Television guidance

Television guidance is a type of missile guidance system using a television camera in the missile or glide bomb that sends its signal back to the launch platform. There, a weapons officer or bomb aimer watches the image on a television screen and sends corrections to the missile, typically over a radio control link. Television guidance is not a seeker because it is not automated, although semi-automated systems with autopilots to smooth out the motion are known. They should not be confused with contrast seekers, which also use a television camera but are true automated seeker systems.
The concept was first explored by the Germans during World War II as an anti-shipping weapon that would keep the launch aircraft safely out of range of the target's anti-aircraft guns. The best-developed example was the Henschel Hs 293, but the TV-guided versions of this weapon did not see operational use. The US also experimented with similar weapons during the war, notably the GB-4 and Interstate TDR. Only small numbers were used experimentally, with reasonable results.
Several systems were used operationally after the war. The British Blue Boar was cancelled after extensive testing. A separate line of development led to TV-guided versions of the Martel missile to fill the anti-shipping role. The US AGM-62 Walleye is a similar system attached to an unpowered bomb, the Soviet Kh-29 is similar.
Television guidance was never widely used, as the introduction of laser guided bombs and GPS weapons have generally replaced them. However, they remain useful when certain approaches or additional accuracy are needed. One famous use was the attack on the Sea Island oil terminals during the Gulf War, which required pinpoint accuracy and was attacked by television-guided GBU-15 bombs.

History

German efforts

The first concerted effort to build a television-guided bomb took place in Germany under the direction of Herbert Wagner at the Henschel aircraft company starting in 1940. This was one of several efforts to produce usable guidance systems for the ongoing Hs 293 glide bomb project. The Hs 293 had originally been designed as a purely MCLOS system in which flares on the tail of the bomb were observed by the bomb aimer and the Kehl-Strassburg radio command set sent commands to the bomb to align it with the target. The disadvantage of this approach is that the aircraft had to fly in such a way to allow the bomb aimer to view the bomb and target throughout the attack, which, given the cramped conditions of WWII bombers, significantly limited the directions the aircraft could fly. Any weather, smoke screens or even the problems of viewing the target at long range made the attack difficult.
Placing a television camera in the nose of the bomb appeared to offer tremendous advantages. For one, the aircraft was free to fly any escape course it pleased, as the bomb aimer could watch the entire approach on an in-cockpit television and no longer had to look outside the aircraft. It also allowed the bomb aimer to be located anywhere in the aircraft. Additionally, it could be launched through clouds or smoke screens and then pick up the target when it passes through them. More importantly, as the bomb approaches the target the image grows on the television screen, providing increased accuracy and allowing the bomb aimer to pick vulnerable locations on the target to attack.
At the time, television technology was in its infancy, and the size and fragility of both the cameras and receivers were unsuitable for weapon use. German Post Office technicians aiding the Fernseh company began the development of hardened miniaturized cameras and cathode ray tubes, originally based on the German pre-war 441-line standard. They found the refresh rate of 25 frames per second was too low, so instead of using two frames updating 25 times a second, they updated a single frame 50 times a second and displayed roughly half the resolution. In the case of anti-ship use, the key requirement was to resolve the line between the ship and the water, and with 224 lines this became difficult. This was solved by turning the tube sideways so it had 220 lines of horizontal resolution and an analog signal of much greater resolution vertically.
In testing carried out by the Deutsche Forschungsanstalt für Segelflug starting in 1943, they found one major advantage of the system was that it worked very well with the 2-axis control system on the missile. The Kehl control system used a control stick that started or stopped the motion of the aerodynamic controls on the bomb. Moving the controls to the left, for example, would move the controls to begin a left roll, but when the stick was centred it left the controls in that position and the roll continued to increase. Not being able to see the control surfaces after launch, the operators had to wait until they could see the bomb begin to move and then use opposite inputs to stop the motion. This caused them to continually overshoot their corrections. But when viewed through the television screen, the motion was immediately obvious and the operators had no problem making small corrections with ease.
However, they also found that some launches made for very difficult control. During the approach, the operator naturally stopped the control inputs as soon as the camera was lined up with the target. If the camera was firmly attached to the missile, this happened as soon as enough control was input. Critically, the missile might be pointed in that direction but not actually travelling in that direction, there was normally some angle of attack in the motion. This would cause the image to once again begin trailing the target, requiring another correction, and so on. If the launch was too far behind the target, the operator eventually ran out of control power as the missile approached, leading to a circular error probable of, too far to be useful.
After considering several possibilities to solve this, including a proportional navigation system, they settled on an extremely simple solution. Small wind vanes on the nose of the missile were used to rotate the camera so it was always pointed in the direction of the flight path, not the missile body. Now when the operator maneuvered the missile, he saw where it was ultimately headed, not where it was pointed at that instant. This also helped reduce the motion of the image if they applied sharp control inputs.
Another problem they found was that as the missile approached the target, corrections in the control system produced ever wilder motion on the television display, making last-minute corrections very difficult despite this being the most important part of the approach. This was addressed by training the controllers to ensure they had taken any last-minute corrections before this point, and then hold the stick in whatever position it was once the image grew to a certain size.
Sources claim that 255 D models were built in total, and one claims one hit a Royal Navy ship in combat. However, other sources suggest the system was never used in combat.

