The effort to stop the aircraft is known as the Mobile Force Protection Program and is overseen by the Defense Advanced Research Projects Agency, which examines ways that technology can help the U.S. military. DARPA anticipates awarding contracts within weeks for the first of three phases of testing and research, said J.C. Ledé, who oversees the program.

“Right now, the best way of detecting that there is an unmanned airplane is by listening for that radio signal,” Ledé said. “Once they stop emitting that radio signal, they’re going to get a lot harder to find.”

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Early stages of the research were launched in October with a solicitation to industry, and final proposals for the first phase are due in January, according to DARPA documents. The program is focused specifically on going beyond using electronic jamming to stop unmanned planes and helicopters of to 200 pounds. Each company picked is expected to get about $3 million in the first phase, with the possibility of continuing on to two subsequent phases of work that are longer and more lucrative.

Ledé said he and his team focused on defending a convoy with important cargo aboard, because it is more complicated than defending a stationary target and because what is learned will apply in other circumstances. Unmanned aircraft are now “sufficiently inexpensive” that the U.S. military must anticipate some of them may be flown directly into U.S. troops or vehicles as part of an attack, he said.

“If you are going to attack a high-value convoy, I think they would be willing to commit the hardware to it,” he said. “At most, it’s a few thousand dollars worth of hardware for a UAV.”

The effort comes as the U.S. military more broadly examines an array of ways to take out potential enemy drones. Marine Lt. Col. Dave Sousa, who examines the problem for his service, said shotguns, sniper rifles, water cannons, mini-rockets and lasers all have been considered, and the services increasingly are working together on the problem.

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“When you’re more than a couple hundred meters out, you can’t tell what that thing is carrying,” Sousa said of unmanned aircraft. “You can’t tell if it has a GoPro camera. . . . You don’t know what it is. So you’ve got to detect, track and ID, and then there’s how you’re going to deal with that threat.”

In Iraq and Syria, Islamic State militants have loaded grenades on small drones and used them to attack civilians and local forces working to drive them out. In Ukraine, pro-Russian separatists and Ukrainian soldiers have used small unmanned aircraft to find and track opposing forces. The United States sent Ukraine some mini-drones last year, but Russian-backed separatists were able to easily jam them, rendering them relatively useless, according to a Reuters report.

Army officials said in a strategy paper published in October that enemy drones could be stopped in some cases with cyber or electronic warfare or concealment through methods such as smokescreens but that small units of U.S. troops will probably face them at some point. In those cases, direct fire may be necessary, the paper said.

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“Some threat UAS will be difficult to defeat left-of-launch or at standoff ranges,” the paper said. “These lines of effort will be focused largely on the individual and small unit levels.”

The new DARPA project countering autonomous drones acknowledges those concerns but specifically forbids any option that could cause harm to U.S. troops or civilians as incoming drones are engaged.

The prohibited options include high-powered directed-energy weapons, high-caliber weapons with “uncontrolled projectile trajectories,” live animals and anything that does not fit on a tactical vehicle such as a Humvee or on a small naval riverine craft.

Ledé said Phase 1 of the project could begin in May. It is expected to take about a year and will be followed by an 18-month phase with the top two competitors in the first part of the project. The third and final phase of the project will take about 21 months and focus specifically on countering a large raid of autonomous, unmanned drones.