Apparatus for processing a sequence of samples of a received signal R reflected by a target surface and having a delay and a frequency shift relative to a reference signal D to obtain a delay Doppler map for the reflected signal, the apparatus comprising: a first correlation module (9) for obtaining a plurality of partial correlations z(n', k) between the sequence of samples x r (n) corresponding to the reflected signal R and a sequence of samples x d (n) corresponding to the reference signal D, across a set of delays, the first correlation module being configured to obtain a number of partial correlations z(n', k) of said plurality of partial correlation, providing one partial correlation sample for each delay, by obtaining a DFT of at least a first sequence of samples and a DFT of at least a second sequence of samples, the first sequence comprising at least one subsequence of samples of the reflected signal and the second sequence comprising at least one associated subsequence of samples of the reference signal, and to obtain an inverse DFT of a sequence of samples derived from at least said DFTs of the first and second sequences; a memory (10) for storing a plurality of sets of partial correlation samples, the samples of each set corresponding to the partial correlations for a different delay; and a second correlation module (11) for obtaining a DFT of the samples in each set to determine the correlation between the sequences of samples of the reflected and the reference signals across a plurality of frequency shifts at each delay of the set of delays.