Nobody knows exactly where “Rat 55" lives or precisely what technology it uses to accomplish its mission, but it sure is an exotic beast. The USAF’s secretive NT-43A testbed is a highly modified 737-200 that goes far beyond ground-based radar and infrared signature testing on stealthy aircraft by plying its trade high up in the sky in their target aircraft’s natural operating environment.

The aircraft’s massive radomes and structural modification were said to have been designed by Lockheed’s Skunk Works and were installed by the Goodyear Aerospace plant in Arizona around the turn of the century. Before that, USAF serial number 73-1155 lived its life as a humble T-43A navigational training aircraft from 1974 to 1997. It was then retired to the boneyard and stored there for over two years before being chosen for its new unique role. The aircraft’s first flight in its monstrous NT-43A form was on March 21, 2001.

According to Globalsecurity.com, the NT-43A’s genesis may have deep roots dating back to the origins of stealth technology itself. It seems that the company behind the project has direct links to the creator of the famous Skunk Works’ ECHO-1 software program that resulted in the stealthy ‘Hopeless Diamond’ which eventually turned into the Have Blue demonstrator and then the famous F-117A Nighthawk:

In mid-2000 the Lockheed Martin Advanced Prototype Center, part of the Advanced Development Programs’ (ADP) organization, handed over its first major deliverable — a flight-worthy composite structure — on the center’s first design and fabrication contract. The structure, a 19-foot long, 6.2-foot diameter fairing and bulkhead assembly, was delivered to customer DENMAR Inc. The prime contractor Denmar is a company specializing in stealth technology. The “Den” stands for President Denys Overholser, the former Skunk Works engineer credited with devising the shape of the first stealth aircraft. The assembly provides an aerodynamic transition from the existing aft fuselage of a T-43 aircraft, a modified Boeing 737-200, to an oversized radome. The aircraft serves as a Radar Test Bed (RTB) for future Air Force programs. The design, fabrication and machining of the structure’s components were all performed at Palmdale. The structure is made of a 90-percent carbon epoxy/honeycomb sandwich material, with machined aluminum parts, and houses an airborne radar assembly. The contract with DENMAR also included the design and fabrication of three radomes for the RTB aircraft. Each radome is more than 6.5 feet in diameter and 9 feet in length.

The NT-43A radar aircraft testbed (probably where the “Rat” call-sign comes from) is most likely the last stop in advanced low-signature testing and validation for new stealthy aircraft, new radar absorbent coatings and heat mitigation technologies, as well as exotic and stealthy structural modifications to existing aircraft. In other words, whether it be a whole new aircraft or just a new radar absorbing material applied to an existing stealthy aircraft, the NT-43A evaluates it under realistic flight conditions with its powerful radars and infrared energy detecting devices. Even repairs and depot-level work on existing stealth aircraft may be validated in flight via the NT-43A.

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The NT-43A’s massive radars are most likely able to take incredibly detailed synthetic aperture ‘pictures’ of their target aircraft, as well as collecting sensitive ‘raw’ radar measurements and associated data. Its front and rear pods that give the jet such an ungainly appearance allow it to collect radar and infrared data (the IR sensors sit atop the radar fairings) from every angle around the target aircraft in flight without having to fly at oblique angles towards and away from it. The radar system may have some bistatic capabilities when both front and back arrays are used in conjunction with one another.

Beyond evaluating a design’s low observable qualities in flight, data collected by “Rat 55" is most likely also used to better understand a stealthy aircraft’s strengths and weaknesses. For instance, an aircraft may have very little radar signature head-on, but at certain angles its radar cross section grows significantly. Once these intricacies are very well documented, the performance of enemy radars, infrared search and track systems and anti-air missile systems can be better theorized. This allows for a much more accurate understanding of how close a ‘stealth’ aircraft can get to different threats without being detected or without being engaged. Mission planners can then build elaborate routes around known enemy air defense systems and give crews good advice on how to present the best aspect of their aircraft to unplanned for ‘pop-up’ threats so that they have the best chance of surviving a mission. This route of lowest possible detection and greatest possible survival is known as the ‘Blue Line’ among stealth aircraft attack crews.

