Optical frequency standards offer a step improvement of one to two orders of magnitude in the future ability to realise the SI second [1].

The NPL strontium ion optical clock project [2] is targeted on creating systems with better performance than microwave clocks, for both space [3] and terrestrial environments.

Recent activities have centred on compact transportable system development and long-term unattended operation with quantum-limited performance.

One ring and two end-cap 88Sr+ trap systems based on the 2S1/2 - 2D5/2 674 nm optical-clock quadrupole transition are under development.

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