Absolute frequency readout derived from ULE cavity for next generation geodesy missions

Abstract

The next generation of Gravity Recovery and Climate Experiment (GRACE)-like dual-satellite geodesy missions proposals will rely on inter-spacecraft laser interferometry as the primary instrument to recover geodesy signals. Laser frequency stability is one of the main limits of this measurement and is important at two distinct timescales: short timescales over 10-1000 seconds to measure the local gravity below the satellites, and at the month to year timescales, where the subsequent gravity measurements are compared to indicate loss or gain of mass (or water and ice) over that period. This paper demonstrates a simple phase modulation scheme to directly measure laser frequency change over long timescales by comparing an on-board Ultra-Stable Oscillator (USO) clocked frequency reference to the Free Spectral Range (FSR) of the on-board optical cavity. By recording the fractional frequency variations the scale correction factor may be computed for a laser locked to a known longitudinal mode of the optical cavity. The experimental results demonstrate a fractional absolute laser frequency stability at the 10 ppb level (10−8) at time scales greater than 10 000 seconds, likely sufficient for next generation mission requirements.

Scale Factor Determination for the GRACE Follow-On Laser Ranging Interferometer Including Thermal Coupling

“…The principle has been demonstrated in laboratory experiments with an accuracy of roughly 3 MHz ≈ 10 ppb. It is noteworthy that the scale factor tone and its sidebands have little influence on the conventional PDH readout [30,31]. The advantage of this technique is that only minor changes to existing flight hardware are needed, e.g., the use of GHz electro-optical modulators (EOMs) instead of MHz.…”

Scale Factor Determination for the GRACE-Follow On Laser Ranging Interferometer including Thermal Coupling

“…After determination of the USO frequency during precise orbit determination, this technique is able to provide an estimate for the FSR frequency with high accuracy. The principle has been demonstrated in laboratory experiments with an accuracy of roughly 3 MHz ≈ 1 ppb [30,31]. The advantage of this technique is that the changes in existing flight-hardware are only minor.…”

Short- and long-term variability of the Antarctic and Greenland ice sheets

“…A GRACE-FO successor mission led by GFZ (Deutsches GeoForschungsZentrum, German Research Centre for Geosciences)-DLR and NASA-JPL ( Jet Propulsion Laboratory) is scheduled for launch in 2027. This mission will fly an upgraded laser-ranging interferometer, which will improve intra-satellite distance measurements by two orders of magnitude 216 , leading to corresponding enhancements in the spatial and temporal resolution of mass change observations 218,219 . In parallel, the ESA Ministerial Council is working to launch an additional GRACE-FO-type satellite pair in 2031.…”