On February 11th the U.S. Navy had its first flight test of a new IRST (Infa-Red Search & Track) pod. IRST uses a high resolution infrared (heat sensing) 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.
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 can identify ground targets from 20 kilometers away, and the latest IRST systems 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 a hostile radar.
The navy is 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 IRST technology is pitched as being much more effective, giving warnings (that something is out there) at long range (several hundred kilometers). The new generation IRST is also able to spot targets on the ground or at sea.
The U.S. Air Force is not as enthusiastic about IRST. 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 includes 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, thus it is undetectable by the enemy.
IRST has its limitations. The main ones are 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 gives pretty vague data. Still, it's believed that just having an indication that someone is out there, more than a hundred kilometers away, gives you an edge.
The F-18E IRST will be mounted in a modified centerline drop tank, which will contain the IRST 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. Other aircraft, like the F-22 and F-35, have the IRST built into the fuselage. American manufacturers have added IRST to F-15Es exported to South Korea and Singapore.
Pilots have already found that they could use their targeting pods for spotting aircraft, which prompted air forces to hustle up the equipping of more aircraft with IRST (which, while designed especially to spot other aircraft, can also be used to detect surface, land or sea, targets.) The air force has not given up on IRST, it is just not as eager to buy the new and improved IRST as the navy.