Aug 25, 2016 Dan Katz | Aviation Week & Space Technology
The Radar Strikes Back
This is the second article in a series. Since the advent of stealth technology, claims have abounded about ways low-observable aircraft can be detected. Chief among these are radars that operate at lower frequencies than those stealth aircraft are designed to defeat. With digital electronics technology overcoming some of the performance limitations inherent in VHF and other low-frequency radars, can they render stealth obsolete?
To understand the current balance of stealth versus counter-stealth as the Lockheed Martin F-35 joins the F-22 in operational service requires a closer look at how radars work, at the effect of wavelength on radar reflection, and at the capabilities of advanced lower-frequency systems now being deployed.
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Lower-frequency radars are better for detecting stealth aircraft because of their longer wavelengths, which are inversely proportional to frequency (see table, Radar Band Frequencies and Wavelengths). Most fire-control radars operate in X-band (8-12 GHz), although some short-range systems use higher-frequency Ku-band (12-18 GHz). Search radars are typically S-band (2-4 GHz), for longer range. Some surface-to-air missile (SAM) systems use C-band (4-8 GHZ) for both search and fire-control, as a compromise between range and resolution. Long-range early warning radars typically operate in L-band (1-2 GHz) or lower and it is these frequencies that have counter-stealth properties. The reason lies in the behavior of radar waves as they reflect off structures, which can be divided into three regimes based on the size of the structure relative to the wavelength.
High-Frequency Scattering
A high-frequency regime (not to be confused with the HF radio band) applies when the structure is at least 10 times longer than the incident radar wave. In this regime, specular mechanisms dominate the radar, in other words the angle of reflection equals the angle of incidence, like billiard balls colliding. “Backscatter” – reflection towards the emitting radar – is reduced by angling surfaces so that they are rarely perpendicular to radars and suppressing the reflections from re-entrant structures such as engine intakes and antenna cavities with combinations of internal shaping, radar absorbent material (RAM) or frequency selective surfaces.
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In this regime, “surface wave” mechanisms are small contributors to RCS, but are still present. These are the electromagnetic waves created by the currents induced in a surface when radar energy strikes it. As these currents move back and forth across the surface, they emit electromagnetic energy known as “traveling waves.” If the wavelength is small relative to the surface, these waves are weak and their overlap will generate maximum backscatter when the radar signal is at grazing angles.
When these currents encounter discontinuities, such as the end of a surface, they abruptly change and emit “edge waves.” The waves from different edges interact constructively or destructively due to their phases. The result is they strengthen the reflection in the specular direction and create “sidelobes” – a fan of returns around the specular reflection which undulate rapidly and weaken as the angle deviates from the specular direction. The currents can also swing around to a structure’s back side, becoming “creeping waves” that shed energy incrementally and contribute to backscatter when they swing back toward the radar.
While small at high radar frequencies, surface waves still require attention on stealth aircraft. Aligning discontinuities to direct traveling waves towards angles of unavoidable specular return, such as the wing leading edge, can limit their impact at other angles. Designing airframe facets with non-perpendicular corners and so radars view them along their diagonals, at low angles and across from the facets’ smallest angles, limits the area of edge-wave emission. At high relative frequencies, surface waves can also be suppressed with RAM.
They can also be reduced by blending facets. The first stealth aircraft, the F-117, was designed with a computer program that could only predict reflections from flat surfaces, necessitating a fully faceted shape, but all later stealth aircraft use blended facets. Shapes composed of blended facets are more aerodynamic, but also allow currents to smoothly transition at their edges, reducing surface-wave emissions. Therefore, blended bodies have the potential for a lower RCS than fully faceted bodies. And blending the curves around an aircraft in a precise mathematical manner can reduce RCS around the azimuthal plane by an order of magnitude. The penalty is often a slight widening of the specular return at the curves, but in directions at which threat radars are less likely to be positioned. This was one of the great discoveries of the second generation of stealth technology.
