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HIGH-TECH RADARS FOR MODERN FIGHTERS

Since World War II no fighter can fulfill its missions without an onboard radar. It enables the detection of hostile aircraft by night, during bad weather and over great distances. However, the Lichtenstein equipment from Telefunken, which was installed in fighters like the Me 110 or the Me 262, has as much in common with today's ECR90 for the Eurofihghter as the first Benz has with a Formula 1 racing car.

During the last five decades scientists and engineers have optimised every radar component and have now almost reached the physical boundaries. In contrast, the principle architecture of the radar (Radio Detecting and Ranging) has remained the same over those years:

Firstly a so-called Exciter (frequency generator) creates high frequency signals. In the case of today's fighters they are mostly in the X-band, i.e. around 10 Gigahertz (10 000 000 000 Hertz). This corresponds with a wavelength of three centimetres. Decisive factors for the quality are stability (the pulses should possibly be identical, give or take a few Hertz), as well as the agility of the frequencies in a bandwidth of maybe five per cent.

The signals are then amplified in the transmitter a million fold - the more the better, because, as can be concluded from the radar equation, the transmission performance is proportional to the fourth root of the distance between aircraft and target. In other words, to double the range the output has to be increased sixteen times!

Nowadays TWT (travelling wave tubes) are employed. These work with high voltage of 30 to 50 kV. They are fitted with a generator creating electronic radiation and are surrounded by an electronic or permanent magnet. Because of the high output the transmitter has to be kept very cool (often with the help of liquids). However, it is often the most delicate part of the radar.

From the transmitter the radar waves reach the antenna via rectangular hollow ducts. The aerial consists of a level disc with hollow duct "grooves" to spread the energy. It is covered with a protective shield. This shield has high precision slots with 1/100 mm tolerance. Energy is emitted from these slots. It is not let out evenly in all directions, but in a sharply focused ray. This is achieved through the clever distribution of the electromagnetic energy over the entire area of the antenna. However, some loss through so-called sidelobes cannot be avoided.

With a beam opening angle of perhaps 1.5 to 3 degrees only a small area of the surroundings can be illuminated. It means, in concrete terms, that with a beam angle of two and a half degrees the area in question at 60 kilometres distance has an expanse of three kilometres.

For this reason one needs to be able to swivel the antenna. In the case of the ECR90 it is moved around with strong electric motors which combine precision with a high turning torque. After all, about four kilograms have to be moved swiftly.

The length of the radiated radar impulses is in the microsecond area, (0.000 001 sec). Depending on the pulse frequency, (high, medium, low), the antenna is then available as receiver for the rest of the time. If for example an impulse lasting 0.2 microsecond is being radiated at an interval of a thousandth of a second, it is possible to receive for the length of 999.8 microseconds afterwards.

The recaptured energy is obviously a lot less than that which was radiated. With maximum ranges and standard targets factors of 10-12 (million millionth = 0.000 000 000 001) are no exception. This has something to do with the distance covered by radar impulses, which is after all twice as great as the target distance. One must also consider the dampening effect of the atmosphere, which is less in the X-band.

Finally the radar cross section of the target is also of some importance. It is measured in square meters and has nothing to do with the physical size but a lot with the shape and the materials used. Although the exact numbers are strictly secret, one can assume that the Panavia Tornado has a radar cross section of six to eight square meters, while that of the F-117 is markedly lower at certain radiation angles (maybe 0.1 sqare metres).

The antenna passes on weak signals through the hollow ducts onto the receiver. There they can only be interpreted as target signals, if they are stronger than the constant physical electronic "hissing" or noise of the radar components. The noise ratio, which is roughly three dB for the entire receiver, depends on the materials that are used and on the construction of the amplifiers. Mostly several stages are connected to each other with the least noisy element at the beginning.

An important criterion for the receiver is also its high dynamic range, because the echo signal increases to the power of four with the approach of another aircraft. At a distance of ten km it can be 10,000 times stronger than at a distance of 100 km.

The signals then go through an analogue/digital converter, which is able to differentiate between around 32,000 energy levels. High performance computers take over the interpretation of the data from that point. The performace of the signal processor is crucial here, because it is being bombarded constantly with vast amounts of information. Specially designed chips are used for this. These are very often connected in parallel. This is how roughly three billion calculations (3 Gigaops) can be carried out per second. The number of these calculations is still rising.

The formula for analysis of these signals are extremely complex. Received pulses need to be added and correlated with the transmission pulses for the intended range. One needs to calculate the speed of the target with the help of the Doppler effect or try to identify the target with a check on frequency shifts.

The targets that have been recognised after all these operations are finally passed on to the data processor. It is responsible for managing the radar and connects with the remaining aircraft systems. According to the modus operandi, which are either chosen by the pilot or automatically, it also controls the search movements of the antenna.

Apart from this the data processor carries out prioritisation of targets. To put it simply, the distance to the target and the approach speed are of the utmost significance.

How the pilot perceives everything is a matter of software. Big colour displays enable a variety of display formats. An army of experts is currently working out how the man-machine-interface can be created simply and clearly: Should aircraft be displayed as rectangles or circles, are enemies red, and how will the operator get through the menus with as few buttons to push as possible?

The trend definitely is towards higher automation. Information from other aircraft sensors, (Infrared, ECM, IFF), are coupled with the radar data. Furthermore the fighter has at its disposal via data links, data from other fighters in the formation or from early warning systems. Thus a more and more detailed picture of the situation is created, although it is still up to the pilot to draw the correct conclusions.

See also our listing of important radar modes!

From page 70 of FLUG REVUE 12/98