RCS Measurements for the Implementation in Radar Target Simulators

Automotive radar sensors play a vital role in the development of autonomous driving due to their ability to detect objects even under adverse conditions [1] . Besides that, radar sensors offer long range capability and compact size, which enables the implementation in tight spaces. The environmental sensing task in autonomous vehicles must be validated due the severity of an operational failure that endangers human life. Carrying out these validation tests in the field however, presents a risk to other traffic participants and involves a great effort since distances in the order of several hundred million kilometer have to be covered to ensure the proper functioning of the system [2] . Also, these test are not repeatable. For this reason, a so-called Vehicle-in-the-Loop (ViL) approach is considered using a end-of-line vehicle placed on a steerable chassis dynamometer with the ability to simulate real world test scenarios. Such Radar Target Simulators (RTS) are capable of performing end-of-line, over-the-air validation tests by looping back a received signal and afterwards modified radar signal. It is essential, that these simulated targets are very close to reality and can also be compared to test drive data. However, this requires that all involved phenomena are investigated and implemented correctly. The radar signal is emitted from the sensor, travels through the radar radio channel, is reflected at an object, and travels back through the radio channel to the receiver, which in the case of radar is typically at the same location as the transmitter. Wave propagation must therefore take into account multiple reflections and scattering from objects in addition to free space propagation. To take the scattering or the backscattering into account, the so-called RCS pattern is usually used. However, since modern automotive radars are capable of resolving individual scattering centers well in angle and distance, considering the target as a point scatterer is far from sufficient. Therefore, a measurement system is presented that is able to measure road users with high resolution. This is done by using a mmW FMCW Radar Sensor working in the frequency range from 76-81 GHz which is equipped with a highly-directive lens antenna. Several traffic participants like cars, bikes, motor bikes and vulnerable road users are measured in detail. Also, measurements are compared to different full wave simulations. However, the results of these measurements cannot be implemented into a Radar Target Simulators, since the number of antennas that are necessary to generate the scattering points is limited, e.g. four antennas [3] . For this reason, the measured scattering points must be combined into relevant scattering clusters, which can then also be simulated with the limited number of antennas of the radar target simulator. The used radar target simulators in this approach is capable of generating echoes by using eight antennas. Each of the scattering clusters is consequently implanted in the wave propagation by its specific RCS pattern.