A method to detect and characterize a frequency hopping (FH) signal is proposed. The context is Spectrum Surveillance. Hence, no a priori information is available: the parameters of the modulation must be automatically determined. Spread spectrum (SS) modulations are well-known for their low probability of interception (LPI). Nevertheless the aim of this paper is to show that, using a time–frequency representation and ad-hoc eigenanalysis techniques, it is possible to characterize such signal, even in realistic conditions (low SNR). Once characterized, the signal can be demodulated. Many tools are used for this characterization, in particular a precise definition of characteristic parameters and mathematical methods, such as data compression, 3D rotations and principal components analysis (PCA).