Active Electronically Scanned Array radars explained
Like most great ideas, radar employs an elegantly simple principle. Radio waves are transmitted from an antenna, they collide with an object, properly known as a target, and are reflected back at the antenna as echoes. Stand in a tiled cellar or cave and shout ‘hello’. Your greeting will echo back to you from the walls. Radar uses the same principal.
Two fundamental laws of physics let a radar do its work. Firstly, radio waves travel at light speed – 161,595 knots, or 299,274 kilometres per second. Most radars send out pulses of radio waves. This is akin to being in a dark room equipped with a torch. Switch it on for a fraction of a second and the light briefly illuminates a door in front of you. In that quick glimpse, your brain will subconsciously compute how far the door is from where you are standing.
A radar works in much the same way. Let us suppose we have a radar that has detected a ship. The radar’s pulse takes 0.2 milliseconds to be transmitted, hit the ship, and be reflected at the antenna as an echo. The 0.2ms comprises the time taken for the transmission, collision, and echo round trip. Because we know the radar pulse travels at 161,595kps, in 0.2ms the radar pulse will have travelled 32.4 nautical miles/nm.
The radar does not need to know the distance of the round trip, just how far away the target is. So all the radar needs to do is halve the time for the round trip. It is then just a matter of calculating how far a radar pulse can travel in 0.1ms, which is 16.2nm, and we can therefore deduce that the ship is 16.2nm from the radar.
That is range taken care of, but what about the target’s speed? This brings us to our second law of physics.
Radars use the Doppler effect. In the early 19th century, Austrian physicist Christian Doppler deduced that, for an observer, a wave’s perceived frequency is dependent on two things – the speed of the frequency’s source and the speed of the observer.
What does this mean? The oft quoted analogy is a police car siren. You stand at the kerb waiting to cross the road. In the distance, you hear the police car’s siren. As the car drives towards you, the tone of its siren seems to rise. As it passes, the tone changes and now appears to fall.
In reality, the frequency of the tone is unchanged as it leaves the siren, but from the observer’s perspective it appears to rise in tone as it approaches and fall in tone as it leaves. Why? The siren generates sound waves and, like radio waves, these have peaks and troughs.
The circular movement of a wave from its peak to its trough and back to its peak is called a cycle. Frequency is simply a measurement of how many cycles per second a particular wave performs. For example, radio waves transmitted by an X-band radar on a frequency of 8.5 gigahertz (GHz) transmit a breath-taking one billion cycles per second. Why does the frequency appear to increase as it approaches you? The cycles take progressively less time to reach you as the police car approaches. In essence they become more ‘frequent’, hence for you the frequency seems to increase. Conversely, they become less frequent as the police car drives away, hence the frequency lowers.
This phenomenon is equally applicable to radar and is known as Doppler Shift. By calculating the difference in frequency between the radar’s original transmission and the echo, the radar can tell if the target is moving. By matching Doppler Shift with range, the radar not only calculates the target’s speed, but also the direction in which it is moving relative to the radar.
This takes care of speed and range, but how does a radar calculate a target’s altitude?
Time for some trigonometry. A radar can tilt its antenna to look up at the sky. So, let us suppose the antenna is tilted at an angle of 30 degrees and the radar detects an aircraft at a range of 30nm. Imagine a straight line drawn from the radar’s 30-degree tilted antenna to the aircraft 30nm away. This is the hypotenuse of an imaginary right-angled triangle.
Now imagine an adjacent line drawn horizontally from the antenna at a zero-degree angle. Finally, we need a third line. This is drawn directly downwards from the aircraft until it hits the horizontal adjacent line. This is the opposite line and by calculating its length, we can deduce the aircraft’s altitude.
At this point, we need a scientific calculator. We take the 30-degree angle, type 30 into the calculator and hit the sin key – the sum is 0.5. This tells us that the opposite line is 0.5 times as long as the hypotenuse which we know to be 30nm. All that is left to do is to multiply 30 by 0.5. Thus, the length of the opposite line is 15nm. To make this more useful, we convert this figure into feet and, thus, our aircraft is at an altitude of 91,207ft.
