The AN/GMQ-13 was a cloud height measuring system that worked from the surface to about 5,000 feet up. Two sensors were placed about 500 feet apart near the runway. The main sensor was a light beam that swept from 0 to 90 degrees and the receiver would pick up the refracted beam. The display was similar to an oscilloscope. If clouds were detected below 5000 feet the base of the clouds would show up as a blip on the screen.