  We consider the probe of astrophysical signals through radio interferometers
with small field of view and baselines with non-negligible and constant
component in the pointing direction. In this context, the visibilities measured
essentially identify with a noisy and incomplete Fourier coverage of the
product of the planar signals with a linear chirp modulation. In light of the
recent theory of compressed sensing and in the perspective of defining the best
possible imaging techniques for sparse signals, we analyze the related spread
spectrum phenomenon and suggest its universality relative to the sparsity
dictionary. Our results rely both on theoretical considerations related to the
mutual coherence between the sparsity and sensing dictionaries, as well as on
numerical simulations.
