Radar versus Stealth Passive Radar and the Future of U.S. Military Power
aced with the prospect of aerial By A r e n d G . W e s t r a stealth proliferation, states in the 21st century are looking for F antistealth defense options. One such alternative, passive radar, appears a cost- effective counter to stealth. Passive radar is a receive-only system that uses transmitters of opportunity.1 Integrating a system of netted receivers, passive radar can detect, track, and target piloted and unpiloted stealth systems and provide cuing for antiair weapons systems. A passive radar system emits no radio energy and can be well camouflaged in both urban and rural landscapes. The threat system produces no indications on friendly radar warning receivers and is difficult to locate and target. Faced with a passive radar threat, the United States may find itself unable to achieve air superiority at an acceptable cost. As this article shows, ongoing advances in passive radar will deny traditional means to defeat enemy air defenses, make air supe- riority difficult to achieve against a passive radar opponent, and require changes in thinking to maintain U.S. power projection capability. In developing this central idea, this article describes the history of the battle between aircraft and radar, the rise of stealth and counterstealth, and the ongoing surge in passive radar and how it relates to advances in signal processing and sensor fusion. Addi- tionally, this article assesses the passive radar threat to stealth, posits implications for future U.S. military power, and recommends a U.S. course of action regarding passive radar.
Aircraft versus Radar “The defensive form of warfare is intrinsically stronger than the offensive”—so argued Carl von Clausewitz in On War.2 The static warfare of the late 19th century and the
Lieutenant Colonel Arend G. Westra, USMC, is a Marine Air-Ground Task Force Plans Officer at 3d Sailor monitors radar screen in Combat Direction Marine Aircraft Wing, Marine Corps Air Station Center aboard USS John C. Stennis
U.S. Navy (Walter M. Wayman) U.S. Navy (Walter Miramar, San Diego, California.
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Great War of 1914–1918 appeared to validate transmitter and receiver via a shared antenna, a always get through.” Indeed, stealth aircraft this idea. In 1921, however, Giulio Douhet configuration known as monostatic, thus creat- have maintained the overwhelming advantage asserted that the airplane changed warfare ing the conventional radar configuration most in recent conflicts, including Operation Allied “by magnifying the advantages of the offense commonly used thereafter. Force and the invasion of Iraq in 2003. and at the same time minimizing, if not nul- Historically, radar has been the corner- Despite the overall success of the U.S. lifying, the advantages of the defensive.”3 stone of air defense. For example, during the stealth program, in 1999 an F–117 was shot Douhet did not envision the many sur- Vietnam War, North Vietnamese air defense down in the Balkans by a Serbian SAM face-to-air threats that would evolve over the radars targeted U.S. aircraft, which, in turn, battery.15 Although some considered the decades after his work was published. Neither countered with jamming and antiradiation downing an anomaly, the incident created did airpower critics. As Sir Stanley Baldwin missiles. Due to the success of North Viet- much controversy. While the Air Force informed the British parliament in 1932, “I namese air defenses, the United States was assessed tactical lessons learned, others saw think it is well also for the man in the street only able to establish temporary air superior- evidence that stealth could be defeated.16 The to realize that there is no power on earth that ity over local areas of North Vietnam. Over incident illustrated what stealth designers can protect him from being bombed, what- the course of the war, the North Vietnamese already knew: stealth technology does