Riding the crest of a wave

From peering into the heart of a volcano to stopping terrorists in their tracks - millimeter-waves promise to influence many areas - but can the technology deliver? Leila Sattary has some answers

IMAGINE if you could see through a terrorist’s clothes to detect hidden weapons, or look to see how a volcano is changing underneath all the cloud and smoke, or have a car that can see a crash coming. Millimetre-wave imaging allows you to do just that, providing not just an image but also the size and shape of the object in view along with its distance and speed, regardless of the viewing conditions.

Millimetre-waves are a relatively unknown part of the electromagnetic spectrum but technology that exploits these wavelengths has increased rapidly in recent years. In the spectrum, they sit between microwaves and the far infrared and are generally accepted as light a wavelength between 1 to 10 mm, hence the name. 

Initially mm-wave generators and detectors were produced for military applications, but investment opportunities in security imaging and automotive radar have stimulated further research in the area. Until recently mm-wave components have remained expensive but an important breakthrough for reducing the cost of the electronics was the capability to make GaAs and InP integrated circuits at mm-wavelengths. These have allowed the size and weight of the components to be considerably reduced.

Position of mm-waves in the spectrum

The advantage of imaging mm-waves is that they can also pass through a large thickness of atmosphere unlike many other parts of the spectrum. Certain frequencies are subject to relatively strong attenuation due to molecules in the atmosphere absorbing the radiation.

Small particles in the atmosphere can cause scattering, which is dependent on the wavelength of the incident light, size, shape and refractive index of the scattering particle. Unlike the infrared and visible regions, mm-waves exhibit comparatively small attenuation due to fog, smoke, rain or clouds. Although the microwave region has even less attenuation than the mm-wave region, the latter has other advantages for example they require smaller antennae to achieve the same resolution. For this reason the mm-wave region is often more useful for imaging through adverse weather.

It is intuitive to compare the advantages and disadvantages of mm-waves in comparison to its neighbours, microwaves and IR. The shorter wavelength in comparison to microwaves provides better spatial resolution and more compact components but penetration through obscurants is slightly less. The longer wavelength in comparison to IR gives much better penetration through obscurants, such as the atmosphere and fabric but the spatial resolution is not as good and the greater wavelength leads to bulkier optics.
 
Metal detectors have been used to identify objects about a person and rely on detection of a mass of metal however their effectiveness depends on size of the object, the type of metal and its orientation. Also metal detectors, which detect magnetic field variation, cannot distinguish between metal objects such as belts or keys and threatening objects and this can slow down the flow of people through security.

With the increased threat of terrorism there is a need for more effective techniques to detect hostile objects. A new type of imaging is needed as modern weapons include plastic or ceramic guns and plastic and liquid explosives, all of which cannot be detected with conventional metal detectors.
Millimetre-waves may provide a solution; they pass through clothing but are partially reflected by the human body, metals and plastics all at different intensities. Effectively they can see through clothes. Such scanners are being tested in UK airports and are posing the public privacy concerns.
Millimetre-waves have a very small penetration depth into biological samples unlike longer wavelength sources. Typically mm-waves are absorbed in the first few hundred microns of media containing water, such as biological structures. If low energies are used then heating of sample is negligible and the photon energy is generally not adequate to break chemical bonds.
Subcutaneous heating or cooling could be an early indicator for many diseases including arthritis, rheumatism and skin cancer. The ability to create a thermal map of sub-surface body temperature would be a useful medical technique. Passively imaging in the mm-wave regime can produce such pictures. The application of mm-waves here is at the initial research stage but has considerable potential for commercialisation.
Millimetre-waves have the property of being able to pass through materials that make up bandages, which allows passive imaging of body parts which hopes to help assess ulcers, burns or wounds without disturbing the dressing and thus avoiding infection. The University of St Andrews is currently trialling this technology at Ninewells Hospital in Dundee.
 
Urban centres have increasingly become sites of conflict and locating threats amongst innocent people is essential. With their ability to “see” people through certain materials like plasterboard and adverse weather, mm-waves have useful military applications for imaging in such situations.
Conventional missile guidance systems employ lasers for guidance but these are not useable in bad weather. With the capability to see through fog, rain, smoke and dust, missile guidance systems at mm-wavelengths provide a viable alternative. The military require all-weather, low-visibility, compact systems that are effective in cluttered or difficult scenes and seekers must be able to obtain as much information as possible about the target and its surroundings. Millimetre-wave systems can only operate at distances of a few kilometres and so can only be used for short range missiles or when the missile is almost at its target to give precision aiming.

