Radiometric correction in range-SPECAN SAR processing

• Main Author: Hobooti, Haleh
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/3709

This thesis investigates the reasons for, and proposes correction methods to reduce the scalloping inherent in the range dimension in images processed using the SPECAN SAR processing algorithm. These corrections methods are successfully tested using ERS-1 satellite data. The SPECAN algorithm was developed in 1979 by MacDonald Dettwiler and Associates, as a multilook version of the deramp/FFT method of pulse compression. The algorithm provides an efficient method of producing spaceborne SAR imagery of modest resolution, which makes it ideal for quicklook imaging. Widespread use of the algorithm in quicklook imaging, however, is hindered by the scalloping present in both the range and azimuth dimensions of the image. This thesis concentrates on correction of range scalloping by presenting explanations for the two types of scalloping present in the range dimension of SPECAN processed images. The basic scalloping present in all scenes is due to the time variation in the envelope of the transmitted signal. The extra scalloping seen in some scenes is due to clipping of the ERS-1 raw data. Because of the geometry of the SPECAN processing cycles, the radiometry of the output is sensitive to the transmitted chirp amplitude. The 0.5 dB power difference between the beginning and end of the transmitted chirp is reflected in each compressed output segment, creating a periodic lightening and darkening throughout the image. This banding effect is especially noticeable in low contrast scenes. This problem is corrected using the chirp replica which is embedded in the data header as an estimate of the pulse shape. The extra scalloping observed in some high reflectivity scenes is attributed to the clipping of the ERS-1 raw data in these scenes. Clipping causes attenuation in the output power. This power loss varies along range according to the degree of saturation, and is different for each compressed data block, thereby creating a discontinuity between adjacent compressed segments which adds to (or subtracts from) the basic scalloping effect. This problem is corrected using the relationship between power loss and degree of saturation in the Gaussian distributed raw data.