not ever people may tell him. The bomber will shot down 190 U.S. aircraft using 1950s-era make an aircraft invisible. As a submariner always get through.”4 Russian surface-to-air missiles (SAMs).10 once aptly noted, “Stealth is a zero-sum game. Yet a few decades earlier in 1904, German engineer Christian Hülsmeyer had the overwhelming offensive power of the airplane was largely patented the telemobilskop, an early form of radar. But it was not until 1935 that radar first mitigated by the deployment of radar and modern air defenses showed significant operational promise. In the now famous Daventry experiment, Sir A third paradigm shift began in the In a given encounter, one platform has it Robert Watson-Watt used radar to detect a 1970s in the “Skunk Works” of Lockheed and the other does not. The tactical advan- British Heyford bomber at a range of 8 miles.5 Martin, where stealth pioneers first created tage accrued by being able to detect, close, Notably, the Daventry experiment tested a the F–117 “stealth fighter” (more bomber than and attack from a covert stance completely passive radar system using the BBC Empire fighter in usage).11 Made operational in 1983, dominates all other factors in any encounter broadcast as a transmitter of opportunity.6 the F–117 saw combat in Panama in 1989 and algorithm.”17 In Serbia in 1999, a SAM battery Watson-Watt went on to develop the British again in the Gulf War in 1990.12 During the commander attacked from a covert stance and Chain Home radar that played a critical role Gulf War, the F–117 was employed against won the tactical advantage. It was a missile in defeating the German Luftwaffe during the Iraq’s most heavily defended targets. In spite shot heard around the stealth world.18 Battle of Britain in 1940.7 of Iraq’s robust air defenses, not a single F–117 World War II served as catalyst for a was lost or damaged during the conflict.13 By The Future of Stealth second paradigm shift. The overwhelming comparison, 32 nonstealth aircraft were lost If anything, the downing of an F–117 offensive power of the airplane was largely to antiaircraft artillery (AAA) and SAMs.14 If over Serbia only highlighted to the United mitigated by the deployment of radar and Baldwin had witnessed the Gulf War, he might States the importance of stealth. Increasingly, modern air defenses. Airpower did not prove have concluded, “The stealth bomber will the U.S. military has made stealth one of its Figure 1. Bistatic Geometry an all-powerful offensive weapon that could not be countered, and the bomber did not Figure 1. Bistatic Geometry always get through. Air defenses of both the Axis and Allied opponents proved complex and resilient, and combatants obtained air TARGET superiority only locally and for limited dura- tions through the costly reduction of enemy air defenses. This paradigm held firm through Rtx ß Rrx World War II and for the duration of the Cold War. For the time being, it seemed that Clausewitz had caught up with the airplane. Despite Watson-Watt’s breakthrough at Daventry, the experiment highlighted passive L radar’s difficulties, including intermittent signal TRANSMITTER RECEIVER strength and, at the time, irresolvable locating and tracking ambiguities due to the passive Note: L = distance between transmitter and target ("bistatic baseline"); radar geometry.8 Passive radar is bistatic, Rtx = transmitter range to target; Rrx = receiver range to target; ß = bistatic angle. meaning the receiver is located at a distance from the transmitter.9 Bistatic radar geometry Source: C.J. Baker and H.D. Gri ths, Bistatic and Multistatic Sensors for Homeland Security (London: University College, 2005), 4, available at
Figure 2. Conventional and Stealth Aircraft Radar Cross Section Signature
dBsm CONVENTIONAL AIRCRAFT NOTIONAL STEALTH AIRCRAFT dB 35 40 30 30 25 20 20 10 15 0 10 -10 5
Note: dBsm = decibels per square meter; dB = decibels Source: David Lynch, Introduction to RF Stealth (Raleigh, NC: SciTech, 2004), 6.
