A Bearing Clock, was a variable speed drive that could maintain a bearing to a target provided the bearing rate was nearly constant (as it often was). It was similar to but less important in a fire control solution than a Range Clock.
The compass bearing to the target was needed to maintain a proper setting of a Dumaresq or an equivalent rate solving instrument. However, the compass bearing did not have to be very accurate at all, as deviations of several degrees would not introduce significant errors in calculating Range Rate or Speed-Across. A bearing clock was, therefore, a convenience appliance helpful in reducing workload in the Transmitting Station.
Bearing clocks were generally not standalone devices, but appeared only in Fire Control Tables, without much fanfare. For instance, the Dumaresq Mark VI in a Mark III Dreyer Table had a bearing clock that could be set to rates varying as much as 15 degrees per minute left or right. Its function was simple: it caused the dial plate which recorded target bearing on the dumaresq to rotate at the bearing rate provided. In this way, the conversion of the target's relative motion would be properly mapped to a range rate and speed-across for a considerable time, and the work and effort required to maintain the proper bearing would be greatly reduced. This permitted TS crew to focus on more sensitive aspects of the fire control problem.
- Pollen Aim Correction System, Part I. Fig. 1.
- Admiralty, Gunnery Branch (1918). Handbook of Captain F. C. Dreyer's Fire Control Tables, 1918. C.B. 1456. Copy No. 10 at Admiralty Library, Portsmouth, United Kingdom.
- Dreyer, Frederic; Usborne, Cecil through Gunnery Branch, Admiralty. (1913). Pollen Aim Corrector System, Part I. Technical History and Technical Comparison with Commander F. C. Dreyer's Fire Control System. P. 1024. in Admiralty Library, Portsmouth.