Reduced complexity multi-frequency imaging using active aperture synthesis

Active imaging utilizes an array of transmitting elements to illuminate a scene and an array of receiving elements to capture and process the reflections. The narrowband coarray definition provides a convenient framework for understanding active imaging, and designing schemes to reduce the number of transmitting and receiving elements required. Wideband signals have been used for active aperture synthetic imaging to obtain range resolution, as well as to improve angular resolution. The step-frequency method achieves the effect of short duration pulses through frequency synthesis, and the coarray idea can be applied in this situation also. We investigate approaches to reduce the signal processing time and hardware complexity of multi-frequency imaging schemes, while still maintaining a reasonable image quality. In one approach, we randomly drop points in the frequency-coarray space. Another scheme uses narrow band transmissions (groups of frequencies) from multiple transmitters and results in a pseudorandom placement of frequency-coarray points. Orthogonal transmission of multi- frequencies allows simultaneous detection at receivers and reduces image acquisition time. Simulation results show good performance and hardware complexity and acquisition time trade-offs.