In order to improve the resolution of imaging radars, electrically large arrays and a high absolute modulation bandwidth are needed. For radar systems with simultaneously high range resolution and very large aperture, the difference in path length at the receiving antennas is a multiple of the range resolution of the radar, in particular for off-boresight angles of the incident wave. Therefore, the radar response of a target measured at the different receiving antennas is distributed over a large number of range cells. This behavior depends on the unknown incident angle of the wave and is, thus, denoted as range-angle coupling. Furthermore, the far-field (FF) condition is no longer fulfilled in short-range applications. Applying conventional signal processing and radar calibration techniques leads to a significant reduction of the resolution capabilities of the array. In this article, the key aspects of radar imaging are discussed when radars with both large aperture size and high absolute bandwidth are employed in short-range applications. Based on an initial mathematical formulation of the physical effects, a correction method and an efficient signal processing chain are proposed, which compensate for errors that occur with conventional beamforming techniques. It is shown by measurements that with an appropriate error correction an improvement of the angular resolution up to a factor of 2.5 is achieved, resulting in an angular resolution below 0.4 • with an overall aperture size of nearly 200 λ 0 .

In order to improve the resolution of imaging radars, electrically large arrays and high absolute bandwidths are required. At a large off-boresight angle of the incident wave, the difference in path length at the antennas becomes a multiple of the range resolution of the radar. Consequently, the radar responses of the targets at the different receiving channels are distributed over a large number of range bins, depending on the direction-of-arrival (DoA) of the target echo. Applying conventional signal processing, the resolution of the radar is considerably reduced. In this paper, the key aspects of DOA estimation are discussed when radars with both a large aperture size and a high absolute bandwidth are employed. A signal processing method is proposed avoiding errors that occur in conventional DOA estimation techniques. As shown by measurements, an angular resolution of 0.4 • can be achieved with an array size of 200 λ0 and a bandwidth of 10 GHz.