138 JFQ / issue 55, 4 th quarter 2009 ndupress.ndu.edu WESTRA nance that enables VHF and UHF radar to MMW antiair missile seekers, and other off-the-shelf technology makes them attrac- detect stealth aircraft. Poor resolution in angle countries are following suit.37 tive for nonpeer countries as well. and range, however, has historically prevented While the aforementioned technologies Passive radars use transmitters of these radars from providing accurate target- offer important capabilities, they possess opportunity. Potential waveforms include FM ing and fire control.32 limitations that restrict their effectiveness and AM radio, television, digital audio/video Since the Gulf War, the Russian defense for air defense. Conventional radar is vulner- broadcast, and cellular phone networks.38 radar industry has put considerable effort into able to detection and attack by electronic Today, passive radar is often configured as digitizing its VHF and UHF radar systems warfare and air-delivered weapons; listening a “multistatic” system using three or more to improve counterstealth capability. Russia’s systems do not provide tracking information; transmitters and receivers. older model radars now have improved reso- and IR/EO/MMW is limited in surveillance Passive radar locates and tracks targets lution and signal processing, and newly devel- capabilities. through a combination of methods, greatly oped models, such as the Nebo surface vehicle In contrast, passive radar is covert, all simplified here for the sake of discussion. unit, which is a VHF adaptive electronically weather, and capable of medium- to long- First, the radar measures the time difference steered array radar, likely present significant range surveillance, and shows strong potential of arrival between the direct signal from the counterstealth capability.33 in detecting, tracking, and targeting stealth transmitter and the reflected signal from the Other recently developed conventional aircraft. It is thus emerging as a solid competi- target to determine the bistatic range. Bistatic radars likely to have counterstealth capabil- tor in the counterstealth game. range, expressed as an ellipse, is shown in ity include Lockheed Martin’s theater high- altitude area defense radar and the Israeli a new paradigm is emerging, enabled by advances in Green Pine radar (recently sold to India), systems with both long range and high networked computing and passive radar technology resolution in the UHF L-band.34 The Signal Multi-beam Acquisition Radar for Tracking Passive Radar figure 3. The radar uses the intersection of the (L) naval radar manufactured by Thales is A new paradigm is emerging, enabled by receiver-to-target bearing and the bistatic range yet another system with reputed counter- advances in networked computing and passive ellipse to estimate approximate target location. stealth capability.35 radar technology. Because of their potential to In a multistatic system, the radar refines target Passive listening systems, such as elec- counter stealth-based airpower advantage, the location based on intersecting bistatic range tronic support measures (ESM) and direc- use of these technologies by peer competitors ellipses. The radar further measures Doppler tion finding (DF), attempt to detect stealth is highly likely. That these systems are both shift—wavelength compression or expansion aircraft radar, radio, and data link emissions low cost and, in part, based on commercial- caused by relative motion—to determine target and pass this information to surveillance radars. LPI techniques of stealth are designed Figure 3. Bistatic Range to reduce or deny ESM and DF, but systems Figure 3. Bistatic Range such as the Russian Kolchuga remain formi- dable threats that are likely being updated with digital processing.36 TRANSMITTER Another counter to stealth is IR/electro- 0 optical (EO) systems, which include IR search Rtx TARGET and track and high magnification optics. Such systems, however, are limited in the ability to scan large volumes of airspace and usually must be cued by other sensors. In addition, L Rrx most of this spectrum is degraded by clouds, low illumination, and low visibility. Stealth aircraft counter IR/EO through heat signature management, stealthy flight profiles, and LO paint schemes. Constant range 0 Growing in potential as a counterstealth ellipse RECEIVER technology is millimeter wave (MMW) imaging, which uses the radiometric signa- ture naturally emitted by all objects. MMW penetrates clouds and low visibility. The wave- Note: Rtx = transmitter range to target; Rrx = receiver range to target; L = distance between transmitter and receiver. Bistatic range, expressed as Rtx + Rrx – L, form can also be transmitted by radar, which remains constant at all points on the ellipse. then receives and processes the return echo. The A–64 Apache Longbow/Hellfire system Source: NationMaster Encyclopedia, “Bistatic Range,” available at
F–117A Nighthawk stealth fighter while passive radar can perform detecting, locating, and tracking functions, it may also be able to perform target identification
SAMs with a command guidance mode, the passive radar could provide midcourse guid- ance via data link. In keeping with the passive radar system, a passive missile seeker—IR, EO, MMW, or perhaps multisensor—would likely be used for end-game guidance in order to complete the kill chain.
