Preprocessing is applied to the Level-0 products before any parameter estimation and image formation steps are performed. Pre-processing includes four main components:
- raw data analysis
- internal calibration
- downlink header validation
- terrain height function.
Raw Data Analysis
The SENTINEL-1 SAR instrument's receive module performs demodulation in the digital domain. Therefore, unlike ENVISAT ASAR which requires corrections to the I and Q channels of the raw signal, for SENTINEL-1, I and Q channel gain imbalance and non-orthogonality corrections are not necessary.
Internal calibration is based on information extracted from the calibration and noise measurements associated with a data-take as part of the initial, interleaved and final internal calibration sequence performed by the instrument. Internal calibration consists of:
- calibration pulse extraction
- replica reconstruction
- reference replica derivation
- instrument drift derivation
- replica whitening
- replica and PG product validation
- noise measurement processing.
Noise Measurement Processing
Noise measurements are recorded during each data-take as part of the initial and final internal calibration packet. Noise measurements are recorded separately for each beam by switching off the TX signal for a sufficient number of TX pulses. In addition, noise-equivalent measurements are represented by the travelling echoes after each interleaved calibration sequence all along the data-take. The noise level is computed as the mean power of the available noise packets for each beam.
In this equation, M is the number of noise packets and N is the number of samples.
Downlink Header Validation
The downlink header validation step is performed to validate the fields of the downlink. It will detect bit errors, missing lines and gaps in the data.
The validation also checks transmission patterns to determine whether the downlink is as expected for such things as the order of swaths, length of bursts and sequence of echo versus calibration and noise packets.
Terrain Height Function
The terrain height function assembles a vector of terrain height values along the azimuth direction of the scene. Each terrain height value is obtained for a particular azimuth time by averaging over a DEM for an azimuth block and covering the entire range dimension of the scene. The azimuth block can be configurable to any number of azimuth lines or for every single azimuth line.
The IPF uses the Global Earth Topography And Sea Surface Elevation (GETASSE30) DEM version 2. The GETASSE30 DEM is a composite of four other data sets: the SRTM30 data, the Altimeter Corrected Elevations (ACE) DEM, the Mean Sea Surface (MSS) data and the EGM96 ellipsoid. The resulting GETASSE30 DEM represents the earth topography and sea surface elevation with respect to the WGS84 ellipsoid at 30 arc second resolution.
In the case of terrain height values being averaged over several azimuth lines, if later processing requires an intermediate value that value is obtained by linear interpolation.
A single terrain height vector is used for different polarisations and for all sub-swaths in IW and EW modes. In particular in the TopSAR case this vector is computed considering only the coverage of the first sub-swath.