The Resonance Region
As the radar wavelength grows, non-specular reflections intensify and specular reflections widen. For flat surfaces, traveling waves grow with the square of wavelength and their angle of peak backscatter rises with the square root of wavelength: at 1/10th the surface length, it is over 15 deg. Tip diffractions and edge waves from facets viewed diagonally also grow with the square of wavelength. Specular reflections from flat surfaces decrease with the square of the wavelength, but widen proportionally: at 1/10th the surface length, they are almost 6 deg. wide. In addition, most RAM types become less effective as wavelength increases. For all these reasons, stealth specialists say the RCS of a stealth aircraft grows approximately with the square of wavelength from the lowest frequency for which it was designed, and that above-mentioned effects become significant when the wavelength reaches about 1/10th the size of a structure.
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But aircraft RCS does not necessarily grow linearly. As surface-wave effects grow, their phases can interfere constructively or destructively with specular reflections. This phenomenon is illustrated in simple form with a sphere (see figure below). As wavelength grows relative to the circumference, the creeping wave circling the sphere grows continuously, but its phase interference with the specular return varies. This causes the sphere’s RCS to undulate, with successively higher peaks corresponding to phase matches between the specular return and the strengthening creeping wave. This phenomenon is known as “Mie scattering” and this regime —where the wavelength is between one and 1/10th the size of the structure—is known as the “resonance region.” Maximum RCS is often reached when the wavelength reaches the approximate size of the structure.
Rayleigh Scattering
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Once the wavelength grows past this point, the specifics of target geometry cease to be important and only its general shape affects reflection. The radar wave is longer than the structure and pushes current from one side of it to the other as the field alternates, causing it to act like a dipole and emit electromagnetic waves in almost all directions. This phenomenon is known as “Rayleigh scattering.” At this point, the RCS for many shapes will then decrease with the fourth power of the wavelength.
Net Effects
These effects occur individually for every shape on an aircraft and their reflections interact with those of every other shape. Smaller shapes exhibit the behavior before larger ones, but also have a lower maximum RCS. The behavior can also vary with changes in aspect angle.
No RCS figures for fighters outside of X-band are publically available but the above phenomena make low-observable aircraft more detectable as shaping and most RAM become less effective. The sizes of wings and tails on fighter aircraft are on the order of one to several meters. This means these shapes might enter the resonance region in L-band and reach Rayleigh scattering in VHF, although the specific angle, frequency and geometry can still matter.
Lower-Frequency Systems
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So why not build every radar for lower bands? Because they are less accurate at lower frequencies. Every antenna generates a beam pattern with a central cone called a main lobe within which most of its energy is emitted and reflected energy detected. The main lobe’s width depends on the ratio of the antenna’s aperture size to its wavelength. Longer wavelengths require bigger apertures, increasing cost and decreasing mobility, and even large antennas struggle to generate fire-control-level accuracy. Early in the Cold War, the Soviets developed the first mobile VHF systems, such as the P-12 “Spoon Rest,” but its accuracy was so poor that target handoff to higher-band fire-control radars was difficult. Fighter radars have been largely restricted to X-band due to the need to fit in small noses.
But with the advent of active, electronically scanned array (AESA) antennas and improvements to computers and signal processing, lower-band radars have become more accurate and their range has increased. The state-of-the-art ground-based VHF system is now Russia’s 55Zh6UME, produced by Nizhniy Novogorod Research Institute of Radio Engineering (NNiiRT). And the radar suite in Russia’s new Sukhoi T-50 fighter includes N036L-1-01 L-band AESA antennas in the wing leading edges. These could be integrated into Sukhoi’s Su-35 as well.
The 55Zh6UME may be able to detect stealth aircraft at far longer ranges than contemporary higher-band search radars. NNiiRT states a VHF detection range of 265 mi. for a 1m2 RCS target, albeit at the curious altitude of 98,000 ft. No reference range has been released for the N036L-1-01. L-band might put the wings and tails of the F-35 and F-22 in the upper resonance region and possibly generate greater returns from their engine intakes and certain small shapes. The N036L-1-01has a smaller aperture and likely less power than nose-mounted radars, but the advantages of L-band could be enough to detect stealth fighters farther away than the main radar.
