|
|
|
Subject:
Jed article
french stratege
8/6/2005 11:39:38 AM
|
| Journal of Electronic defense article
Fighter EW: The Next Generation
by Bill Sweetman
Jul. 1, 2000
The fourth Lockheed Martin F-22 Raptor, aircraft 4004, is due to make its first flight from Marietta, GA, in late July. As the first F-22 to carry offensive avionics, its task is to demonstrate that a stealthy aircraft can be a fighter. Under a deal struck with Congress last year, the F-22 has to prove this key technology by the end of this year if the next ten aircraft are to be authorized.
The F-22 represents a radical departure from the traditional approach to EW. Passive systems, once considered to be defensive in nature, are now critical to detecting, tracking and even attacking the target. The active radar, while still a primary sensor, is used sparingly for specific tasks. Active jamming in the traditional sense has disappeared. The F-22 approach is echoed to some extent in most of today's advanced fighter programs, including the Dassault Rafale, Eurofighter Typhoon and Saab JAS Gripen. It is also fundamental to the future of the Joint Strike Fighter (JSF).
The F-22's EW philosophy is rooted in some of the earliest work on stealth. As the US Air Force (USAF) defined requirements and operational doctrine for the F-117 stealth strike aircraft and B-2 bomber, in 1980-81, a "Red Team" headed by Dr. Paul Kaminski was charged with looking for weaknesses and vulnerabilities in stealth technology. One of the Red Team's most important conclusions was that a stealth aircraft could not survive by low radar cross-section (RCS) alone, but by stealth and tactics.
In the case of the F-117, the Red Team's recommendation resulted in the development of one of the first automated mission-planning systems, but this left the aircraft dependent on a pre-programmed flight plan. The B-2 was designed to feature a sophisticated defensive management system (DMS) which would allow the crew to respond to threat radars not anticipated by the mission plan. The initial DMS was abandoned in the late 1980s. Its successor is the APR-50, developed by IBM Federal Systems (later acquired by Loral and now part of Lockheed Martin).
The USAF's Advanced Tactical Fighter project, which led to the F-22, presented greater challenges. In the air-to-air regime, the primary threats are airborne and move rapidly, making identification, location and tracking more complex. The F-22's sustained speed also shortens engagement timelines by as much as 40 percent.
At the same time, the fighter's classic tool for situational awareness ? a powerful search radar ? can render its stealth characteristics moot. Low-probability-of-intercept (LPI) techniques are not very compatible with continuous searches over a large volume. The fighter's stealth is also of little use if it has to close to visual range in order to identify its targets. Passive search and track and non-cooperative target recognition (NCTR) are not luxuries for a stealthy air-superiority fighter.
The solution to this problem on the F-22 is sensor fusion. The principal sensors are the Northrop Grumman APG-77 radar and the Sanders ALR-94 passive receiver system. The fighter also has two datalink systems: one using the standard VHF/UHF radio frequencies and the other, the intra-flight datalink (IFDL), a low-power LPI link which connects two or more F-22s at close range. The sensors are apertures connected to the fighter's Common Integrated Processor (CIP) banks in the forward fuselage.
The data from the APG-77, ALR-94 and the datalinks are correlated according to their azimuth, elevation and range. Data is combined into a track file, and the final target picture is obtained by choosing the read-out from the most accurate sensor. For example, the passive system may provide the best azimuth data, while the radar produces the most accurate range.
CIP software controls the APG-77 according to emission-control principles. The radar's signals are managed in intensity, duration and space to maintain the pilot's situational awareness while minimizing the chance that its signals will be intercepted. More distant targets get less radar attention; as they get closer to the F-22, they will be identified and prioritized; and when they are close enough to be engaged or avoided, they are continuously tracked.
Sensor fusion and emission control are closely linked. The more the datalinks and ALR-94 can be used to build and update the tactical picture, the less the system needs to use the radar. The IFDL provides another layer of protection against tracking, because any one F-22 in a flight can provide radar data to the others.
The APG-77 and ALR-94 are unique, high-performance sensors. The APG-77 has an active, electronically scanned array (AESA) comprising some 1,200 transmitter and receiver modules. One vital difference between an AESA and any other radar that has a single transmitter (including a passive electronically steered array) is that the AESA is capable of oper |
| |
|
|
|
