Biomass system calibration and verification by mean of an active polarimetric transponder

Abstract

Biomass mission was selected as the 7th Earth Explorer mission within the frame of the ESA Earth Observation Programme. It's primary objective is to provide the worldwide distribution of the above-ground biomass, enabling a more accurate knowledge of carbon stocks and fluxes associated with the biosphere. In particular the mission will provide global information about the above-ground biomass, top canopy forest height and disturbance map indicating forest degradation and re-growth. In order to achieve the goal, a spaceborne fully-polarimetric P-band system has been designed, which represents an unprecedented and challenging objective from many points of view. Furthermore, the requested estimation accuracy, entails quite stringent requirements in terms of measurements quality that need to be verified and monitored during the mission lifetime. In particular, radiometric and polarimetric accuracies are of primary importance for the mission objectives and thus the system needs to be calibrated accordingly. To cope with this issues, an active fully-polarimetric transponder will be used. It will be exploited extensively during the commissioning phase (i.e. calibration and verification) and sporadically during the operational phase (i.e. monitoring and eventual calibration). During the commissioning phase, a particular orbit will be selected, enabling Biomass overpass over transponder each three days with a ground track drifting of 3 Km approximately. The first two months will be dedicated to antenna patterns (for both the polarisations) and antenna pointing characterisation with a total of 21 transponder overpasses. Afterwards, one other month will be dedicated to Radiometric and Polarimetric calibration, Ionospheric correction validation and other system performance parameters verification (i.e. TAR, PSLR, NESZ, ENL, Geo-location accuracy) with a total of 11 overpasses. The Biomass Calibration Transponder (BCT) is an active fully-polarimetric equipment. During each overpass, the transponder will track Biomass (azimuth and elevation mechanical steering) in order to receive and transmit with its maximum gain. It operates according to three different modes. These modes can be enabled separately or in any combination of them. In Signal Acquisition Mode (SAM), the transponder records the pulses received from Biomass. In the Echo Generation Mode (EGM), the detection of the pulse received from Biomass triggers the delayed re-transmission of the received H and V signal components according to the four possible polarisation signatures (i.e. HH, HV, VH, VV). Hence for each received pulse, four delayed pulses are re-transmitted to Biomass after an amplification corresponding to a fixed RCS. Finally, the Self-Generated Echo Mode (SGEM), transmits to Biomass delayed H and V pulses generated within the transponder. The transmission can be triggered by the reception of pulse from Biomass or according to a given timeline synchronized with the Biomass receiving window (Beacon mode). In the final paper, all the issues related to the Biomass system calibration and verification will be addressed, as long as, the established solutions. The commissioning phase baseline plan will be described and more details about the transponder characteristics and its planned usage will be provided.