Walls have eyes

On 5 May 1980 the eyes of the world’s media were trained on the Iranian Embassy in London’s Kensington as the six-day hostage crisis was nearing its bloody end. The turning point came when the body of a hostage was unceremoniously dumped through the front door and the order was finally given for the SAS to storm the building.

Key to the team’s successful entry was the information that had been gained about the six gunmen and their movements with the use of fibre-optic cameras that were threaded through holes drilled in the walls of an adjoining building. This was a risky plan and one which — if uncovered — could have had serious consequences for the 20 surviving hostages trapped inside the embassy.

UK technology expert, Cambridge Consultants, has recently launched a product which — had it been available then — would have made life far easier and safer for the SAS team. The Prism 200 system is a portable radar ‘through-wall’ sensing device that can accurately detail the position of any moving occupants inside a room.

The briefcase-sized box is designed to give special forces a crucial insight into what they may be about to face, and will help reduce the risks involved in hostage or siege situations, it is claimed.

‘Imagine being outside a building as a member of special operations or the special police force,’ said Cambridge Consultants’ Prism 200 project manager Alan Wiltshire. ‘You would want to know how many people are inside and what their movements are. We’ve talked to special forces all around the world and they say that the more information they have about what’s in the room, the better prepared they are to make an entry and reduce risk to their own lives and the occupants.’

The device transmits ultra-wide band (UWB) radar pulses that can pass easily through most solid objects, including concrete, wood and bricks. Its 100Hz frequency means that the waves are short enough to penetrate even reinforced concrete. The radar signal bounces off any objects in the room and the in-built processor analyses these reflections for any tell-tale signs of movement. The key to the system’s reliability and success rate is the sophisticated signal processing, which is used to filter out all unnecessary information.

‘We use the analogy that it’s like looking into a room full of mirrors,’ said Wiltshire. ‘Although you just want to track the person, you get lots of reflections from everything else. To get round this we’ve had to do an awful lot of work on the signal processing.’

Emitting from the box at 100 pulses a second the Prism 200’s signals are bounced back to the system and update the display screen at a rate of around 10 times a second. An array of antennas inside the device give the user an extremely wide field of view — sufficient to comprehensively cover every inch of the interior of a room.

The antennas measure the angle of the signal’s arrival which the system then quickly analyses to provide detailed positional information in the form of a highly-accurate graphical representation. A moving object is indicated as a red dot moving inside a simplified green box representing the room.

Extremely fast digital signal processing is then able to distinguish between the objects that remain the same — frame to frame — and those that have changed. The system does this by keeping a ‘memory’ of signals as a database and cancelling out any that are repeated in the next series of returned pulses.

This sophisticated layer of software means the system can track a moving object in a room up to 20m away and — crucially — is able to distinguish between a moving human target and the many reflections of light that could confuse a normal radar system.

An important factor in the product’s development, according to Wiltshire, is the extensive consultation period the system has undergone over the past year.

Cambridge Consultants produced a working prototype in 2005 which they then demonstrated to the various police and special forces departments from countries around the world. The feedback from this process resulted in a number of design changes that were taken on board in the design of the finished product, explained Wiltshire. As well as making the end product extremely robust and durable, one of the most obvious modifications was the change to the system’s ergonomics which allow it to be used one or two-handed.

The original prototype was developed with handles on either side of the box which would mean that to hold it against a wall the user would have to use both hands.

‘Those we consulted pointed out that if they were ever that close to a possible threat they would need to have their other hand free to carry a weapon in case someone suddenly comes out of a door,’ said Wiltshire. ‘So we gave that problem to our designers. A system like this has to be really user-friendly — if it’s difficult to use there is no point having it.’

The Prism 200 can be used in a couple of different ways. The colour display screen is designed to blink into life within two seconds of being switched on, meaning it can be used to quickly check how many occupants there are in a room. However it can also be used for long-term surveillance and — with the addition of a tripod — can record and track a target’s movements over longer periods of time.

The tripod is crucial, Wiltshire explained, because if the device is moving the returning radar echoes become confused, making it seem as if every object in the room is a target. For detailed surveillance over a period of time a number of Prism 200 devices can also be linked via Ethernet and location and movement patterns inside the building saved to a remote PC.

The device also gives special forces a number of different views that are easily interchangeable depending on the information required. With a click of a button on the easy-to-use control panel the system can switch between a plan, side elevation and 3D viewpoint and the range can be toggled all the way up to the maximum of 20m.

According to Wiltshire there is a real need for such a tactical tool among police and special forces, and Cambridge Consultants revealed that it has already taken a number of advance orders prior to launching the product.

‘I was speaking with a police officer from the Met who told me that this system would have been very useful recently when he broke through a door during a raid and a man was standing behind it with a samurai sword,’ said Wiltshire. ‘Quite simply, if you know what is behind walls and doors then you can make more intelligent, informed decisions.’

When The Engineer saw a demonstration of the system prior to launch, the results were indeed impressive. Behind a thick breeze block wall, the movements of a figure were precise and rapidly updated on the colour screen. In the side elevation view, even crouching figures can be distinguished from those who are standing — a key distinction to make when trying to differentiate between hostages and terrorists in a crisis situation.

However, good as the system is, it would appear that there are a couple of instances when the Prism 200’s ability to see through walls could well be compromised.

One worry might be that even UWB radar signals cannot pass through steel, and so metal doors or furnishings can leave gaps in the system’s readout. While the sophisticated signal processing is capable of working around the problems caused by metal furnishings such as desks, a badly-placed filing cabinet could render the system impotent. In an attempt to solve this problem, Cambridge Consultants has set up a ‘practice room’ filled with difficult-to-process materials.

As well as metal-plated rooms, another possible way to avoid detection is to remain absolutely motionless. The system only picks up on movement, so if a subject was to remain entirely still it is possible that he could escape detection. However, Wiltshire was adamant that the slightest movement — even heavy breathing — would be enough for the sensitive instrument to give away a human’s position.

‘We have tested this extensively in real-world environments and we are confident that its level of accuracy will make it a key tactical tool for special forces,’ said Wiltshire. ‘So far, the feedback from potential users has been fantastic.’