Hewlett Packard HP 5061A Caesium Beam Frequency Standard, originally serving as one of a suite of ‘atomic clocks’ from the mid 1960’s onwards by the Greenwich Time Service, when it was based at the Royal Greenwich Observatory, Herstmonceux, Sussex.

The Greenwich Time Service also provided a synchronised time signal around the Royal Greenwich Observatory for the site’s other astronomical activities, including its Satellite Laser Ranging (SLR) facility. The SLR had been set up in the 1980’s to accurately measure the rate of the Earth’s rotation and is still in operation at Herstmonceux today. The SLR’s ability to measure rotation precisely lay in it receiving a highly accurate time signal, and after the disbanding of RGO at Herstmonceux in 1990, with the Time Service relocated to the National Physical Laboratory, the SLR was suddenly without an accurate time-feed. Some of the caesium standards, of which this was one, remained onsite to continue providing the SLR with this service.

Clockmakers' Museum No. 1519