External calibration derives the calibration constant by measurement of corner reflector targets and homogeneous areas such as rainforest with exactly known backscatter coefficients. This is necessary as it will not generally be possible to know all parameters with sufficient accuracy prior to the in-flight measurements.
External calibration comprises five steps as shown below.
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. Due to the high demand on the radiometric accuracy of 1 dB (3σ) in all four operational modes, it is recommended to measure at least one beam of each mode against the SENTINEL-1 transponders deployed at different locations. Each selected beam will be measured during two passes (ascending and descending). Furthermore, two receive polarisation combinations per operation mode are to be measured simultaneously. By measuring SENTINEL-1 against the three transponders for selected beams, the radiometric calibration can be performed within a limited number of repeat cycles. The absolute calibration factor of all other beams is then derived by applying the antenna model.
Antenna Model Verification
Antenna model verification ensures the provision of precise reference patterns of all operation modes and the gain offset between different beams. Verification of the antenna model is performed for selected beams, at least one with low, one with mid- and one with high incidence angle, all with the same polarisation condition. In addition, some of the beams are selected for measuring the second polarisation condition. Assuming acquisitions for each of the selected beams, using the receiver mode of the transponders and by using acquisitions over rainforest, antenna model verification can be performed within a few cycles.
Geometric calibration is applied to assign the SAR data to the geographic location on the Earth's surface. Using well surveyed reference targets, the internal delay of the instrument and systematic azimuth shifts can be derived. For this purpose the acquired scenes are measured simultaneously against point targets deployed and precisely surveyed.
Antenna Pointing Determination
Antenna pointing determination is performed to achieve correct beam pointing of the antenna. The determination of the antenna pointing by the receiver mode of the transponders is performed using notch patterns in azimuth with different incidence angles (near, mid- and far). Using three transponders with a receiver function within one cycle (two passes) sufficient measurements can be acquired to derive the required accuracy. The appropriate antenna pattern is measured across the rainforest and using ground receivers.
Inter-Channel Phase Calibration
As the signal travels through different receive channels for H and V polarisation, it may experience different gains, phase offsets and even different time delays. Inter-channel phase accuracy is calibrated using the SENTINEL-1 transponders that return the signal with H and V polarisation components, and which therefore allow a direct phase comparison between H and V channels. The antenna model to be derived on the ground describes the antenna patterns with high accuracy. This antenna model is verified for a limited set of elevation beams via measurements over a homogeneous target, i.e. over rainforest. The azimuth beams will be measured using the receiver function of the SENTINEL-1 transponder.