The aim of the absolute calibration is to ensure that the σ° value measured from a target is correct for all incidence angles. To achieve this, the absolute gain and the pointing of each antenna need to be established in flight, and this is done with the help of ground transponders. Transponders are active devices that, after receiving a pulse from the Scatterometer instrument, send a delayed and shifted in frequency pulse back. The Wind Scatterometer receives and detects the transponder pulses from within its receiver and stores data on board.
Once data is downloaded on ground, the measured transponder echo from the sensor and the measured pulse form the transponders device are processed by a calibration algorithm to derive a gain correction value for each antenna and a set of mispointing angles. This allows a reference calibrated system to be established, against which the system performance can be evaluated and monitored. The accuracy that can be reached by this methodology with the ERS-2 Transponders is about 0.5 dB.
The aim of the relative calibration is to assess the values of the 0 measured by the instrument for all incidences by comparing σ° measured at another time and/or location. This calibration is typically based on the usage of a natural target with specific geophysical properties like: isotropy, homogeneity and relative constant in time. For the ERS-2 relative calibration the Amazon rain forest is used to compute the gain correction and to monitor the system performances. The accuracy that can be reached by this methodology is about 0.2 dB. Other natural target (like sea-ice) or model (ocean calibration) can be also used to perform the relative calibration.
More details focussed on the ERS-2Wind Scatterometer mission and the calibration activities for the mission reprocessing can be found in the following papers:
Further information are available on the SCIRoCCO web site.