The line‑of‑sight distance between two satellites flying in a known orbital configuration (often tandem) is measured at up to nanometer precision. Mass anomalies on or within Earth (ice sheets, groundwater, oceans, lithosphere) slightly accelerate or decelerate the leading satellite relative to the trailing one. Over time, this allows scientists to reconstruct a global map of how strong gravity is in different places and how that pattern is changing with time (melting ice, shifting water, etc.).
