A method of processing data signals suitable for providing positioning information comprises sequentially recording blocks of data samples of a satellite broadcast from the beginning of a period of time of interest, and storing them in memory. When the memory is full, previously stored blocks of data samples are overwritten, such that the average quantity of sample data decreases, and wherein the memory content includes blocks of data samples covering substantially the full period of time of interest. This method enables blocks of data samples to be kept in memory for a full period of interest (for example a journey) but allows a limited memory capacity. The use of the memory is intelligent and dynamic.