Television guidance

Television guidance (TGM) 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 allow attacks on ships while the launch aircraft remained safely out of range of the target's anti-aircraft guns. The best developed was the Henschel Hs 293. The US also experimented with similar weapons during the war, notably the GB-4 and TRD-1. Only small numbers were used experimentally and with poor results. One of the first TV guided weapons to see widespread service was the Anglo-French Martel missile, which came in radar-seeking and TV guided versions. 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 platform during the Gulf War, which required pinpoint accuracy.

History

German efforts

The Hs 293 was produced in a number of versions, like this early experimental A model (V4). The D model had an extended nose carrying the camera, and a Yagi antenna at the back to send the signal to the launch aircraft.

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.[1] This was one of a number of efforts to provide guidance 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[lower-alpha 1] was used 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.[2]

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. Additionally, it could be launched through clouds or smoke screens and pick up the target again. More importantly, as the bomb approaches the target the image grows, providing increasing accuracy and allowing the bomb aimer to pick vulnerable locations on the target to attack.[3]

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.[3] German Post Office technicians aiding the Fernseh company began development of small 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.[4]

In testing carried out by the Deutsche Forschungsanstalt für Segelflug (DFS) starting in 1943,[5] 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, the position of the control stick did not directly represent the position of the controls (as is more common today). 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 stop the controls, causing them to continually overshoot their corrections. However, when viewed through the television screen the motion was immediately obvious, and the operators had no problem making small corrections with ease.[6]

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 not yet be flying in that direction. This would cause the image to once again begin tailing 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 (CEP) of 16 m, too far to be useful.[7]

After considering a number of 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 just that instant. This also helped reduce the motion of the image with sharp control inputs.[6]

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 in spite of this being the most important part of the approach. However, this was addressed simply 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.[8]

Sources claim that 255 D models were built in total, and one claims one hit a Royal Navy ship in combat.[9] However, other sources suggest the system was never used in combat.[10]

US efforts

The US had been introduced to the glide bombing concept by the Royal Air Force just before the US's entry into the war. "Hap" Arnold had Wright Patterson Air Force Base begin development of a wide variety of concepts under the GB and related VB ("vertical bomb") programs. These were initially 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 1,000 yards (910 m) 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, the 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 50 kilometres (31 mi) 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 (40 fields a second). A film recorder was developed to allow post-launch critique.[1]

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.[1] 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.

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 image orthicon, and began Project MIMO, short for "Miniature Image Orthicon".[11] 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.[12] Development continued after the war, and it was in the inventory for a time in the post-war period.[13][14]

Martel

The AJ.168 Martel was the Royal Navy's primary naval strike weapon on their Buccaneer fleet in the 1970s and 80s.

In the early 1960s, Matra and Hawker Siddeley Dynamics began to collaborate on a long-range high-power anti-radar missile known as Martel. The idea behind Martel was to allow an aircraft to attack Warsaw Pact surface-to-air missile sites while well outside their range, and it carried a warhead large enough to destroy the radar even in the case of a near miss. In comparison to the US AGM-45 Shrike, Martel was far longer ranged, up to 60 kilometres (37 mi) compared to 10 miles (16 km) for the early Shrike, and a 150 kilograms (330 lb) warhead instead of 145 pounds (66 kg).[15]

Shortly thereafter, The Royal Navy began to grow concerned about the improving air defense capabilities of Soviet ships. The Blackburn Buccaneer had been designed specifically to counter these ships by flying at very low altitudes and dropping bombs from long distances and high speeds. This approach kept the aircraft under the ship's radar until the last few minutes of the approach, but by the mid-1960s it was felt this would offer enough time for an attack and a new weapon was needed to keep the aircraft even further from the ships, ideally never rising above the radar horizon.[15]

This meant that the missile would have to be fired blind, while the aircraft's own radar was unable to see the target. At the time there was no indigenous active radar seeker available so the decision was made to use television guidance and data link system to send the video to the launch aircraft. The Martel airframe was considered suitable, and a new nose section with the electronics was added to create the AJ.168 version.[15]

