"Within the scope of some projects carried out in coordination with the Presidency of Defense Industries, F-16 aircraft and other aircraft belonging to the [Turkish] Air Force will carry out low and high altitude test flights on Monday and Tuesday in the skies of Ankara," an official statement from Governorate said. There are no details yet on the exact test objectives. However, video footage of the tests so far show F-16s and F-4s flying over Murted and examples of the 91N6E surveillance and acquisition radar and the 96L6E air search and acquisition radar, the latter elevated on an 40V6M mast, clearly in operation below. The mast-mounted version of the 96L6E is also designed to be better able to detect low-flying targets that a radar positioned right on the ground might not be able to detect through the surface clutter.

These initial tests may just be to ensure that the radars are working properly or to see if they can adequately distinguish Turkish aircraft as friendlies. U.S. military officials, as well as those from other NATO members, have previously warned that the S-400 does not meet the alliance's interoperability requirements and therefore would not be able to be integrated with other allied air defense networks during an actual crisis. This, in turn, creates the risk that the Russian surface-to-air missile system might not be able to properly tell friendly and hostile aircraft apart when it matters most.

RIA Novosti A diagram showing various typical S-400 battalion components, including the 91N6E and 96L6E radars, as well as the 40V6M mast system.

Whether or not these tests, or additional evaluations in the future, are collecting data on how jets, such a the F-16, appear on the S-400's radars is unclear. The 92N6E fire control radar is notably absent from the pictures and video of the tests, as well. The United States had feared that the air defense system could gather details about the F-35's stealth signature, or other information about that aircraft's capabilities, which Russian technicians assisting Turkey could then have potentially relayed back to the Kremlin. The U.S. government's position has been and remains that Turkey can have either the S-400 or the F-35, but not both.

Turkish Ministry of Defense A Turkish Air Force Block 50 F-16C Viper. At least one of these aircraft took part in the tests of the S-400 radars.

Whatever the case, the testing of the S-400 radars, and doing so with American-made aircraft, sends a defiant message to the United States and makes clear that Turkey, and its President Recep Tayyip Erdogan, has no intention of giving up the air defense system, at least any time soon. It also dares U.S. President Donald Trump and his administration, as well as Congress, to act in regards to sanctions under the Countering America's Adversaries Through Sanctions Act, or CAASTA. Unless Turkey were to get a waiver, this law requires the U.S. government to take significant action against its Turkish counterparts over the purchase of the Russian-made system. However, the White House and members of Congress have deferred on this repeatedly, suggesting they would only act if Turkey "activated" the S-400's radars. This was clearly meant to provide room for Ankara to reverse course and abandon the purchase, keeping open a potential path for the country to rejoin the F-35 program. "Turkey’s acquisition of sophisticated Russian military equipment, such as the S-400, creates some very serious challenges for us, and we are talking about it constantly," Trump had said alongside Erdogan during the latter's visit to Washington, D.C. earlier in November. "We talked about it today. We’re talking about it in the future. Hopefully, we’ll be able to resolve that situation."