2.1. Automotive Radar Technology

It is well known that Radar is based on the spread of electromagnetic waves. The frequency ranges from 3–30 GHz (centimeter waves) to 30–300 GHz (millimeter waves). There are two commonly used frequencies in the automotive sector, one for short-range applications with 24 GHz and the other one for long-range with 77 GHz. Both have different performances in terms of resolution, due to smaller wavelength in the latter case.

For detecting different landforms and assess surface processes, we considered the use of the radar sensors ARS-408 and ARS-404 (Advanced Radar Sensor) from Continental [48] (Figure 1). Besides Continental, other automotive supplier such as Bosch, Hella or TRW produce radar sensors for automotive applications as well. Further, companies such as Sick and Innosent offer radar sensors with focus on industrial applications. From the wide range Continental is the unique enterprise that meets our desired requirements to achieve our goals: (i) low price; (ii) low weight and power consumption; (iii) high robustness; (iv) small size; (v) high range option; and (vi) suitable and easily usable interface.

Radar sensors ARS-40X showing. On the right side of the sensors, are the CAN interface connector. (a) ARS-408 and (b) ARS-404.

The ARS-400 radar sensor series originally comes from the automotive industry. These radar sensors use to be built in the front bumper of cars and trucks to observe the area in front of the vehicle. Both are long-range sensors and use 77 GHz with a wavelength of about 3.9 mm. The sensors have different sizes and therefore, different performances whereby the ARS-408 is the larger one with higher performance. Due to a special software adaptation from the subsidiary Continental Engineering Services (CES), the radar sensor can be used for industrial purposes such as area monitoring or collision avoidance for large construction vehicles. Data can be exchanged via the Controller Area Network (CAN) interface. The larger ARS-408 is on the left-hand side (Figure 1a) and the smaller ARS-404 on the right-hand side (Figure 1b). On the right side of the sensors are the CAN and power connector situated.

The ARS-400 series has a real aperture and uses the frequency modulated continuous wave (FMCW) process. It has a phased array antenna and the measurement is performed in two dimensions. The detected reflections of the radar are issued as clusters via the CAN interface. Relevant data obtained from a cluster are longitudinal and lateral distances as well as the radar cross-section (RCS) values which represent the reflected power. Contrary to other radar sensors, it is not necessary to start the evaluation of the data at a raw level. The radar output is pre-processed, which means that the process of the raw data and filtering evaluation is already done. Figure 2 shows the coordinate system of the sensor with a cluster and the naming of the axis. Extraordinary for the sensors is that they have two scan areas. Each sensor has two antenna field setups, one for the near-field and one for the far-field measurements. Both fields have naturally different tasks and specifications. The near field is characterized by a short-range and a large opening angle. Therefore, it is used in the automotive field for slow situations such as driving in a city. On the other hand, the far-field is characterized by a long-range and a small opening angle for fast and straight driving modes. Figure 2 shows the field of view of the ARS-408. In the blue color, the near-field is represented and in the red one, the far-field.

Based on R. Liebske [49] the coordinate system from the radar sensor with one cluster data is described in the longitudinal and lateral axis. In the blue color, the near-field view is represented and in the red, the far-field.

To compare the performance of the two sensors, we present Table 1, which shows the most important specifications.

Specifications of the ARS-404 and ARS-408 radar sensors [49].

Though the sensors look different, the mounting is the same. This has the advantage that only one bracket is necessary to fit the system for both sensors. This is the reason why the small ARS-404 has a large size in comparison to other specifications such as the weight. However, because of a lower weight, the ARS-404 is more appropriate for the application in a UAV. In contrast to the ARS-408, the performance can be considered something more limited. To compare both sensors, a recording system was developed as follows.