Any technology can be a two-edged sword. Cell phones are a good example. They're clearly a great convenience, even a life-saving tool during natural disasters and medical emergencies. And they should save many more lives under the Federal Communications Commission's enhanced 911 mandate, which requires that cell phones relay their precise locations to 911 operators.
Yet some people are appalled that their cell phone will always give away where they are. That makes it possible to compile a "road map" of a user's movements. So imagine the outcry when privacy worriers learn that cell-phone systems can be used to locate and track a car, boat, or plane -- even if no one inside is carrying a wireless phone. But outcry or not, the technology seems certain to be deployed, given its myriad civilian and military applications.
The technology is called Celldar, from "cellular" plus "radar." Under development since 1997 at Roke Manor Research Ltd (SI ). in Romsey, England, it differs from conventional radar in several key respects. When radar tracks planes for air traffic controllers -- or maps the surface of Venus from a space probe -- the same set of equipment both sends out radio signals and picks up the returning echoes. So computers can easily calculate an object's distance and relative size based on how long it takes the returning signals to bounce back and how strong they are.
In contrast, Celldar takes a so-called passive approach: It watches and interprets how signals from cell-phone base stations interact with objects such as cars, trucks or planes. The hardware required for this is much simpler than existing radar systems. A Celldar prototype built in 1999 consisted of a PC and the insides of two cell phones, and cost just $3,000, says Peter Lloyd, head of Roke Manor's Celldar program. The flip side is, the signal-processing software is complex: It must allow for the varying travel times for signals between two or more cellular base stations and a Celldar receiver, as well as the times from the different base stations to the target. Lloyd says Roke's clever program is based on "$10 million worth of expertise in writing software" for cellular systems and military radars. One big plus to the military: passive radar systems are invisible to anti-radar weapons because they don't have their own transmitters.
Celldar's implications are exciting -- but also troubling to some. Even though the technology can't be used to identify cell-phone users, since it "sees" only radio waves echoing off hard surfaces, it and similar approaches are evolving quickly. In addition to Celldar, which is sopping up $1.5 million a year for development, a dozen other passive-radar projects are under way in the U.S., Europe, and Asia. As the technology bears fruit, it should give the world's police and homeland security agencies new tools for monitoring shipments of illegal weapons and drug smuggling operations. Highway officials could gain a detailed window on traffic flows, helping them to minimize congestion. But because passive-radar systems could be cheap enough for hobbyists to buy -- or cobble together themselves -- the technology could also become the next fad among people who own police-radio scanners or who enjoy snooping on their neighbors' comings and goings.
This month marks something of a passive-radar milestone. In late October, Roke will show off the latest Celldar system to officials of Britain's Defense Ministry at an army exercise on the Salisbury Plain. During the field maneuvers, Celldar will detect the movement of ground vehicles and determine whether cell-phone signals are bouncing off a tank, truck, or armored personnel carrier.
Also in late October, radar researchers from around the world will gather in Seattle to discuss recent advances, including passive systems that use FM radio or TV broadcasts instead of cell-phone signals. The invitation-only meeting will be hosted by John D. Sahr, a University of Washington electrical engineer. Since 1997 he has operated a passive-radar system unshrouded by military secrecy. It harnesses an FM station's signals to study particles in the ionosphere -- the top layer of the atmosphere, over 300 miles up. Sahr decided to go with passive radar, he says, "because it's incredibly cheap" -- $20,000 vs. $25 million for a comparable active system. "You could probably do an amateur system for under $5,000," Sahr adds. A system for small airports might cost as little as $15,000. That's important because of the 5,280 public airports in the U.S., only about 300 currently have radar.
STRONG SIGNALS. Lockheed-Martin Corp. (LMT ) is perhaps the best-known passive-radar champion, but others include Avtec Systems, Dynetics, and ONERA, the French counterpart of NASA. Lockheed-Martin's system is dubbed Silent Sentry. Last fall, in a demo for the U.S. Air Force, a third-generation Silent Sentry radar tracked all the air traffic over Washington, D.C., by picking up FM and TV echoes. Because FM and TV transmissions are more powerful than their cell-phone cousins, Silent Sentry can detect planes as far away as 135 miles, roughly 10 times the reach of an individual cell-phone tower.
However, because cell-phone towers are scattered far and wide in many countries, an airborn Celldar system "could covertly monitor a whole country" by flying along its borders, says Lloyd. At the Seattle meeting, Roke Manor and British aerospace giant BAE Systems (BAESY ), which signed on as a Celldar partner in July, 2002, will reveal details on their progress toward systems for small robot spy planes as well as Airborne Warning & Control Systems (AWACS). With BAE on board, Lloyd declares, "we could field an AWACS model in two years."
That claim prompts some skeptical head-scratching by Benjamin J. Slocumb, a senior research engineer at Numerica Corp., a small Fort Collins (Colo.) defense contractor that develops advanced target-tracking algorithms. A receiving antenna that's moving, he says, "injects a whole bunch of difficult problems" in collecting and processing cell-phone echos. But Roke's Lloyd stands firm: "We'll show results in Seattle that are seven times better than [past studies] have said is possible."
Despite Celldar's military potential, Lloyd predicts the first applications will come in the civilian sector. He says transportation officials are eager to use Celldar to monitor road traffic because it would avoid the expense of installing either sensors in roads or TV cameras overhead. And police cars equipped with Celldar could follow a car driven by a suspected crook or terrorist from a safe distance, without danger of being seen.
TRACKING MADE CHEAP. Celldar might also provide an alternative to the global positioning satellite (GPS) systems now being explored by insurance companies and governments for monitoring vehicles. Their goal is to set premiums individually, based on how much and how fast each car or truck is driven -- or to levy a road-use toll determined by the distance a vehicle travels, over which types of roads, and at what times of day. In Ireland, AXA Insurance is testing a GPS gadget called Traksure. It continuously checks a car's speed and location, then compares that data with the local speed limit, obtained from digital maps. But Celldar might do the job more cheaply.
And it might support schemes by Oregon and other states regarding "pay-for-use" road taxes. Many transportation experts assert that taxing actual driving distances would be a more equitable way of funding highway upkeep than today's tax on gasoline and diesel fuels. That's why the European Commission wants every vehicle in Europe to be fitted by 2010 with a black-box device that can be tracked by satellite. Germany is now testing such a system on trucks, and Britain plans to require it on trucks by 2006.
Once the passive-radar cat is out of the bag, there's even a chance it could evolve into a means of tracking people on the street. "But there'd be a lot of technical challenges," because the human body is a poor reflector of radio signals, says Shawn M. Herman, a researcher at Numerica whose 2001 PhD thesis was on passive radar. Still, classified research is under way in Europe to create a hybrid surveillance system, using both passive radar and images from the TV cameras that now monitor many urban intersections and streets, airports, and other public places. Suspected terrorists and other bad guys could then be watched more closely.
The Big Brother implications of all this might unleash a massive public backlash. But just as plausibly, people may decide to put up with technology's double-edged sword to regain a measure of the security they have lost.
By Otis Port in New York