# Fingerprints of the Automotive Radar Scattering of Passenger Cars and Vans
## 2 Materials and Methods  
The test vehicle (W213 E-class, Figure 1a) was equipped with an ARS510, a commercial automotive long-range radar [17], mounted at the center of the front bumper at a mounting height of \(H_{\text{R}}=0.6\) m. The measurements were conducted at the Mercedes-Benz proving ground [18] in Immendingen, Germany (Figure 1b), to ensure well-defined measurement conditions without disruptive effects from other traffic participants or infrastructure (e.g., guardrails or light poles). There are more than 30 different test circuits with 68 km tracks over a total surface area of 520 hectares. The tests were conducted in the Bertha area, which is used to test driver assistance systems and autonomous vehicles. This area is 950 m long and up to 650 m wide. Figure 2 illustrates the selected test scenario: the test vehicle drives at a constant speed of 20 km/h towards a static vehicle in the same lane. The distance \(R\) varies from 105 m to 5 m, while the resulting aspect angle remains approximately constant at a value of 0\({}^{\circ}\). Under these conditions, it is expected that the influence of underbody reflections is most clearly visible, because the vehicle is located in several range cells of the radar sensor. Therefore, multiple detections per vehicle can be measured due to the range resolution of the sensor. Additionally, the symmetry of vehicles in the longitudinal direction is beneficial to evaluate the reflectivity of different regions of the underbody. Ten repetitions were recorded for each set of measurements. Each recording was truncated at a distance of \(R=15\) m to  
Figure 1: Images of the test vehicle (**a**) and the proving ground (**b**). eliminate undesirable effects caused by the braking maneuver of the test vehicle. The object list provided by the sensor was used to extract all radar detections from the target vehicle and transform them into the target coordinate system. To evaluate the contribution of underbody detections to the scattering characteristics of vehicles, pyramidal microwave absorbers with a height of 10 cm were placed for different measurement runs in different positions at the target vehicle (Figure 3). In the first setup depicted in Figure 3a used as a reference, the target detections were recorded without microwave absorbers. Then, absorbers were positioned on the rear window as shown in Figure 3b, to identify the influence of reflections from the C-pillar and the roof. In the third setup depicted in Figure 3c, additional absorbers were placed underneath the rear bumper to obstruct the propagation path under the car. Finally, the absorbers underneath the rear bumper were repositioned to cover the tailpipes as well, as illustrated in Figure 3d. Next, the target detections from different vehicle types were recorded. Six different vehicles from Mercedes-Benz were used as target vehicles (Figure 4), they were selected to represent a variety of body shapes, ground clearances and powertrains: Two limousines--one with a combustion engine (S-Class S580, model year 2020) and one battery-electric vehicle (EQE 350, model year 2022), one SUV (GLS 580, model year 2020), one off-road vehicle (G-Class G500, model year 2018) and two Vans (V-Class V250, model year 2014, and Sprinter Mobility 23, model year 2018). With the exception of the Sprinter, all vehicles have roughly similar lengths \(L\) and widths \(W\) (Table 1), but different heights \(H\) and body shapes. The space between the road surface and the underbody influences potential multipath propagation underneath the car. The ground clearance \(Z\) is defined by the height of the lowest part of the underbody except for the wheels, which might be different than the height of other underbody parts, bumper and side skirts. Therefore, it is difficult to identify a single parameter that characterizes the geometric boundary conditions for multipath propagation.