Local oscillator and focal distance measurement system for the Square Kilometre Array

A system which simultaneously provides coherent local oscillator signal and measures focal length for the Large Adaptive Reflector is discussed. The Local Oscillator subsystem provides a phase-stable signal to an airborne platform over a radio link. A phase stability of 5°rms at 22 GHz is the goal for the system. Feedback is employed to readjust the phase of the delivered signal, to compensate for changes in the transmission medium. The dynamic properties of the Local Oscillator system are controlled primarily by a phase-locked loop. A Focal Length Measurement subsystem employing the Chinese remainder theorem for ambiguity resolution is proposed. The subsystem will measure the focal distance of 500 m with an accuracy of 70 mum. Both systems are based on interferometric techniques and are combined into one. A prototype of the Local Oscillator subsystem has been built and tested to demonstrate feasibility of the concept.