Spy satellites could soon be able to identify someone from space by looking at their shadow.

They would use a computer program that searches for the movement of shadows on the ground, and then identifies their owners from the way they walk.

The technique  -  called gait analysis  -  relies on the fact that someone's walking style is very difficult to disguise.

The satellite or spy planes will detect the movement of people's shadows to identify them

It could be used to monitor known criminals and suspected terrorists using satellites or spy planes. It could even be used in the hunt for Osama Bin Laden.

There is however a significant catch. The system, being developed by Nasa, is useless once the sun goes in.

And, although there has been an explosion in satellite imagery and technology in recent years, it is still impossible to recognise someone with confidence using pictures taken in orbit.

Images from high-altitude aircraft and spacecraft show only the tops of their heads.

Experts say aerial shots are no good for monitoring someone's stride length and walking rhythm.

However, that is not true of shadows. According to Dr Adrian Stoica of Nasa's Jet Propulsion Laboratory in California, video from space could provide enough data to confirm a suspect's identity  -  as long as details of the person's walking pattern were on file.

He has created computer software that can seek out and recognise the shadows of individuals in aerial video footage, reports New Scientist magazine.

It isolates moving shadows and uses data on the position of the sun and camera angle to 'correct' the shadows if they are foreshortened or elongated.

Dr Stoica, who presented his research at a security conference in Edinburgh, said the software then applies regular gait analysis to the corrected images.

In tests on video footage taken from the sixth floor of an office building, the software spotted shadows and extracted information that could be used to identify someone.

The technique is still at the earliest stages of development, and it could be many years before it is used by military, police and intelligence services.

It also has potential flaws. While satellite gait analysis might be useful in countries such as Pakistan and Iraq, trying to identify a suspected terrorist from a shadow in a rainy Manchester is likely to be fraught with difficulties.

The quality of satellite imagery may also not be good enough to reveal a sharp image.

Space imaging expert Dr Bhupendra Jasani at King's College London says the sort of geostationary satellites currently pointing their cameras down to the Earth simply do not have the resolution to provide useful detail.

'I find it hard to believe they could apply this technique from space,' he said.

Aerial surveillance - a key tactic in military spying

Minor differences in gait that can help identify individuals include the length of steps, walking speed, the wiggle of hips and the angles of the knee, ankle and hip.

Aerial surveillance has long been a key part of military spying - ever since 19th century armies began using hot air balloons to observe enemy positions on the battlefield - but in recent years the speed of technological advance has accelerated.

As well as orbiting satellites, spy planes such as the RAF's Nimrod can circle at high altitude watching the movements of individual enemy gunmen on the ground, by day or night, using high definition thermal cameras.

At the same time unmanned aerial vehicles, or UAVs - ranging from huge jets the size of an airliner to tiny hand-launched drones a couple of feet across - are now widely used in Iraq and Afghanistan, where commanders on the ground increasingly rely on the images beamed from flying cameras to allow them to track approaching enemies or spot dangers in hostile built-up areas.