Contents I. Introduction
The assessment of the volume of bulk solids in silos by means of radar is a challenging task, due to the complex spatial structure [1]. MIMO radar systems are emerging in this application and are appropriate to improve the accuracy of the measurement [2]–[4]. These systems are utilizing two-dimensional (2D) antenna arrays, connected to a radar device with multiple transmitting and receiving channels, in order to measure the three-dimensional (3D) distribution of scatterers, allowing for a reconstruction of the surface profile. Since the dimensions of the antenna array are typically small compared to the distance to the surface, the far field approximation can be applied. The surface profile is scanned by tilting the antennas main beam, by means of digital beam forming (DBF) [5]. In order to allow for an accurate reconstruction of the surface profile, the angular resolution has to be sufficiently small and the side lobe suppression SLS has to be large enough to suppress nearby disturbers. The performance of a MIMO radar system with a given number of transmitting and receiving antennas can be improved regarding the SLS and the angular resolution by optimizing the topologies of the transmitting and receiving arrays. Different approaches have been investigated for both, far field and near field imaging applications [5]–[9].
