To find hidden bombs, the U.S. military has tried everything from sniffing their chemical scent to shooting lasers into the ground. The Pentagon’s blue-sky researchers think they have a better paradigm: search for them the way physicians locate tumors.
Darpa recently launched a program called Methods for Explosive Detection at Standoff, or MEDS. The agency wants companies to “rapidly develop and demonstrate non-contact methods to detect explosives embedded or packaged in opaque media with high water content (e.g., mud, meat, animal carcasses)” using ultra-wideband microwaves, that “have shown promise for breast cancer detection,” according to a project solicitation.
In recent years, breast cancer researchers have had success using ultra-wideband, or UWB, to detect malignant tumors in breast cancer patients. It’s complicated, but involves zapping tissue at close range with signals from across the radio spectrum, at much wider range than conventional X-rays. To oversimplify it, the UWB waves bounce back and highlight healthy tissue while contrasting it with a darker-colored malignant tissue. If it’s possible to detect a tiny tumor with it, then it stands to reason that it may be possible to detect explosives hidden inside some wet mud or even a dead dog. (It happens.)
Improvised bombs remain the primary weapon of insurgent warfare, and they’re proliferating beyond the warzones. Meanwhile, the number of IED attacks against U.S. troops has continued to increase, though the Pentagon’s Joint Improvised Explosive Device Defeat Organization says that the bombs are now fortunately killing fewer troops. Locating the bombs before they explode is tremendously complex.
The U.S. built expensive bomb jammers for a time — until insurgents switched to bombs that detonate by wire; and then to bombs that detonate by pressure. Metal detectors used in Afghanistan provoked insurgents to build bombs out of wood and fertilizer. The military has built a plane-mounted radar to track militant bomb teams from above. But a slew of expensive directed-energy weapons have come to little — including one truly ridiculous lightning gun — and the military has been so desperate to stop bombs planted in drainage ditches that the Army allowed itself to get ripped off by shoddy Afghan contractors.
And it won’t be easy to build ultra-wideband detectors. A recent declassified NATO report (.pdf) on work with UWB bomb detectors was optimistic about method, but clutter like rocks and other debris “degrade[d] the signal to clutter ratio and reduce[d] the system performance.” Handheld UWB detectors, such as the Army’s Minehound scanner, were also limited by a single pair of antennae and — with few exceptions — couldn’t draw an image of what they saw.
Darpa’s MEDS program isn’t requiring its detectors to use imaging — it really wants something that can simply pinpoint a buried bomb — but considers the option “desirable, if feasible.” The agency doesn’t specify if the devices will be handheld or carried by vehicles or robots. Regardless, the devices will have to get pretty close to the ground for an antenna to pick up a signal, as the NATO report considered the range on potential systems to max out around 20 to 30 centimeters “in ideal conditions.”
The program also points toward other “potentially relevant areas” such as non-linear acoustics — distorted sound waves, basically — and “mixed modality mechanisms” like electron-photon beams. What Darpa doesn’t want are devices that use X-rays “with the possible exception of X-ray backscatter,” due to the risk of contamination. One thing about finding tumors: you don’t want to poison your patient as you search, as any doctor could tell Darpa.