Counterstealth technologies, intended to reduce the effectiveness of radar cross-section (RCS) reduction measures, are proliferating worldwide. Since 2013, multiple new programs have been revealed, producers of radar and infrared search and track (IRST) systems have been more ready to claim counterstealth capability, and some operators—notably the U.S. Navy—have openly conceded that stealth technology is being challenged.

These new systems are designed from the outset for sensor fusion—when different sensors detect and track the same target, the track and identification data are merged automatically. This is intended to overcome a critical problem in engaging stealth targets: Even if the target is detected, the “kill chain” by which a target is tracked, identified and engaged by a weapon can still be broken if any sensor in the chain cannot pick the target up.

The fact that some stealth configurations may be much less effective against very-high-frequency (VHF) radars than against higher-frequency systems is a matter of electromagnetic physics. A declassified 1985 CIA report correctly predicted that the Soviet Union’s first major counterstealth effort would be to develop new VHF radars that would reduce the disadvantages of long wavelengths:  lack of mobility, poor resolution and susceptibility to clutter. Despite the breakup of the Soviet Union, the 55Zh6UE Nebo-U, designed by the Nizhny-Novgorod Research Institute of Radio Engineering (NNIIRT), entered service in the 1990s as the first three-dimensional Russian VHF radar. NNIRT subsequently prototyped the first VHF active electronically scanned array (AESA) systems.

VHF AESA technology has entered production as part of the 55Zh6M Nebo-M multiband radar complex, which passed State tests in 2011 and is in production for Russian air defense forces against a 100-system order. The Nebo-M includes three truck-mounted radar systems, all of them -AESAs: the VHF RLM-M, the RLM-D in L-band (UHF) and the S/X-band RLM-S. (Russian documentation describes them as metric, decimetric and centimetric—that is, each differs from the next by an order of magnitude in frequency.) Each of the radars is equipped with the Orientir location system, comprising three Glonass satellite navigation receivers on a fixed frame, and they are connected via wireless or cable datalink to a ground control vehicle.

One of the classic drawbacks of VHF is slow scan rate. With the RLM-M, electronic scanning is superimposed on mechanical scanning. The radar can scan a 120-deg. sector mechanically, maintaining continuous track through all but the outer 15-deg. sectors. Within the scan area, the scan is virtually instantaneous, allowing energy to be focused on any possible target. It retains the basic advantages of VHF: NNIRT says that the Chinese DF-15 short-range ballistic missile has a 0.002 m2 RCS in X-band, but is 0.6 m2 in VHF.

The principle behind Nebo-M is the fusion of data from the three radars to create a robust kill chain. The VHF system performs initial detection and cues the UHF radar, which in turn can cue the X-band RLM-S. The Orientir system provides accurate azimuth data (which Glonass/GPS on its own does not support), and makes it possible for the three signals to be combined into a single target picture.

The higher-frequency radars are more accurate than VHF, and can concentrate energy on a target to make successful detection and tracking more likely. Using “stop and stare” modes, where the antenna rotation stops and the radar scans electronically over a 90-deg. sector, puts four times as much energy on target as continuous rotation and increases range by 40%.

Saab’s work on its new Giraffe 4A/8A S-band radars points to ways in which AESA technology and advanced processing improve high-band performance against small targets. Module technology is important, maximizing the AESA’s advantages in terms of signal-to-noise ratio. The goal is signal “purity” where most of the energy is concentrated close to the nominal design frequency, which makes it possible to detect very small Doppler shifts in returns from moving targets.

New processing technologies include “multiple hypothesis” tracking in which weak returns are analyzed over time and either declared as tracks or discarded based on their behavior. China is taking a similar approach to Russia, as seen at last November’s Zhuhai air show. Newcomers included the JY-27A Skywatch-V, a large-scale VHF AESA closely comparable to Russia’s RLM-M, developed by East China Research Institute of Electronic Engineering (Ecriee), part of the China Electronics Technology Corp. (CTEC). Two alternative UHF AESAs and a YLC-2V S-band passive electronically scanned array radar were also on show.

CETC exhibits indicated a focus on combining active and passive detection systems, including the flight-line display of a large-area directional, wideband passive receiver system identified as YLC-20. It appears to be used as an adjunct to the CETC DWL-002, which is a three-station passive coherent location (PCL) system similar to the Czech ERA Vera series, using time difference of arrival processing to locate and track targets. Also shown on a wall chart was the JY-50 “passive radar,” which operates in the VHF band.