Like the earlier German and US weapons, the Martel required the weapon officer to guide the missile visually while the pilot steered the aircraft away from the target. Unlike the earlier weapons, Martel flew its initial course using an autopilot that flew the missile high enough that it could see both the target and the launch aircraft (so the data link could operate). The television signal would not turn on until the missile reached the approximate midpoint, at which point the weapons officer guided it like the earlier weapons. Martel was no a sea skimming missile, and dove on the target from some altitude.[15]

The first test launch of the AJ.168 took place in February 1970 and a total of 25 were fired by the time testing ended in July 1973, mostly at Aberporth in Wales. Further testing was carried out until October 1975, when it was cleared for service. It was used only briefly by the Royal Navy before they turned the remainder of their Buccaneers over to the RAF. The RAF used both the anti-radar and anti-ship versions on their Buccaneers, with the anti-ship versions being replaced by the Sea Eagle in 1988, while the AS.37 anti-radar versions remained in use until the Buccaneers were retired in March 1994.[15]

Walleye

The original Walleye looked more like a missile than a bomb. It was a primary weapon of the A-7 Corsair II.
Walleye II had a larger warhead, much larger wings, and an extended range data link.

US interest in television guidance essentially ended in the post-war period. Nevertheless, small scale development continued, and a team at the Naval Ordnance Test Station (NOTS) developed a way to automatically track light or dark spots on a television image, a concept today known as an optical contrast seeker. Early use of the AGM-12 Bullpup demonstrated that the "silver bullet" was to difficult to use and exposed the launch aircraft to anti-aircraft fire, and the need for a fire-and-forget weapon became clear. In January 1963, NOTS released a contact for a bomb and guidance system that could be used with their tracker. In spite of being a glide bomb, this was confusingly assigned a number as part of the new guided missile numbering system, becoming the AGM-62 Walleye.[16]

As initially envisioned, the system would use a television only while the missile was still on the aircraft, and would automatically seek once launched. However, this quickly proved infeasible, as the system would often break lock for a wide variety of reasons. This led to the addition of a data link that sent the image back to the aircraft, allowing guidance throughout. This was not a true television guidance system in the classic sense, as the operator's task was to continue selecting points of high contrast which the seeker would then follow. In practice, however, the updating was almost continuous, and the system acted more like a television guidance system and autopilot, like the early plans for the Hs 293.[16]

Walleye entered service in 1966, and was quickly used in a number of precision attacks against bridges and similar targets. This revealed that it did not have enough striking power, and more range was desired. This led to the introduction of an extended range data link (ERDL) and larger wings to add range from 16 nautical miles (30 km; 18 mi) to 24 nautical miles (44 km; 28 mi). Walleye II was a much larger version based on a 2,000 pounds (910 kg) bomb in order to improve performance against large targets like bridges, and further extended range to as much as 32 nautical miles (59 km; 37 mi).[16] These were widely used in the later portions of the war and they remained in service through the 1970s and 80s. It was an ERDL equipped Walleye that was used to destroy the oil pipes feeding Sea Island and help stop the Gulf War oil spill in 1991. Walleye left service in the 1990s, replaced largely by laser guided weapons.

Notes

  1. ↑ Kehl was the transmitter, Strassburg the receiver in the bomb.

References

Citations

  1. 1 2 3 Abramson 2003, p. 6.
  2. ↑ Münster 1956, p. 136.
  3. 1 2 Münster 1956, p. 137.
  4. ↑ Münster 1956, p. 138.
  5. ↑ Münster 1956, p. 143.
  6. 1 2 Münster 1956, p. 147.
  7. ↑ Münster 1956, p. 144.
  8. ↑ Münster 1956, pp. 150-151.
  9. ↑ Kopp, Carlo (April 2012). "The Dawn of the Smart Bomb". Air Power Australia.
  10. ↑ Münster 1956, p. 159.
  11. ↑ Abramson 2003, pp. 7-8.
  12. ↑ Abramson 2003, p. 9.
  13. ↑ ""Roc," New Sky Terror". Popular Science: 120. February 1946.
  14. ↑ Yenne, Bill (2005). Secret Gear, Gadgets, and Strange Gizmos. Zenith Imprint. p. 24.
  15. 1 2 3 4 5 White 2006.
  16. 1 2 3 Parsch 2002.

Bibliography

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