Radars also need to calculate bearing – where the target is in relation to the radar. When the radar is calibrated before use it will determine north, a point of zero degrees on the compass. Modern radars will often use an internal compass or a GNSS (Global Navigation Satellite System) to determine this.
Radars bear many similarities with humans – they must look at something to see it. Let us suppose that you are standing in a room looking straight ahead northwards. You hear a noise and turn your head to see the source of the noise which you realise is 30 degrees to your right, and just slightly below a north-northeast bearing.
Radar works in the same way. An antenna must point at an object to see it. Returning to the aircraft example above, let us suppose our aircraft is flying south of the radar. As the antenna turns, the transmitted RF (Radio Frequency) energy hits the aircraft and bounces back when the antenna is at an angle of 180 degrees. The radar correctly determines that the aircraft is on a southerly bearing.
For much of their history radars have had a restricted world view. Their ability to detect using radio waves revolutionised our world, from warfare to navigation and even parking. Yet they saw the world as a human would if looking through a soda straw. If you were standing in a field looking through a pair of binoculars, your field of view would be limited as you are in effect looking through two tubes. If you wanted to know what is happening to your right or left, you would need to physically turn in these directions.
This is the same for radars. The antenna must be steered towards the area the operator wishes to see. The next time you go to an airport, look at the air traffic control radar. This rotates continually through 360 degrees. This is because air traffic controllers need to see what is happening in the airspace around the airport.
Fighter planes have radars mounted in their nose which prohibits them from rotating the antenna through 360 degrees. Conventional fighter radars have antenna mountings which can be tilted around the antenna’s axis which, while all well and good, means the antenna must be physically steered.
This creates two problems – by tilting the antenna in one direction, you might not see what is happening in another. As well, physically tilting the antenna takes time. While this may be less than a second, in high-tempo air combat this elapsed time could mark the difference between death and survival.
Cognisant of these challenges, radar engineers developed the phased array antenna. By exploiting the phase shift phenomenon, these radars can electronically steer their beams.
As we noted above, a radio wave moves through a cycle. If you have two RF waves being transmitted by the same antenna at the same frequency, the distance between each wave’s peaks and troughs is known as their phase. If their peaks and troughs occur at the same moment, they are in phase. If there is a delay between the peaks and troughs they are out of phase.
In a conventional radar, radio waves are generated by the radar and sent to the antenna. When they arrived via a wave guide at the antenna they are transmitted as a steady stream of pulses.
Let us suppose that instead of having one transmitter mounted on the antenna we have 10 mounted across the antenna face, from left to right, with the radar’s RF energy split and transmitted by all 10 transmitters on the antenna face. The radar is no longer transmitting a single beam but transmitting 10.
In our hypothetical radar, the phase of the transmission is one millisecond. That is, it takes one millisecond for the transmission to perform a full cycle. If each of the 10 transmitters send out their RF energy at the same time, all these transmissions will be in phase. Their peaks and troughs will happen at the same moment and all these transmissions will move in a straight line in front of the antenna.
However, phased array radars have a clever bit of kit called a phase shifter which slightly delays the transmission. Our hypothetical radar sends its RF energy to the antenna with its 10 transmitters. Moving from left to right, the first transmitter does not alter the phase of the incoming energy while the second delays its transmission by 0.5ms, and the one next to this for the same amount of time, and so on.
Splitting the RF energy and shifting the phase will cause all the RF energy being transmitted from this antenna to sweep in a particular direction – left to right or top to bottom – without having to move the antenna. Software controls the precise timing required for this, but the phase shifters give the radar a potentially wide field of view without having to physically move the antenna. Furthermore, the software can change the direction of the beam far quicker than physically moving the antenna.
