ALOS PALSAR product verification
The Advanced Land Observing Satellite - ALOS (© JAXA) |
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Corner reflectors of this type with 1.5 and 3 m side length will be used for PALSAR product verification |
ALOS, an enhanced successor of the Japanese Earth Resources Satellite 1 (JERS-1), was launched from JAXA’s Tanegashima Space Center in January 2006. ALOS operates from a sun-synchronous orbit at 691 km, with a 46-day recurrence cycle carrying a payload of three remote sensing instruments: the Panchromatic Remot e Sensing Instrument for Stereo Mapping (PRISM) , the Advanced Visible and Near-Infrared Radiometer type 2 (AVNIR-2) and the polarimetric Phased Array L-band Synthetic Aperture Radar (PALSAR). The PALSAR sensor has the capacity to operate with a wide range of off-nadir angles and resolutions in a single-, dual-, and quad-pol mode.
An important contribution to the ALOS mission is the verification and validation of PALSAR products distributed by the European ADEN node. Under a contract with ESA/ESRIN, the Institute has to assess ALOS/PALSAR data quality during the commissioning phase of the instrument and to provide a set of algorithms for quality control throughout the mission lifetime.
Software tools to be developed and implemented include point target analysis, distributed target analysis, geometric analysis, antenna pattern estimation, polarimetric analysis and the estimation and analysis of propagation effects. The framework for this tool set is the in-house developed CALIX software, which is our standard calibration and verification environment also being used within the IOCS for TerraSAR-X.
During calibration and verification campaigns, active and passive calibrators (transponders, dihedral and trihedral reflectors) will be deployed as external references, providing well-defined point target responses for product quality assessment.
Since PALSAR is the first fully polarimetric spaceborne L-band sensor, propagation effects are important new issues to be addressed. The main challenge is to assess the influence of the ionosphere on the polarisation, known as Faraday rotation. This effect depends on the Total Electron Content (TEC) in the ionospheric layer below the ALOS spacecraft and is proportional to the wavelength squared. Being negligible for X-band sensors, Faraday rotation is significant in L-band and can reach up to 100° with a pronounced diurnal variation and a strong dependence on solar activity during the 11-year solar cycle.
After the successful launch JAXA is currently performing early functional check-outs. First acquisitions over our Pol-InSAR and calibration test sites are envisaged for April and the detailed planning of the campaigns has started. First results are expected in summer 2006. The PALSAR product verification activity for ESA will be finished early 2007.