On-board data reduction for the Envisat Advanced Synthetic Aperture Radar : evaluation of the impact on interferometric and wave mode applications
Creators: name: McLeod, Ian
Publisher: name: University of British Columbia
Year: 2009

description: The ENVISAT remote sensing satellite, to be launched by the European Space Agency in 1998, will carry the Advanced Synthetic Aperture Radar (ASAR) microwave sensor. In order to ease the severity of data storage and downlink restrictions, an on-board SAR data reduction algorithm was designed by MacDonald Dettwiler and Associates (MDA) for EN VIS A T in 1993 [14]. This algorithm, called Flexible Block Adaptive Quantization (FBAQ) was tested extensively during development, to determine the impact of the algorithm on SAR image quality. However, many common uses of SAR data involve further processing of the SAR images. Two very common examples of this are Interferometric SAR (InSAR) where phase differences between two SAR images of a common scene can be used to estimate topography, and Wave Mode SAR, where the power spectrum of SAR images of ocean waves are used to determine wave characteristics such as direction and wavelength. For these applications, various qualities of the SAR data are exploited which may not be directly related to image quality. Therefore, in this study, an extension of the previous work performed at MDA, the evaluation of the FBAQ algorithm was expanded to include the SAR applications of Interferometry and Wave Mode. The goals of the work were first to quantitatively assess the degradation of InSAR and Wave Mode results due to FBAQ, and secondly to determine if the quality of the results were acceptable. To accomplish the above goals, Wave Mode and InSAR processing was performed on ERS- 1 SAR data (modified to reflect the properties of ENVISAT data [16]) that had been encoded using FB AQ. Error measures taken against results produced using un-encoded data were used to quantify the impact of FBAQ on both InSAR height estimation and Wave Mode ocean parameter estimation. For InSAR, all possible data reduction ratios were used to determine which was acceptable for precision generation of Digital Elevation Models (DEMs). For Wave Mode, only 8- bits/sample to 2-bits/sample FBAQ was used due to storage limitations during acquisitions over oceans. To determine whether the quality of the FBAQ Wave Mode results were acceptable, they were compared to 2-bits/sample linearly truncated Wave Mode data, which represents one of the current methods used to compress Wave mode data aboard ERS-1 and ERS-2. The results of the study were as follows. For InSAR, only 4-bit FBAQ, with an average RMS height estimation uncertainty increase of less than 3 %, was found to produce topographical estimation acceptable for precision DEM generation. The 4-bit FBAQ also presented no problems for image registration and phase unwrapping, two important processing steps in InSAR processing. It was verified, however, that InSAR processing was also possible for 3-bit and 2-bit FBAQ encoding levels, which may find use in lower precision mapping. As well, important insights were gained into the characterization and spatial distribution of FBAQ encoding noise and coherence magnitude degradation, important factors in interferometric quality. For Wave Mode, the 2-bit FBAQ was found to produce results of better quality than those currently available using the 2-bit linear truncation option of ERS-1 and ERS-2. Location of spectral peaks in the ESA Wave Product were on par with 8-bit data, while the spectral peak magnitudes were degraded by about 10 %. This compares favorably, however, to the current 2-bit Wave Mode data which was found to experience degradation as high as 20%. As well, insight was gained into the spectral error distribution of FBAQ data, the impact of ships within the wave scene, the importance of normalization procedures in wave mode analysis, and the impact of data saturation on FBAQ data quality. Overall, the FBAQ algorithm was found to perform acceptably well for InSAR and Wave Mode applications.