SPOT5 THX: a 2.5m fused product

SPOT5 was launched on the 4th of May 2002 by an Ariane 4 rocket from Kourou, French Guyana. Like previous SPOTs, SPOT5 is able to acquire both high resolution panchromatic data and low resolution multispectral images (green-red-infrared images), with a 2.2 s time delay between the two acquisitions. The swath remains equal to 60 km but panchromatic resolution has been increased from 10 m to 2.5 m using an optimal quincunx sampling scheme named THR mode, while multispectral sampling was improved from 20 m to 10 m. Like other high resolution remote sensing systems, SPOT5 makes it possible to produce high resolution multispectral images by merging panchromatic and multispectral data. The main difficulty consists in panchromatic/multispectral geometric registration since the misregistration depends on attitude perturbations and local relief, which leads to use automatic correlation. To merge registrated multispectral and panchromatic images, CNES selected a simple but efficient method, based on intensity/hue/saturation transform. This method was successfully compared to a set of other merging techniques through an experiment including quantitative and qualitative assessment by experts. The final false colored product, named THX, is a three band multispectral image covering a 60 km × 60 km area per scene with a 2.5 m sampling interval, which may be converted into a pseudo colored using a matrix transformation. Panchromatic/multispectral fusion finally proves to be of utmost importance for global system optimization, since it allows to generate on ground a product that could not be produced on board, mainly for transmission and data storage limitations.