Level-1 Post Processing Algorithms
Post-processing generates the output SLC and GRD products as well as quicklook images. Post-processing consists of post-processing range processing, post-processing azimuth processing and post-processing output processing as shown in the figure below.
The processing is applied to each sub-swath for IW and EW and for each azimuth block consisting of a burst for IW and EW and an entire vignette for WV.
Post-Processing Range Processing
Post-processing range processing is performed on each range line within an individual burst of a sub-swath for TOPSAR or vignette for WV.
Post-Processing Azimuth Processing
Post-processing azimuth processing is performed on each azimuth line for each range look within an individual burst of a sub-swath for TOPSAR or vignette for WV.
Post-processing GRD Output Processing
For GRD products, the post-processing output processing consists of optionally removing the thermal noise, de-burst and merge for TOPSAR mode, generation of quicklooks and writing to the output file format.
TOPSAR Debursting and Sub-Swath Merging
The TOPSAR modes acquire data in bursts and for several sub-swaths. For GRD products, the bursts are concatenated and sub-swaths are merged to form one image.
Bursts overlap minimally in azimuth and sub-swaths overlap minimally in range. Bursts for all beams have been resampled to a common grid during azimuth post-processing.
In the range direction, for each line in all sub-swaths with the same time tag, merge adjacent sub-swaths. For the overlapping region in range, merge along the optimal sub-swath cut. The optimal cut is defined from the Noise Equivalent Sigma Zero (NESZ) profiles between two sub-swaths.
If the two NESZ profiles intersect inside the overlapping region, the position of the intersection point is the optimal cut.
If the two profiles do not intersect, all the points in the overlapping region are taken from the sub-swath that has the lowest NESZ over the overlap region.
In the azimuth direction, bursts are merged according to their zero Doppler time. Note that the black-fill demarcation is not distinctly zero at the end or start of the burst. Due to resampling, the data fades into zero and out. The merge time is determined by the average of the last line of the first burst and the first line of the next burst. For each range cell, the merging time is quantised to the nearest output azimuth cell to eliminate any fading to zero data.
Application LUT Scaling
Application LUTs are used to apply a range-dependent gain function to the processed data prior to generation of the final image output. The application LUT scaling is used to optimise the radiometric scaling of the main feature of interest, while optimising the available dynamic range in the output product and to compensate for changes in the radar backscatter with changing incidence angles. LUT's that could be used include:
Level-1 SLC and GRD images are then scaled to 16 bit and saved in GeoTIFF file format.
Quicklooks are lower resolution images of the product data used to preview the data. Quicklooks are generated by power detecting, averaging and decimating in both azimuth and range directions by a configurable amount. IW and EW SLC product quicklooks are first de-burst and merged.
Single polarisation products are represented with a grey scale image. Dual polarisation products are represented by a single composite color image in RGB with the red channel (R) representing the first polarisation, the green channel (G) representing the second polarisation and the blue channel (B) representing an average of the absolute values of the two polarisations.
Quicklook images are scaled to 8 bit and saved in PNG file format.