Radar

When asked, the Air Force spokesperson said they could not provide any additional information about how this unique aircraft might be utilized in the future. TWZ has also reached out to NASA to ask whether it has any further plans for this jet.

To be clear, getting a statement about RAT55 like the one above is already highly unusual. The aircraft is often referred to as the most secretive Boeing 737 in the world. It is understood to be based at the Tonopah Test Range Airport (TTR) in Nevada, a remote and highly secure facility long used to conceal shadowy aircraft programs. When the NT-43A has been seen, it has usually been from afar, as it has flown around the U.S. military’s flight testing hubs at Area 51, also in Nevada, and Edwards Air Force Base in neighboring California. The jet has only very rarely appeared anywhere else.

Still, it has become instantly recognizable from its heavily modified nose and huge radome protruding from the rear of the fuselage behind the tail. RAT55 is festooned with other bits and bumps to support its primary mission of signature measurement in support of stealthy aircraft programs. The two huge radar arrays at the front and back of the aircraft allow it to precisely measure the radar signatures of stealthy aircraft flying nearby. This information is used to validate low-observable (stealthy) coatings and other design elements. Electro-optical and infrared sensors are also fitted above the two main radomes. Being able to collect signature data mid-air offers advantages over doing so on the ground, since the subject can be continually observed from all angles, including overhead. You can read more about what is known of the NT-43A’s capabilities and role here.

TWZ has been talking for years about the prospect that RAT55, which is now more than five decades old, could just be getting closer to being retired entirely. The jet is a heavily modified conversion of a T-43A trainer aircraft, which is itself a militarized version of the Boeing 737-200 airliner. The Air Force retired the last of its standard T-43As in 2010. Usage of first-generation 737s in any configuration is dwindling globally, and the remaining examples will only become ever-more challenging to support.

With all this in mind, TWZ has also been watching closely for a replacement for RAT55 to appear. However, to date, no explicit successor to the NT-43A has definitely emerged.

In the meantime, NT-43A could now end up being a key aircraft for support space launches and recoveries. It could also perform other testing and development duties unrelated to space launches and low-observable capabilities. If it becomes available to more customers, its unique services could be in high demand, especially with a new stealth boom on the horizon with new fleets of Collaborative Combat Aircraft drones.

If RAT55 does now come more into the light after decades of largely being a ‘ghost’ within the U.S. military flight test community, we may start to learn more about the aircraft, in general, as well as what the future holds for it and any potential successors.

Contact the author: joe@twz.com

Stockholm’s transfer of two Saab 340 AEW&C aircraft to Ukraine was announced in May 2024, as we reported at the time. The donation was part of the biggest Swedish military aid package for Ukraine up to that point, worth around $1.25 billion.

At the time, it was said that it would take around a year to train the aircrew and maintenance personnel, as well as prepare the ground facilities for the new aircraft.

The Saab 340 AEW&C aircraft is known in Swedish military service as the ASC 890 and alternatively as the S 100D Argus. The Swedish Air Force operated two of these aircraft, with both understood to have been provided to Kyiv. Another two ex-Swedish Saab 340 AEW&C aircraft are operated by Thailand, and another pair were sold to Poland, having previously been operated by the United Arab Emirates.

Royal Thai Air Force Gripen and Saab 340 Erieye AEW

For Ukraine, the significance of the Saab 340 AEW&C is hard to overstate.

It brings an entirely new capability for the Ukrainian Air Force, which has never operated any type of AEW&C platform.

The core of the aircraft is its Saab Erieye active electronically scanned array (AESA) radar. Carried atop the fuselage, this can detect air and sea targets at ranges of up to around 280 miles, with the aircraft typically operating at an altitude of 20,000 feet. Reportedly, the AESA system can track up to 1,000 airborne and 500 surface targets simultaneously.

Erieye explained

All this data is handled by three mission crew: a mission control officer, a combat control operator, and a surveillance operator. It can also be downlinked to ground stations and to other aircraft, at least in theory, of which more later.

Unlike a surface-based air defense radar, the Erieye provides a ‘lookdown’ capability. Without the line-of-sight limitations of terrain, the radar will be especially useful for detecting low-flying Russian drones and cruise missiles. These are otherwise notably tricky targets, due to the low altitudes at which they fly and their small radar signatures. With that in mind, the aircraft should provide Ukraine’s air defenses with a huge situational-awareness boost.

Within its layered air defense network, Ukraine relies heavily on crewed fighter jets (as well as other aircraft platforms) to intercept drones and cruise missiles. Adding the Saab 340 AEW&C to the equation, the radar plane should be able to work as a fighter controller, detecting targets, prioritizing them, and then assigning them to the fighters for interception. Equally, this data could be provided to other air defense assets.

Especially useful in this regard would be the NATO-standard Link 16 datalink communications system. This would, in theory, be compatible with Ukraine’s F-16 and Mirage 2000 fighters, as well as Western-supplied ground-based air defense systems.

Український пілот про ефективність Mirage2000/Ukrainian Pilot on the Effectiveness of the Mirage2000

However, in late 2024, it was reported that the F-16s donated to Ukraine have had Link 16 systems removed or disabled, due to U.S. worries that they could end up in Russian hands.

This would deprive those fighters of a real-time air defense ‘picture’ sourced from the Erieye radar, and, at this point, it’s unclear if the Link 16 connectivity is available to Ukraine.

As of March 2025, it was reported that the delivery of the radar planes was on track, and they would be able to operate effectively with Ukraine’s F-16s.

“The timing of ASC 890 deliveries is linked to when certain modifications to F-16 fighters will be ready. There is no delay in the transfer of airborne early warning aircraft to Ukraine,” the Lithuanian Delfi news agency reported. This sounds very much like a reference to enabling the Link 16 connectivity.

However, even without Link 16, the Saab 340 AEW&C is able to provide much-enhanced air and sea surveillance for Ukraine.

Later versions of the Erieye radar also have a synthetic aperture radar and ground moving target indication (SAR/GMTI) capability, although it’s not clear if Ukraine has received that either. While SAR provides detailed image-like mapping of the ground at standoff ranges, GMTI detects and tracks movements on the ground over time, which would allow monitoring of Russian troop movements.

Should Ukraine’s ambitious plans to buy Saab Gripen fighters in the future materialize, the combination of these jets and the Saab 340 AEW&C could make a tailor-made solution for air defense and other missions.

For all its capabilities, the Saab 340 AEW&C will also be a prime target for Russia.

Russia’s own A-50 Mainstay AEW&C aircraft have been repeatedly targeted by Ukraine, in recognition of their value as force multipliers.

Russia was estimated to have nine A-50s in active service at the start of the full-scale invasion. Since then, there have been two combat losses, and another of these aircraft was damaged in a drone attack while on the ground at a base in Belarus, and its current status is unknown. More recently, the General Staff of the Ukrainian Armed Forces claimed an A-50 had been hit in a strike on a maintenance facility in the Novgorod region of Russia.