 



A Software Package for the Quality Assessment of the IPAF Low Bit Rate Mediterranean Scatterometer and Altimeter Products: ERS1.WSC.DWP and ERS1.ALT.MPR
Abstract
IntroductionThe Low Bit Rate data processing chain installed at the IPAF Station generates, on a monthly basis, the offline products ERS1.WSC.DWP and ERS1.ALT.MPR relative to the Mediterranean area. These products are distributed to the PI’s and to the interested users. The ERS1.WSC.DWP (Scatterometer Dealiased Wind and Pressure) product contains the offline processed wind vector fields (referred at a height of 10 meters in a neutral stable atmosphere) retrieved from the s _{0} backscattering coefficients measured by the ERS1 scatterometer for each node of the illuminated grid constituted by 19x19 cells covering an area of 475x475Km^{2}. Moreover the product contains the pressure field generated from the computed wind field on the same grid of measure . The ERS1.ALT.MPR (Altimeter Mediterranean Product) contains the following quantities obtained by processing the ERS1 Radar Altimeter raw data:
The product contains 80 1 Hz measurements, each measurement corresponds to a 7 Km along track extension so that the product coverage corresponds to an extension of 500 Km along track. A validation activity has been performed in order to verify the consistency of the scatterometer and altimeter data processing algorithms for the generation of the ERS1.WSC.DWP and ERS1.ALT.MPR products using validation campaigns data (Bartoloni et. al., 1994, 1997) The software package LBRQC has been designed in order to perform the quality control of the LBR products on a routine basis. The user interface of the package has been implemented using the IDL language, the data handling and computation routines have been written in C and FORTRAN. It can run on a DEC ALPHA Station; moreover a PC version implementing some basic functionalities for the product quality control is available for the distribution to the users. The logical architecture of the LBRQC is shown in Fig.1. Fig.1: LBRQC Logical Model The main functionalities provided by the package are:
Data LoadingThe Data Loading functionality gives the possibility to ingest several data types on different support media:
Graphical AnalysisThe Graphical Analysis allows to display the product on a geographic map, determined by the product geographic coverage, by showing the satellite passes contained in the product (Fig.2). It allows to select one or more frames contained in the product according to different selection criteria:
The selected frames may be displayed by showing the main quantities contained in the product such as wind and pressure fields for the scatterometer product, satellite height with respect to the sea surface and mean sea level with respect to the reference ellipsoid for the altimeter product. Concerning the scatterometer product the Graphical Analysis gives the possibility to perform different operations on the selected frames:
Fig.2 ERS1.WSC.DWP scatterometer frames Fig. 3: Wind and Pressure Fields Fig. 4: Wind Speed Histogram Analysis The Graphical Analysis of the altimeter product allows the following operations:
Fig.5: MPR SWH graphical display Fig.6: Crossover differences histogram
Fig.7: Ionospheric correction scatter plot (OPR vs MPR) Product Numerical AnalysisThe LBRQC package gives the possibility to perform different numerical analysis, according to the considered Scatterometer or Altimeter product, in order to determine numerical and statistical quantities necessary for the product quality assessment. The Scatterometer Numerical Analysis mainly consists in:
The parameters regularization consists in the exploitation of physical properties of the processed quantities aimed at the determination of mathematical relationships to be used as regularizing filters. The filtering techniques implemented in the LBRQC package are the free divergence projection and the manual dealiasing of the wind data contained in the scatterometer product. Moreover the Scatterometer product analysis allows to generate wind derived quantities such as pressure field, divergence and vorticity of wind field and divergence and vorticity of stress field. The Altimeter Numerical Analysis consists in the following activities:
The filtering of parameters is essentially based on the development of selection criteria for the determination of spurious measurements ("outlayers"). The determination of such measurements is important in order to eliminate fictitious errors in the processed data before performing the correlation analysis with external data for the product quality assessment. Fig.8: MPR 1Hz SSH data editing The evaluation of the power spectral density relatively to the altimeter tracks (that is the Fourier transform of the autocorrelation function computed on different data sequences) is important in order to show peculiar features of the processed data error and to subsequently determine a tailored error treatment. The crossover analysis consists in the analysis of the sea surface height difference in correspondence of the crossover points with several geophysical corrections. Finally quality parameters may be obtained, for both the scatterometer and altimeter product, performing the correlation analysis of all the product data with respect to externally provided data by computing the root mean square difference, the vector difference mean value and the correlation and bias coefficients between the considered data sets.
ReportingThe reporting functionality gives the possibility to print the results of all the performed graphical and numerical analysis on paper and on winword compatible files. Each graphical ouput shall be accompanied by a sheet containing the textual information provided by the user during the product analysis. ReferencesA.Bartoloni, C.D’Amelio, F.Zirilli, 1995: "Wind reconstruction from Kuband or Cband scatterometer data", IEEE Transactions of Geoscience and Remote Sensing, Vol.31, No.5, September 1995. A.Bartoloni, C.Celani, 1993: "A new algorithm for the reconstruction of the time delay from sea echo data of a radar altimeter, Proceedings of IGARSS '93 Symposium, Vol. IV, pg. 15701573, 1993. A.Bartoloni, C.D’Amelio, G.Milillo, F.Nirchio, 1994: "ERS1 scatterometer data processing over the mediterranean sea: algorithm development and validation activity at IPAF", Proceedings of the 14th EARSeL Symposium, Goteborg, Sweden, pg. 431437, 68 June 1994. A.Bartoloni, C.Celani, G.Milillo, F.Nirchio, 1997: "Processing and validation of the ERS1 radar Altimeter data at the Italian PAF", to appear on 3rd ERS Symposium’97 Proceedings. Keywords: ESA European Space Agency  Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry 

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