Contrast seeker

Analog television cameras scan an image as a series of horizontal lines that are stacked vertically to form a grid or "frame".

The camera's progression through the frame is carefully controlled by electronic timers, known as time base generators, that produce smoothly increasing voltages.

The series of varying voltages from the sensor forms an amplitude modulated (AM) signal that encodes the brightness variations along any given scan line.

If the change is greater than a selected threshold, it triggers a second circuit that sends the output of the two scanning time base generators into capacitors.

The missile is initially brought onto the target manually, normally using a small cueing input on the pilot's control stick, or by the weapons officer in a two-seat aircraft.

For instance, a tank on a roadway might provide a very high contrast tracking spot, only to have that disappear when it drives off the road into low bush.

This is very effective against vehicles like tanks, where the engine produces an excellent high-contrast image to seek on, and few other objects in nature will generate a similar signal.

This means the aircraft has to carry two types of missiles, ones with IR for vehicles, and ones with visible light seekers for attacking other targets like bridges or bunkers.

[3] Early contrast seekers thus used a second system that noticed the target spot beginning to spread over several pixels and lock the approach angle into a coasting phase once this happened.

This was essentially a bomb equipped with short straight wings and small tail surfaces with a tracking system from Hammond-Crosley called the B-1.

Unlike later examples, this used a mechanical scanning system, with two photocells examining changes in contrast as the seeker oscillated left and right.

In some early combat uses in the Vietnam War, pilots on their very first mission "vaporized" a truck with a direct hit, only to be admonished by their commanding officer for using a $25,000 weapon against a $500 target.

This differed primarily in that it sent the image back to the launch aircraft while the missile was in flight, allowing the weapon officer to correct its trajectory in a fashion more similar to other television-guided systems.

Because of the environment they flew in, contrast seekers were generally not suitable to launch from helicopters,[11] which led to experiments using wire guidance (TOW) or laser homing (Hellfire).