4 Advanced Radar Systems
4.6 Low Observability
The importance of a radar system in detecting, identifying and engaging a target is undeniable.
However, for years the aircraft designer has been attempting to minimise the ability of a radar Figure 4.36 SAR picture of tanks with 1 ft resolution (Sandia National Laboratories)
170 ADVANCED RADAR SYSTEMS
to detect the aircraft. This art is known as low observability or, more colloquially, stealth, and has been utilised for many years, although perhaps with more prominence with the introduction into service of the US Air Force stealth aircraft: F-117 Nighthawk, B-2 Spirit and F-22 Raptor (Figure 4.37).
Table 4.2 Baseline air-to-ground mapping radar specification
Frequency 10 GHz
Antenna Flate plate; 3 3beamwidth; 30 dB gain;30 dB sidelobes Transmitter Travelling wave tube (TWT); 10 kW peak power; 5% duty cycle Receiver 5 dB noise figure; dual conversion; RF preamplifier; STC; AGC A/D conversion I/Q 8 bits; 120 MHz maximum rate
Signal Processor 100 MFLOPS/s; 8 Mbytes RAM Radar data processor 2.5 MIPS; 64 kBytes RAM Communications 1553 data bus
Display 512 512 8 bits; monochrome
Exciter Up to 250:1 pulse compression; variable CHIRP rate; 0.01 ms minimum to 10 ms maximum
Table 4.3 Comparison of ground-mapping modes
Parameter RBGM DBS SAR ISAR
Azimuth
Centre Heading Selectable Selectable 60 Selectable
stabilised 60 60 60
Swath 60(max.) 45 2:5 nm 2:5 nm N/A
Scan rate 60 deg/s Varies with Spotlight mode Spotlight mode azimuth 60–5
deg/s
Resolution Real beam 20:1 beam 25 ft cross-range Target motion sharpening resolution dependent 512 azimuth bins 512 azimuth bins 128 doppler
bins, 0.25–2.0 Hz resolution Range
Scale Selectable 5– Selectable 5– Range/azimuth Range/azimuth
160 nm 40 nm centre is centre is
designated designated Resolution Rmax/256 : 950 ft R¼ R az 25 ft, 5 ft, 512 range
at 40 nm 325 ft for 512 range bins bins 30 nm
PRF PRF¼ 2 kHz, Variable PRF: Variable PRF: PRF¼ 800 Hz
P-to-P 2.5 kHz to 1.5–3 kHz, at 100 nm
frequency agility 500 Hz PRF¼ 1700 Hz at 30, 25 nm, 300 knots
Signal STC, AGC, NCH 32-point FFT 512-point FFT 128-point CFT processing integration
The history of the use of stealth techniques stretches back before these aircraft were developed (Figure 4.38). This shows that the development of stealthy techniques stretches back to the mid-1950s. The evolution of stealthy aircraft can be characterised by three distinct generations:
1. First generation. The original strategic reconnaissance aircraft developed by the Lock-heed ‘Skunk Works’ – the U-2 and the SR-71 Blackbird.
Figure 4.37 US Air Force stealth aircraft.
F.117 A Nighthawk (US Air Force photo by Staff Sgb D Allmen 11)
B-2 Spirit (US Air Force photo by Master Sergeant Val Gempis)
F.22 (US Air Force photo)
1950 1960 1970 1980 1990 2000 2010
1st 2nd 3r d
Stealth Generations & Milestones
Source : AIAA & AW&ST
Figure 4.38 History of the development of stealth aircraft.
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2. Second generation. In the early 1970s the concept of ‘faceting’ was invoked, first on the two Have Blue demonstrator aircraft and then applied in production on the F-117. This technique involved the use of angular faceted areas designed to redirect or deflect the radar energy away from the emitter.
3. Third generation. This generation was enabled when computational techniques had developed such that the entire aircraft could be predicted and evaluated using numerical means. These aircraft are soft blended shapes quite unlike the faceted approach. The Tacit Blue demonstrator proved these techniques which were subsequently adopted on the US Air Force advanced tactical fighter (ATF) fly-off aircraft: the Lockheed Martin YF-22A and the Northrop YF-23A. Subsequently, these techniques have been applied to the Northrop B-2 Spirit stealth bomber which is in service and the Lockheed Martin F-22A Raptor which is just entering service. The F-35 [previously the joint strike fighter (JSF)]
also applies similar techniques, looking very similar in appearance to the F-22.