From Detection to Engagement
Using lower frequencies can extend detection range against stealth aircraft, and provide early warning, but to engage them an adversary has to guide a missile accurately enough to put the target within the lethal radius of its warhead. The volume available inside missiles restricts onboard radars to higher C-, X- or Ku-band, so how to guide them?
One approach is to use VHF command terminal guidance. The idea is to link the 55Zh6UME search radar to the S-300/400 weapon system and use its data to direct the missiles all the way to their targets. According to data released by NNiiRT, however, the 55Zh6UME is not accurate enough for this. The manufacturer claims a root mean square error of 0.25 deg. in azimuth and elevation against a 1 m2 RCS target. This means for targets only 20 mi. away it could be off by more than 460 ft., and proportionally more for more distant targets. This is inadequate to guide a missile. As for the N036L-1-01, Sukhoi does not claim the T-50 can engage targets with it and, being restricted in height to the thickness of the wing, the system likely has poor elevation accuracy.
Another approach is to use lower-frequency systems to cue fire-control radars and extend their range against stealthy targets. This theory digs into how radars detect aircraft. A radar must discern a target’s return from environmental clutter and noise generated by its own electronics. Designers chose a ratio between signal and noise (S/N) at which the radar has an acceptable probability of detecting real targets, typically 90%, and an acceptable rate of false alarms, usually one per minute.
To improve S/N ratio, radars integrate the returns from numerous pulses. Since a target will be present at every pulse, but noise varies randomly, the signal builds up until the S/N ratio is achieved and the computer declares a target. Therefore, if a radar knows roughly where to look, it can send more pulses into a restricted search cone and increase the S/N ratio from farther away.
Theoretically, this technique can increase fire-control radar ranges up to those of the cueing sensors, but in practice it has limitations, such as signal processing hardware. A radar must generate enough pulses to cover its entire field of view, which means several thousand combinations of azimuth and elevation for regular search and even dozens to hundreds for a restricted search. For each angle, the radar must break up every return into dozens of range bins and each range-bin must be broken up into many velocity bins. Complex mathematics must also be performed for the bins and their resulting values before a target can be declared. So processing and memory requirements build up quickly.
In addition, signal processing is best done digitally, but that requires quantizing the analog signal into series of bits called words. The sensitivity of this analog-to-digital converter must be set so that above-average signals do not saturate the converter. But this means that low-end signals can register as zero, and stealth fighters reflect less than 1/1000 the energy of conventional fighters. Larger words can be used, but every bit increases processing and memory requirements, increasing cost, size, weight and complexity.
While the processor details for the S-400 SAM and Su-35 fighter are not known, the manufacturers’ information suggests the ranges of their X-band fire-control radars cannot be extended significantly. Almaz-Antey’s quoted range for the S-400’s Gravestone radar of 250 km for a 4m2 RCS target is specifically stated as with designation from the Big Bird search radar. The S-400’s Big Bird can detect 1m2 targets at 338 km (equivalent to 478 km for a 4m2 target) and designate 4m2 targets at 390 km, and still Gravestone’s detection range is less. As for the Su-35’s Irbis-E, it only detects a 3m2 target at 400 km in a special narrow-angle, maximum-power search mode; detection range in standard search is half that. This suggests the higher figures for both systems are achieved only when the radar already receives external cueing.
Furthermore, extending radar range with external cueing would apply to conventional as well as stealth targets. The RCSs of conventional aircraft also grow with longer wavelengths and increasing signal integration time would be effective for a non-stealthy target. Therefore, this capability would likely be reflected in a greater detection range against higher RCS targets.
A third approach to engaging stealth aircraft is to combine VHF-command mid-course guidance and X-band active terminal guidance. In this scheme, a lower-frequency radar directs a missile towards a stealthy aircraft until the onboard X-band radar acquires the target. The U.S. Navy, for example, plans to use UHF-band AESA radars on its E-2Ds to provide mid-course guidance to SM-6 SAMs.
