Geodesy is the study of Earth's size, shape, orientation, and gravitational field, and the variations of these qualities over time. Nowadays this field is concerned with changes in the shape of Earth's surface, because small detectable changes are associated with issues with impactful events on society, such as ice melting, sea level rise, land subsidence, and aquifer depletion. Twenty-first-century geodetic studies are dominated by geodetic measurements from space. Current space geodetic measurements can detect small movements of the Earth's solid and fluid surfaces as well as changes in the atmosphere and ionosphere. Space-based geodetic observations can be categorized into four basic techniques: positioning, altimetry, interferometric synthetic aperture radar (InSAR), and gravity studies. Precise positioning is the fundamental geodetic observation required for surveying and mapping. Instead of using traditional triangulation and leveling networks that require line of sight between measurement points, space geodetic methods are use line of sight between the measurement points and celestial objects or satellites. Relative positioning can be achieved over very large distances in which the precision is almost independent of the distance between the two measurement points. Altimetry, rather than measuring 3-D changes by positioning techniques, involves only changes in surface elevation. Altimetry measurements are conducted by releasing pulses towards the Earth's surface every several milliseconds, resulting in circular ground measurements along the satellite track. Because of these large circular measurements, altimetry measurements are useful for measuring flat surfaces. InSAR is a powerful method to detect surface change. This method compares pixel-by-pixel SAR phase observations of the same area acquired from roughly the same location in space to produce digital elevation models. Satellite orbits are very sensitive to lateral variations in the Earth's gravity field. Precise measurements of satellite orbits by ranging (distance) and other technologies yield accurate determination of the Earth's shape and its variations over time.
Wdowinski, S., & Eriksson, S. (2009). Geodesy in the 21st Century. EOS,90(18), 153-155.