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UNDERGROUND communications for first responders don’t work well on many frequencies, but research into a broad range of communications issues, which is being carried out by the National Institute of Standards and Technology (NIST), offers insight into how the problem can be solved. In a recent report on their preliminary findings, the scientists confirmed a frequency “sweet spot” for tunnels and provided data sets and formulas for calculating what that sweet spot is in different environments.

The idea of a sweet spot in tunnels has actually been around since at least the 1970s. Normally, lower frequencies transmit well through building materials, but in an underground tunnel, those lower frequencies experience a lot of loss. Unfortunately, high frequency signals also experience loss and do not transmit as well as lower frequencies, according to the report’s lead author, Kate Remley.

In a disaster, such as a fire or terrorist attack, where first responders might have to communicate underground in an area where the existing communication infrastructure was damaged, they would need to determine the optimum middle-ground frequency. Remley’s report helps provide models with which to make that measurement.

The NIST researchers based their data on tests done in two mine tunnels. They’re hoping to conduct future tests in subway tunnels.

The sweet spot varies depending on factors such as the dimensions of the underground structure where communications occur. “The bigger the dimensions of the tunnel, the lower in frequency the sweet spot moves,” says Remley. “So, what that means is basically for a tunnel with bigger dimensions, you’re able to use one of your lower frequency bands, which is nice—and because of other sorts of transmission effects, lower frequencies do tend to go farther.”

Some of the frequencies allocated by the government to first-responder communications, such as the low 450 MHz and the high 5 GHz, are not optimum. More recently, the Federal Communications Commission allocated 700 MHz bands for first-responders, which Remley deems a “promising” frequency.

In addition to the radios that are commonly used by first responders, the frequency data will also be helpful in the use of wireless communications devices and the transfer of data such as video. Also, robots and other devices that use wireless technology are often deployed in an emergency.

“There is no base station, and communications are point to point. So a team deploying a robot on scene is a communication system unto itself,” says Remley, adding that, “knowledge of the sweet spot could help in deciding which frequencies to employ to optimize communications.”

The work NIST is doing in this area is “extremely important,” says Alan Caldwell, former fire chief and senior government relations advisor to the International Association of Fire Chiefs. “There’s no incident management without command and control. The linchpin of command and control is communications.”

In addition to determining optimal first-responder frequencies, the research team is hoping to use its findings to develop open technology standards that will improve first-responder communications.