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

The delay induced by the Earth's atmosphere on the Global Positioning Syste m (GPS) signal has been exploited in the last decade for atmospheric remote sensing. Ground-based GPS measurements are traditionally used to derive co lumnar water vapor content, while space-based GPS measurements, obtained by a receiver in a low-Earth orbit tracking GPS satellites occulting behind t he Earth's atmosphere, yield accurate, high-resolution profiles of refracti vity, 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 inve rsion scheme where spherical symmetry is assumed for the atmosphere, and th e refractivity is modeled as piecewise exponential, with scale height chang ing from one atmospheric layer to the next. Additional refractivity data, d erived from a model, might be introduced above the receiver as an a priori constraint, and are treated as properly weighted additional measurements. T he exponential scale heights and a normalizing value of refractivity are re trieved by minimizing, in a least squares sense, the residuals between meas ured bending angles and refractivity and those calculated on the basis of t he exponential model and ray-tracing. As a first validation step, we illust rate results comparing refractivity and temperature profiles obtained by th is generalized ray-tracing scheme against those derived via the Abel invers ion for the GPS/MET experiment. Additionally, we present results for a hypo thetical 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 retri eval both below and above the receiver at a set of locations in the atmosph ere ranging from middle to tropical latitudes. The main objective is that o f 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 he ights ranging from the Earth's surface to slighly above the receiver locati on can be derived by GPS data collected from within the atmosphere.