| Popis: |
Many data compression systems rely on a final stage based on an entropy coder, generating short codes for the most probable symbols. Images, multispectroscopy or hyperspectroscopy are just some examples, but the space mission concept covers many other fields. In some cases, especially when the on-board processing power available is very limited, a generic data compression system with a very simple pre-processing stage could suffice. The Consultative Committee for Space Data Systems made a recommendation on lossless data compression in the early 1990s, which has been successfully used in several missions so far owing to its low computational cost and acceptable compression ratios. Nevertheless, its simple entropy coder cannot perform optimally when large amounts of outliers appear in the data, which can be caused by noise, prompt particle events, or artifacts in the data or in the pre-processing stage. Here we discuss the effect of outliers on the compression ratio and we present efficient solutions to this problem. These solutions are not only alternatives to the CCSDS recommendation, but can also be used as the entropy coding stage of more complex systems such as image or spectroscopy compression. |
