Waves: Part 11 - Phased Arrays

Here we look at more recent developments in radar such as phased arrays and synthetic apertures.

The classic radar system determines the time of flight of the return from the target and uses this to establish the range, but not the direction or height. The early Chain Home system used a broad transmitted beam and relied on a highly directional receiver.

Fig.1 - Any detail in an object must result in wave fronts at an angle to the optical axis. If these cannot be captured by a device having a suitable aperture, resolution is lost.

Fig.1 - Any detail in an object must result in wave fronts at an angle to the optical axis. If these cannot be captured by a device having a suitable aperture, resolution is lost.

As the wavelengths in use became shorter with the advent of microwave electronics, it became possible to use smaller antennas that could work for both the transmitter and receiver. The entire assembly would rotate so that transmitted pulses were scanned over the surrounding area. A target would only produce a return when the antenna was pointing at it, so direction could be obtained.

In early systems, the return would produce a blip on the display screen, which showed that there was an object at a particular range and bearing, but said nothing more about it. It is fundamental to all wave-based imaging systems that the resolution is diffraction limited. Fig.1 shows that detail in an object can only result in wave fronts that are off-axis and the resolution limit comes when these wave fronts pass outside the aperture of the lens or mirror of the receiver.

It follows from transform duality that for a given wavelength, the smaller the item to be resolved, the bigger the aperture has to be. Large radar antennas that have to be rotated become a nuisance. During the cold war, mechanically steered parabolic antennas were used for long range early warning. These would often be protected from the elements by spherical fiberglass covers. A set of these was installed in the nineteen sixties in Yorkshire, England and could be seen for miles. Locally they were dubbed “The Golf Balls”.

One of the drivers in the development of fiberglass was its radar transparency and it became the standard material for radomes. Later it was found suitable for molding small boats and unsurprisingly, these were found to have very low radar signatures and needed to be fitted with reflectors to avoid being run down by larger vessels.

Ultimately the Yorkshire golf balls were replaced by a set of three flat fixed antenna arrays that were steered electronically. Fig.2 shows that if a number of small antennas are fed though a system of delays, the direction of the resulting wave front can be steered by changing the delays. That is how phased array radar works. The beam can be steered left and right and up and down with no large moving parts. This means that in many applications a larger effective aperture can be used in less space. Most modern warships have such antennas.

In airborne radar applications, the phased array technique can be used to make a flat antenna and to optimize directivity, but a single rotating antenna would still be lighter and smaller than three fixed antennas. The rotodomes in some AWACS airplanes contained planar phased arrays. The rotodome was axially symmetrical so it had the same aerodynamic effect on the airplane no matter which way the antenna was pointing. The rotodome on the Boeing Sentry was thirty feet across and weighed six tons.

Fig.2 - In the phased array, multiple small antennas radiating at different times can form a wave front whose direction can be changed at will by altering the time delays.

Fig.2 - In the phased array, multiple small antennas radiating at different times can form a wave front whose direction can be changed at will by altering the time delays.

Another way of improving resolution is to increase the effective aperture using motion. A sideways looking radar in an airplane moves transversely to the target. If the returns from a number of transmitted pulses with the antenna in different places can be combined, the effect is almost as good as if the antenna was much larger.

The widespread adoption of radar reduced the chances of the success of an airborne attack by providing advanced warning. Naturally, the military mind turned to ways in which the radar returns from airplanes could be reduced, prevented or confused. One of the first methods was to air drop thin strips of metal foil, having a length of approximately half of the wavelength of the radar in use, between the radar set and the airplane(s) to be protected. This technique came to be known as “window” and was first used in WWII with considerable effect.

An airplane being pursued could launch its own window to confuse any following airplane or missile. During the cold war, bombers were fitted with highly developed window systems that could measure the wavelength of any radar that illuminated them and cut the strips of window to the appropriate length before releasing them.

Active systems were also developed, by which an airplane could generate a false return to an incoming radar pulse, making the airplane appear to be somewhere else.

The most successful approach to avoiding radar detection has been what came to be called stealth technology. This has a long history and the principle was discovered by accident. An airplane designer by the name of Jack Northrop argued that an airplane in which all of the necessary parts were enclosed within the wing could dispense with a body and a tail and thus suffer less drag. What Northrop said was true of the bomber airplanes of the time that were driven with piston engines at moderate speed.

Some of these large machines were built and tested and naturally the test flights would be tracked by radar. It was then discovered that the radar sets couldn’t see the flying wings. At first this was used by the crews of the wings to make fun of the radar stations, but eventually it was realized that an important discovery had been made.

Stealth airplanes are designed so that practically nothing in their shape presents a surface at right angles to an arriving radar beam, such that the beam is deflected anywhere but where it came from. Like the earlier flying wings, stealth airplanes are tailless with the body blended into the wing. They are mostly comprised of flat mirror surfaces joined up vaguely to resemble an airplane. Steps are taken to make the shape as incoherent as possible. Even the undercarriage doors are irregular to prevent the shut lines from being straight.

The radar signature of these machines is vanishingly small, but so is their stability, with aerodynamics compromised by the odd shape. This is overcome with artificial stabilization. Jack Northrop lived to see these machines enter service.

In Doppler radar, if any target in the beam moves with a component of motion towards the antenna, the frequency of the received signal will be shifted according to the Doppler effect. In a simple system, the return signal is mixed with the transmitted signal and the difference appears as a beat signal whose frequency is proportional to the relative speed between the radar and the target. The distance travelled is proportional to the number of cycles of the beat frequency in the appropriate time.

As the transmitted and received signals are narrow bandwidth sine waves, which are readily interpolated, the transmission did not need to be continuous and if it was pulsed, the same antenna could be used for the receiver when the transmitter was off.

If the radar beam is pointed obliquely forward and down from an airplane, a component of the airplane’s speed over the ground could be measured. This is an important point, because airplanes fly in moving air masses and the air speed and heading is seldom the same as their ground speed and track.

Doppler radar became an important contributor to airborne navigation because once calibrated from a known location it was self contained and needed no external signals that could be jammed or disabled. In a typical system four beams were radiated from the airplane, two forward and outward, two to the rear and outward. Processing the differences between the returns allowed for changes in height, roll and pitch of the airplane to be cancelled out. The equipment could then produce the true ground speed and track as well as computing the wind direction.

When flying over the ocean, if the sea surface should be flat calm, the beam would be reflected away and the return would be very small. This seldom happens. As has been seen in an earlier part, waves in water cause points on the surface to rotate and a component of that motion would cause an error in the Doppler return. A compensating adjustment could be set into some Doppler navigators to oppose the error.

An airplane with Doppler and inertial navigation was in good shape as it could survive a failure of either system. Such an approach was extremely useful when flying in polar regions, where the magnetic compass becomes a source of entertainment rather than a navigational aid. 

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