Selection of the SPOT5 image compression algorithm

Improving the ground resolution of SPOT 5 compared with SPOT 4 involves multiplying the data rate by 4. This made it necessary to seek a new image compression algorithm able to significantly increase the compression ratio while complying with the image quality requirements of SPOT users. We finally selected a DCT based algorithm embedded in a regulation loop in order to obtain a constant rate at the compressor output. This algorithm was first tested using simulated images. Quantitative and qualitative analyses were carried out at different development stages. Finally, a wide range of images chosen from the SPOT image database was used for validation purpose. This led us to propose several optimizations which have now been thoroughly tested. The result is an almost lossless compression algorithm, which will be used on both SPOT5 and HELIOS II, the French space agency's forthcoming Earth observation missions.