Threat Employment A future adversary will look increasingly to counter the U.S. stealth advantage with passive radar, either as a stand-alone system or in conjunction with active surveillance
U.S. Air Force (Andy Dunaway) U.S. Air Force radars. Passive radar is relatively cheap, and
140 JFQ / issue 55, 4 th quarter 2009 ndupress.ndu.edu WESTRA its covert stance lends itself to a strategy of Electronic warfare (EW) offers the Building new generations of stealth striking from concealment. Moreover, our potential to temporarily neutralize passive aircraft may be feasible, but efforts to most likely future opponent—an authoritar- radar. Standoff noise jamming would have improve stealth will eventually reach a point ian state—already possesses tight control over an effect, but because the location of the of diminishing returns. Advantages will its commercial media, a situation that requires receivers is unknown, the jammer would grow more difficult and expensive to achieve a relatively small step to optimize broadcast- need to emit across a wide sector, unavoid- as counterstealth technologies concurrently ing parameters for passive radar use. ably reducing jamming signal density.51 grow more advanced. This same adversary will build a passive multistatic receiver network in the VHF and North Vietnamese surface-to-air missile UHF bands, blending the system into the crew in front of SA–2 launcher vertical buildup of urban terrain.50 In remote areas not served by media broadcast, the adversary may disperse a network of inex- pensive throw-away transmitters to function as the surveillance area illuminators. He will integrate passive radar and other sensors for rapid, efficient command and control. It is likely that such an adversary will make efforts to develop or acquire passive SAMs with low observable launch signatures and procure and deploy high- and mid-altitude unmanned aerial vehicles—“missile trucks”—to deny flight at those altitudes.
Countering Passive Radar Countering passive radar will prove difficult. What are the signs that an opponent is using passive radar? Forehand knowledge of the threat may provide an idea of general capabilities. Are friendly air forces losing U.S. Air Force aircraft to ground fire with little or no threat warning indications? With no radio frequency basic stealth techniques will be less effective than they once electronic intelligence available, locating the were against passive radar systems that benefit from bistatic passive radar receivers will be challenging. Intelligence will face a difficult task of using geometry and the use of counterstealth waveforms indirect methods—human intelligence, ground surveillance, computer network Moreover, deception jamming may be of Implications operations, and nodal analysis—to collect on limited use against passive radar, also due to Passive radar has many implications for sparse information. the unknown receiver location.52 Other types future U.S. military power. Stealth will con- If the command and control nodes and of jamming, however, may prove highly effec- tinue to be a critical feature of tactical military receivers cannot be found, targeting planners tive. Overall, the lack of known threat loca- aircraft, particularly as a defense against pres- could focus on destroying suspected trans- tion bolsters the argument for a robust EW ently fielded weapons systems. As is evident mitters—for example, FM radio, television, capability that is integral to friendly multirole in the continued proliferation of conventional and HD television networks. Depending on aircraft. Advocacy for or against a dedicated radar SAMs and AAA, these threats are not their location and the potential for collateral EW platform, however, is beyond the scope of going away any time soon. damage, however, destroying these targets this article. Stealth airframes require long design may result in undesirable strategic conse- Other means of countering passive and procurement processes, whereas avion- quences, particularly in urban areas. radar include special operations and computer ics and software are more readily modified. At the tactical level, friendly forces could network attack. In the end, targeting passive This phenomenon is driving a philosophy employ reactive defenses against SAM launch radar systems may fall in the “too hard” cat- in tactical aircraft design that basic stealth and fly-out and conduct immediate counter- egory for limited warfare. Missile systems— techniques are the critical solid foundation attack of associated threat systems through mobile SAMs, UAVs, and even man-portable upon which the aircraft’s more malleable electronic attack, standoff weapons, directed air defense systems—may be easier to find offensive and defensive capabilities—sensors, energy, or other means. This approach, than passive radar. The adversary will likely weapons, and communications—are built. however, would consume time and resources deploy substantial passive air defense assets, The concept of a layered defense will be criti- and would likely fail to achieve low- to mid- and U.S. forces will face a long, tedious cal to the survivability of stealth aircraft in altitude air superiority. process of locating and attriting them. the future. ndupress.ndu.edu issue 55, 4 th quarter 2009 / JFQ 141 FEATURES | Passive Radar and the Future of U.S. Military Power
