Bi-static Nearfield Calibration for RCS Measurements in the C-V2X Frequency Range

Automotive radar systems are indispensable for advanced driver assistance systems. Beside existing monostatic millimeter-wave radars, distributed passive radar systems using multi-static constellations provide additional options to augment the radar visibility of road users. Especially radar sensing concepts which use communication signals, like cellular vehicle-to-everything (C-V2X) communication around 5.9GHz, move into this focus. Due to electrically very large radar targets and small distances between the radar systems and the crossing target on the road, these road users are usually detected under nearfield conditions. Hence, the bi-static radar cross-section (RCS) of these radar targets must also be characterized under comparable conditions. This paper describes two bi-static RCS calibration methods, using a reference object or the radar range equation. Following this initial step, a further calibration approach is presented which takes the nearfield effects of the antennas into account. The bi-static electromagnetic scattering of a bicycle at 5.9GHz was measured under nearfield conditions and calibrated with these different calibration approaches. Upon comparison with numerical simulations of the far-field RCS, the measured and calibrated RCS results show significant, angle-dependent differences which provide evidence that the measurements include nearfield effects. The comparison between the different calibration methods shows differences up to 8dB depending on the bi-static angles and demonstrates the importance of the consideration of nearfield effects in the RCS calibration.