The concept holds promise, but would first require the lower-frequency radar to be able to localize the target enough for the missile to detect it. Missile sensors cannot match the range of fighter radars because they have far less power and gain. They only have to acquire targets towards the end of the flight, but against an F-35 or F-22 they will be looking at aircraft detectable at less than a fifth of the usual range. In addition, even if detected by the missile, stealth-fighter electronic countermeasures are made more effective by their low observability. This is because spoofing techniques, such as range- or velocity-gate pull-off, require the jamming signal to overwhelm the aircraft’s real radar return, which is smaller for a stealth fighter.
When questioned about lower-frequency radar, some F-35 program officials concede detection is possible, but dismiss the possibility of engagement. This assessment appears to accurately reflect the state of the stealth-counterstealth balance – for now. But faster processors, smaller memory chips, stronger transmitters, better signal processing and superior antenna technology all have the potential to erode the advantage current stealth aircraft enjoy. When it comes to the state of stealth, neither side can claim final victory yet.
Anatomy of a Stealth Fighter Shootdown
Perhaps the best cautionary tale against assuming stealth fighters are invulnerable is the story about how one has already been shot down. Four days into NATO’s air campaign over Serbia, an F-117A was brought down by an SA-3 northwest of Belgrade. The alliance’s air forces assumed Serbia’s outdated equipment posed a minimal threat to the Nighthawk. They didn’t even mind the crowds, which are believed to have included Serbian agents, outside their airbases watching planes takeoff.
The stealth fighters flew the same routes every night on their way to Belgrade. On the ground, Lt. Col. Zoltan Dani, commander of the 3rd Missile Battalion, 250th Air Defense Missile Brigade, was able to eavesdrop on the unencrypted radio traffic between fighter pilots and the E-3 AWACS directing them. Colonel Dani had studied the F-117’s technology and positioned his unit where he determined to be the optimum position from which to detect it.
On the night of March 27, 1999, weather had forced the cancellation of all NATO strike missions with the exception of eight F-117s. A little after 8pm, radar units in northern Serbia reported they had detected a target with a small RCS. At 26,000 ft., an F-117 was heading northwest from Belgrade after striking its target.
Col. Dani ordered his P-18 search radar (a 1970s upgrade of the P-12) activated. Initially, it detected nothing, but then he instructed the operator to activate an “innovation” and a target appeared on the screen at 31-37 mi. Colonel Dani has declined to detail the “innovation” but it’s believed to have enabled operation at an even lower frequency than normal. When the target closed adequately, the SA-3 operators began turning on their radars for 20-second intervals, to minimize exposure to NATO’s anti-radar missiles. On the third try, they locked on a target from 8-9 mi. away and fired off a pair of missiles at its 4 o’clock. The first flew over the F-117, failing to detonate, but the second struck, blowing off its left wing and sending it uncontrollably towards the ground.
The first lesson of this incident is that survivability is a combination of technology and tactics. When militaries use advanced technology without regard for tactics, a tactically skilled opponent can exploit a weakness, particularly if combined with a bit of technical ingenuity. Col. Dani knew the F-117’s flighpath and the Nighthawks were the only aircraft around. That makes detection a lot easier than when an aircraft that can approach from any direction in a crowded sky. Hence the importance of tactics and also an underrated part of stealth technology: the electronic receivers that detect radar emissions and the computers that chart courses which minimize the chances of detection.
The second lesson is the continuing importance of combined arms operations. Stealth fighters might be able to do some jobs alone but they are more effective, and survivable, when combined with broadband stealth aircraft, jamming, anti-radar missiles, decoys and stand-off weapons. After the F-117 was shot down, it is believed U.S. EA-6B electronic attack aircraft began supporting the F-117s and strike aircraft gave more attention to search radars.
The third lesson is the potential vulnerability of stealth aircraft to lower frequencies. It is possible though that the F-117 is more susceptible to them than its successors. While it has a flat bottom, its fully faceted airframe might be more vulnerable than a blended shape at lower frequencies, because of surface wave effects, and the modified P-18 may have caught it an angle to exploit that. It also used early RAM. On the other hand, today’s lower frequency radars have far greater detection ranges than the P-18 and if they can solve the engagement problem they may be able to engage modern stealth fighters. To paraphrase the words of Col. Dani: there is no such thing as “invisible to radar,” there is only varying degrees of visibility.
