Passive Radars A game-changer for Air Defense

Ever since Britain integrated radar technology into its revolutionary air defense system during the Second World War‭, ‬radars have had a profound influence over the conduct of warfare‭, ‬especially the back-and-forth‭, ‬techno-adaptive dialectic between aerial‭ ‬systems and those ground-based systems designed to detect and engage them‭. ‬Fast-paced developments in passive radar technology may once again be set to bring the advantage to air defense over air offense in this long-running tussle‭.‬

This article examines recent developments in passive radar technology‭, ‬especially specific projects in Russia‭, ‬Germany‭, ‬China and elsewhere‭. ‬Unlike other passive detection systems‭ ‬–‭ ‬like those for Electronic Intelligence‭ (‬ELINT‭) ‬and those Communications Intelligence‭ (‬COMINT‭), ‬which can detect‭, ‬locate‭, ‬and track aircraft based on receiving their radar emissions or signals from onboard communications systems‭ ‬–‭ ‬this new breed of passive radar system holds out the promise of achieving the same effects against non-emitting targets‭ ‬–‭ ‬at the high-end of sophistication scale‭, ‬stealth aircraft‭; ‬at the lower-end‭, ‬small‭, ‬off-the-shelf drones‭. ‬

In simple terms‭, ‬passive radar systems compute an aerial picture from reading how civilian and commercial communications signals‭ ‬bounce off airborne objects‭. ‬The advantage is that the technique works with any type of signal present in airspace‭: ‬these could‭ ‬be radio or television broadcasts or the ubiquitous emissions from mobile phone stations‭. ‬Passive radars use these emissions to‭ ‬detect targets moving through an area of the sky instead of by means of an active radar emitter‭. ‬

It should not be forgotten‭, ‬however‭, ‬that the passive radar concept has been around for a long time‭; ‬indeed‭, ‬it dates back to the very beginning of military radar systems‭, ‬with passive radars in operational service during the Second World War‭. ‬In recent years‭, ‬the passive radar concept has generated considerable enthusiasm across a multitude of nations and within their defense production bases and research communities‭. ‬Indeed‭, ‬as two experts in the field have noted‭: ‬“In recent years‭, ‬the research and development activities focused on Passive Radar have been astonishing in number‭, ‬diversity‭, ‬and worldwide distribution‭.‬”‭ ‬Multiple weapons manufacturers in various countries are now rapidly pursuing the technology and believe that such systems might‭ ‬be so-called‭ ‬“game-changers”‭ ‬in warfare as well as in dealing with the evolving peacetime security threat posed by commercially available drones‭. ‬

As with many newly emerging and sensitive defense programs‭, ‬there is little in the way of detailed information about cutting-edge passive radar projects in the public domain‭. ‬However‭, ‬sufficient details exist to begin making inferences‭, ‬albeit speculative‭ ‬ones‭, ‬about what the eventual emergence of this new breed of passive radars as operational systems might mean for the military balance in the air domain as well as for addressing a range of evolving security challenges‭. ‬

A cat-and mouse game‭ ‬

In great part‭, ‬commentary about passive radars has centered on their prospective ability to negate much of advantages afforded by stealth technology‭. ‬This potential attribute of passive radars is highly significant in the cat-and-mouse game between stealthy combat aircraft‭ (‬designed‭, ‬of course‭, ‬to be undetectable by radar‭) ‬and those sensors that attempt defeat this advantage‭. ‬Perhaps the most pronounced selling point of the hugely expensive U.S‭. ‬F-35‭ ‬is that its designers and advocates promise that its invisibility is so complete that it is fair to tout the platform as an invincible weapon‭ ‬–‭ ‬with all the ramifications for employment doctrine that follow from this belief‭. ‬

But might passive radars‭, ‬which are vastly cheaper to build and deploy than stealth aircraft‭ (‬and this is not to mention that huge sums that have gone into research and development‭ (‬R&D‭) ‬for stealth technology‭), ‬negate the F-35’s most distinguishing attribute‭? ‬German radar-maker Hensoldt believes its system can‭ ‬–‭ ‬and has‭. ‬Hensoldt claims that passive radar system‭, ‬named TwInvis‭, ‬tracked two F-35s for 150‭ ‬kilometers following their appearance at the 2018‭ ‬Berlin Air Show in Germany in late April of that year‭. ‬

