to the scene. The handsets were linked to establish a mesh network that finally allowed rescuers to communicate with each other, no matter how deep in the cave they went.
With communications established all along the route through the cave, rescuers were able to map the twists, turns, and steep grades along the way to the boys. Also, with reliable communications, the rescuers were able to stockpile oxygen tanks along the route and confirm their locations to the rescue coordinators at the cave's entrance, in order to help them bring the boys out of the cave.
Even so, it still took 11 days for rescuers to reach the trapped soccer team, and three more days to extract them—17 total days from the beginning of their ordeal.
"In the cave," explained Jake Saunders of Oyster Bay, NY-based market research firm ABI Research, "there were a range of physical obstacles that foiled standard line-of-sight walkie-talkies, especially the water blocks and the minerals in the cave walls themselves. However, using software-defined radio, a mesh network was able to be formed by changing the parameters of each handset's radio dynamically, enabling them to surmount these obstacles."
Saunders said the crux of software-defined radio is not really the radio, but its hardware. Conventional handsets have many dedicated components, chips that perform fixed functions, which software-defined radio replaces with configurable software that allows the handsets to be much smaller, yet able to demonstrate dynamic responsiveness to changing circumstances. Instead of Radio Frequency (RF) components such as filters, amplifiers, and modulators, SDR handsets utilize software algorithms that can instantly change frequency band (such as VHF to UHF to ISM), protocol (such as TDM or FDM or SS), data rate, and more.
Power consumption is also more efficient, since each handset added to the mesh lowers the power required by each handset to maintain contact with all the others, increasing battery life and reducing interference (in contrast to traditional walkie-talkies, which typically operate at full power constantly).
The Maxtech handsets used in the rescue each contained an agile transceiver chip from Analog Devices Inc. (ADI), as well as an FPGA for fast reconfiguration. Using multiple (ultimately 11) handsets allowed their firmware to form a dynamic ad hoc mesh network, which permitted full communications throughout the entire process of reaching and retrieving the soccer team location from the flooded underground cave.
Uzi Hanuni, chief executive officer of Maxtech, said the SDR handsets used "cognitive radio algorithms that continuously optimize communications between all the radios in the network through automatic frequency hops in real time."
R. Colin Johnson is a Kyoto Prize Fellow who has worked as a technology journalist for two decades.
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