The U.S. Navy is introducing an upgraded version (Block 2) of its IRST21 (Infrared Search and Track) long-range heat sensing device. The original (Block 1) IRST21 looked good on paper but insufficient testing during development resulted in an IRST that pilots found less useful than advertised when put to work in the air under realistic combat conditions. IRST21 was described as a major upgrade of IRSTs developed during the 1990s for the F-14. The upgrade fell short in practice and Block 2 is an upgrade of the original 1990s IRST design upgrade. To make matters worse European (including Russian) air forces have been using a more effective (and recent) QWIP IRST technology. American and European engineers have spirited debates about which tech is actually superior. American pilots believe IRST21 Block 1 is not effective and reliable enough to succeed in air-to-air combat. Block 2 is supposed to make it all better but pilots have not been able to give Block 2 a reality test. Meanwhile European pilots describe their QWIP IRST systems as quite satisfactory, providing a reliable passive (no electronic signals) sensor with a range of up to 80 kilometers.
IRST21 Block 1 was available in early 2014 and the U.S. Navy carried out its first flight tests. Like older IRST systems IRST21 uses a high-resolution infrared (heat sensing) sensor to act like a radar to positively spot and identify a potential aerial target. This is done by comparing what IRST sees to 3-D models of known aircraft stored in the pod computer. Most IRST21s are mounted in the ATFLIR (Advanced Targeting Forward Looking Infrared) pods used to spot surface targets. IRST21 was a notable improvement in IRST capabilities building on the IRST system used successfully in the retired F-14s. The major advance in IRST21 was that it was accurate enough at long range to not only identify the aircraft type but provide targeting information for long-range radar homing missiles (like AMRAAM) carried on American fighters. These missiles have their own short-range radars built in but until ITST21 came along required the firing aircraft to turn on its targeting radar to get the location of the target before launching the long-range missile. While launching the missile gives away your position you still get in the first shot which in air combat is the single most important factor in defeating an opponent.
Back 2014 IRST Block 2 was already under development. One new Block 2 feature was that it allowed IRST21 equipped aircraft to identify, track and attack multiple targets with missiles. In practice that often did not work with Block 1 and now there are doubts that performance will be improved sufficiently with Block 2.
FLIR (Forward Looking Infrared Radar) has been around since the 1980s, and as the technology became more powerful, it became possible to spot and identify targets at longer ranges. IRST has been around since the 1960s but it took half a century to develop an IRST technology that was competitive with radar. The current navy targeting pod, ATFLIR, includes the IRST21 and is supposed to get Block 2. Before IRST21 the IRST used could identify targets from 20-30 kilometers away, and the latest IRST systems (IRST21) have an even longer range. It eventually became possible to use FLIR, in the form of IRST, as a sensor for spotting aircraft. This was a big advantage because FLIR/IRST is passive. It doesn't broadcast, like normal radar, thus the target cannot detect those radar transmissions and be alerted that it is being "painted" by hostile radar. When used by a stealthy aircraft (as F-22 and F-35 do) this is a lethal combination.
The navy was playing catch-up by equipping some of its F-18E fighters with the new generation of IRST. Russian and European fighters (MiG-29, Su-30, Eurofighter, Rafale) have long had IRST and were introducing the more effective QWIP IRST tech. For the Russians, this was seen as a way to deal with stealthy American aircraft. The U.S. stealth warplanes were built to defeat radar. But these stealthy aircraft still gave off heat and IRST works by seeking out heat. The navy already has some aircraft equipped with an IRST pod based on the technology used in the older F-14D IRST. But newer IRST21 technology was pitched as being much more effective, giving warnings (that something is out there) at long range (several hundred kilometers) and accurate targeting data at about 50 kilometers. The new generation IRST21 was also able to spot targets on the ground or at sea. The problem is that these contacts aren’t persistent or reliable enough to substitute for using a radar. The earliest (1960s) IRST tech only claimed to warn the user that something was out there. Those early IRSTs could only do that at short (under 20 kilometers) ranges. That “something out there” range has greatly increased, to over a hundred kilometers. But now pilots want to use IRST to accurately locate targets for their long-range missiles. For IRST21 Block 1 that has proved to be more of a work-in-progress.
The older IRST had acknowledged limitations. The main ones are the range. The early IRST tech eventually delivered up to about 30 kilometers for accurate detection but much farther for "something is there." There were still problems with clouds distorting the heat signature of the target. The short range means that another aircraft using its radar (which has a range of over 100 kilometers for precise identification) has an obvious edge. The distortion problems were slowly being solved by improved computer analysis of the detected image. Since many warplanes like to operate "quiet" (without any electronic transmissions), IRST becomes the best way to spot the other guy and open fire first. At longer ranges, IRST still gave pretty vague data. It was found that just having an indication that someone is out there, more than a hundred kilometers away, gave you an edge.
The U.S. Air Force was always not as enthusiastic about IRST for the F-15C. In 2011, as an economy move, and because of unspecified "technical problems", the U.S. Air Force dropped all efforts to equip any of its F-15C fighters with IRST. This included an effort, begun in 2009, to equip a hundred F-15Cs with heat-sensing pods once used to equip navy F-14Ds (which were retired in 2006). The refurbished navy IRST pods would have enabled the F-15s to detect and track aircraft, over a hundred kilometers away from the heat the target aircraft give off. IRST is a passive (it does not broadcast) sensor, and thus it is undetectable by the enemy. It was known that the tech in the F-14 pods was being upgraded as IRST21 but the air force wanted to see just how effective the new tech was before placing orders.