GCA is largely obsolete now - it was also used on aircraft carriers. Used mainly because it needed no extra equipment on the aircraft other than nerves of steel by the pilot who probably was entirely blind flying!! Most often a truck was parked at the side of the runway at the touchdown point - two radar antennas either electronically or mechanically swept an arc up and down and left-right - the controller in the van had two displays, both with a fan shaped scales, one of elevation and one of azimuth with the hopeful landing spot at the left bottom for elevation and centre left for azimuth. The aircraft was steered by the normal radar into roughly the right location and when the DCA controller saw him on his screens he started talking down the aircraft on another frequency by just giving the pilot his position relative to the desired flight profile " slightly high and left of path, 8 miles to touchdown, coming on to path, still slightly left, 6 miles, " etc etc then "wheels down, 100 feet" and hopefully a small thud from outside the truck - job done.
