At the beginning of the Vietnam War, the state of the art for most aircraft weapon delivery was just about as advanced as it had been in World War II. That is, the visual dive bomb pass was the normal method of delivery. Given the higher speeds, shallower dives, and higher release altitudes, accuracy was arguably lower than in World War II.
Worse still, dive bombing was essentially a day-only tactic. Previous aerial interdiction campaigns in Italy and Korea suffered from the fact that a day only interdiction campaign allowed the enemy as many hours of freedom of movement as it did hours of observation and attack. Some attack aircraft, notably the A-6 Intruder, were equipped to perform “precision” strikes using radar, but that meant the target had to be a significant radar target, or a fixed position offset from such a radar target. Radar technology simply couldn’t pick out discrete targets such as trucks along the Ho Chi Minh trail.
Recognizing this shortcoming, the defense establishment put a lot of time, money and effort into devising alternative methods of target acquisition for night time use. One popular method was image intensification. The “starlight” scope took ambient illumination and magnified it to provide an image. Several platforms used such Low Light Level Television (or LLTV). But LLTV suffered from white-out if there was too much illumination (such as from weapons exploding) or poor image if there was little or no ambient light, such as on moonless overcast nights.
Forward Looking Infra Red, or FLIR, used the heat given off by various objects to provide a picture. Oddly, many FLIR systems are turret mounted, but for some reason, still retain the term “Forward Looking.”
Early FLIR systems were, by the standards of today, rather crude. But they gave airmen for the first time an effective way to pierce the darkness, and acquire non-radar significant targets on the ground. And if you can see it, you can kill it.
The Marines, presumably impressed with the side firing capability of the AC-119 and AC-130, modified two OV-1oA Broncos into what became knows as the YOV-10D* NOGS, or Night Observation Gunship. Basically, they added a FLIR system to the nose of a Bronco, with associated displays in the cockpits, and then added a tri-barrel XM197 20mm cannon on a turret to the aircraft belly.
The service test in Vietnam was pretty successful. The problem was, with the 20mm turret, the Bronco could not carry its sponsons, which on the A model had five stores stations, as well as mounting four fixed forward firing 7.62mm M60D machine guns. The tradeoff in conventional wasn’t worth it.
Eventually, with some modifications, the idea of a FLIR turret in the nose of the Bronco was accepted, and the OV-10D, without a gun turret, but with its sponsons, would enter production, serving with the Marines through Desert Storm.
Today, virtually every aircraft with a ground attack role carries FLIR, either on a pod or as an integral part of the aircraft.
*The “Y” in the designation is to denote its use as a service test.