English: An interior shot of the GL Mk. III(B) radar, taken by Bill Wallace. The two displays in the center of the image show the range to a selected targets, the upper display showing everything within 32,000 feet, and the lower one showing the 1.8 km within a selected area on the upper display. The shaft protruding from the panel in the lower right sets a bias control in the electronics which allows the operator to line up the target blip with a wire stretched across the front of the lower display, which can be seen glinting in this photo. The wheel in the extreme lower left is used to rotate the antenna to track in azimuth, a similar station off-frame to the left controls the altitude.

The GL Mk. III(B) was a Gun Laying radar, which provided highly accurate angle and range information on a single target. This data was sent, sometimes fully automatically, to a mechanical computer known as a "predictor", which used this information to calculate a location in space where the target aircraft would be in the future. The output of the predictor drove mechanical pointers that were displayed on a number of Anti-Aircraft Artillery (AAA) guns. Operators of the guns simply had to operator their controls so their own pointers were on top of the ones from the predictor, and load shells. Modern systems automate this completely.

The Mk. III(B) was the British version of the Mk. III, there was also a similar Canadian version, the Mk. IIIc. Both would be paired with a longer-range, wider-pattern radar to pick up the targets, which would then be tracked by the GL radar sets.