Operational SAR satellite missions impose new requirements to on-board data compression such as a higher data reduction ratio, more flexibility, and faster data throughput. A novel approach is Entropy-Constrained Block Adaptive Quantisation (ECBAQ). This method outperforms currently used Block Adaptive Quantisation with respect to Signal-to-Quantisation-Noise-Ratio and equals the performance of more complicated methods such as Vector Quantisation and Trellis Coding variants. The ECBAQ algorithm can be implemented using an architecture that is essentially not more complicated than that of a BAQ encoder and suitable for high-speed implementations. Moreover, the method features bit rate programmability with non-integer rates. This allows the SAR information throughput to be optimised for different types of applications. It is suitable for the application of region-of-interest coding and can be cascaded with frequency filtering to achieve even more data reduction.