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By Charlie Gao, The National Interest
Ever since the development of stealth technology for aircraft, many different systems have been advertised as “stealth killing.” One of the more innovative solutions is the Russian Struna-1/Barrier-E bistatic radar system developed by NNIIRT, a division of the Almaz-Antey Joint Stock Company. Almaz-Antey is the premier air-defense and radar manufacturer in Russia; they make the Tor, Buk and S-400 anti-aircraft systems, as well as their respective search radars. The Struna-1 was originally developed in 1999. A further evolution of Struna-1, the Barrier-E system was later showcased for export at MAKS 2007. While it is not part of Almaz-Antey’s online catalog, it was shown alongside other radars at MAKS 2017. The system is rumored to be deployed around Moscow.
The Struna-1 is different than most radars in that it is a bistatic radar, meaning it relies on the receiver and transmitter of the radar to be in two different locations as opposed to conventional radar technology where the receiver and transmitter are located in the same location. Normal radar systems are limited by the inverse fourth power law. As the radar target goes further away from the transmission source, the strength of the radar signal decays as per the regular inverse square law. However, radar detection works by receiving reflections of the radar signal. With a conventional radar, this results in the received signal being four times weaker than that put out. Stealth works because at a distance, an aircraft can mitigate its radar returns to be small by scattering them and absorbing them using radiation-absorbent materials. This degrades the quality of the radar track so it is harder to distinguish precise information about an aircraft.
(This first appeared in October 2017.)
The Struna-1 solves this problem by positioning the transmitter in a different location than the receiver. The link between the transmitter and receiver has increased power relative to a conventional radar, as it falls off according to the inverse square law as opposed to the inverse fourth power law. This allows the radar to be more sensitive, as it is effectively acting as a radar tripwire. According to Russian sources, this setup increases the effective radar cross section (RCS) of a target by nearly threefold, and ignores any anti-radar coatings that can scatter the radio waves. This allows the detection of not only stealth aircraft, but other objects with low RCS such as hang gliders and cruise missiles. As many of ten receiver/transmitter tower pairs—each tower is called Priyomno-Peredayushchiy Post (PPP) in Russian publications—can be placed. Sources vary in potential configurations of the towers, but the maximum span between two single towers is 50km. This leads to a maximum theoretical perimeter of 500km.
These individual towers have relatively low power consumption, and they do not emit as much energy as traditional radars, making them less vulnerable to anti-radiation weapons. The towers are mobile, allowing for forward deployment in times of conflict. They rely on microwave data links to communicate with each other and a centralized monitoring station, which can be located at a significant distance from the system. The distributed nature also allows the system to keep operating if one node goes down, albeit with less precision. The low height of the transmitter and receiver towers (only 25m off the ground) make Struna-1 very good at detecting low altitude targets, a target set that conventional radars often have trouble with.
Limitations of the Struna-1 include a low detection altitude. The nature of the system results in the detection range being a rough biased parabola between the receiver and transmitter. This limits the detection altitude to around 7km at the tallest point, with the maximum detection range going down as one gets closer to the transmitter/receiver towers. The transverse size of the detection zone is likewise limited, being around 1.5km close to the towers to 12km at the optimal point between the towers. The small size of the detection zone limits the use of the Struna-1 system as a tripwire, it cannot replace traditional radars as an overall search mechanism. However with its high precision tracks of stealthy aircraft, it would serve as a good counterpart to other longer-band radar systems such as Sunflower, which provide less precise tracks of planes. The Struna-1 cannot act as a targeting radar due to its inability to provide constant radar illumination tracking a target, so it cannot be used to guide in semi-active surface-to-air missiles.
While the Struna-1 bistatic radar is not a be-all end-all detection solution for stealth aircraft, it could pose a significant threat to stealth NATO aircraft in a future conflict. Strike aircraft with stealth features are particularly vulnerable, the strike role tends to favor flight profiles that might cause aircraft to fly into the Struna-1’s detection range. In tandem with other modern “stealth-defeating” radar systems, the Struna-1 could provide critical information to an adversary on the position and movement of stealth aircraft.
Charlie Gao studied Political and Computer Science at Grinnell College and is a frequent commentator on defense and national security issues.