Title: Laser Frequency Stabilization for GRACE-2
Author: William Folkner
Organization: Jet Propulsion Laboratory
Co-Authors: G. deVine, J. Dickson, S. Esterhuizen, W. M. Klipstein, K. McKenzie, D. Shaddock, R. Spero, R. Thompson, N. Yu, M. Stephens, J. Leitch, R. Pierce

The GRACE mission monitors changes in the Earthís gravity field by measuring changes in the distance between spacecraft induced by that changing field. The distance variation is measured with a microwave ranging system with sub-micron accuracy. The ranging measurement accuracy is limited by the signal-to-noise ratio and by the frequency stability of the microwave signal referenced to an ultra-stable oscillator (USO). For GRACE-2 a laser ranging system is envisioned with accuracy better than the GRACE microwave ranging system. A laser ranging system easily provides improved signal-to-noise ratio over the microwave system. Laser frequency stability better than the GRACE USO stability has been demonstrated in several laboratories using thermally stabilized optical cavities. We have developed a prototype optical cavity and associated optics and electronics to provide a stable frequency reference for the laser ranging system, with performance a factor of 20 better than the GRACE USO, which has passed environmental tests to establish suitability for use in orbit.