Radar and signal processing versus Stealth Planes

Russia is putting up more radar stations for detecting stealth planes.

What’s perhaps most impressive about Russia’s Sunflower radar isn’t its ability to detect stealth fighters, but its comparative compactness. Many low-frequency radars installations are huge and power-intensive — and big, fat targets during a shooting war.

Sunflower, by contrast, is small and portable, according to media reports. “The system could be put online in 10 days and needs a team of just three people to stay operational,” Sputnik explained. “It does not need much power, it is easy to operate and it does not have much equipment.”

Quality of the detection depends upon pulse compression and signal processing

Low-frequency radars operating in the VHF and UHF bands can detect and track low-observable aircraft aka stealth planes.

Traditional limitation of VHF and UHF-band radars is that their pulse width is long and they have a low pulse repetition frequency [PRF]—which means such systems are poor at accurately determining range. As Mike Pietrucha, a former Air Force an electronic warfare officer who flew on the McDonnell Douglas F-4G Wild Weasel and Boeing F-15E Strike Eagle once described to me, a pulse width of twenty microseconds yields a pulse that is roughly 19,600 ft long—range resolution is half the length of that pulse. That means that range can’t be determined accurately within 10,000 feet. Furthermore, two targets near one another can’t be distinguished as separate contacts.

Signal processing partially solved the range resolution problem as early as in the 1970s. The key is a process called frequency modulation on pulse, which is used to compress a radar pulse. The advantage of using pulse compression is that with a twenty-microsecond pulse, the range resolution is reduced to about 180 feet or so. There are also several other techniques that can be used to compress a radar pulse such as phase shift keying. Indeed, according to Pietrucha, the technology for pulse compression is decades old and was taught to Air Force electronic warfare officers during the 1980s. The computer processing power required for this is negligible by current standards, Pietrucha said.

With a missile warhead large enough, the range resolution does not have to be precise. For example, the now antiquated S-75 Dvina—known in NATO parlance as the SA-2 Guideline—has a 440-pound warhead with a lethal radius of more than 100 feet. Thus, a notional twenty-microsecond compressed pulse with a range resolution of 150 feet should have the range resolution to get the warhead close enough—according to Pietrucha’s theory.

The directional and elevation resolution would have to be similar with an angular resolution of roughly 0.3 degrees for a target at thirty nautical miles because the launching radar is the only system guiding the SA-2. For example, a missile equipped with its own sensor—perhaps an infrared sensor with a scan volume of a cubic kilometer—would be an even more dangerous foe against an F-22 or F-35

Russia sunflower radar

Sunflower clearly trades performance for portability. Where larger low-frequency radars can detect targets thousands of miles away, the new Russian system reportedly has a range of just 300 miles or so.

But there are clear tactical advantages in being able to quickly set up large numbers of smaller, low-frequency radars. Russia could, on short notice, deploy batteries of Sunflowers on the periphery of conflict zones in order to begin getting a vague idea of where U.S. and allied stealth fighters are operating.

That’s hardly a sure-fire way of defeating stealth. But it’s not nothing.

SOURCES- National Interest, War is Boring

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