VMADS mounted on a Humvee vehicle

The US military have also adapted mm-waves into weapons themselves by developing VMADS (Vehicle Mounted Active Denial System) which they plan to use as a non-lethal weapon in conflict. VMADS fires intense mm-waves at the target person which heats their skin to temperatures up to 55ËšC causing them pain comparable to getting a blast from an oven. The pain ends when the target moves out of the line of site of the instrument, and so could potentially be used to disperse crowds or on the front line to deter the enemy.
This new technology has met with some criticism. The main concern is that the effects intensify on sweaty skin and may produce harmful hotspots as a result of a radiation build up. This has brought up debate of how functional the weapon would be in hot or humid conditions. VMADS has not yet been deployed and is undergoing further testing. If VMADS proves to be safe and effective then it could provide a useful half way point between verbal deterrents and shooting.
 
 
Millimetre-wave technology has been applied for collision avoidance radar by companies such as NavTeq. Adaptive cruise control works by detecting the presence of other automobiles by using radar. When there are no vehicles ahead the system keeps a steady speed controlled by the driver but the car approaches other vehicles ahead the speed is controlled to keep a safe distance away.

Commercial and military aircrafts are often required to land in limited visibility conditions. Millimetre-waves give an image on landing that a pilot can use to guide the aircraft. The image below, from BAE systems, shows the pilot’s view of foggy conditions and the image at a mm-wavelength. Using the image of the runway enables the pilot to land safely even though the runway could only be seen a few metres before landing.

The Air France Concorde disaster in 2000 that led to the eventual grounding of Concorde was caused by a 16 inch piece of metal on the runway. Since then there has been a need for a device to detect foreign object debris on airport runways. Purpose built radars such as Tarsier, made by QinetiQ, consist of a pair mm-wave radars mounted at appropriate heights to gather data from different locations around the airfield. The radar gives an update on the debris between every take off; approximately once a minute. The Tarsier system has been purchased by many airports including, Heathrow, Dubai and Vancouver.
Clouds play an important role in our climate and their behaviour greatly affects weather systems. Knowledge about the 3D large-scale distribution as well as the structure of clouds on a smaller scale is essential for accurate weather forecasts and climate change predictions. Uncertainty about cloud structure leading to errors in weather predictions provided an impetus to develop more accurate tools for studying clouds.

Artist’s impression of CloudSat orbiting Earth

The only mm-wave radar in space, CloudSat, developed by JPL, measure the power backscattered by the clouds as a function of distance from the radar which orbits above the clouds. This gives a vertical profile of the cloud distribution and the apparatus is also capable of Doppler measurements to aid in the identification of particle types to find the locations of melting layers.
The University of St Andrews has also applied mm-wave imaging for volcanoes. Active volcanoes can evolve rapidly often changing size and shape on a daily basis and volcanologists can predict eruptions by these changes. However the atmosphere around volcanoes is often full of dust or covered in cloud as water condenses when it hits the side of the volcano. Millimetre-waves have the ability to see through cloud and dust and so can be used to image the volcano when there is no visibility. This tool has great potential to improve volcano predictions and give volcanologists a deeper understanding of their behaviour.
 
The first radar of its kind, AVTIS can monitor lava domes of volcanoes irrespective of the weather conditions or time of day. When these domes collapse they form pyroclastic flow which is essentially an avalanche of ash and rock. These flows demolish anything in their path and in areas where there are settlements nearby, accurate predictions are essential to prevent disaster.
 
Millimetre-wave observations of astronomical objects have contributed to new understanding of a variety of areas from the chemistry of molecular clouds to the cosmic microwave background (CMB).  Observing at mm-wavelengths allows detailed study of the chemical, physical and dynamical states of atmospheres and the interstellar medium, which gives essential clues to how young stars are formed.
 
Fusion reactors attempt to force nuclei together at very high temperatures (100,000,000K) to overcome their Coulomb repulsion. Under the right conditions this reaction creates more energy than it consumes and unlike nuclear fission, produces no highly radioactive or fissile products apart from eventually the reactor itself. The fuel is composed of deuterium and tritium which exist abundantly on earth and are easily extracted from water using electrolysis.
 
Reaching the temperature required for fusion while also controlling the instabilities in the resulting plasma are the main challenges that must be overcome before fusion energy can sustainably produce energy. When this becomes a reality, fusion power could solve the energy crisis and cut carbon emissions to a small fraction of their current levels.
Heating by electric currents are often not enough to heat the plasma to the temperature required to maintain thermonuclear reactions. It is essential to control detrimental instabilities, in the plasma to maximise efficiency. Millimetre-waves are used to locally heat the plasma to achieve greater stabilisation. These mm-wave techniques are being tested as part of the heating mechanism in fusion reactors such as ITER (International Thermonuclear Experimental Reactor).

Millimetre-wave research now receives high levels of funding and impacts on modern applications yet the technologies are often poorly understood or appreciated. With one of the largest mm-wave research groups, the University of St Andrews has set up an outreach project funded by EPSRC called Vision for the Future. More details of it can be found at www.vision4thefuture.org

By Leila Sattary. Leila is a final year physics Masters student at the University of St Andrews. She works with the millimetre-wave group and is heavily involved with their outreach project 'Vision for the Future.'