Ch 33. GNSS Radio Occultation

Chapter Overview:

Atmospheric sounding using GNSS signals has proved to be a useful, multifaceted‚ and robust method for measuring properties such as temperature, pressure and humidity in the atmosphere. GNSS-based sounding is a widely-used satellite-based observation to improve weather forecasts. It is particularly effective for measuring subtle changes in Earth’s climate, due to its high accuracy. The soundings, which produce vertical profiles of temperature, pressure and water vapor from Earth’s surface to the stratosphere, are obtained via a technique called “radio occultation” (RO). RO uses a specially designed GNSS receiver in low-Earth orbit to receive GNSS transmissions after they have traversed the Earth’s atmosphere and ionosphere in a limb-viewing geometry. The name “radio occultation” is derived from the fact that the GNSS transmitters occult, or “hide”, behind the Earth after the signal is transmitted. In this chapter, we provide a detailed technical description of the RO technique as it applies to sounding the atmosphere and the ionosphere. In the ionosphere, RO retrieves the density of ionospheric electrons at altitudes above 90 km. We provide the reader with a detailed understanding of how the GNSS signal is affected as it propagates through the atmosphere in the occultation geometry, and show how atmospheric properties are retrieved from the GNSS signal received in orbit. We adopt a unique requirements-based “system design” perspective so that an engineering team can design a receiver and antenna system that meets specified precision requirements for the retrieved atmospheric variables. Numerous references to source material and detailed derivations provide a self-contained “handbook” for RO that new and experienced scientists and engineers will find useful. A brief survey of scientific and weather applications of RO is provided to conclude the chapter.

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