Passive Detection and China's Counter-Stealth Radar Capabilities
How Beijing is Leveraging Passive Radar to Challenge America's Stealth Advantage
The ability to operate within an adversary’s airspace with a low probability of detection has been a cornerstone of U.S. military strategy, enabling freedom of action in contested environments. In the People’s Republic of China, however, a concerted national effort is underway to develop technologies that directly challenge this advantage. This effort, a critical component of China’s broader Anti-Access/Area Denial (A2/AD) strategy, is centered on passive radar (无源雷达), a technology China is aggressively developing as a key component of its counter-stealth (反隐身) doctrine.
This development represents a significant shift in the dynamics of electronic warfare, which I touched upon briefly in my post on China’s SIGINT/ELINT satellites. Let’s return to this topic now and examine the PLA’s efforts to create a layered, resilient sensor network capable of detecting, tracking, and ultimately targeting even the most advanced low-observable aircraft.
The Underpinnings of Passive Coherent Location
At its core, Passive Coherent Location (PCL), also known as Passive Covert Radar (PCR), is a departure from traditional radar. Instead of emitting powerful radio waves and listening for the echo—a process that makes active radars a prominent and vulnerable target for anti-radiation missiles—PCL systems are entirely passive. They are silent receivers, exploiting the ambient electromagnetic environment.
These systems operate in two primary modes. The first, Passive Emitter Tracking (PET), functions by detecting and geolocating a target’s own electronic emissions, such as its radar, communications, or IFF transponder signals. The second, more advanced method uses “illuminators of opportunity” (IoO)—existing, non-cooperative energy sources. These can include a wide variety of signals, from commercial FM radio broadcasts and Digital Video Broadcasting-Terrestrial (DVB-T) signals to cellular base station transmissions (4G/5G) and even satellite signals (GPS, Beidou, LEO internet constellations). The PCL receiver simultaneously captures the direct-path signal from the illuminator and the faint reflections of that signal off a target. By measuring the minuscule Time Difference of Arrival (TDOA) between these two signals across multiple, spatially diverse receiver stations, the system can triangulate the target’s precise location.
The geometric advantage is key. Modern stealth aircraft are meticulously designed to defeat monostatic radars, where the transmitter and receiver are co-located, by deflecting radar energy away from the source. This creates “cones of silence” where the aircraft is exceptionally difficult to detect. PCL systems, by being inherently bistatic (one transmitter, one receiver) or, more powerfully, multistatic (multiple transmitters and/or receivers), create a network of listening posts that can be positioned to catch these deflected signals. From the perspective of a receiver far from the transmitter, the stealth aircraft’s primary defensive feature is turned into a detectable signature.
From Emulation to Innovation
China’s intense interest in passive radar was catalyzed by events in 1999: the downing of a U.S. F-117 stealth fighter over Yugoslavia, reportedly aided by a Czech Tamara passive surveillance system, and the accidental U.S. bombing of the Chinese embassy in Belgrade. These incidents served as a wake-up call, demonstrating both the potential vulnerabilities of stealth aircraft and the strategic imperative for China to develop its own advanced military technology.
Initially, China pursued a dual-track approach: attempting to acquire foreign systems while investing in indigenous research. In 2004, a deal to purchase six advanced Czech VERA-E passive surveillance systems was scuttled by U.S. diplomatic pressure. For Beijing, this was a critical lesson in the dangers of relying on foreign suppliers for core military capabilities. The setback only strengthened China’s resolve to achieve self-sufficiency.
The state-owned China Electronics Technology Group Corporation (CETC) became the nexus of this effort. Key institutes under its umbrella, such as the 14th Research Institute (NRIET) in Nanjing (a famed radar house) and the 29th Research Institute (SWEEI) in Chengdu (an electronic warfare specialist) were tasked with mastering PCL technology. They were supported by a robust academic-military research ecosystem. Institutions like Xidian University, the National University of Defense Technology (NUDT), and Wuhan University began producing cutting-edge research on advanced algorithms, novel clutter suppression techniques, and the use of new illuminators.
Key Operational Systems
By the mid-2010s, this concerted effort had yielded a portfolio of operational systems. Among the most significant are:
YLC-20: Developed by NRIET, the YLC-20 is primarily a PET system, designed to detect and locate targets by their own emissions across a wide frequency band (reportedly 380 MHz to 12 GHz). Conceptually based on systems like the VERA-E, it can serve as a long-range, strategic “tripwire” to provide early warning. Its primary limitation, however, is its ineffectiveness against a target observing strict emissions control (EMCON).
DWL002: A product of the 29th Institute, the DWL002 is a more advanced, multi-station system first revealed around 2009. It is explicitly described as a true PCL radar designed to counter stealth aircraft. With a reported detection range of up to 400 km against fighter-sized targets and 600 km against larger aircraft, a single DWL002 system deployed on the coast could provide persistent, covert surveillance over the entire Taiwan Strait. Its architecture, consisting of one master and multiple client stations deployed tens of kilometers apart, is essential for the TDOA multilateration required for accurate 3D tracking.
