Search

We describe and present preliminary results from an independent bending-angle algorithm implementation for the calculation of radiowave bending angles propagated from a Global Positioning System (GPS) to a Low Earth Orbiting (LEO) satellite. The algorithm utilizes raw atmospheric excess phases and satellite kinematics to determine Doppler frequency shifts of the GPS signals from which bending-angle profiles are derived. The intent is twofold: (a) perform a series of sensitivity studies to investigate the effect of Doppler frequency shift and satellite velocity variations on bending-angle profiles; and (b) examine and compare our results against the bending-angle profiles provided by the Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) Data Analysis and Archive Centre (CDAAC). In this paper, we employ the same GPS observations as by CDAAC to analyze various occultation events in 2006, 2007, 2008, and 2009. Our principal finding is that radiowave bending angles exhibit an inversely proportional behaviour to orbital velocity variations while they change proportionally to the variations of Doppler frequency shift. Our studies also reveal that bending angles are more sensitive to orbital velocity than to Doppler frequency shift variations and show 'wave-like' structures in the upper part of the atmosphere, unlike COSMIC-derived profiles. We discuss the differences between our derived bending-angle profiles and the COSMIC-derived profiles, emphasizing the advantages of our algorithm implementation. Nous decrivons ici les resultats preliminaires de l'implantation d'un algorithme independant de modulation d'angle pour le calcul de la modulation d'angles radioelectriques propages d'un systeme de positionnement global << GPS >> vers un satellite d'orbite basse terrestre << LEO >>. L'algorithme utilise les phases atmospheriques excedentaires brutes et la cinematique du satellite pour determiner les decalages Doppler des signaux GPS a partir desquels sont derives les profils de modulation des angles. Le but est double : (a) effectuer une serie d'etudes de sensibilite afin de connaitre l'effet du decalage Doppler et des variations de vitesse du satellite sur les profils de modulation d'angles et (b) examiner et comparer nos resultats par rapport aux profils de modulation d'angles fournis par le Constellation Observing System for Meteorology, Ionosphere & Climate << COSMIC >> Data Analysis and Archive Centre << CDAAC >>. Dans cet article, nous utilisons les memes observations GPS que le CDAAC pour analyser divers evenements d'occultation survenus en 2006, 2007, 2008 et 2009. Notre principale decouverte a ete que les modulations d'angles montrent un comportement inversement proportionnel aux variations de vitesses orbitales, alors qu'elles changent proportionnellement aux variations du decalage Doppler. Nos etudes demontrent aussi que les modulations d'angles sont plus sensibles a la vitesse orbitale qu'au decalage Doppler et elles demontrent des structures a << formes d'ondes >> dans la partie superieure de l'atmosphere, a l'oppose des profils derives des donnees COSMIC. Nous discutons des differences entre nos profils de modulation d'angles et les profils COSMIC, soulignant les avantages de la mise en oeuvre de notre algorithme. [Traduit par la Redaction], Introduction The Global Positioning System (GPS) is a constellation of 24 satellites orbiting the Earth at an altitude of ~26 600 km, evenly distributed over five orbital planes of 55° [...]

Change detection within non-stationary and unequally spaced remote sensing time series has become a key methodology for a broad range of environmental applications. A new method of analysing vegetation variation over lands is proposed. Four regions in northern Tunisia with various characteristics are selected, and a non-stationary and unequally spaced Normalized Difference Vegetation Index (NDVI) time series is obtained for each region since 2000. The Landsat 7 remote sensing satellite imagery with insignificant cloud-shadow coverage is used to calculate the NDVI after atmospheric correction. The Least-Squares Wavelet (LSWAVE) software is implemented to rigorously analyse each NDVI time series and study the relationship between the vegetation of olive trees and temperature/precipitation in one of the regions. To investigate possible effects of temperature on the green cover caused by increasing water salinity, the coherency between the NDVI and sea surface temperature time series is also shown in the region of Lake Ichkeul in Tunisia. [ABSTRACT FROM AUTHOR]