As more unmanned aircraft go buzzing around overhead, there’s more and more talk that those of us on the ground being watched should, at least, know who’s doing the watching. Joseph Lorenzo Hall, a senior staff technologist at the Washington, D.C.-based think tank Center for Democracy and Technology, said that when he first contemplated how to go about ID-ing drones, he thought of “N-Numbers,” or the identifying characters affixed to the tails of airplanes in the U.S. Perhaps a “D-Number” for drones?

The problem with the D-Number, Hall said at last week’s Drones & Aerial Robotics Conference (DARC) at New York University, is that even with binoculars it might be impossible to make them out from the ground.

“These things,” he said, referring to drones, “are so freakin’ tiny.”

So Hall and his colleagues switched gears. He floated an alternative plan in an April blog post, and then turned it into a formal proposal to the Federal Aviation Administration. The agency must, per a congressional mandate, figure out how to integrate unmanned aircraft into the “national airspace system” — a mainstreaming that some say the FAA has dragged its heels on, while at the same time handing out fines like the $10,000 penalty given to a man who used a drone to photograph the University of Virginia campus, allegedly maneuvering through car tunnels and nearly bumping into statues.

There should, Hall said, be an ID number bolted onto drones, something like the fireproof plates attached to voting machines. But more than that, his plan would use radio frequencies, sent by on-board transmitters, to identify where a drone came from. ID numbers could be tied to registries in an FAA database, like how manned aircrafts work. And those frequencies could be read on a cell phone or a tablet computer, making it possible to scan the skies, fire up an iPhone or iPad app and know, in real-time, whose drones are flying over your head.

Military drones aren’t really Hall’s worry, nor is it the hobbyist experimenting below 400 feet. It’s the scores of other “mundane, civilian users of drones,” like those tracking the weather, building maps and monitoring traffic. One possibility is that the FAA drone registry let anyone know what “sensing package” is aboard the drone. It’s not just cameras — drones today can carry readers for Synthetic Aperture Radar, capable of forming ultra-high-resolution terrain maps; LIDAR, equipped to measure everything from the speed at which something on the ground is moving to its chemical composition; and much more.

But since writing his post and filing his comments last spring, Hall had come to believe that it was not going to be easy. And so he’d come to DARC for help.

The biggest problem is that for the system to work, it needs both an on-board GPS system to pinpoint the drone’s location and a transponder to send that data back down to earth. The cost for materials alone would run $6,000, or about 10 times what an entry-level drone costs. Another option: Put lower-cost transmitters on base-stations instead, given that most drones can’t go all that far. But Hall worries that natural and man-made obstacles, like hills and buildings, might get in the way. If we decide that drones have to constantly ID themselves or risk getting yanked from the sky, he reasons, there can’t be hiccups.

Hall found the conference crowd receptive. Noted Seton Hall information law expert Frank Pasquale even highlighted the idea from the main stage. But two days spent discussing the idea raised more questions than answers. Do drone operators have privacy rights? If they’re being tracked in real time, might, say, a journalism outfit complain that it’s losing a competitive advantage? Would drones spoof their identity? Should some sort of chip be embedded in the factory to cut down on that sort of thing? Might it be enough for the FAA to hold drone identities in a “privacy escrow” that you can petition when something goes wrong?

Cities, Hall said, pose particular challenges. Drones are becoming part of big cities’ surveillance packages. Mayor Michael Bloomberg said on the radio in March that the use of drones by everyone from law enforcement to terrorists is both “scary” and probably unavoidable. But if you use radio frequencies to track drones, Hall said, it matters that “New York City is a bunch of steel valleys.” Earlier this month, a man walking near Grand Central Station in Manhattan said a crashing drone nearly missed him. The police, he said, declined to pursue the case. Said Hall, “there’s a lot of people worried about drones falling out of the sky.”

Hall started his career as an astrophysicist before switching over to security work. “As a privacy nerd,” he said, “there’s a concept of notice, of being aware of how people are using information about you, and we’re already seeing that being a problem on the Web. And with the drone, you can buy the most beautifully secluded home ever to get away from it all, and still not understand the circumstances under which you can be completely free.” At the conference, Hall said, he got the sense that lawyers, researchers and industry representatives in attendance thought his idea had legs, and that perhaps the $6,000 price tag wasn’t such a bad thing. (Though, he added, those who sell drones and drone accessories might see dollar signs.)

“Maybe there should be some barriers,” Hall said, “when every yahoo can pilot a commercial-grade drone on the cheap.”

Nancy Scola is a Washington, DC-based journalist whose work tends to focus on the intersections of technology, politics, and public policy. Shortly after returning from Havana she started as a tech reporter at POLITICO.