Reminds me of GRACE, which consists of two spacecraft in the same orbit, one following the other. By measuring the separation distance between the two spacecraf

Reminds me of GRACE, which consists of two spacecraft in the same orbit, one following the other. By measuring the separation distance between the two spacecraft (which I believe is done through interferometry), the gravity field of the Earth can be measured precisely. In turn, this allows us to measure mass transport in the world (mainly of water). Very cool experiment. (Many of my coworkers at the Albert Einstein Institute in Hannover, Germany were working on this project. The focus at the institute was on optical interferometry for the detection of gravitational waves, i.e. LIGO and LISA. LISA would be a formationa-flying interferometer in heliocentric orbit.) https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO