  This paper presents a quadrature compressive sampling (QuadCS) and associated
fast imaging scheme for synthetic aperture radar (SAR). Different from other
analog-to-information conversions (AIC), QuadCS AICs using independent
spreading signals sample the SAR echoes due to different transmitted pulses.
Then the resulting sensing matrix has lower correlation between any two columns
than that by a fixed spreading signal, and better SAR image can be
reconstructed. With proper setting of the spreading signals in QuadCS, the
sensing matrix has the structures suitable for fast computation of
matrix-vector multiplication operations, which leads to the fast image
reconstruction. The performance of the proposed scheme is assessed using real
SAR image. The reconstructed SAR images with only one-fourth of the Nyquist
data achieve the image quality similar to that of the classical SAR images with
Nyquist samples.
