Before pulling the trigger, a sniper planning to assassinate an enemy operative must be sure the right person is in the cross-hairs.

To help with this Western forces commonly use software that compares a suspect’s facial features or gait with those recorded in libraries of biometric data compiled by police and intelligence agencies. However, this technology can be foiled by a disguise, head-covering or even an affected limp.

For this reason America’s Special Operations Command (SOCOM), which oversees the units responsible for such operations in the various arms of America’s forces, has long wanted extra ways to confirm a potential target’s identity.

Responding to a request from SOCOM, the Combating Terrorism Technical Support Office (CTTSO), an agency of the defence department, has now developed a new tool for the job. This system, known as Jetson, is able to measure, from up to 200 metres away, the minute vibrations induced in clothing by someone’s heartbeat.

How Does it Work?

Since hearts differ in both shape and contraction pattern, the details
of heartbeats differ, too. The effect of this on the fabric of garments produces a “heartprint” – a pattern reckoned sufficiently distinctive to confirm someone’s identity.

To measure these heartprints remotely employs gadgets known as laser vibrometers, which work by detecting minute variations in a laser beam that has been reflected off an object of interest. They have been used
for decades to study things like bridges, aircraft bodies, warship cannons and wind turbines – searching for otherwise-invisible cracks, air pockets and other dangerous defects in materials.

However, only in the past five years or so has laser vibrometry become good enough to distinguish the vibrations induced in fabric by heartprints.

According to experts the best laser vibrometers can measure a displacement of just ten picometres (trillionths of a metre). However, such precision cannot be obtained by the obvious means of measuring changes in the time it takes reflected light to travel to and from a surface. Instead, laser vibrometers detect changes, caused by the Doppler effect, in the reflected beam’s frequency.

• If it is bouncing off something moving towards the detector, its frequency will be shifted to the blue end of the spectrum.
• If the reflective surface is moving away, the shift is to the red.

Although SOCOM has received its first batch of Jetson heartprint readers they are, understandably, reluctant to reveal too many details:

• The beam involved is infrared, and therefore invisible to the human eye.
• The devices work best when a suspect is neither running nor in a moving vehicle.
• And they have a gimbal that helps keep the beam focused on the suspect for the 30 or so seconds it takes to obtain a good heartprint reading.
  • However, SOCOM has not released the process by which heartprint libraries might be built up in the first place.
  • One starting point, presumably, would be to catalogue the heartbeats of detainees in the way that fingerprints and DNA samples are now taken routinely.

Other Uses

• Confirming the identity of enemy targets is not the only job imaginable for this high-precision form of laser vibrometry.
  • The stress of telling a falsehood can alter someone’s heartbeat, so it could improve lie-detection technology, according to experts.
• Another use for laser vibrometry is at he border, as the heartbeats of nervous smugglers are likely to differ from those of ordinary travellers.
• Finally, it has potential medical uses, with plans to run a clinical trial employing it for the detection of arrhythmia and other heart problems.

Is It Restricted to Optical Frequencies?

In short, no.

A device is currently being designed that emits radio waves and captures return signals that have bounced off, and been modified by, a person’s vibrating chest cavity.

The advantage of this approach is that it can penetrate materials opaque to light waves, such as walls and heavy clothing.

A prototype has been able to read heartprints from as far away as 50 metres.

Potential markets for the device include

• Cardiac diagnostics;
• Identification of authorised users of computers and vehicles; and
• Identify masked protesters, although this can be stymied by wrapping tinfoil around the torso.