Figure 4.38 also shows the milestones of the campaigns in which the stealth aircraft have been successfully deployed: the F-117 in the first and second Gulf Wars and Kosovo and the B-2 during Kosovo and the second Gulf War. Despite flying thousands of missions, the only casualty was an F-117 shot down near Belgrade during the Kosovo campaign.
Recalling the basic radar range equation discussed in Chapter 3, the radar range is proportional to 1=4 where is the radar cross-section (m2). Reducing the cross-sectional area therefore affects radar range, although only according to the fourth root. However, by carefully designing an aircraft, the value of may be reduced by many decades (and the reflected signal by many dB), and therefore reduction in the radar cross-section is an attractive and effective approach to reducing the detection range of an aircraft by radar.
4.6.1 Factors Affecting the Radar Cross-section
The radar cross-section may be thought of as comprising three elements:
Geometric cross-section;
Directivity;
Reflectivity.
There is a limit to what can be done regarding the first component since the size of an aircraft will be dictated by the role, weapons payload and range. However, a combination of low-directivity and low-reflectivity techniques has been successfully developed, as the degree of stealth offered by some of the aircraft testifies.
The major areas on conventional fighter aircraft that contribute to a high radar cross-section are shown in Figure 4.39. The radar, cockpit, engine intakes, drop-tanks, engine exhaust and rudder/elevator combination can all produce large returns, primarily because they can act as radar reflectors. External carried ordnance and the wing leading edge can produce large reflections on occasions. The aircraft fuselage and small blended inlets like a gun muzzle produce relatively low radar reflections, as do minor air inlets or blended inlets on top of the aircraft.
For a stealthy shape to be achieved, these factors need to be taken into account at the design stage. The design features needed to ensure a low-observability aircraft are shown in Figure 4.40. These include:
1. Blended wings and fuselage, and the use of blended chines at the side of the fuselage.
These features were originally incorporated on the SR-71 and also utilised on later aircraft, particularly on the Northrop YF-23A and B-2, minimising all necessary bumps and protuberances including external antenna.
2. Swept leading edges, not necessarily linear, to reduce ‘end-fire’ array effects or to ensure that constant angles are maintained.
3. A low-profile ‘blended cockpit’ to avoid the cockpit and pilot acting as a corner reflector.
Engine (large if straight) Air Inlet
Figure 4.39 Areas contributing to a high radar cross-section.
Shielded Eliminate Bumps & Protrusions
Low RCS Performance
Trade Off
Figure 4.40 Low-observability design objectives.
174 ADVANCED RADAR SYSTEMS
4. Ordnance. On the most stealthy aircraft the weapons carriage is internal.
5. Bandpass radome and other radar RCS reduction techniques which will be described later.
6. Canted rudders such as those used on the YF-22A, YF-23A, F-22A and F-35A/B/C assist in not providing a corner reflector effect. The Northrop YF-23A was an extreme example of this feature, and its stablemate, the B-2, being a flying wing, has no rudders at all.
7. Shielded engine nozzles. Note the B-2 overfuselage engine intakes and exhausts.
Unfortunately, many if not all of these features mitigate against aircraft performance, so in reality the RCS and performance have to be the subject of trade-offs. Many of the stealthy aircraft are unstable and require high-integrity sophisticated flight control or fly-by-wire (FBW) systems. In most other respects the aircraft systems are fairly conventional and have often borrowed and adapted major subsystems from non-stealthy combat aircraft.
The benefits conferred by the design principles outlined above are augmented by the use of radar-absorbent material (RAM) to improve the low-observability features. An example of the combination of both techniques may be gained by examining the intake design on the F-117 and F-35 fighters.
