1. A method for generating a high-availability inverse synthetic aperture radar (ISAR) image, the method comprising: forming a range resolved data cube over subapertures, fine range bins, and coarse Doppler bins, the range resolved data cube comprising a plurality of first data vectors in the subaperture direction; forming a plurality of product vectors from the first data vectors and a plurality of quadratic phase vectors; performing a fast Fourier transform (FFT) of each of the plurality of product vectors to form a plurality of two-dimensional arrays; forming a plurality of sparse two-dimensional arrays by calculating a coherency metric for each element of each of the plurality of two-dimensional arrays; and stacking the plurality of sparse two-dimensional arrays to form a sparse 3-dimensional (3-D) image.