Minimize Calibration Activities

Level-1 product calibration encompasses:

  • absolute and relative radiometric calibration
  • polarimetric calibration
  • geometric calibration

These steps are applied during the SENTINEL-1 in-flight commissioning phase at the following six milestones:

  • T0: start of antenna pointing determination campaigns
  • T1: start of geometric calibration as well as antenna model verification campaigns and end of antenna pointing determination campaigns
  • T2: end of geometric calibration campaigns
  • T3: end of antenna model verification campaigns and start of radiometric calibration campaigns
  • T4: end of radiometric calibration campaigns
  • T5: end of commissioning phase.
Calibration Schedule for In-Orbit Commissioning Phase Versus Repeat Cycles of 12 days

Radiometric Calibration

The radiometric calibration consists in estimating the elements of the radar equation allowing transforming the SAR Pixel values into physical units.

See External Calibration

Polarimetric Calibration

The objective of the S-1 polarimetric calibration is to determine:
  • polarimetric coregistration: residual sptatial offset in range between the co- and cross-polarisation. This is measured using the S-1 transponders that are sending back to the SAR instrument a signal in the polarisation at the same time such that the IRF peak position of both Rx channels is to be used for co-registration assessment. This is assessed as function of the mode and for swath.
  • Tx Channel cross-talk: not measured
  • Rx channel cross-talks: The Rx cross-talks are measured using regular corner reflector (trihedral) providing a system response only in co-polarised channel. The Rx cross-talk is defined as the ratio between the peak of the IRF (visible in co-polarisation) with the value located at the same position in the cross-pol channel (compensated by the polarimetric delay)
  • Polarimetric channel imbalance: Cannot be easily measured as there is no VV/HH measurement possible. During phase E2 it is proposed to investigate further the usage of PS calibration techniques for the estimation of the channel imbalance. Alternative methods using more specific calibrators can also be investigated

Geometric Calibration

The process of geo-referencing consists in associating to a SAR sample determined by its azimuth and range timing a location on the earth. The role of the geometric calibration consists in estimating accuracy (offset) and precision of the pixel localization process. The geometric precision is referred as Absolute Location Error (ALE).

SAR geometric calibration is performed by over calibration ground equipment (e.g. transponders or corner reflector) which actual position is known with a very high accuracy (using differential GPS). The determination of the geometric accuracy is obtained by comparing their measured position against their expected (true) position. This way it is possible to determine systematic azimuth and range offset by adjusting the IPF accordingly.