The purpose of external calibration is to enable the pixel intensity of a feature in an image to be converted to a physical quantity such as radar cross-section. This is achieved through the calculation of a calibration constant by the radiometric measurements from transponders and corner reflector targets. These measurements are supplemented by measurements from homogeneous areas such as rainforest. The use of these calibration devices is necessary as it will not generally be possible to know all parameters with sufficient accuracy prior to launch. So a specific exercise is performed during the satellite commissioning phase with subsequent monitoring throughout the routine phase.
There are various aspects related to and associated with external calibration:
Radiometric calibration is applied to correct for the bias of SAR data products. The required absolute calibration factor is derived by measuring the SAR system against reference point targets with well-known radar cross section (transponders and corner reflectors) together with reference distributed targets such as the Amazon rainforest. As the requirement on the radiometric accuracy is 1 dB (3σ) in all four operational modes, it is necessary for multiple measurements to be made for each SM swath and for each IW/EW sub-swath and for each polarisation.
Antenna model verification ensures the provision of precise reference elevation antenna patterns of all operation modes and the gain offset between different beams. Verification of the antenna model is performed by comparison with Elevation Antenna Patterns (EAPs) derived from imagery of the large isotropic distributed target of the Amazon rainforest. Verification is performed for all possible polarisation combinations.
External calibration devices can also be used to measure polarimetric co-registration and polarimetric channel distortion (cross-talk). The co-registration assessment is based on the IRF peak pixel position of the transponders in both polarisations of dual polarisation products. Unlike transponders, a standard type of corner reflector (trihedral) will only have a response in HH or VV and so an IRF response will be seen in only one of the polarisations in the dual-polarisation L1 products. Although unsuitable for co-registration analysis, an assessment of cross-talk can be made using this type of calibration device.
Geometric calibration is required to accurately locate a pixel position in an image to the geographic location on the Earth's surface. This is achieved using the surveyed location of reference targets such as corner reflectors. Comparison with the surveyed and measured location of the reference targets are used to measure both range and azimuth timing biases that can subsequently be included in the SAR processor. To enable this comparison to be performed various corrections, such as the atmospheric path delay, are required using various external information (meteorological, plate tectonic and solid Earth tide).
Antenna pointing determination is performed to achieve correct beam pointing of the antenna in both azimuth and elevation. The azimuth pointing is determined using the receiver mode of transponders while in elevation a special notch acquisition is performed over the Amazon rainforest.