A Laser Focus for Gravity Measurements
Prototype assembly and thermal enclosure (approximately 10” tall) for the advanced laser cavity. The cavity itself is made using glass with a low coefficient of thermal expansion to maintain a constant distance between end mirrors. (Credit: W. Folkner)
The twin satellites of the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), developed in partnership with the German Research Center for Geosciences and launched in 2018, provide a dramatic view of Earth’s variable gravity and mass distribution and spurred new insights in a variety of scientific fields. Like its predecessor, GRACE (launched in 2002), GRACE-FO is producing discoveries with far-reaching impacts on society, from hydrology and geophysics to ice mass estimates and global sea levels.
The measurement technique pioneered by GRACE, and utilized by GRACE-FO, works by detecting the slight changes in the movement of the twin satellites relative to each other as they orbit Earth. Subtle variations in gravity can be detected using careful range measurements between the satellites.
Like GRACE, GRACE-FO uses a microwave ranging instrument, referenced to a stable quartz clock and coupled with precise GPS tracking, to measure these minute changes to within a few microns. GRACE-FO also demonstrated an advanced laser instrument that has shown significant improvements in the accuracy of inter-spacecraft ranging. The laser system owes its advantages largely to the shorter wavelength of the laser, compared to the microwave wavelength, as well as a thermally isolated optical cavity, which is more stable than the quartz clock.
Key technologies for the laser demonstration – the optical cavity, cavity assembly, and ranging electronics – were developed at the Jet Propulsion Lab in collaboration with Ball Aerospace with funding from ESTO’s Instrument Incubator Program funding in 2002 and 2007 (Principal Investigator: William Folkner, JPL). The German Space Program provided measurement optics and the steering mirror assembly along with instrument integration.
To date, the laser system on GRACE-FO has operated flawlessly as a technology demonstration alongside the primary microwave interferometry system and is widely regarded as the likely primary instrument for the next gravity mission. Data from the laser demonstration are available at the Physical Oceanography Distributed Active Archive Center (PO.DAAC).