Focusing Bistatic SAR Data Using the Nonlinear Chirp Scaling Algorithm

Bistatic synthetic aperture radar data are more challenging to process than the common monostatic counterparts because the flight geometry is more complicated and the data are usually nonstationary. Whereas time-domain algorithms can handle general bistatic cases, they are very inefficient; therefore, frequency-domain methods are preferred. Several frequency-domain monostatic algorithms have been modified to handle a limited number of bistatic cases, but a general algorithm is sought, which can handle cases such as nonequal platform velocities, nonparallel flight tracks, and high squints. In this paper, we modify the nonlinear chirp scaling (NLCS) algorithm to handle a general case of bistatic data. The key is to use a linear range cell migration correction to reduce the range-azimuth coupling, an NLCS to precondition the data for azimuth compression, and a series expansion to obtain an accurate form of the signal spectrum. The azimuth nonstationarity is handled through the use of invariance regions. Simulations have shown that the modified NLCS algorithm can handle data with more complicated bistatic geometries than the previous algorithms.

I. Introduction

Monostatic synthetic aperture radar (SAR) data compensated to a straight line path are azimuth invariant because point targets with the same range of closest approach have the same azimuth phase history. As a result, efficiency can be gained by processing the data in the azimuth frequency domain (FD). Bistatic data, however, are often azimuth variant because both the transmitter and the receiver can assume different motion trajectories. An additional complication is that the range history of a bistatic target is the sum of two hyperbolic range equations, giving a double square-root term in the range equation (called a flat-top hyperbola) [1]. Therefore, the analytical simplicity of the single square root of the monostatic case is no longer available. As a result of these two properties, monostatic SAR processing algorithms must be modified if they are used to handle bistatic data.