No-fly zones

After calls for the international community to consider imposing a no-fly zone over Libyan airspace, we look at some of the physics-based technology involved in enforcing them.

F14 Fly past
Photo: US Navy

No-fly zones are sometimes set up in areas of conflict to prevent military aircraft from being used against civilian populations. Notable examples are the two over Iraq from 1991–2003, and that over Bosnia and Herzegovina in 1993–5. To police the zones effectively, any aircraft violating them first need to be detected, and then, if necessary, shot down.  

Pilots whose planes are damaged may need to eject from their aircraft. All of these aspects of no-fly zones make use of physics.

Spotting and tracking aircraft violating a no-fly zone is usually accomplished using radar.

The basic principle is a simple one: emit a radio signal and time how long it takes to be reflected back. The distance to the target can be easily calculated from that time and the constant speed of the wave – the speed of light.

The line-of-sight velocity of the object detected can be worked out from the change in wavelength of the radio signal due to the Doppler Effect.

The term “radar” was coined, as an acronym of RAdio Detection and Ranging, by the US Navy in 1940, but the development of radar goes further back than that.

During the end of the 19th century and the beginning of the 20th, several physicists independently came up with the idea of using electromagnetic waves to detect objects such as ships.

That idea was taken more seriously in the UK from 1934 with the increasing threat of war with Germany, and by the time World War 2 began in 1939 there were 19 radar stations along the south and east coasts helping to defend British airspace.

Modern scanning of the skies for aircraft that shouldn’t be there is often conducted from the air rather than the ground. The Airborne Warning And Control System, or Awacs, is based on airliner designs with the addition of a radar dome mounted on the back.

As well as detecting unknown or “bogey” aircraft themselves, Awacs can share data with friendly planes, which can allow them to avoid using their own radar systems for greater stealth.

Although initially based on radio, many radar systems now use the microwave and very high infrared parts of the spectrum, with wavelengths around 1 mm to 1 m. The shorter wavelength gives a better resolution.

This can be crucial when there might be a need to separate targets only yards apart – in the 1986 movie Top Gun, Tom Cruise’s character gets into trouble when his radar fails to spot until too late that a contact is not just one aircraft but two. The system used by the F14 fighters shown in the film operates in the X-band, at a frequency of 8–12 GHz (a wavelength of around 3–4 cm).

Some higher-resolution radar systems can also create images of a target object. The principle is similar to that of an ordinary flash camera. This makes it possible to identify the type of aircraft that has been detected from long range.

In areas where there may be friendly aircraft operating, a final identification check is often performed visually. This is not perfect either. Two US Army Blackhawk helicopters were shot down by the US Air Force over one of the Iraqi no-fly zones in 1994, and this led to greater caution over Bosnia and Herzegovina the following year, when Bosnian and Croatian helicopters were often painted with the markings of the Red Cross or of UN aid-giving organisations.

If a detected aircraft is positively identified as enemy, or “bandit”, a decision may be taken to intercept it.

The weapon of choice is usually a missile. Unlike bombs, they are powered by their own internal propulsion – a rocket or jet engine. These are usually either solid-fuel rockets or turbojet engines for ease of maintenance and simple design respectively.

As well as detecting hostile aircraft, radar is often used to guide missiles to their targets. This is sometimes from the radar in the aircraft with instructions on course changes relayed from there to the weapon, but more modern missiles carry their own radar on-board.

The other commonly used method is for the missile to lock onto a particular heat signature.

Many use accelerometers and gyroscopes to calculate their own position and velocity, and to track the target, they can change course in one of two ways.

Using “thrust vectoring”, the direction of thrust from the engine is changed by altering the nozzle angle. Using aerodynamic maneuvering, the missile’s fins are rotated to change the air pressure, and therefore net force, on one side – the principle is the same as that by which planes themselves manoeuvre, and similar to that which creates the lift that gets them airborne in the first place.

Using both its own position and velocity and that of its target, along with the basic equations of mechanics. The missile’s guidance or targeting system then plots a collision course.

Bailing out
A pilot whose plane is hit by a missile may need to eject.

Early ejection seats used an explosive charge of solid propellant under the seat to launch them clear of a stricken aircraft. As they could damage a pilot’s spine, the Royal Air Force imposed a limit of three bail-outs before permanent reassignment to duties on the ground.

Modern seats are rocket-propelled, and many move an ejecting pilot out of the way of his crashing plane by deploying a drag chute – a small parachute designed to create drag – into the airflow.

Ejecting is now safer for the pilot in a range of circumstances from zero speed on the ground to high speed and high altitude – as long ago as 1996 two crew members ejected at 80 000 ft from an aircraft travelling at more than three times the speed of sound.

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