Andhra politics

Agile radar beams
Active electronically scanned arrays energize fighter performance

By Michael Peck and Glenn W. Goodman Jr.
May 09, 2005

Source : http://www.isrjournal.com/story.php?F=750341

Advanced digital radar systems are giving new U.S. fighter planes the unprecendented ability to search for aircraft, track ground vehicles and map terrain simultaneously — at greater ranges and with better reliability than conventional systems.

Key to the capabilities in the new systems, called active electronically scanned array (AESA) radars, is an antenna consisting of 500 to 1,000 transmit/receive (T/R) modules, each the size of a candy bar or even smaller, instead of a central transmitter and receiver. Each module acts like a small individual radar.

Unlike a conventional mechanically steered array (MSA), the antenna array of T/R modules is fixed, with no moving parts. The radar can steer its agile beams electronically — at nearly the speed of light — and redirect them instantaneously from one target to another.

In MSA radars, a circular or elliptical antenna plate in the nose of the aircraft is moved rapidly using a gimbal system with three or four drive motors to scan an area of airspace or on the ground, a single flashlight-like beam at a time. AESA radars can track significantly more targets than current mechanical systems and can operate in multiple modes simultaneously, such as air-to-air search and ground mapping.

The first AESA radar that will be produced in large numbers for a fighter aircraft is Raytheon’s APG-79, slated to become operational on new U.S. Navy single-seat F/A-18E and two-seat F/A-18F Super Hornets in September 2006. It will offer up to three times the aerial target detection range and five times the reliability at only about 40 percent of the operating and support costs of Raytheon’s predecessor APG-73 radar on F/A-18C/D Hornets and those Super Hornets already produced, the company says. AESA radars also are being developed by Northrop Grumman Electronic Systems in Baltimore for the U.S. Air Force’s new F/A-22 Raptor and the planned Joint Strike Fighter.

PASSIVE VS. ACTIVE

Electronically scanned array antennas have been around since the 1950s in land-based and shipboard radar applications, but were slow to take hold in airborne applications due to volume and cost constraints.

Passive ESA radars, such as the U.S. Army’s Patriot and U.S. Navy’s Aegis, use a central transmitter and receiver like MSAs to feed their radiating elements, but steer the beam using an electronically controlled phase shifter placed immediately behind each radiating element. In the AESA radar, a small, low-power T/R module is placed immediately behind each radiating element, eliminating the central transmitter and receiver and the signal power losses that occur in the passive ESAs when the central transmitter distributes signals to the radiating elements and return signals are combined in analog form and sent to the central receiver. Historically, the central (traveling wave tube) transmitter and its high-voltage power supply have accounted for a large percentage of failures experienced in airborne radars.

Both passive and active ESAs offer higher reliability than MSAs because of their lack of moving parts and the fact that the phase shifters in the passive ESAs and the T/R modules in AESAs are inherently reliable. In addition, as many as 6 percent of the phase shifters or the T/R modules can fail without seriously impairing the radars’ overall performance. Both passive and active ESAs also offer more agile beam steering. For example, to jump the antenna beam from one target to another separated by 100 degrees, an MSA takes roughly a second. An ESA can do it in less than a millisecond. An AESA can even simultaneously radiate multiple, independently steered beams on different frequencies.

Aircraft that use passive ESA radars include the U.S. Air Force’s B-1B bomber and E-8 Joint Surveillance Target Attack Radar System and the French Rafale fighter.

Breakthroughs in T/R module manufacturing and miniaturization in recent years — Northrop Grumman and Raytheon use what they call sixth-generation T/R module technology — at last have made it feasible to fit large numbers of the modules in a lightweight AESA antenna in the nose of a fighter aircraft.

“To populate a radar with many hundreds of these T/R modules and getting them to act together is revolutionary. In fact, taking all the moving parts out of airborne radars is revolutionary,” said Scott Porter, director of aerospace business development at Northrop Grumman-Baltimore.

