While they are mostly associated with fighter aircraft, IRST sensors are also used by surface forces, including navies. In naval use, IR sensors are used primarily as early warning devices, which means that shipboard IRST is integrated with ship’s self defense / point defense system. IRST detects targets and sends data to weapons systems, which then engage targets based on pre-determined priorities. In naval use at least, IRST will not replace radars because high humidity at sea level significantly reduces its range. However, it is still an important sensor, especially during the times of radar silence (e.g. threat from anti-radiation missiles). Also of importance is IRST’s immunity to jamming and passive operation, which makes them extremely important in electronically degraded environment. IRST’s shorter operational wavelength means greater resolution, and thus far greater ability to detect small targets as well as to distinguish between the low-flying targets and the background clutter. For these reasons, IRST and IR targeting systems are already in widespread use in air forces for purposes of both air-to-air and air-to-ground combat.
Primary role of shipboard IRST is to detect anti-ship sea-skimming missiles, and to designate the targets so as to enable defensive response. Due to a combination of powerful rocket engine, surface heating due to aerodynamic friction and hot shock cone (a Mach 4 missile creates >650*C shock cone), even small missiles can be relatively easily detected and tracked. Other roles of a shipboard IRST include short-range surveillance, target identification, IR imaging of a shore and nighttime navigation. These systems can be used in modes ranging from manual to fully automatic, in which case IRST device is under control of ship’s fire control system. Naval IRST systems are also used for anti-air warfare and detecting terrorist-class threats such as surfaced divers, rafts, small boats and light aircraft. Modern shipboard IRSTs are evolving into dual-use surveillance and targeting sensors. This alleviates wear and tear on dedicated sensors, and allows quicker response. Anther trend are reductions in size and weight of sensors. Modern naval IRSTs are light enough to be mounted on ship masts, allowing 360* coverage.
IRST technology has several key advantages. Passive, electromagnetically silent operation allows the ship to reduce its electromagnetic signature and eliminate the threat of anti-radiation missiles. Usage of IR spectrum also leads to higher resolution than radar, as well as the multipath effect close or on the sea surface. However, IR systems are also limited by atmospheric attenuation to windows in 3-5 and 8-14 μ, and both atmospheric windows are susceptible to degradation in certain sea conditions. More specifically, 8-14 μ window is much more sensitive to temperature differences between the target and the background, enabling far greater range in optimal conditions. However, it is also far more sensitive to the effects of water vapour. On the other hand, 3-5 μ window is less sensitive to water vapour, but can be affected by the sun glitter. Consequently, most IRST sensors are dual-band.
DAS IRST has demonstrated the ability to monitor the activity of small boats, rubber rafts, and swimmers within several kilometers of the ship. Neither visual nor radar surveillance have that capability. This makes IRST critical for shipboard situational awareness against assymetric threats. Importance of this was demonstrated in the USS Cole incident, when a small surface craft delivered explosives which tore a 40-by-40 ft gash in the destroyer’s port side, killing 17 sailors. This also makes IRST ideal for air cushion vehicles, designed to conduct day/night off-shore, near-shore, and on-shore missions. These missions require very high level of situational awareness in many different conditions – night, sea spray, haze, smoke, sand, dust etc. The current system of radar, night-vision devices and visual watches is not adequate. IRST can fill this gap, providing the ability to discern land terrain, sea and waterway features as well as contacts such as small boats and patrol craft, in near-shore and shore environment where performance of surface radar is limited. This includes detection of beach and land based threats with low visual and radar signature.
Hrvatski Vojnik Broj 45., godina III., 27. kolovoza 1993. (Croatian Soldier No.45, year III, 27. August 1993.)
Real-Time Fleet Protection, L.N. Smith and J.R. Waterman, Optical Sciences Division, 2007 NRL Review, pg 196-197