SAPPHIRE RESONATOR TRANSDUCER ACCELEROMETER FOR SPACE GRAVITY GRADIOMETRY

Techniques developed for state-of-the-art microwave displacement trans ducers may be incorporated into a gravity gradiometer array suitable f or 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-cryoge nic 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-ord er (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 elemen t is coupled to a spring-mass accelerometer, with a mechanical resonan ce near 16.8 Hz, and perturbs part of the evanescent field of the reso nator. The resonant frequency of the mode is an exponential function o f the spacing between the resonator and the tuning element. The presen t 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 requi re df/f0 < 5 x 10(-10) to resolve mug acceleration. The residual phase noise characteristics were also determined by measurement of the sing le side band (SSB) noise power spectral density.