US efforts

The US had been introduced to the glide bombing concept by the Royal Air Force just before the US entered into the war. "Hap" Arnold had Wright Patterson Air Force Base begin the development of a wide variety of concepts under the GB and related VB programs. These were initially of low importance, as both the Army Air Force and US Navy were convinced that the Norden bombsight would offer pinpoint accuracy and eliminate the need for guided bombs. It was not long after the first missions by the 8th Air Force in 1942 that the promise of the Norden was replaced by the reality that accuracy under was essentially a matter of luck. Shortly thereafter the Navy came under attack by the early German MCLOS weapons in 1943. Both services began programs to put guided weapons into service as soon as possible, a number of these projects selected TV guidance.
RCA, then a world leader in television technology, had been experimenting with military television systems for some time at this point. As part of this, they had developed a miniaturized iconoscope, model 1846, suitable for use in aircraft. In 1941 these were experimentally used to fly drone aircraft and in April 1942 one of these was flown into a ship about away. The US Army Air Force ordered a version of their GB-1 glide bomb to be equipped with this system, which became the GB-4. It was similar to the Hs 293D in almost every way. The Army's Signal Corps used the 1846 with their own transmitter and receiver system to produce an interlaced video display with 650 lines of resolution at 20 frames a second. A film recorder was developed to allow post-launch critique.
Two B-17's were fit with the receivers and the first five test drops were carried out in July 1943 at Eglin Field in Florida. Further testing was carried out at the Tonopah Test Range and was increasingly successful. By 1944 the system was considered developed enough to attempt combat testing, and the two launch aircraft and a small number of GB-4 bombs were sent to England in June. These launches did not go well, with the cameras generally not working at all, failing just after launch, or offering intermittent reception that generally resulted in the images becoming visible only after the bomb had passed its target. After a series of failed launches, the team returned home, having lost one of the launch aircraft in a landing accident. Attempts to use the system to produce an air-to-air missile using command guidance failed due to issues with closing speed and reaction time.
By the end of the war, advances in tube miniaturization, especially as part of the development of the proximity fuse, allowed the iconoscope to be greatly reduced in size. However, RCA's continued research by this time had led to the development of the greatly improved image orthicon, and began Project MIMO, short for "Miniature Image Orthicon". The result was a dramatically smaller system that easily fit in the nose of a bomb. The Army's Air Technical Services Command used this in their VB-10 "Roc II" guided bomb project, a large vertically dropped bomb. Roc development began in early 1945 and was being readied for testing at Wendover Field when the war ended. Development continued after the war, and it was in the inventory for a time in the post-war period.