GNSSRO

GNSS radio occultation (GNSSRO) is a satellite-based sensing technique that uses signals from global navigation satellite systems to probe the Earth's atmosphere. As a GNSS signal passes through the atmosphere and is occulted by the Earth, its path is bent and delayed in a way that depends on atmospheric refractivity. Low-Earth-orbit satellites equipped with GNSS receivers capture the occulted signals from several GNSS constellations, enabling vertical profiling of atmospheric properties.

The measurements include bending angles, Doppler shifts, and phase paths across multiple frequencies. By combining dual- or multi-frequency observations, the ionospheric delay can be separated from neutral-atmosphere effects. Retrieval algorithms use the refractivity profile to infer temperature, pressure, and humidity in the neutral atmosphere, and electron density in the ionosphere, typically with high vertical resolution.

GNSSRO data support numerical weather prediction, climate research, and atmospheric science. They provide near-global vertical profiles with high precision, including regions with sparse radiosonde coverage and under all weather conditions, contributing to improved atmospheric analyses and forecast skill.

Key missions include CHAMP, FORMOSAT-3/COSMIC, and COSMIC-2, among others. The technique remains a foundational method for atmospheric sounding, with ongoing developments to increase data coverage, sampling density, and integration with other remote-sensing systems.

Limitations of GNSSRO involve uneven geographic sampling dictated by occultation geometry, reliance on precise satellite orbits and clocks, and model-dependent assumptions in the retrieval process. These factors can introduce uncertainties, particularly in highly structured or rapidly changing weather systems, though continued improvements in instrumentation and processing mitigate many challenges.