A novel approach to atmospheric profiling with a mountain‐based or airborne GPS receiver

Abstract

The delay induced by the Earth's atmosphere on the Global Positioning System (GPS) signal has been exploited in the last decade for atmospheric remote sensing. Ground‐based GPS measurements are traditionally used to derive columnar water vapor content, while space‐based GPS measurements, obtained by a receiver in a low‐Earth orbit tracking GPS satellites occulting behind the Earth's atmosphere, yield accurate, high‐resolution profiles of refractivity, temperature, and water vapor. A GPS receiver on a mountain top or an airplane with a “downward looking” field of view toward the Earth's limb is a novel concept presented here. We describe a generalized ray‐tracing inversion scheme where spherical symmetry is assumed for the atmosphere, and the refractivity is modeled as piecewise exponential, with scale height changing from one atmospheric layer to the next. Additional refractivity data, derived from a model, might be introduced above the receiver as an a priori constraint, and are treated as properly weighted additional measurements. The exponential scale heights and a normalizing value of refractivity are retrieved by minimizing, in a least squares sense, the residuals between measured bending angles and refractivity and those calculated on the basis of the exponential model and ray‐tracing. As a first validation step, we illustrate results comparing refractivity and temperature profiles obtained by this generalized ray‐tracing scheme against those derived via the Abel inversion for the GPS/MET experiment. Additionally, we present results for a hypothetical situation where the receiver is located within the atmosphere at a height of 5 km. For the last case we investigate the accuracy of the retrieval both below and above the receiver at a set of locations in the atmosphere ranging from middle to tropical latitudes. The main objective is that of establishing whether the bending measurements have sufficient strength to allow for retrieval of refractivity below and possibly above the receiver location. Our findings suggest that accurate profiles of refractivity at heights ranging from the Earth's surface to slighly above the receiver location can be derived by GPS data collected from within the atmosphere.