Geodetic Observations

Geodetic observations are fundamental to establishing and maintaining the Essential Geodetic Variables (EGVs). Without observations, there would be no data, geodetic products, or scientific results to underpin these variables. Observations from space-geodetic techniques are particularly important for establishing accurate and long-term stable global reference frames and for monitoring and understanding the dynamics of the Earth system. Each space-geodetic technique is sensitive to different Earth system signals, operates at different spatial and temporal resolutions, and provides geodetic products with different levels of latency, accuracy, and quality. Satellites at medium Earth orbit (MEO) and geostationary orbit (GEO) are particularly advantageous for positioning because they are less affected by gravitational and atmospheric drag perturbations. In contrast, low Earth orbit (LEO) satellites are particularly well suited for determining the Earth’s gravity field, as they are more sensitive to short-wavelength gravity signals, and for observing changes at the Earth’s surface, including topography, oceans, ice sheets, lakes, rivers, and soil moisture. A major advantage of satellite observations is their ability to provide homogeneous and consistent measurements over large areas of the Earth’s surface, including regions that are difficult or impossible to access with terrestrial techniques. LEO satellite observations are therefore essential for monitoring sea level, ice-sheet mass changes, terrestrial water storage, atmospheric water vapour, high-resolution surface motion, and temporal variations in the Earth’s gravity field. Terrestrial observations complement satellite-based techniques by providing higher spatial and temporal resolution, supporting the validation and calibration of satellite observations, and enabling the detection of short-wavelength and high-frequency signals that may not be resolved by satellite missions. The combination of space- and terrestrial-based observations is therefore essential for developing comprehensive, consistent, and sustained geodetic information across the full range of spatial and temporal scales.