YLC-29: Publicly unveiled in 2017, the YLC-29 represents a further evolution. It reportedly uses civilian FM radio signals as its illuminator of opportunity. Operating in the VHF band (around 100 MHz), the YLC-29 exploits the physical principle that stealth shaping and radar-absorbent materials, typically optimized for higher-frequency X-band and C-band radars, are less effective against longer wavelengths. This allows it to generate a larger radar cross-section from stealth aircraft. Chinese media has touted it as superior in performance to both the older YLC-20 and the foreign VERA-E.
These systems do not operate in isolation. Chinese military doctrine, particularly its focus on “informatized warfare” (信息化战争), emphasizes a layered, integrated air defense system (IADS) where different sensors with complementary strengths work in concert.
A “Passive-to-Active” Engagement Scenario
Consider a potential conflict scenario. A flight of U.S. stealth aircraft approaches the Chinese coast, maintaining strict radio silence to evade detection. A YLC-20 system sees nothing. However, a network of YLC-29 and DWL002 systems are passively listening, using signals from powerful FM broadcast towers on the mainland. They detect the faint, distorted echoes from the approaching aircraft and generate a track file.
This track, while perhaps not precise enough for a direct missile lock due to inherent resolution limitations, provides a crucial “covert cue.” The data is securely networked via high-bandwidth links to a regional air defense command center, which fuses it with other intelligence. The track is then passed to a SAM battery, such as the HQ-9. The battery’s powerful but vulnerable active engagement radar remains inactive and does not transmit, betraying no sign of its readiness. Only when the stealth aircraft enters the missile’s engagement envelope does the active radar activate for a very short burst—just long enough to acquire a fire-control-quality track and guide the missile. This “passive-to-active” doctrine maximizes the survivability of the entire air defense network by minimizing the transmission time of its most vulnerable components.
Challenges and U.S. Countermoves
Despite these advances, China’s PCL capabilities are not without weaknesses. The “illuminator dilemma” remains a key challenge. A reliance on third-party broadcast signals means that a PCL system’s performance can be degraded if a station ceases broadcasting or is destroyed in a conflict. To mitigate this, Chinese researchers are intensely focused on leveraging their own sovereign, space-based assets—like the Beidou navigation satellite system and future LEO communication constellations such as GuoWang—as controllable illuminators.
Furthermore, the immense computational burden of PCL signal processing is a non-trivial engineering hurdle. The core technical task involves canceling the overwhelmingly powerful Direct Path Interference (DPI) from the illuminator to detect the target echo, which can be many orders of magnitude weaker. This requires sophisticated adaptive filtering algorithms and significant processing power (FPGAs, GPUs), which can constrain the mobility and affordability of the systems.
These systems also have inherent performance limitations. They generally exhibit poorer resolution and accuracy, particularly in the elevation (height-finding) dimension, than high-frequency active radars. This reinforces their doctrinal role as a cueing and surveillance sensor rather than a primary engagement sensor. They are also not invulnerable. While they cannot be targeted by anti-radiation missiles, their receiver sites can be located through SIGINT targeting their communications and can then be kinetically attacked. Advanced U.S. electronic warfare capabilities could also be used to spoof illuminator signals, creating a flood of false targets to saturate the Chinese network’s processing capacity.
The Future Battlespace
Looking ahead, China’s PCL development is moving in several key directions. The first is a determined push into space, aiming to transform PCL from an opportunistic, geographically limited capability into a persistent, global surveillance system. The second is the deep integration of Artificial Intelligence and Machine Learning to automate and optimize the PCL processing chain. This includes using AI for advanced clutter suppression (distinguishing a stealthy drone from a flock of birds) and even for automated target recognition (ATR), classifying a detected target based on its unique multistatic scattering characteristics.
The proliferation of these systems also presents a strategic challenge. The export of relatively low-cost, survivable, and covert PCL systems to other nations could significantly complicate U.S. air operations in theaters far beyond the Indo-Pacific, eroding a key technological advantage upon which American airpower has long relied.
The development of PCL signals a fundamental shift in air combat. The contest is no longer one of simple invisibility, but a far more complex battle of signatures, processing, and control of the electromagnetic spectrum. As China continues to develop and field these passive detection systems, maintaining air superiority in the 21st century will depend on the ability to understand its capabilities and devise effective counters.
Thanks for reading! I hope you enjoyed this post. The plan is to tackle more ISR topics in the next few weeks, such as high-frequency, direction-finding (HF/DF). As always, please subscribe to Orders and Observations and share my newsletter if you can, and follow me on X/Twitter and LinkedIn if you’d like to connect or collaborate.
Very detailed and clear analysis!
Amazing article, rare to hear such detailed and balanced account