Foster scanner

Foster scanners were widely used in post-World War II radar systems used for artillery and mortar spotting.

The resulting system has the basic size and shape of a large pizza box with the rear side rounded.

The feed to the antenna is via a rectangular slot in a waveguide running the length of the box in front of the parabolic section.

When sent to the antenna, this causes the wavefront to scan across the area between the two horizontal plates, "snapping" to the initial position when the inner cone rotates back to the original point.

The Foster scanner was used in a number of post-World War II counterbattery radar systems, the first examples reaching service in the late 1950s and early 1960s.

The location of the shell horizontally was revealed by the timing of the returned signal compared to the rotation of the scanner at that instant.

The UK Green Archer radar used a manual switch to flip between the vertical angles.

In this role the scanner would be used to make two or more rapid observations of the shell in flight, and then plotted to calculate the initial position.