Several subsequent media reports about TwInvis’‭ ‬success in tracking the stealthy F-35‭ ‬billed the portable German system‭ ‬–‭ ‬with a collapsible antenna‭, ‬it so compact that it can packed be into a SUV‭ ‬–‭ ‬a potential game-changer in aerial defense‭. ‬Putting the Hensoldt’s 2018‭ ‬claim under closer scrutiny‭, ‬however‭, ‬suggests caution in reaching the conclusion that passive radars have made stealth technology obsolete‭. ‬First and foremost‭, ‬the F-35s were installed with Luneburg lenses on their wings for their journey to Berlin‭. ‬These little knobs are placed on stealthy aircraft as they pass through friendly air space so that they can be detected by local air traffic control‭.  ‬They artificially create a radar cross section in the frequency bands in which airspace-deconfliction radars operate so that traditional‭, ‬defense radar systems know what they are dealing with‭. ‬In short‭, ‬they‭ (‬the F-35s‭) ‬were not flying under stealthy conditions in Germany in 2018‭. ‬

Nonetheless‭, ‬minimizing specular reflection at their front sector most stealth aircraft have been optimized for monostatic type‭ ‬active radars operating at higher frequency bands‭. ‬Passive radars involving lower frequency bands and multi-static type scattering may still prove a viable approach against stealth‭. ‬Although it remains to be proven whether such passive radar technology can‭ ‬be fully effective against stealthy aircraft designs‭, ‬such systems will be an improvement on traditional radar emitters whose signals are broken up or absorbed by stealthy aircraft designs so that little reflects to ground-station sensors‭, ‬leaving defensive-radar operators in the dark‭. ‬

Passive radars’‭ ‬biggest ultimate impact in international affairs maybe how they shift the advantage from stealthy aircraft to air defenders‭, ‬but their potential effects in the air domain might reach far beyond this‭. ‬This is certainly why many defense companies‭ ‬–‭ ‬both private and publicly owned‭ ‬–‭ ‬are attempting to get into the passive radar business‭. ‬

New generation of passive radars‭ ‬

Hensoldt’s passive radar system is but one of an emerging generation of sensors and processors that promise to find previously undetectable activities in any given airspace‭. ‬Russia appears to be developing one of the most impressive passive radar systems‭. ‬Moscow’s new passive radar‭, ‬called Tropa‭ (‬Russian for‭ ‬“path”‭), ‬uses communication towers to locate signals without transmitting one itself‭. ‬The manufacturer‭, ‬OKB-Planeta‭, ‬owned by Russia’s private defense solutions holding company RTI‭, ‬claims the radar also has civilian uses‭ ‬–‭ ‬to stop drones spying on people‭, ‬for example‭. ‬

More ambitiously‭, ‬China claims to have revealed a prototype of an‮ ‬advanced quantum radar‮ ‬that is‮ ‬resistant to jamming‮ ‬and may be‭ ‬able to‮ ‬detect stealthy aircraft‭. ‬Although the system’s technological architecture is rooted in proven science and could be game-changing‭, ‬the Chinese still face significant development challenges in turning it into an operational capability‭. ‬

Other states are working on systems that make use of non-co-operative transmitters‭ ‬–‭ ‬FM radio stations or Digital Audio Broadcasting‭ (‬DAB‭) ‬towers‭ ‬–‭ ‬to create an air situation picture using non-emitting target detection‭. ‬As mentioned above‭, ‬reflections from emissions are received by one‭, ‬or a network of‭, ‬antennas‭, ‬providing 3D real-time omnidirectional coverage for tracking of multiple targets in congested airspace‭. ‬Marketing information about such systems typically claim their company’s system will be able to detect‭, ‬locate‭, ‬and track non-emitting targets such as drones‭. ‬

Used independently or within a larger air control system‭, ‬such passive radar systems could prove highly significant in countering the drone threat to critical infrastructure‭, ‬such as commercial airports‭. ‬In the past few years‭, ‬security analysts have repeatedly warned that the challenge of protecting critical infrastructure‭, ‬and especially airports‭, ‬against drones is an insurmountable challenge‭. ‬Passive radar systems may provide an ability to better defend against multiple non-emitting small drones because detecting and tracking them is the first necessity in the ability to destroy or incapacitate them‭.