“For the same amount of real estate on an aircraft, especially fighters, you can cram a lot more of the T/R modules into an antenna and fill up more of the aircraft than you can with an MSA. Instead of one moving antenna with a transmitter black box behind it trying to pump out power, you now have many, many T/R modules mounted together in the same space all staring at the same place at the same time.”

AESA life-cycle costs are expected to be significantly lower than those of MSAs, Porter said, because their electronics will be more reliable and easier to fix than the moving parts in an MSA assembly. Indeed, Northrop Grumman is so confident in the reliability of the Joint Strike Fighter’s APG-81 radar that it may recommend that the nose radome be sealed. Though this would make it harder to repair the system, Northrop Grumman engineers say the radar will function properly for years, and that it could lose up to 6 percent of its T/R modules without affecting performance. “We don’t expect many radomes to be removed after our AESA radars are installed,” Porter said.

Similarly, Raytheon says the mean time between critical failures of its APG-79 radar going on the Navy’s Super Hornets is in excess of 15,000 hours of operation, and claims its AESA antenna might require no maintenance for 10 to 20 years.

FIRST OF THEIR KIND

The only operational fighter aircraft currently equipped with an AESA radar are 18 U.S. Air Force F-15Cs with the 3rd Fighter Wing at Elmendorf Air Force Base, Alaska. They have flown for about five years with the APG-63(V)2 developed by Raytheon Space and Airborne Systems, El Segundo, Calif. That radar, no more of which will be built, is a predecessor to the company’s more advanced Navy APG-79. It was an AESA antenna upgrade to the F-15’s APG-63(V)1 MSA radar designed to add a capability to target small cruise missile-size targets. The APG-63(V)1 MSA remains in full-rate production.

Raytheon has been developing a lighter-weight, more maintainable AESA radar for the Air Force’s other 161 F-15Cs — the APG-63(V)3 — and has built a prototype that will be tested by the service. However, due to budget constraints, those aircraft may never get the upgrade. The Air Force firmly plans to modernize the MSA radar on its 224 newer F-15E ground-attack models, beginning around 2010, likely with an APG-63(V)4 radar from Raytheon that will use the AESA antenna from the (V)3, as well as processors from the Navy’s APG-79.

Drawing on APG-79 technology, the (V)3 AESA uses more compact “tile” T/R modules compared with the (V)2’s larger “brick” modules. The tiles reduce the number of required T/R modules by a factor of four and the depth of the antenna array from nine inches to four inches, said Michael Henchey, Raytheon’s director of strategy and business development for Air Combat Avionics. They also reduce the weight of the array significantly.

Raytheon, following a year of flight testing, began delivering the first low-rate initial production versions of the APG-79 in January to F/A-18 manufacturer Boeing Integrated Defense Systems, St. Louis. The radar is a key element of Block II upgrades preplanned for the Navy’s Super Hornets, which became operational in 2001. The service will conduct operational testing of the LRIP radars on Super Hornets in October and November. Full-rate production of 415 APG-79s is scheduled to begin in 2007.

Bill Gardner, Raytheon’s APG-79 engineering, manufacturing and development program manager, said the radar will detect and track twice as many targets at greater distances than the APG-73, permitting the aircrew to “persistently observe targets and launch air-to-air missiles from their maximum range.” The radar system automatically establishes tracking files for each detected target, reducing pilot workload.

Another key feature of the APG-79 will be its ability to conduct air-to-air and air-to-ground operations essentially simultaneously because it can switch modes so rapidly. The pilot will be able to conduct ground mapping with the radar while it continues searching for and tracking aerial targets.

“With interleaved air-to-air and air-to-surface cockpit displays, the aircrew will be able to maintain situational awareness while executing air-to-surface missions,” Gardner said.

