Qinetiq's spy in the sky

A technique astronomers use to search for gamma ray bursts in the depths of the Universe could be adapted to find terrorists responsible for car bombs in Iraq.

UK defence specialist

is developing coded aperture imaging, once reserved for astronomical purposes, for sensor systems to be used in high-altitude airships and unmanned air vehicles (UAVs).

The surveillance technology would detect and track large numbers of moving vehicles in dense urban areas 24 hours a day, from an altitude of 12 miles (20km).

Qinetiq is leading a team that recently secured a 33-month, £11m ($22m) contract from the

(DARPA) to continue its research on the technology for its Large Area Coverage Optical Search While Track and Engage (LACOSTE) programme.

Traditional imaging systems use lenses to focus light on to a detector, whereas coded aperture imaging replaces the lenses with a flat mask containing multiple pinholes (the coded aperture).

Astronomers use the technology to create images of non-visible radiation, such as gamma rays and X-rays. Points of light radiating from the cosmos pass through the pinholes of the mask and cast shadows onto the detector, resulting in multiple overlapping images. A built-in central processing unit then decodes this pattern and reconstructs the original image.

Unlike the systems used by astronomers, Qinetiq's technology would be capable of imaging longer wavelengths such as visible and midwave infrared (IR), which would allow the system to create images of scenes on the ground during both day and night. The system would also be unique in its ability to look at several directions at once.

Chris Slinger, Qinetiq's principal investigator on the LACOSTE programme, said the new system uses an adaptable coded aperture. The Qinetiq mask consists of a micromachined silicon array of pixels. Each pixel can be individually controlled and shut with a zap of voltage. With this sort of technique a variety of patterns are possible.

Slinger said the most challenging part of the research was developing a way to create images of wavelengths as long as visible light and infrared. Astronomers use shadow patterns to create images of gamma rays and X-rays, but longer wavelengths are subject to diffraction, making images difficult to reconstruct.

To address this problem, the Qinetiq team developed a light modulation method that uses interference to gather image data.

Each pixel in the array acts as an interferometric modulator that transmits light from the scene according to its position. Light waves pass through each pixel and then position one on top of the other to create a new wave pattern on a cooled infrared focal plane array. The information is then patched through and processed with a standard CPU using coded algorithms.

Slinger said diffraction problems had previously stopped engineers from considering coded aperture technology as a solution for longer wavelengths, but Qinetiq's efforts will prove valuable because there are no other options for constructing images of moving vehicles 24 hours a day in urban areas.

Other techniques such as radar are good for indicating moving targets, he added, but in built-up areas, buildings can obscure the radar systems 'line of sight'. Also, unlike surveillance systems that just work with visible light, this technology also works at night.

Slinger said use of midwave infrared technology makes it possible to track vehicles for long periods and identify certain vehicles with accuracy.

'Different car paints would give different intensities in the infrared,' he said. 'Cars would also be different shapes and temperatures, with engines located in different places. You can use some of those cues to provide a level of discrimination.'

Slinger gave an example of how the technology could be used for anti-terrorism operations in places such as Baghdad.

'If you're tracking all the cars in the city and suddenly there's a car bombing, you could look at your tracking data and see which car exploded, the route it had been driven to get there and perhaps where it had started from,' he said. 'Perhaps there might be a building where the bomb makers loaded the car up with explosives.'

He added: 'At the same time you could look on top of the vehicles that were at that building and use their independent tracking history to see where they came from.

'You could get very powerful information that would help you gain valuable intelligence about where the bomb makers and their associates came from and their movements.'

Slinger said Qinetiq is planning a field trial for the tracking technology within the next 33 months, but it will not be ready to release the technology for use until a few years after the project's completion.