202 The Open Atmospheric Science Journal, 2010, 4, 202-209 1874-2823/10 2010 Bentham Open Open Access Atmospheric Profiling in the Inter-Tropical Ocean Area Based on Neural Network Approach Using GPS Radio Occultations Fabrizio Pelliccia 1 , Stefania Bonafoni *,1 , Patrizia Basili 1 , Piero Ciotti 2 and Nazzareno Pierdicca 3 Università degli Studi di Perugia, Dipartimento di Ingegneria Elettronica e dell’Informazione, via G. Duranti,93 – 06125 Perugia, Italy Università di L’Aquila, Dipartimento di Ingegneria Elettrica e dell’Informazione, Poggio di Roio – 67040 L'Aquila, Italy Università di Roma “La Sapienza”, Dipartimento di Ingegneria Elettronica, via Eudossiana, 18 – 00184 Roma, Italy Abstract: In this study we have proposed a method based on neural networks to retrieve refractivity, temperature, pressure and humidity profiles by using FORMOSAT-3/COSMIC GPS radio occultation data. To overcome the constraint of an independent knowledge of one atmospheric parameter at each GPS occultation, we trained three neural networks with refractivity profiles as input computed from the geometrical occultation parameters relative to the FORMOSAT- 3/COSMIC satellites, while the targets were the dry and wet refractivity profiles and the dry pressure profiles obtained from the contemporary European Centre for Medium-Range Weather Forecast data. We have considered 1041 available satellite radio occultations covering the entire ocean area spanning within the Tropics during July-August 2006. We have used 937 profiles for training the neural networks, the remaining 104 ones for the independent test. Keywords: GPS, radio occultation, neural network, atmospheric profiling. 1. INTRODUCTION Global Positioning System (GPS) radio occultation (RO) is considered as a global sounding technique for providing atmospheric profiles useful for numerical weather prediction and climate change studies. The radio occultation system employs GPS receivers placed on Low-Earth Orbit (LEO) satellites to sound the Earth’s neutral atmosphere and ionosphere evaluating the additional delay affecting a radio signal when passing through the atmosphere due to the refractivity index magnitude and its variations [1, 2]. Since 1995, when the first LEO satellite Microlab-1 (GPS/MET mission) was operational [3, 4], several GPS-RO satellite missions have been launched, which include Stellenbosch University Satellite (SUNSAT), Challenging Mini-Satellite Payload (CHAMP), Satelite de Aplicaciones Cientificas-C (SAC-C), Gravity Recovery and Climate Experiment (GRACE) and Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC). GPS occultations provide an active probing of the atmosphere working under all-weather conditions due to the insensitivity of the GPS signal wavelength to scattering by clouds, aerosols, and precipitation, with relatively high vertical resolution throughout the depth of the atmosphere associated with the limb-viewing geometry. A limiting factor for this technique is the horizontal resolution. It is set by the Fresnel diffraction-limited pencil-shaped sampling volume *Address correspondence to this author at the Università degli Studi di Perugia, Dipartimento di Ingegneria Elettronica e dell’Informazione, via G. Duranti,93 – 06125 Perugia, Italy; Tel: +39-75/585 3663; Fax: +39-75/585 3654; E-mail: stefania.bonafoni@diei.unipg.it of each measurement which has a horizontal resolution of about 200 km in the direction along the occulted link and a resolution of 1 km or better in the cross-link and vertical directions [5]. The GPS-RO technique is exploited to obtain profiles of refractivity, temperature, pressure and humidity in the atmosphere at global scale, and several investigations have demonstrated that the retrieval accuracies are comparable to traditional atmospheric remote sensing techniques [6, 7]. Even though the atmospheric refractivity profiling by radio occultation is a well-defined problem, care must be taken to analyze factors affecting the occulted signal (multipath, satellite motion etc.) and to compute temperature and particularly humidity profiles from refractivity [5]. The accuracy of atmospheric profile estimation is affected by the use of proper boundary conditions and by the presence of water vapour in the atmosphere, that complicates the interpretation of the refractivity [8]. Refractivity profiles can be converted in a straightforward way into pressure and temperature profiles in regions where water vapour is negligible such as in the upper troposphere. In the middle and lower troposphere additional information is necessary, therefore GPS refractivity measurements are employed to derive profiles of water vapour partial pressure or specific humidity given an independent knowledge of temperature obtained from independent observations (i.e. radiosoundings or data from atmospheric numerical modeling). For example, an iterative method exploiting GPS RO refractivity measurements and temperature profiles obtained from radiosoundings or models to derive profiles of water