SAPPHIRE RESONATOR TRANSDUCER ACCELEROMETER FOR SPACE GRAVITY GRADIOMETRY

Techniques developed for state-of-the-art microwave displacement transducers may be incorporated into a gravity gradiometer array suitable for spatial inertial navigation and geodesic research applications. To make such a system attractive for placement into a small satellite, it should be small in size and weight, rugged, and operate at non-cryogenic temperature. One promising technique is based on the modulation of the resonant frequency of a shielded sapphire microwave resonator. In the present experiment, the sapphire resonator operates in a high-order (azimuthal mode number 12) mode near 9.66 GHz. The unloaded quality factor of 1.0 x 10(5) indicates that the mode is probably not a pure whispering gallery mode, but rather a hybrid. A sapphire tuning element is coupled to a spring-mass accelerometer, with a mechanical resonance near 16.8 Hz, and perturbs part of the evanescent field of the resonator. The resonant frequency of the mode is an exponential function of the spacing between the resonator and the tuning element. The present experiment yields a maximum Q(df/dx) = 3.91 x 10(2) MHz mum-1 at 310 K. At this level of sensitivity the current accelerometer would require df/f0 < 5 x 10(-10) to resolve mug acceleration. The residual phase noise characteristics were also determined by measurement of the single side band (SSB) noise power spectral density.