# Survey on Ranging Sensors and Cooperative Techniques for Relative Positioning of Vehicles
## 3 Non-Cooperative Relative Positioning
### Radio Ranging
The usage of the 24 GHz UWB band has been prolonged until January 2018 [21]. However, mutual interference of radar sensors is also becoming a growing issue in radar technology [22; 23]. Typically, a distinction is done between short-range radar up to 20 or 30 m, medium-range radar up to 100 m and long-range sensors that measure beyond this distance up to 250 m [24]. Short-range radars are used for collision avoidance and object detection and need higher range resolution and, consequently, more bandwidth (4 GHz at 77 GHz to 81 GHz). They usually have also a wider field of view up to \(\pm\)30\({}^{\circ}\). Long-range radars, on the other hand, are mainly used for ACC, can live with a lower range resolution and are designed with smaller bandwidths (500 MHz between 76 GHz and 77 GHz). Their field of view is usually smaller and around \(\pm\)10\({}^{\circ}\). In general, the range and speed resolution of a radar is determined by its bandwidth. Products available on the market state range accuracies of 10 cm up to 1% to 5%, while the speed accuracies are around 0.2 m/s. Generally, the across-track accuracy of radars is low. The approach used to estimate the angle of arrival of the reflected signal is to implement an array of patch antennas and compare the amplitude and phase of signals in partially overlapping beams. Some radar sensors provide moderate angular resolution by using mechanical sweeping radar scanners [19]. Angular accuracies between 0.5\({}^{\circ}\) and 5\({}^{\circ}\) are typical, yielding lateral uncertainties from 87 cm-4 m at 50 m, respectively. Targets can be easily resolved when they are located in different ranges and velocity cells [25]. For instance, commercial radar sensors claim to have a correct resolution of objects when their distance difference is above 1 m or their speed difference is above 2 km/h. The target resolution for objects at approximately the same distance and speed relies on lateral resolution, which is dependent on the lobe width, which, at the same time, depends on the wavelength. Larger wavelengths yield broader lobes or the same antenna dimension [26]. Therefore, moving from 24 GHz to 79 GHz yields a better lateral resolution and the availability of the 4 GHz bandwidth enhances the overall longitudinal resolution of target range and velocity. Other characteristics of radar sensors are their invisible integration behind electromagnetically-transparent materials, as for instance, behind the front bumper. Besides a few exceptions, radars usually have no moving parts and are, therefore, more robust and less prone to mechanical failures than laser scanners. In contrast to vision-based and laser scanners, radars are robust against environmental conditions, such as changes in light or fog and rain. In recent years \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sensor** & **Frequency** & **Bandwidth** & **Range** & **Azimuth Angle** & **Accuracy** & **Cycle** \\ \hline Bosch LRR3 & 77 GHz & 1 GHz & 250 m & \(\pm\)15\({}^{\circ}\) & 0.1 m, 0.12 m s\({}^{-1}\), - & 80 ms \\ Delphi ESR & 77 GHz & - & 174 m & \(\pm\)10\({}^{\circ}\) & 1.8 m, 0.12 m s\({}^{-1}\), - & 50 ms \\ Continental ARS30x & 77 GHz & 1 GHz & 250 m & \(\pm\)8.5\({}^{\circ}\) & 1.5 \%, 0.14 m s\({}^{-1}\), 0.1\({}^{\circ}\) & 66 ms \\ SMS UMRR Type 40 & 24 GHz & 250 MHz & 250 m & \(\pm\)18\({}^{\circ}\) & 2.5 \%, 0.28 m s\({}^{-1}\), - & 79 ms \\ TRW AC100 & 24 GHz & 100 MHz & 150 m & \(\pm\)8\({}^{\circ}\) & -, -, 0.5\({}^{\circ}\) & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Radar sensors. the price of radar sensors has dropped substantially, and they are available in middle and high class vehicles.