“…The above variables are all 3D matrix with size of ( K , M and N are the numbers of resolution cells in X , Y and Z directions, respectively). In microwave imaging system, the sampling interval d is large with respect to working wavelength, causing the initial phase to randomly vary in different resolution cells [22]. Due to the existence of , the frequency spectrum covers the whole frequency band, which indicates that when sparsely sampled, the broad‐band energy will be mixed with side‐lobe noise, and the direction of maximum variance will not completely cover the desired signal.…”