Decca ASMI (Airport Surface Movement Indicating Radar) used at Heathrow from 1967-1989. The object was used to give air-traffic controllers a high-definition picture of the entire airfield. The object was situated on top of the control tower. By giving controllers a pin-point picture of the airfield, including even individual aircraft and vehicles, the airfield could operate in darkness and fog.

The high resolution resulted from the very short wavelength used- 8mm.