Biomass External Calibration Plan

Kurzfassung

Biomass is a fully-polarimetric interferometric P-Band SAR mission with a centre frequency of 435 MHz and a 6 MHz bandwidth. The primary objective of Biomass is to determine the worldwide distribution of forest aboveground biomass in order to reduce the major uncertainties in calculations of carbon stocks and fluxes associated with the terrestrial biosphere. The Biomass satellite is planned to be launched in June 2024. This talk will present an overview of the Biomass key performance parameters and performance drivers and the in-Orbit calibration and performance verification approach. The Biomass P-band SAR will operate in a quad-Pol mode where the V- polarisation and H-polarisation pulses are transmitted alternatively while backscattered signals are received simultaneously in both polarisation. The critical polarimetric performance parameters are the combined Tx and Rx Channel Imbalance between V and H polarisations (required to be better than - 25dB at 3 sigma) and the Cross Talk (required to be better than -30 dB at 3 sigma). Another key performance parameter directly linked to the mission's primary objective of Biomass retrieval is the radiometric stability which is required to be better than 0.5 dB at 1 sigma. The in-orbit commissioning and performance verification activities are currently under definition. The in-orbit commissioning will rely on a dedicated fully polarimetric active transponder which will be installed at the ESA New Norcia Site, Western Australia. In addition, Targets of Opportunity will be imaged. Three commissioning parts with different activities are currently foreseen. Following the platform and instrument in-orbit tests, the in-flight antenna pattern characterisation using the transponder begins. This activity focusses on acquiring 21 azimuth antenna pattern cuts at different elevation angles as the Biomass satellite transits over the transponder in a drifting orbit with a revisit every 3 days. In this phase, the transponder will be operated in the Signal Acquisition Mode. The Antenna pattern characterisation consists of the measurement of the patterns generated by each single patch of the Biomass satellite Feed Array (doublet). Thus, for each overpass, 4 different antenna pattern azimuth cuts are collected. The 21 overpasses provide doublet patterns measurements at different elevations. For each doublet, the 2D antenna pattern (only main lobe) is determined by means of a polynomial fitting. The resulting antenna patterns are then averaged with their corresponding reference patterns obtained from on-ground characterisation (RASP) and the final H and V patterns are obtained combining the doublet characterised patterns. Following the antenna pattern characterisation, the satellite will be put in a zero-drift orbit which allows to perform repeated acquisitions over selected target areas with the same geometry. The focus is to characterise the radiometric stability and other performance parameters by means of the Persistent Scatterer calibration technique. Also, information on the temporal de-correlation of natural targets (as acquired during the operational phase) will be obtained. Finally, the satellite will again be placed in a drifting orbit with the transponder being revisited every 3 days at different elevation angles and in each one of the 3 swaths. In this phase, the calibration and verification of the key performance parameters will take place primarily based on transponder measurements, supplemented by Targets of Opportunities. The transponder will be operated in the echo generation mode. This mode enables the re-transmission of four echoes for each received pulse from Biomass corresponding to the four possible polarimetric signatures.