GOAS [Global Positioning System (GPS) Occultation Analysis System] is a computer program that accepts signaloccultation data from GPS receivers aboard low-Earth-orbiting satellites and processes the data to characterize the terrestrial atmosphere and, in somewhat less comprehensive fashion, the ionosphere. GOAS is very robust and can be run in an unattended semi-operational processing mode. It features sophisticated retrieval algorithms that utilize the amplitudes and phases of the GPS signals. It incorporates a module that, using an assumed atmospheric refractivity profile, simulates the effects of the retrieval processing system, including the GPS receiver. GOAS utilizes the GIPSY software for precise determination of orbits as needed for calibration. The GOAS output for the Earth's troposphere and mid-to-lower stratosphere consists of high-resolution (<1 km) profiles of density, temperature, pressure, atmospheric refractivity, bending angles of signals, and water-vapor content versus altitude from the Earth's surface to an altitude of 30 km. The GOAS output for the ionosphere consists of electron-density profiles from an altitude of about 50 km to the altitude of a satellite, plus parameters related to the rapidly varying structure of the electron density, particularly in the E layer of the ionosphere.
This program was written by George Hajj, Emil Kursinski, Stephen Leroy, Byron Iijima, Manuel de la Torre Juarez, Larry Romans, and Chi Ao of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Software category. This software is available for commercial licensing. Please contact Don Hart of the California Institute of Technology at (818) 393-3425. Refer to NPO-30596.
This Brief includes a Technical Support Package (TSP).
Processing GPS Occultation Data To Characterize Atmosphere
(reference NPO-30596) is currently available for download from the TSP library.
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Overview
The document outlines the GPS Occultation Analysis System (GOAS), a software package developed by NASA's Jet Propulsion Laboratory (JPL) for remotely sensing the atmosphere using Global Positioning System (GPS) occultation data. GOAS is designed for post-processing atmospheric variables, which include refractivity, density, temperature, pressure, water vapor, and electron density in the ionosphere. The software aims to achieve high accuracy and resolution in these variables, which are crucial for weather prediction, climate research, and atmospheric science.
GOAS operates using a one-dimensional variational (1DVar) approach to separate dry and wet contributions to refractivity. It utilizes a first guess profile derived from weather analyses to estimate temperature and water vapor profiles. The accuracy of the derived atmospheric variables is challenging to ascertain due to the lack of absolute observations, but predictions suggest temperature accuracy within a few tenths of a Kelvin and geopotential accuracy of approximately 10 meters. Water vapor accuracy is estimated to be between 0.2 to 0.5 g/kg, depending on the atmospheric conditions.
The system processes a high volume of occultation data daily, employing optimal smoothing and filtering techniques to enhance the quality of the results. GOAS is capable of detecting and correcting erroneous integer phase cycle shifts, ensuring the reliability of the data produced. The output includes various atmospheric profiles, such as bending angles, refractivity, density, temperature, pressure, and water vapor, as well as electron density profiles in the ionosphere.
GOAS was first demonstrated in 1995 with data from the GPS/MET mission, marking a significant advancement in the use of radio occultation techniques for atmospheric analysis. The software has evolved over time, incorporating contributions from researchers worldwide and expanding its capabilities to run on different operating systems, including LINUX.
The document emphasizes the importance of GOAS in advancing our understanding of the atmosphere and its dynamics, highlighting its role in improving weather forecasting and climate modeling. It serves as a technical support package under NASA's Commercial Technology Program, aiming to disseminate aerospace-related developments with broader scientific and commercial applications.
