Radio occultation (RO) is a remote sensing technique that uses the propagation of radio waves through the Earth’s atmosphere to probe its temperature and refractive index structure. The method was first proposed by V. A. Kotelnikov in 1948 and was developed for use in meteorology and ionospheric physics. RO has since been used for atmospheric sounding on a variety of platforms, including satellites, aircraft, rockets, balloons, and ground-based facilities.
The basic principle of RO is simple: as radio waves propagate through the atmosphere they are bent or refracted by variations in the atmospheric refractive index. The amount of bending depends on the gradient of the refractive index along the ray path; therefore, measurements of the bending can be used to infer vertical profiles of temperature and/or moisture in the atmosphere. In practice, RO measurements are usually made using radiosondes (instrumented weather balloons) or GPS signals from artificial satellites.
The first successful demonstration of RO occurred during a rocket launch from White Sands Missile Range in 1961. This early experiment showed that RO could be used to measure electron density profiles in the upper atmosphere with good accuracy. Since then, numerous other studies have demonstrated the utility of RO for measuring various atmospheric parameters such as temperature, humidity, pressure, winds, and composition. More recently, advances in satellite communications technology have led to the development of space-based instruments capable of making continuous RO measurements from orbit. These instruments have revolutionized our ability to study global climate change and monitor dynamic processes in Earth’s atmosphere on a near-continuous basis.
RO measurements offer several advantages over traditional airborne sounding techniques such as radiosondes. First, because they utilize naturally occurring radio waves (e.g., GPS signals), they do not require expensive payloads or launch vehicles. Second, because they can be made continuously and remotely (from space or ground), they provide dense coverage over large areas without incurring high costs associated with deploying many radiosondes simultaneously. Finally, due to their small footprint (~1 km2), multiplexing capability (i