It is amazing that in almost 15 years of continuous operation, so few pictures exist of Rat 55. The few grainy side-on images that have come out in recent years show additional modifications to the already bulging 737, including a new large dome fixture located on the aircraft’s upper spine.

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The vast majority of pictures that do exist of the NT-43A are taken from thousands of feet below as the ungainly jet plies its trade in formation with various aircraft over and around Death Valley. The image to the left, taken by photographer Jeff Swearingen on February 24th, 2009 is of one of those flights (check out more of Jeff’s awesome photo work here).

Death Valley in particular provides a natural radar testing laboratory of grand proportions. Its very low elevation, sparse population and its buffer of mountain ranges on all sides add up to make it ideal for all types of radar-based testing and evaluating. This, along with it being at the very center of America’s flight test world, make it an ideal testing environment for the NT-43A.

Aside from the historic flight test corridors and aviation mecca over and around the Mojave Desert and Death Valley, the NT-43A also seems to frequent the incredibly desolate Tonopah Test and Training Range airport. Located towards the northern edge of the Nevada Test and Training Range (NTTR), the shadowy base that once housed an entire fleet of then top-secret F-117 Nighthawks, a full stable of Soviet-built fighter aircraft and now the RQ-170 Sentinel drone, along with who on earth knows what else, is rumored to be the home of the NT-43A.

Rat 55 seen taxing at Ton0pah Test Range Airport recently. The NT-43A has been spotted at Tonopah regularly and it is very possible that there or Area 51 is the jet’s secretive home base.

Groom Lake/Area 51 is also another possible candidate for being a permanent home of the NT-43A, although space is rumored to be at a premium when it comes to parking aircraft at the top-secret installation. Area 51 hosts a similar ground-based system used for measuring and evaluating a stealthy aircraft’s radar signature as it flies through the air. This system of large radar dishes is known as the Dynamic Coherent Measurement System, or DYCOMS for short. Obviously, the NT-43A has a great advantage over DYCOMS in that it can stay in tight formation with its target aircraft, instead of constantly moving toward or away from it at much larger distances. Additionally, it can take measurements from above even large aircraft, which a ground-based system cannot.

The NT-43A is said to be flown by Pentagon super-contractor EG&G, the same group that provides many services to places like Area 51 as well as non-classified sites around and outside of the Nevada Test and Training Range. EG&G is probably most famous for operating ‘Janet Airlines’ that ferries contractors into secretive and non-secretive installations around the American Southwest, including Area 51 and the Tonopah Test Range Airport. The group operates a formal high-security terminal at Las Vegas McCarran International Airport for a good majority of its flight activities and its aircraft are easily recognizable in their all white paint with a single red stripe down their sides. Before upgrading to 737-600 models in the last few years, Janet 737s were older -200 models of which the NT-43A is based. In fact, some of the older Janet jets were converted T-43As themselves and are now stored at Davis Monthan Air Force base’s sprawling boneyard.

A Janet Airlines 737 blasts out of Las Vegas on another trip, most likely to Area 51 or Tonopah Test Range Airport just hundreds of miles to the north. EG&G, the contractor who operates the Janet network is said to also operate the USAF’s only NT-43A radar signature testbed.

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So if you ever find yourself in Death Valley, the Mojave Desert, or the expansive federal lands of southern Nevada and look up to see what looks like a flying Tylenol capsule, pinch yourself because you have witnessed a rare sight indeed. What is even more exciting is that there is probably an even crazier looking aircraft very close by as Rat 55 seems to deal only in close proximity to aircraft of the most exotic and advanced nature.

And here’s to the men and women who actually operate this thing. Their work leads to lower detectability which results in better survivability and greater lethality for America’s most deadly combat aircraft, some of which we have not even met yet.

Not bad for 40 year old jet with a bad nose job and no official place to call home!

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Tyler Rogoway is a defense journalist and photographer who maintains the website Foxtrot Alpha for Jalopnik.com You can reach Tyler with story ideas or direct comments regarding this or any other defense topic via the email address Tyler@Jalopnik.com