The Gravity Recovery and Climate Experiment (GRACE) Follow-On mission is a sequel to the original GRACE space mission, which consisted of two satellites separated by 200 km that monitored their distance from each other via microwaves to map Earth’s gravitational field. The GRACE Follow-On is doing the same, but with the addition of a laser-ranging interferometer (LRI) shared by two satellites separated by 220 km. New data shows that the LRI has tracked phase continuously for more than 50 days and provided range information with noise levels of 10 nm/(Hz)1/2 at 40 mHz and 300 pm/(Hz)1/2 at 1 Hz, which is much less than the noise level of the microwave instrument and well below requirements.
The GRACE missions measure variations in distance of up to a few hundred meters. The gravity signal has a dynamic range of about 10-8 to 1 m/(Hz)1/2, most of which is the static gravity field, but small amounts of which are temporal variations in the field. The LRI’s lasers are fiber-coupled Nd:YAG nonplanar ring oscillators emitting 25 mW at 1064.5 nm—both satellites carry identical optical cavities, with one of them stabilizing the frequency of the laser on the “master” satellite. The LRI’s beam-steering mirror, which steers a beam with a 140 μrad half-cone angle, has a range of several milliradians in two axes and a speed of greater than 100 Hz. The amount of light received by one satellite from the other has a power in the range of nanowatts. The LRI, which is the first laser interferometer to be operated between distant satellites, succeeded as soon as it was turned on in space. One of the mission's achievements is demonstrating interspacecraft interferometry for the future Laser Interferometer Space Antenna (LISA), which will detect gravitational waves at much lower frequencies and higher sensitivity than the existing ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO). Reference: K. Abich et al., Phys. Rev. Lett., 123, 031101 (2019); doi:10.1103/physrevlett.123.031101.