The F-117 intake uses a combination of single reflections, multiple reflections and RAM to lower the engine intake RCS (see Figure 4.41 which shows the intake grill and a diagram
Engine Radar Energy
Radar-Absorbing Material (RAM)
Reflect
Penetrate Absorb
Energy absorbed by multiple
bounces
Soft Absorb
Hard Reflect
Grid Element
Figure 4.41 F-117 intake grill & RCS reduction.
portraying the intake construction). The intake grill is actually coated with RAM and is triangular shaped. Energy incident upon the intake grill may be reflected from one of the triangular grid elements, being dissipated and reflected away from the threat radar. This is indeed how faceting is used across the entire external surface of the F-117 with considerable success. Some incident energy will impinge directly upon the grid element and will be absorbed. A proportion of the energy will pass through the grill and enter the intake. After being reflected within the intake, it will undergo multiple bounces against the RAM-coated rear of the grid element and will mostly be dissipated within the engine intake. Such minor amounts of energy that do escape the intake grill will be of very low power and randomly scattered.
The F-35 intake is reported to be more sophisticated, being a serpentine duct rather than a direct, more conventional intake. Although at first glance the relatively open intake would appear to suffer from a high RCS, sophisticated techniques are used that lower the cross-section over a range of frequencies. The intake is designed to counter radar threats at three wavelengths loosely termed long ( 30 cm), medium ( 10–20 cm) and short ( 3 cm), equating to 1 GHz (long-range surveillance radar), 1.5–3 GHz (AWACS radar) and 10 GHz (fighter radar) respectively.
At long wavelengths the duct behaves as indicated in Figure 4.42. The wavelength is too large to propagate effectively down the inlet and a minimal amount of energy reaches the engine/blocker. Most of the incident energy is attenuated by the RAM coating around the inlet lip and the remainder reflected away from the threat radar. The RAM coating around the inlet lip is optimised to have a maximum effect at 1 GHz.
For medium wavelengths the wave is able to propagate effectively down the inlet and proceeds down the serpentine duct unimpeded. As it reaches the engine, it impinges upon the RAM-coated blocker which is tuned to absorb the maximum amount of energy at this frequency and to attenuate subsequent reflections or bounces. Most of the energy is dissipated at this point; the very small amount of residual energy that does remain is reflected out of the duct (Figure 4.43).
Engine Fan Blocker
Long Wavelength
~ 30cm
Radar Energy
Radar-Absorbing Material (RAM)
Wave is too big to enter inlet and reflects diffusely. Inlet is angled to reflect away from tactical targets RAM coating of inlet lip is tuned to absorb long wavelength and reduce reflection
Very little energy reaches blocker
Figure 4.42 F-35 engine inlet duct response to long wavelengths.
176 ADVANCED RADAR SYSTEMS
@Spy
Short-wavelength energy incident upon the duct travels readily down the duct, reflect-ing off the RAM coatreflect-ing on the inside of the duct walls. This RAM is tuned to achieve maximum attenuation at 10 GHz and therefore heavily attenuates the energy. The energy that does reach the engine becomes trapped and is mostly dissipated in the blocker/engine labyrinth. A small portion of the energy is reflected out of the front of the duct, as shown in Figure 4.44.
Engine Fan Blocker
Medium Wavelength
~ 10 - 20cm
Radar Energy
Radar-Absorbing Material (RAM)
Wave is proper size for duct to act as a waveguide and carry energy unimpeded
Wave is too big to enter spacing of blocker vanes. RAM coating is tuned to this frequency and absorbs most of the energy
Small amount of energy is reflected back out of the duct
Figure 4.43 F-35 engine inlet duct response to medium wavelengths.
Engine Fan Blocker
Medium Wavelength
~ 3cm
Radar Energy
Radar-Absorbing Material (RAM)
Wave bounces of walls of duct and is absorbed by RAM lining tuned to high frequency
Small amount of energy enters blocker which acts as a waveguide
Wave exits blocker and hits fan where energy is lost in fan/blocker labyrinth
Energy escaping the blocker/fan loses more energy as it is reflected back out of the duct
RAM Lining
Figure 4.44 F-35 engine inlet duct response to short wavelengths.
