Page 16 - Royal Observer Corps

Atomic Weapons Detection Recognition and Estimation of Yield known by the acronym AWDREY
was a desk-mounted automatic detection instrument, located at 12 of the 25 Royal Observer
Corps (ROC) controls, across the United Kingdom, during the Cold War. The instruments would
have detected any nuclear explosions and indicated the estimated size in Megatons.

With the display unit mounted in a 3-foot-high (0.91 m) steel cabinet, the system used two sets of
five photosensitive cells within the detection head to record the intense flash of light produced by
the detonation of the weapon followed, within a second, by a second intense flash. This double
flash is characteristic of a nuclear explosion and measurement of the short gap between the two
flashes enabled the weapon's power to be estimated, and the bearing to be indicated. It had a
range of 150 miles (240 km) in good visibility. From 1974 AWDREY units were used together with
a device known as DIADEM (Direction Indicator of Atomic Detonation by Electronic Means) which
measured the electromagnetic pulse (EMP) generated by an explosion. The instruments were in
constant operation (state of readiness) between 1968 and 1992 and tested daily by
full-time ROC Officers.