AESA radars also offer better air-to-ground resolution than MSA systems, particularly using their synthetic aperture radar (SAR) mode. As a March 2004 Government Accountability Office report stated, “The first F/A-18F with the AESA radar installed recently demonstrated high-resolution SAR modes at three times the resolution and 2½ times the range of the currently operationally deployed F/A-18 radar. This capability represents the first step in multiple areas that the AESA radar will greatly improve the F/A-18E/F Super Hornet’s air-to-air and air-to-ground radar capabilities in addition to adding modes not currently available to the fleet.”

OTHER AESA RADARS

Early this year, Northrop Grumman-Baltimore delivered the first APG-81 AESA radar for the Lockheed Martin F-35 Joint Strike Fighter. It is undergoing development flight tests onboard Northrop Grumman’s BAC 1-11 flying test-bed aircraft. Late this year, the radar will go to Lockheed Martin’s Mission Systems Integration Lab for testing to integrate it with the rest of the mission systems suite.

A joint venture of Northrop Grumman-Baltimore and Raytheon Network-Centric Systems, McKinney, Texas, has been developing the APG-77 AESA radar for the F/A-22 fighter for nearly 15 years. The radar flew on a preproduction aircraft for the first time in late 2000. Its T/R modules have been improved over time, and software allowing the radar to perform high-resolution mapping of ground targets is being added.

A fourth-generation variant of the APG-77, with design improvements adapted from the APG-81, flew for the first time last June. “We are inserting our fourth-generation AESA technology into the F/A-22’s Lot 5 of production, and that radar is in flight-test now,” Porter said. “So the F-35 and F/A-22 will have highly common radars at that point.” Pentagon officials approved the F/A-22 for full production at the end of March; the fighter will become operational in December.

AESA radars also have been in development for the three latest European fighters — Sweden’s JAS-39 Gripen, the Eurofighter Typhoon and France’s Rafale. The Gripen and the Eurofighter are equipped with mechanically steered array radars, and Rafale with a passive electronically scanned array, each of which features air-to-air and air-to-ground modes.

The Swedish Air Force’s Saab-built Gripen, also being acquired by South Africa, the Czech Republic and Hungary, became operational in 1997. It carries the PS-05/A radar from Ericsson Microwave Systems, which has been developing an AESA radar to potentially replace the PS-05/A.

Germany, the United Kingdom, Italy and Spain are the four Eurofighter partner countries. Development of the aircraft’s ECR-90 Captor radar by the EuroRadar consortium (BAE Systems, EADS, Spain’s INISEL and Italy’s FIAR) began in 1990; the radar entered production in 1998. Delivery of the first 148 Tranche (Lot) 1 production aircraft, begun in 2003, will be completed in 2007. Production of 236 improved Tranche 2 aircraft with upgraded computers was set to commence soon, and negotiations have been in progress among the partner countries to define the capabilities package for the 236-aircraft Tranche 3.

France’s Dassault is in series production of carrier-based Rafale M and air force Rafale B/C variants for the French military. They carry the RBE2 passive electronically scanning array radar developed by Thales DETEXIS.

In 1993, a BAE Systems-Thales-EADS consortium began development of a new AESA radar to replace the Eurofighter’s ECR-90 and the Rafale’s RBE2. Called the Airborne Multi-mode Solid-state Active array Radar , it could be ready for fielding on Tranche 3 Eurofighters and Rafales around 2010.

For electronics companies accustomed to building conventional MSA radars, fabricating AESA systems presents challenges. “We’ve built MSAs with the same building blocks for the past 30 or 40 years,” Porter said. “To design and manufacture an AESA radar is a totally different process.”

Porter said he foresees AESA systems being simpler to maintain in the future, with ground crews able to replace individual T/R modules without having to remove the entire radar assembly from the aircraft. He also said he sees aircraft radar evolving to the point where it is built directly into the skin of the aircraft.

“You can develop radar antenna arrays that can be structurally incorporated into the skin of an aircraft. However, such ‘smart skin’ would have to be an integral part of the design from the very beginning,” Porter said. “It would be tough to get all those T/R modules to stare at the same space. And remember that instead of the radar being nice and snug inside a radome, it would be exposed to the elements.” •

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