Second harmonic level monitors in cesium atomic frequency standards

The cesium frequency standards (CFS) to be flown on board GPS Block W satellites will carry a second harmonic level monitor (SHLM), the output of which will be the only telemetry datum available on the CFS health The SHLM replaces the beam current monitor (BCM) currently operating in GPS CFS. We have carried out an analysis of the validity and reliability of the SHLM as a CFS health indicator. The results of a rigorous calculation of the cesium beam tube (CBT) output signal under the square-wave microwave frequency modulation scheme to be used in Block IIR CFS show that almost any process causing the output of the BCM to decrease will lead to a proportionate decrease in the SHLM output, but the SHLM output will be more sensitive to changes in some CBT operating parameters than the BCM output. In particular, that is the case for changes in the ionizer temperature and microwave power. We also show that the signal-to-noise ratio at the SHLM will be less than that at the BCM