A Software Package for the Quality Assessment of the I-PAF Low Bit Rate Mediterranean Scatterometer and Altimeter Products: ERS-1.WSC.DWP and ERS-1.ALT.MPR

Abstract

The software package LBRQC has been developed in the framework of the realization of the Italian Processing and Archiving Facility for the scientific assessment of the off-line Mediterranean products obtained by processing the ERS-1 Scatterometer and Altimeter data.

The software package allows the user, through an interactive and graphically oriented interface, to simply exploit the main functionalities provided by the package: product navigation, product graphical and numerical analysis, correlation analysis with external data.

The LBRQC software package can be operated at the I-PAF station by an LBR Quality Control operator in order to produce quality reports to be distributed to the users together with the product or it may be installed through the interested users for a direct exploitation of the functionalities provided by the package

Keywords: Scatterometer, Altimeter, Quality Control.

Introduction

The Low Bit Rate data processing chain installed at the I-PAF Station generates, on a monthly basis, the off-line products ERS-1.WSC.DWP and ERS-1.ALT.MPR relative to the Mediterranean area. These products are distributed to the PI’s and to the interested users.

The ERS-1.WSC.DWP (Scatterometer Dealiased Wind and Pressure) product contains the off-line 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 ERS-1 scatterometer for each node of the illuminated grid constituted by 19x19 cells covering an area of 475x475Km². Moreover the product contains the pressure field generated from the computed wind field on the same grid of measure .

The ERS-1.ALT.MPR (Altimeter Mediterranean Product) contains the following quantities obtained by processing the ERS-1 Radar Altimeter raw data:

• 20 Hz geophisycally corrected distance from the satellite to the sea surface;
• 1 Hz averaged distance from the satellite to the sea surface and the corresponding standard deviation;
• 1 Hz averaged significant wave heigth (SWH) and the corresponding standard deviation;
• 1 Hz wind speed at nadir and the corresponding standard deviation;
• 1 Hz sea specular points elevation probability density function.

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 ERS-1.WSC.DWP and ERS-1.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 Loading;
• Product Graphical analysis;
• Product Numerical Analysis;
• Reporting.

Data Loading

The Data Loading functionality gives the possibility to ingest several data types on different support media:

• I-PAF LBR products in CEOS format (exabyte);
• Fast Delivery data (UWI) (exabyte);
• F-PAF OPR products in CEOS format (CD-ROM);
• ECMWF wind analysis in GRIB format (CCT);
• ECMWF SWH data in GRIB format (CCT).

Graphical Analysis

The 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:

•  
• time period selection;
•  
• mouse-driven geographic selection;
•  
• selection of frames corresponding to different classification parameters.

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:

• to manual dealiase the product wind field in an interactive mode;
• to perform the free divergence projection of the wind field relative to a sub-area interactively selected by the user;
• to plot the wind derived scalar fields (pressure, divergence and vorticity of wind, divergence and vorticity of stress) over the corresponding product or manual dealiased wind field (Fig.3);
• to simultaneously display the wind field for all the different data sets available (product, manual dealiased, free divergence, ECMWF and Fast Delivery);
• to perform the correlation analysis of the product data quantities (wind speed and direction) of all the available data sets with the ECMWF wind analysis through histograms and scatter plots (Fig.4).

Fig.2 ERS-1.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:

• to simultaneously display all the product quantites with the available corresponding external data (see Fig.5);
• to plot the power spectral density function;
• to plot the crossover points according to the sea surface height difference values;
• to plot the histogram relative to the crossover statistical analysis (see Fig.6);
• to produce histograms and scatter plots for the correlation analysis of the product data quantities (wind speed, significant wave height and sea surface height) of all the available data sets with the corresponding ECMWF wind analysis, ECMWF SWH Wave Model and SSH mathematical modelizations (see Fig.7).

Fig.5: MPR SWH graphical display

Fig.6: Crossover differences histogram

Fig.7: Ionospheric correction scatter plot (OPR vs MPR)

Product Numerical Analysis

The 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:

• sigma naught data simulation and reprocessing;
• parameters regularization;
• estimation of wind derived quantities;
• correlation analysis with external data.

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:

• data editing (see Fig.8);
• power spectral density evaluation;
• crossover analysis.

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.

Reporting

The 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.

References

A.Bartoloni, C.D’Amelio, F.Zirilli, 1995: "Wind reconstruction from Ku-band or C-band 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. 1570-1573, 1993.

A.Bartoloni, C.D’Amelio, G.Milillo, F.Nirchio, 1994: "ERS-1 scatterometer data processing over the mediterranean sea: algorithm development and validation activity at I-PAF", Proceedings of the 14th EARSeL Symposium, Goteborg, Sweden, pg. 431-437, 6-8 June 1994.

A.Bartoloni, C.Celani, G.Milillo, F.Nirchio, 1997: "Processing and validation of the ERS-1 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