By Eric Hehs Posted 23 August 2010

The southernmost run station on the Lockheed Martin side of the runway at NAS Fort Worth JRB contains an aircraft that is one part airliner and one part advanced fighter. This curious combination is a Boeing 737-300 transformed into a flying laboratory for the F-35 program.

This aircraft, known as the Cooperative Avionics Test Bed (aka CATB or CATBird), was built in 1986. The aircraft, formerly operated by Indonesian Airlines, now sports a bright red vertical tail with CATB in white block letters. Instead of commercial passengers, the 737 now transports a team of test engineers. It serves as a flying laboratory for developing and demonstrating avionics for the F-35 Lightning II—in particular, avionics related to mission systems.

“Mission systems are best viewed as systems that enhance the combat effectiveness of the aircraft,” notes Tony Nigro, deputy manager for the CATBird. Nigro has been associated with the CATBird since 2005. “This aircraft is all about reducing risk for the program. We want to uncover and resolve issues with the mission systems software as early as we can and before they reach the F-35 fleet. CATBird allows us to test mission systems in a dynamic environment against both fixed and moving targets. We can simulate situations in this aircraft that we can’t simulate in a ground laboratory.”

Mission systems provide the pilot with situational awareness – through the processing, fusion, and display of data from both onboard and off-board sources. The advanced sensor suite on the F-35 collects vast amounts of information that is processed and presented to the pilot through the large color flat panel displays in the cockpit. Certain flight-critical and tactical information is also projected onto the helmet-mounted display. The overall process allows pilots to make faster and more effective tactical decisions. Pilots can also transfer sensor information to other aircraft as well as to maritime and ground forces.

Mission systems-related hardware on the F-35 includes the APG-81 active electronically scanned array radar; electronic warfare; an integrated communications, navigation, and identification system; the integrated core processor; the electro-optical targeting sensor; the electro-optical distributed aperture system; and the pilot’s helmet-mounted display.

Not coincidentally, this same hardware is found externally and internally on the CATBird. In fact, the most noticeable feature from the outside of the flying laboratory is the F-35 integrated forebody, or nose section, mounted on the front of the 737. The nose contains the active electronically scanned array radar and the front top sensor and two side sensors for the distributed aperture system, or DAS. Designated the AN/AAQ-37, the DAS, consists of six electro-optical sensors placed at various locations on the outside of the aircraft. The system, developed by Northrop Grumman, provides situational awareness in a 360-degree spherical field around the aircraft. As such, it warns the pilot of incoming aircraft and missile threats and provides day/night vision, fire control capability, and precision tracking of wingmen and friendly aircraft for tactical maneuvering.

Moving back from the nose of the CATBird to just behind the cockpit, the sides of the forward fuselage sprout a pair of fixed canards. The nose-to-canard spacing is geometrically identical to the nose-to-wing spacing of an actual F-35. The canards contain forward-facing sensors that are part of the electronic warfare system.

A twenty-seven-foot spine, running on top of the fuselage, contains two more of the six total DAS sensors; the global positioning system receiver; antennas associated with UHF, VHF, and satellite communications; and several datalinks, including assemblies and interface units associated with the F-35’s multifunction advanced datalink. The MADL, as this sophisticated datalink is called, allows the aircraft to communicate within and between flights to share a common view of the battle space.

A structure on the underside of the fuselage, called a canoe, contains additional MADL hardware, another UHF antenna, and the remaining two DAS sensors. Two smaller wing-like surfaces, called strakes, protrude from the fuselage between the main 737 wing and the empennage. These structures contain rear-facing sensors that are part of the electronic warfare system as well as part of the F-35 radar altimeters. Less visible appendages are two tail cones of the 737 engine fairings. These fairings contain two more aft-facing electronic warfare sensors.

The flight deck of the 737-300 was left mostly intact with the exception of the addition of an electronic flight bag, which provides time/space position information to ground users via a datalink. The flight bag, a common upgrade for commercial 737s, consists of a color moving map that aids navigation and a system that provides satellite weather downloads, flight charts, manuals, and other files for the pilots.

A large area just behind the cabin is devoted to hardware racks. A high-fidelity F-35 cockpit sits to the right rear section of these racks. Twenty workstations fill the back half of the aircraft. The rear lavatories were removed to make room for additional hardware racks and storage. The aircraft retained the forward lavatory and the galley area.

Minimum pilot crew consists of one pilot and one co-pilot. All four CATBird pilots are Lockheed Martin Aeronautics employees.

The CATBird engineering team spent a lot of time refining the external design additions of the aircraft to ensure that the aircraft retained the standard flying qualities of a commercial airliner. After the 737 was modified, a short flying qualities test program was flown in the spring of 2007 to validate that the CATBird retained those flying qualities.

The pilots fly with flight test engineers involved in the development and testing of the mission avionics. The crew also has a designated test director and a test conductor as well as personnel who monitor the network and instrumentation.

The typical duration for a Mission Systems test flight is 2.5 hours, with the longest Mission Systems flights lasting slightly more than four hours. All three of the four-hour-plus flights were flown on the CATBird’s August 2010 deployment to Edwards Air Force Flight Test Center in California. The CATBird has the ability to fly four-hour missions routinely.

“The most complex mission systems testing we perform involves multiship, air-to-air fusion scenarios,” explains says Ron Kolber, lead for the CATBird mission systems. “These missions require a tremendous amount of coordination and logistics. They are also some of the most challenging for the F-35 systems.

“Sensor fusion testing is a key advantage CATBird brings to the table,” continues Kolber. “While the static infrastructure of fusion software can be tested in ground labs, navigation systems and various sensors require moving inputs to test the fusion algorithms.”

CATBird testing will follow a block buildup for mission systems software through the current System Development and Demonstration phase of the program, which finishes with Block 3 software. Given its unique capability, CATBird can also function as an additional static test laboratory when it is on the ground.

First flight of the modified aircraft occurred at Mojave Air and Space Port in Mojave, California, on 23 January 2007. As of August 2010, it has completed more than 130 flights, most of which have been in direct support of mission systems testing for the F-35.

A majority of the test flights are from the aircraft’s home base in Texas. “In addition to Edwards AFB, we have also visited Eglin AFB in Florida,” notes Bruce Patton, flight test lead for CATBird. “At these government ranges, we can test the F-35 mission systems against high-fidelity threat emitters and various airborne targets. We have also conducted low-level radar altimeter and navigation testing in the mountains near Holloman AFB in New Mexico.”

CATBird has flown against one and two air adversaries so far. The targets, both in the air and on the ground, will increase in number and complexity as the software blocks progress.

The F-35 mission systems are maturing at a rapid pace. CATBird is currently being used to test the Block 1 avionics hardware and software. It has successfully demonstrated air-to-air and air-to-ground target detection and tracking with the radar, the electronic warfare system, and the electro-optical targeting system, both independently and cooperatively with sensor fusion. It has also demonstrated synthetic aperture radar mapping, using that capability successfully to target Joint Direct Attack Munition and GBU-12 guided weapons.

“Even in these early software releases, the F-35 mission systems avionics have proven to be very capable,” concludes Patton. “The capabilities we are refining with CATBird will provide the warfighter with a weapon system that is second to none.”

Eric Hehs is the editor of Code One.
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