Cautious optimism for air defenders‭ 

Modern conflicts‭, ‬such as in Yugoslavia or Middle East region‭, ‬have shown combat active radars with rotating antennas have a low‭ ‬survivability rate against a sophisticated military opponent‭ ‬–‭ ‬perhaps somewhere in the region of 20‭ ‬to 30‭ ‬minutes‭. ‬Indeed‭, ‬active radars are often the first targets to get hit at the outset‭ ‬of a military conflict‭, ‬whether by anti-radiation missiles‭, ‬GPS-guided systems‭, ‬or increasingly so in the future by directed-energy weapons‭ (‬DEW‭). ‬Moreover‭, ‬active radar systems need not be destroyed‭, ‬only taken out of action‭. ‬Because the location of these systems can be clearly identified by satellite‭ ‬–‭ ‬most have remained fixed in the same spot for years‭ ‬–‭ ‬the majority are vulnerable to destruction from reliable precision strike weapons‭. ‬It is against this context that passive radars hold such appeal to air defenders‭. 

Because they possess no emitters‭, ‬passive radars are covert‭, ‬meaning pilots entering a monitored area are unaware they are being‭ ‬tracked‭. ‬Because they have no dedicated transmitters generating heat‭, ‬they cannot be detected by their thermal signatures‭. ‬What‭ ‬is more‭, ‬their broadcast antennas are generally small and quite difficult to spot‭.‬‮ ‬For air defenders‭, ‬this promises a major improvement in their chances of survival should war break out against a capable adversary‭. ‬

Yet there remain severe drawbacks with the technology and limitations in when and where it can be used‭. ‬For one thing‭, ‬passive radars depend on the existence of radio signals and other emissions which may not be present in remote areas of the globe‭. ‬This perhaps makes passive radar systems more suitable for air defense in one’s home territory than as an operational deployable asset‭. ‬Moreover‭, ‬the coverage of such systems is limited up to medium altitudes‭, ‬since there is practically no broadcast at higher altitudes‭. ‬In addition‭, ‬the technology is not yet accurate enough to guide‭ ‬missiles‭, ‬though it could be used to send infrared-homing weapons close to a target‭.‬

As for the techno-adaptative dialectic between stealth and ground-based radars‭, ‬perhaps neither set of proponents of either technology are correct‭. ‬As defense commentator Tyler Rogoway surmises‭: ‬“It‭ [‬stealth‭] ‬isn’t a magical cloak of invisibility‭. ‬It never has been and it never will be‭. ‬And passive radar isn’t a magic stealth detection tool‭, ‬either‭.‬”‭ ‬However‭, ‬ongoing advances in passive radar will deny traditional means to defeat enemy air defenses‭, ‬make air superiority difficult to achieve against a passive radar opponent‭. ‬In the shorter-term‭, ‬passive radar’s impact may be felt in homeland security and critical infrastructure protection against small‭, ‬non-emitting drones than in upsetting the defense-attack dynamic in the air domain‭. ‬Small UAVs pose as a significant threat to both military installations and industrial facilities as more conventional weapons‭. ‬Passive radar could maintain surveillance of the airspace immediately surrounding such strategic sites‭, ‬enhancing security‭, ‬and providing effective protection from these new threats‭. ‬Theoretical coverage studies based on bistatic radar cross section simulations and complex propagation models show initial positive signs about the possibility of drone detection with passive radars‭. ‬

Passive radars are no longer a‭ ‬“sunrise”‭ ‬technology‭; ‬we are on the threshold of seeing more and more systems entering the marketplace and governments utilizing passive‭ ‬radar to confront the increasing threat from drones‭.‬

By‭: ‬Dr‭. ‬Ash Rossiter
Assistant professor of international security at Khalifa University‭, ‬UAE‭)‬


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