The story that the CIA uses tiny homing beaconsto guide their drone strikes in Pakistan may sound like an urban myth. But this sort of technology does exist, and might well be used for exactly this purpose. It might even have been the "secret weapon" that Bob Woodward said helped the American military pacify Iraq.

The military has spent hundreds of millions of dollars researching, developing, and purchasing a slew of "Tagging tracking and locating" (TTL) gear – gizmos designed to keep covertly tabs from far away. Most of these technologies are highly classified. But there's enough information in the open literature to get a sense of what the government is pursuing: laser-based reflectors, super-strength RFID tags, and homing beacons so tiny, they can be woven into fabric or into paper.

Some of the gadgets are already commercially available; if you're carrying around a phone or some other mobile gadget, you can be tracked - either through the GPS chip embedded in the gizmo, or by triangulating the cell signal. Defense contractor EWA Government Systems, Inc. makes a radio frequency-based “Bigfoot Remote Tagging System” that's the size of a couple of AA batteries. But the government has been working to make these terrorist tracking tags even smaller.

Sandia National Laboratories have carried out development on "Radar Responsive" tags, which are like a long-range version of the ubiquitous stick-on RFID tags used to mark items in shops. The Radar Responsive tag stays asleep until it is woken up by a radar pulse. The tags in Wal-mart have a range of a couple of meters, Sandia's tags can light up and locate themselves from twelve miles away.

This document from 2004 describes the tags as being credit-card sized and with a "geolocation accuracy" of three feet. The radio waves penetrate buildings. Suggested application include "search and rescue, precision targeting, special operations." The selection of aircraft used to illustrate the system includes a Predator drone.

The reports from Pakistan suggest that the CIA knew which village to strike, they just needed to locate the exact building (descriptions like "third house on the left" can be dangerously ambiguous, especially when viewing from the air). A Radar Responsive tag would be very handy for guiding a strike from a drone a few miles away.

Nor is this the only technology out there. A 2002 Defense Science Board report on counter-terrorism mentioned, among other things, the possibility of using invisible chemical dye to mark terrorists, so they could be spotted using a suitable viewer.

The 2006 Quadrennial Defense Review – the Pentagon's once-every-four-years grand strategy document – included a section on defeating terrorist networks, which mentioned the importance of tagging and tracking both terrorists and their gear. Two methods suggested are tinier-than-tiny radar tags, and dynamic optical tags. Darpa, the Pentagon's way-out research arm, spent years developing these "small, environmentally robust, retro reflector-based tags that can be read by both handheld and airborne sensors at significant ranges." They rely on small silicon reflectors which return a laser signal – as long as that signal can be seen from the air. "Each Dynamic Optical Tag or DOT is an inch across and based on a 'quantum well modulator,'" the agency explains. "They are read using a laser interrogator, which can be mounted on an aircraft; the laser 'wakes up' the tag, which sends a return signal at over 100 kbps. This can be simply the ID of the tag, or it can be data that it has recorded - for example, details of where it has traveled since last interrogated, or recorded video or audio."

Covert radar tags were descried in a 2004 report by the National materials Advisory Board. Inkode, a company that also provides cheap RFID tags for supermarkets, has developed a means of embedding aluminum fibers in paper and other materials. The fibers are described as 6.5 millimeters long and 1.5 micrometers in diameter.

When illuminated with radar, the backscattered fields interact to create a unique interference pattern that enables one tagged object to be identified and differentiated from other tagged objects," the company says. "For nonmilitary applications, the reader is less than 1 meter from the tag. For military applications, the reader and tag could theoretically be separated by a kilometer or more."

The fibers can be embedded in "paper, airline baggage tags, book bindings, clothing and other fabrics, and plastic sheet." Eight thousand fibres can be embedded in a typical 8½ by 11 inch piece of paper, which could be seen by radar at a similar distance to a meter-square target. So even something as small as a cigarette paper could be detected through walls, uniquely identified and precisely located from a tactically-useful distance in order to direct a missile strike.

This 2007 briefing from U.S. Special Operations Command hints at research into even more exotic ways to keep tabs on a target. Technology goals include spotting a "human thermal fingerprint at long distance," "augmentation of natural signatures: e.g. 'perfumes' and 'stains.'" The presentation also mentions a "bioreactive taggant" that is a "current capability." Next to the words in a picture of a bruised arm.

We do not know if any or all of these technologies are actually in use. After all, mobile phones are also a good way of locating an individual from long range, and there are numerous other sensors that can be used to direct a strike. But technologically speaking, the miniature homing beacon calling in CIA drone strikes is not just another urban myth.

– David Hambling and Noah Shachtman

[Photo: Sandia National Labs]