The American F-15C fighter finally got a new IRST (Infra-Red Search & Track) capability when the first Legion IRST Pods were delivered in 2018. The 250 kg (550 pound) Legion Pod is another version of the existing IRST21, which is built into the forward section of centerline fuel tanks on F-18E aircraft. Legion is the same tech but in a separate pod. In early 2019 the air force ordered the Block II version of the Legion pod, which incorporates new features being added to IRST21. Exact capabilities of new IRST systems are kept secret for as long as possible because once potential competitors know the operational capabilities of IRST they can better develop countermeasures. Like radar, IRST is less accurate at longer distances. The major innovation of recent IRST systems like IRST21 is the use of more efficient software on faster computers that analyze distant and indistinct images and accurately identifies aircraft type and where it is headed and at what speed. This information goes to the aircraft fire control system which updates the long-range missiles with target data and gets the missiles launched quickly. The need for speed no longer refers to just how fast an aircraft is moving.
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.
IRST21 was available for flight testing in early 2014 when 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. This is similar to 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 precise 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. Moreover, IRST, especially with the Block II improvements, allows the IRST21 equipped aircraft to identify and track multiple targets and fire missiles at them.
FLIR (Forward Looking Infrared Radar) has been around since the 1980s, and as the technology became more powerful, it was possible to spot and identify targets at longer ranges. The ATFLIR could identify ground targets from 20 kilometers away, and the latest IRST systems (like 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 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. 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). The new generation IRST21 was also able to spot targets on the ground or at sea.
The older IRST had limitations. The main ones are the range (usually about 30 kilometers for accurate detection but much farther for "something is there") and 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 are 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 F-18E IRST21 were mounted in a modified centerline drop tank, which contained the IRST21 as well as 68 percent of the usual fuel. One problem with this approach is that the F-18E can't jettison this drop tank to make itself more maneuverable for air-to-air combat. Well, you could jettison, but you lose your IRST and an item that costs over two million dollars. Other aircraft, like the F-22 and F-35, have the IRST built into the fuselage. American manufacturers have added IRST pods to F-15Es exported to South Korea and Singapore.
Pilots quickly found that they could use their new generation IRST for spotting aircraft accurately, at long distance and get off the first shot with AMRAAM. This prompted air forces to hustle up the equipping of more aircraft with the improved IRST21 type equipment, especially since this IRST was designed especially to also accurately surface targets on land or sea. The air force took a while longer than the navy to accept the value of the new generation of IRST as exemplified in IRST21, the Legion pod and the internal IRST in the F-35. The F-22 has also had its IRST upgraded although additional upgrades are delayed because of tight budgets.