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F-22 RADAR RANGES PASS EXPECTATION
Testing of new and upgraded U.S. fighter aircraft shows that their next-generation radars are performing even better than required by the Air Force—offering an average of about 15% more range than specified.
The additional radar performance has triggered an unexpected batde in the Air Force and aerospace industry. On one side are those who believe the Boein'g F-15 can soldier on in the air superiority role for many more years with the new long-range radars and proposed long-range air-to-air missiles; on the other side are those eager to field the Lockheed Martin F-22 force before budget cutting reduces the program size any further.
The catalyst in this confrontation is the active electronically scanned array (AESA), basically the front-end of the new radars, which use thousands of elements (each an active transmitter/receiver module) to form an antenna that can simultaneously communicate, jam and search for targets with more than twice the acuity of the radars they replace.
Air Force and Navy requirements called for AESA radars that could reach out 70-120 naut. mi. against small 1-meter-size targets. The radars range depends on the size of the antenna array on a given type of aircraft. But the unexpected dividendput the range of an upgraded F-15's radar at 105-110 mi. instead of the expected 90 mi. or less. The argument of advocates then became, why buy more F-22s with a 120-mi.-range radar when updated F-15s cost a fraction of the price. Lockheed Martin's rebuttal lies deep in the "black world" of classified
programs. "While it's true that the F-22 AESA radars typical operating range is 120 mi. or less, that distance is driven by the need to lower or derate its power output. This reduction is necessary to maintain a low-probability-of-intercept (LPI) capability that is essential to match its performance to stealth operations. By limiting the strength of signal output, dividing the signal into several frequency bands, frequency hopping and limiting the radar's field of view, the F-22 should be able to avoid detection even when using its radar. However, Lockheed Martin contends the Northrop Grum-man-built radar, when operated normally without LPI restrictions, as the F-l 5 would be, has a range of 140-145 naut. mi. The radar ranges of AESA-equipped aircraft stair-step down from a specified 90 mi. for the Joint Strike Fighter, through the F-15 and F/A-18E, to a projected 70 mi. for the Block 60 F-16. All ranges are predicated against a 1-meter-square target. For a standard, non-AESA-equipped F-15, the range against a 1-meter target is only 60 naut. mi.
At maximum radar range, the F-22 is specified to have a 86% probability of detection of a 1-meter target with a single radar hit. A 1-meter target would be about the size of a cruise missile or a fighter with some stealth treatment and no external weaponry. In addition, the signal characteristics of the beam allow the radar to pick out features from the return that provide more of an image than previous radars. Specialists say you can't get target identification at maximum range, but you can start the interrogation process and begin gathering target data.
"For surveillance, F-22 pilots will use the radar's air-to-air moving target indicator [MTI] mode," the Lockheed Martin official said. "That just gives you a blob. For target recognition, you have the spot mode or ultrahigh-resolution [UHR] capability, which usually only requires a sweep or two [by the radar]."
MTI AND UHR ranges are similar, although UHR power requirements are slighdy less. More importantly, the pilot has several radar beams for target identification, which means the system can track some targets while still scanning for others. The number of targets that can be tracked while scanning is more than 100.
The UHR mode has the promise of detecting aircraft types just from the radar signal and not from any other aircraft emissions, such as the radar or identifica-tion-friend-or-foe signal. UHR provides image quality of about 1 ft. at more rhan
100 mi. If the target is 30 yards long, the F-22 will have about 100 image cells or pixels to work with in determining a target's size and shape. .
"It's almost like low-wave infrared," the official said. "You're getting back a spectral return that is characteristic of an aircraft type, depending on the aspect angle. You have to know that, but it can be figured out with only a few hits of the radar. Once you know what direction the threat is coming from [the threat track], you know the aspect angle, and the computer compares the signal to spectral return models of various aircraft until it gets a match."
Whenever foreign aircraft appear at U.S. air shows or participate in military operations like Red Flag, the U.S. is gathering intelligence on the aircraft. "When there are MiG-29s, Su-35s, Mirages, Rafales or Eurofighters flying at air shows, intelligence officers are present measuring those aircraft with radars to build a catalog of information. With a good library of aircraft radar signal characteristics, computer specialists can build the processing into modern radars that permit identification.
Adding to radar capabilities are optical systems and passive electronic intelligence gathering and electronic support measures.
In all, the F-22 has about 2-GHz. bandwidth in forward-looking, high-gain passive listening modes. If an enemy aircraft is sending signals co the ground or an AWACS, the signal can be detected by the F-22 at very long ranges.
"In addition to a huge radar detection and tracking capability, the F-22's radar is electronically scanned, which gives it a tremendous antijam and electronic coun-ter-countermeasures capability and provides enough agility for pulse-to-pulse frequency changes," a Lockheed Martin official said. "Because of the large number [about 2,000] of active transmitter/receiver modules, you can do all sorts of tricky things with the beam to avoid [enemy] jamming in either your main or side lobes."
"GIVEN THE INHERENT design of the [AESA] radar, the F-22 has a tremendous capability for electronic-intelligence gathering," he said. "So, if you could capture those data on a recording, you could build your own electronic order of battle with F-22s. Because they are so close to the threat radars, it could be better than a [RC-135 Rivet Joint] or a [ferret] satellite in gathering detailed information. We could exploit that in tremendous detail just by reading out the F-22's data files."