Level-1 focused data are the generally available products intended for most data users. The Level-0 product is transformed into a Level-1 product by the application of algorithms and calibration data to form a baseline engineering product from which higher levels are derived.
The processing involved to produce Level-1 data products includes pre-processing, Doppler centroid estimation, single look complex focusing, and image and post-processing for generation of the SLC and GRD products as well as mode specific processing for assembling of multi-swath products.
Figure 1: Level-1 Processing Flow
For converting digital pixel values to radiometrically calibrated backscatter, all the required information can be found in the product. A calibration vector is included as an annotation in the product allowing simple conversion of image intensity values into sigma or gamma nought values.
Level-1 products can be either Single Look Complex (SLC) or Ground Range Detected (GRD). Each acquisition mode can potentially generate Level-1 SLC and GRD products. GRD resolutions will depend on the mode and the level of multi-looking.
Single Look Complex
Level-1 Single Look Complex (SLC) products consist of focused SAR data, geo-referenced using orbit and attitude data from the satellite, and provided in slant-range geometry. Slant range is the natural radar range observation coordinate, defined as the line-of-sight from the radar to each reflecting object. The products are in zero-Doppler orientation where each row of pixels represents points along a line perpendicular to the sub-satellite track.
The products include a single look in each dimension using the full available signal bandwidth and complex samples (real and imaginary) preserving the phase information. The products have been geo-referenced using the orbit and attitude data from the satellite and have been corrected for azimuth bi-static delay, elevation antenna pattern and range spreading loss.
Stripmap SLCs contain one image for its single swath per polarisation band. IW, having three swaths, has three images in single polarisation and six images for dual polarisation. EW, having five swaths, has five images for single polarisation and ten images for dual polarisation.
For IW and EW, each sub-swath consists of a series of bursts. Each burst has been processed as a separate SLC image. The individually focused complex burst images are included, in azimuth-time order, into a single sub-swath image, with black-fill demarcation in between, similar to the ENVISAT ASAR Wide ScanSAR SLC products.
For IW, a focused burst has a duration of ~2.75 seconds and a burst overlap of approximately ~0.4 seconds. For EW, a focused burst has a duration of ~3.19 seconds with an overlap of ~0.1 seconds. The overlap slightly increases in range within a sub-swath. Unlike ASAR WSS which contains a large overlap between beams, for SENTINEL-1 TOPSAR products, the imaged ground area of adjacent bursts only marginally overlap in azimuth just enough to provide contiguous coverage of the ground. This is due to the one natural azimuth look inherent in the data.
Images for all bursts in all sub-swaths of an IW SLC product are re-sampled to a common pixel spacing grid in range and azimuth. Burst synchronisation is ensured for both IW and EW products.
The Swath Timing data set record in SLC products contains information about the bursts including dimensions, timing and location that can be used to merge the bursts and swaths together.
Ground Range Detected
Level-1 Ground Range Detected (GRD) products consist of focused SAR data that has been detected, multi-looked and projected to ground range using an Earth ellipsoid model such as WGS84. The ellipsoid projection of the GRD products is corrected using the terrain height specified in the product general annotation. The terrain height used varies in azimuth but is constant in range.
Ground range coordinates are the slant range coordinates projected onto the ellipsoid of the Earth. Pixel values represent detected magnitude. Phase information is lost. The resulting product has approximately square resolution pixels and square pixel spacing with reduced speckle at a cost of reduced geometric resolution.
For the IW and EW GRD products, multi-looking is performed on each burst individually. All bursts in all sub-swaths are then seamlessly merged to form a single, contiguous, ground range, detected image per polarisation channel.