Previous PCL systems, including Vera, are designed to exploit active emissions from the target. However, by teaming PCL and other passive receivers with active radars, the defender creates bistatic and multistatic detection systems, which may reduce the effectiveness of RCS-reduction measures that are primarily monostatic. For instance, highly swept leading edges are designed to deflect radar signals away from the source, but can create spikes detectable by multistatic systems.

Older and smaller VHF radars such as the NNIRTI’s 1970s-era P-18 are being upgraded by at least five teams: Retia in Czech Republic, Arzenal in Hungary, Ukraine’s Aerotechnica, and organizations in Belorussia and Russia. The -Chinese navy has retained VHF radar on its newest air warfare destroyers such as the Type 52C Luyang II and Type 52D Luyang III. The possibility of a more modern VHF radar appearing on the new, larger Type 055 destroyer cannot be ruled out.

The challenge to stealth posed by lower-frequency radars and other detection means has been acknowledged at higher levels since 2013. U.S. chief of naval operations Adm. Jonathan Greenert has publicly expressed doubt as to whether stealth platforms constitute a complete answer to the developing anti-access/area-denial (A2/D2) threat, and a January 2014 paper by the Center for a New American Security noted, “One recent analysis argued that there has been a revolution in detecting aircraft with low RCS, while there have not been commensurate enhancements in stealth.”

Boeing has promoted the EA-18G Growler’s ability to jam in the VHF band, which is built into the current ALQ-99 low-band pod configuration (the most modern part of the system) and the planned Increment 2 of the Next Generation Jammer system. Increment 2 will likely comprise an upgrade to the current pod—the best solution to emerge from an analysis of alternatives conducted in 2012. A contract should be issued in 2017 with initial operational capability in 2024.

A different kind of radar threat is the very-long-wave over-the-horizon (OTH) radar, typified by Australia’s Jindalee OTH Radar Network (JORN), Russia’s Rezonans-NE, and China’s OTH systems. Again, processing is the key to increasing the accuracy and sensitivity of these systems, typified by the Phase 5 upgrade to JORN.

OTH long-wave radars are inherently “counterstealth” because at very long wavelengths that are close to the physical size of the target, conventional radar cross-section measurement and reduction techniques do not apply. Claims by Jindalee’s original designers that the radar could detect the B-2 were published in the late 1980s and were taken seriously by the U.S. Air Force. At the time, however, the service could argue that OTH’s resolution was so poor that it could not represent the start of a kill chain. Today, however, that low resolution can be mitigated by networking multiple radars, and by using OTH-B to cue high-resolution sensors.

Outside the radio-frequency band, the U.S. Air Force (AW&ST Sept. 22, 2014, p. 42) is the latest convert to the capabilities of IRST. The U.S. Navy’s IRST for the Super Hornet, installed in a modified centerline fuel tank, was approved for low-rate initial production in February, following 2014 tests of an engineering development model system, and the Block I version is due to reach initial operational capability in fiscal 2018. Block I uses the same Lockheed Martin infrared receiver—optics and front end—as is used on F-15Ks in Korea and F-15SGs in Singapore. This subsystem is, in turn, derived from the IRST that was designed in the 1980s for the F-14D. 

While the Pentagon’s director of operational test and engineering criticized the Navy system’s track quality, it has clearly impressed the Air Force enough to overcome its long lack of interest in IRST. The Air Force has also gained experience via its F-16 Aggressor units, which have been flying with IRST pods since 2013. The Navy plans to acquire only 60 Block I sensors, followed by 110 Block II systems with a new front end.

The bulk of Western IRST experience is held by Selex-ES, which is the lead contractor on the Typhoon’s Pirate IRST and the supplier of the Skyward-G for Gripen. In the past year, Selex has claimed openly that its IRSTs have been able to detect and track low-RCS targets at subsonic speeds, due to skin friction, heat radiating through the skin from the engine, and the exhaust plume. The U.S. Navy’s Greenert underscored this point in Washington in early February, saying that “if something moves fast through the air, disrupts molecules and puts out heat . . . it’s going to be detectable.”

These detection improvements do not mean the end of stealth, in the view of most industry and government sources, but they do underlie current plans and discussions for the future applications of RCS-reduction and other stealth-related technologies. For example, the long debate over the appropriate level of stealth technology for the U.S. Navy’s Unmanned Carrier-Launched Airborne Surveillance and Strike program has revolved around the development of A2/AD threats. The result is the end of a decades-long misapprehension, widely held in professional as well as public circles, that there is no major difference in stealth performance among various low-observable designs.