A human head provides a good analogy for phased array antennas. Imagine that you cannot move your eyes which are fixed to look straight ahead. You will need to physically move your head to see what is happening around you. This is how a conventional radar works. Now imagine that you cannot move your head, but you can move your eyes. Moving your eyes lets you to see what is happening around you. The way a phased array radar behaves is akin to you being able to move your eyes while your head is fixed.
Having a fixed head or fixed eyes is far from ideal so, for this reason, many phased array antennas are mounted on conventional revolving antennas. This is often the case for ground-based air surveillance or naval surveillance radars with the user getting the best of both worlds: conventional use but seeing 360 degrees around their locale.
Alternatively, they can stop the antenna and look at a particular area while still sweeping their transmissions several degrees off the antenna’s main axis. This may be handy if operators can ignore specific areas like the friendly territory behind them, or if a platform cannot house a conventional antenna.
Therefore phased arrays are mainly used for large, fixed ballistic missile early warning radars as these radars only need to look in a particular direction. Likewise phased arrays have been used for fighter aircraft radars as these cannot be scanned horizontally through 360 degrees because they are in the aircraft’s nose.
Active Electronically Scanned Array radars, known as AESAs, are state-of-the-art. In the military domain, the first such system used on a combat aircraft was the Mitsubishi Electric’s J/APG-1 fire control radar on board the Mitsubishi F-2A/B fighters of the Japan Air Self Defense Force from 1995. Since then, AESAs have proliferated across land, sea, and air domains. They are a step up from phased array radars as they not only harness the same electronic beam steering of phased arrays, but take things further.
AESA antennas have multitudes of Transmit/Receive Modules (TRMs) mounted on their antennas – large radars may have thousands of these. Each TRM is a miniature radar. It generates RF energy, transmits it, and receives the echoes bouncing off the target. Like a phased array, phase shifters accompanying the TRMs can steer the radar’s transmissions in a particular direction without necessarily needing to steer the antenna.
What makes AESA radars so clever is their ability to perform several tasks at once or in rapid succession. For example, a fighter aircraft radar can look in two different directions at once – the radar could be looking straight ahead for air-to-air targets, while some of phase shifters on some of the TRMs could be steering beams downwards to look for targets below the aircraft, or even on the ground. Similarly, the radar could be tracking one target, helping guide an air-to-air missile to another while watching the sky for additional targets, all at the same time.
We have used our own human heads and two eyes – literally – to explain how conventional and phased array antennas work. Now – and arachnophobes beware – some spiders have up to 12 eyes which are used to provide it with the widest field-of-view allowing it to be alert to predators and potential prey.
Image if the spider could task each of its eyes independently – some could look in one direction for food, others could look elsewhere to keep watch for hunters. One problem would be the spider’s brains capacity to process and synthesise the images to prevent it becoming disoriented.
AESA radars get around this problem by having powerful processors which can task different parts of the radar to do different things, filtering the huge amounts of data gathered by the AESA so that only the information the user needs is displayed.
Like phased array radars, AESAs can be mounted on a static antenna. A fixed AESA antenna in a combat aircraft will typically have a field of view of around 120 degrees but, much like phased arrays, AESAs are sometimes mounted on moveable antennas to get the best of both worlds by providing an extra few degrees in the field of view.
The Eurofighter’s planned CAPTOR-E is one example. The antenna is mounted on what Euroradar – the consortium which builds the CAPTOR-E – calls a swash plate which physically moves the antenna expanding the field of view to 200 degrees, a significant increase.
Other AESA radars such as the AN/APG-81 used by the F-35 and the AN/APG-79 on the Block II and Block III Super Hornet are fixed AESA antennas.
But AESAs are not the last word in radar technology. Currently, Gallium Nitride (GaN) is the material of choice for TRMs because it is robust and can tolerate high temperatures. For radars, this means that more power can be sent through the TRMs resulting in longer detection ranges.
Future innovations look towards the use of diamonds in TRM design because of their ability to absorb a lot of punishment and to further improve performance.
AESA technology itself will probably be replaced at some point. When this happens, radar will take another important leap forward and this article will need to be updated.
This article appeared in the May-June 2021 issue of ADBR.