Basic stealth techniques, however, will planners must recognize the counter to U.S. ■■ work hand in hand with key allies to be much less effective than they once were stealth-based air-superiority that is currently develop shared capabilities against passive radar systems that benefit from unfolding, of which passive radar forms a core ■■ explore enhancing parallel technolo- bistatic geometry and the use of counterstealth technology. These self-same leaders must take gies (such as disposable transmitters). waveforms. Increasingly, combatants will use appropriate measures to ensure that the United passive radar and weapons systems to detect, States is not caught off guard by this impending Develop methods of degrading enemy acquire, track, and target aerial stealth plat- shift in the technological landscape. The fol- passive radar. In support of this effort: forms. Against such systems, stealth on its own lowing recommendations are in order. will likely provide inadequate protection for Endeavor to be a leader in the passive ■■ focus on a multilevel EW capability manned aircraft, UAVs, and missiles. radar field. Arguably, the United States has against passive radar54 This article posits that an ongoing race marginalized the passive radar field due to ■■ continue to develop layered defensive between stealth and counterstealth is emerg- a focus on conventional radar systems. The measures for aircraft and UAVs. ing, in which technology will provide only U.S. military must gain an understanding incremental advantage to a combatant until of passive radar, not merely theoretically, or Prepare for military operations without a new counter is found. This assertion does with minor research and development proj- air superiority. In support of this effort: not mean that there are no further opportuni- ects, but with a dedicated effort. But why, one ties to leverage stealth advantages, but that may ask, build a stealth counter when there ■■(again) develop passive radar, but advances in stealth will be more evolutionary is no immediate stealth peer competitor? The in this case to deny enemy air superior- than revolutionary. The future of stealth and answer is that would-be competitors in the ity—future enemy stealth capabilities are counterstealth will more closely resemble the stealth arena are making a dedicated push to ultimately not a matter of if but when technological one-upmanship that occurred develop this technology. We cannot afford to ■■continue to integrate complementary during World War II and the Cold War than spend billions on stealth, only to fail to thor- piloted and unmanned system capabilities the order of magnitude advantage the United oughly understand and counter rival systems. ■■plan and train to the contingency of States enjoyed during the Gulf War and the two In support of this effort: military operations with only local air supe- decades that have followed. Against a passive riority or with air superiority largely denied. radar adversary, air superiority will likely only ■■ build collaboration between key indus- be achieved at significant cost. Forcible entry try and independent electronic engineers Passive radar will play a critical role and amphibious operations will accordingly ■■ increase prioritization of passive radar in future conflict. Ongoing advances in prove much more challenging. Once again, the research and development passive radar will deny traditional means defensive form of warfare asserts itself. ■■ develop and field a passive radar of defeating enemy air defenses, make air system on a U.S. training range—as a train- superiority difficult to achieve against a Recommendations ing tool for U.S. stealth pilots and systems to passive radar opponent, and require changes To best position the United States for test countermeasures and tactics and assess in thinking to maintain U.S. power projec- the future, military strategists and operational performance53 tion capability.
we cannot afford to spend billions on stealth, only to fail to thoroughly understand and counter rival systems
F–35 Lightning II flies over Eglin Air Force Base, future home of Joint Strike Fighter training facility U.S. Air Force (Julianne Showalter) U.S. Air Force
142 JFQ / issue 55, 4 th quarter 2009 ndupress.ndu.edu WESTRA
Will the United States go forward to hq.af.mil/Publications/fulltext/gulf_war_air_power_ 32 Willis and Griffiths, 95. 33 a future that resembles the past—one in survey-vol5.pdf>. “Nebo-SVU Surveillance Radar (Russian 14 which air superiority is gained only through Ibid., 641. Federation)—Jane’s Radar and Electronic Warfare 15 Benjamin S. Lambeth, “Kosovo and the Con- Systems,” available at ndupress.ndu.edu issue 55, 4 th quarter 2009 / JFQ 143