Title: Laser Frequency Stabilization for GRACE-2
Primary Author: Folkner, William
Organization: Jet Propulsion Laboratory
Co-Author(s): W. M. Folkner, G. deVine, S. Esterhuizen, W. M. Klipstein, K. McKenzie, D. Shaddock, R. Spero, R. Thompson, D. Wuchenich, N. Yu - Jet Propulsion Laboratory, California Institute of Technology
M. Stephens, J. Leitch, M. Davis, J. deCino, C. Pace, R. Pierce - Ball Aerospace and Technologies Corporation

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 are developing a space-qualifiable optical cavity and associated optics and electronics for use on GRACE-2 to provide a stable frequency reference for the laser ranging system. Two breadboard units have been developed and tested for performance and ability to survive launch and orbit environments. A prototype unit is being designed using lessons learned from test of the breadboard units.