From the foregoing it can be seen that the shape and size of the inlet, the nature of the serpentine duct, the use of the blocker fan and the judicious use of specialised RAM coating material greatly reduce the intake RCS, thereby enabling an apparently normal engine inlet to have a low RCS response to different types of radar.
The maintenance and preservation of the radar-absorbing coatings on a stealth aircraft pose a major servicing penalty. Every time an aircraft panel is removed for maintenance it has to be resealed – effectively recaulked – in order to preserve the low-observability signature of the aircraft. This takes time enough, but in many cases the sealant used has a long curing time so it can take some time before the aircraft may be used on a mission. Also, certainly during the F-117 production run and possibly with the B-2 bomber, different aircraft were coated with different coatings or sealants as production proceeded and improved treatments became available. This created a configuration control issue with several different variants of coating standard across the fleet. It is understood that modifica-tion programmes are in place to ameliorate this problem.
The stealthy aircraft creates a problem for the radar and radio designers as a vast proliferation of antennas have to be fitted on to the aircraft in order for it to fulfil its mission, each with its own particular system performance goals. In Chapter 2, the wide array of antennas performing the primary radar, CNI, and EW functions on a JIAWG/F-22 RF architecture was outlined, and the trend towards shared apertures for JAST/F-35 applications was described. The radar, being at the front of the aircraft, faces particular problems that need to be addressed to maintain the overall RCS signature of the aircraft.
4.6.2 Reducing the RCS
Some of the particular problems that the radar confronts in minimising the RCS are as follows:
1. Antenna mode reflections. The antenna mode reflections mimic the antenna main beam and sidelobes. Therefore, merely positioning the antenna such that the main beam is not pointing towards the threat radar is insufficient.
2. Random scattering. This is caused if the antenna characteristics are not uniform across the antenna. The solution is to maintain close production tolerances and hence uniformity across the array.
3. Radar antenna edge diffraction. Mismatches of impedances at the perimeter of the antenna can cause reflections called edge diffraction. In effect the outer perimeter of the antenna acts as a loop and reflections tend to be abeam of the antenna rather than fore and aft.
4. Coupling of structural modes into the antenna. In this case the antenna can effectively
‘mirror’ the incident energy from the threat radar, thereby compromising low RCS. In an ESA, active or passive, this can be readily overcome by rotating the array such that it points slightly down and the reflected energy is directed away from the threat radar. This technique, although effective, has the disadvantage of reducing antenna effective area by the cosine of the depression angle.
The subtle and different effects of the antenna reflection effects of antenna mode, random scattering and edge diffraction are shown in Figure 4.45. Antenna mode reflection is minimised by ensuring correct RF matching of the feed elements to avoid reflections.
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Random scattering is minimised by maintaining close production manufacturing tolerances to maintain uniformity across the array. Edge diffraction effects may be reduced by placing RAM material around the perimeter of the array to minimise the loop reflection effect.
This should be a width identically equal to 4 at the threat radar operating frequency, and so a significant penalty may apply as the antenna size and therefore gain is reduced accordingly.
4.6.3 Comparative RCS Values
A crude comparison of the RCS value for a range of objects is shown in Figure 4.46. The comparison is shown with two scales: the upper scale depicts the RCS in square metres while the lower scale depicts dB/m2, that is, dB power reduction of a return related to a reference square meter of target. The 1 m2target is the reference point for a small airborne target such as a cruise missile.
The stealth aircraft B-2, F-22, F-117 and F-35 appear on the left-hand side of the diagram in the 30 to 40 dB region. Conventional aircraft range from the F-18E/F at 0 dB to bombers and transport aircraft atþ30 dB. Ships are a huge target at 104m2or more. Normal everyday objects range from insects at30 dB, through birds to humans at 10 dB. On this diagram it could be deduced that an F-22 has an RCS of 1 107(or one ten-millionth) of that of a B-52 bomber.
Although these comparisons serve some purpose in indicating some of the relative magnitudes involved, they do not present the whole picture. In fact, no aircraft may be made invisible, and stealth aircraft may be detectable at lower than conventional radar frequencies. They may, however, be very difficult to track or engage even if detected.
Figure 4.45 Factors affecting the antenna RCS.