File:GL Mk IIIc radar Accurate Position Finder.jpg

English: The GL Mk. III was a microwave-frequency Gun Laying radar that fed information directly to a predictor to calculate the angle and fuse delays for an associated battery of anti-aircraft guns. Using radars of this sort reduced the number of shells fire to get a "hit" from about 41,000 in the pre-war era, to about 1000 using the Mk. III.

This image shows the Canadian version of the GL Mk. III, the Mk. III(C). It came in two halves, the APF, Accurate Position Finder, shown here, and the ZPI, Zone Position Indicator, not shown. The ZPI was a wide-angle search system that cued the APF operators to the rough location of the target. A generator set, seen on the left, fed power to both radars.

Mk. III was designed in the era before it was possible to quickly switch microwave frequencies, and so both the Canadian and British models of the Mk. III both feature two antennas and their associated parabolic reflectors, one used for transmission and the other reception. One easy way to tell the two models apart is the flat edges of the reflectors, seen only on the Canadian version.

Mk. III was also designed before there was a way to feed microwaves through cables to a rotating antenna. Instead, the entire operators cabin rotated to track targets, and can be seen to be slightly off-centered in this photo. The antennas "nodded" up and down to track in elevation, and for transport the entire assembly could be rotated forward and locked into the bracket seen projecting from the front of the cabin.

Both of these limitations were soon addressed, switching through the use of the soft Sutton tube and rotation through the move to waveguides instead of coaxial cables, leading to the dramatically superior US SCR-584. The -584 became widely used starting in 1944, and the Mk. III's were generally relegated to secondary roles.