# Survey on Ranging Sensors and Cooperative Techniques for Relative Positioning of Vehicles
## 6 Conclusions  
This survey paper has presented a review on different cooperative and non-cooperative relative positioning sensors and technologies for driver assistance systems in current vehicles and future automated vehicles. Table 4 summarizes all presented relative positioning techniques. The requirements reviewed in Section 2 are added in each column to evaluate qualitatively each of the techniques according to the analysis of the previous sections. Five symbols (\(++\), \(+\), \(\circ\), \(-\), \(--\)) are used to symbolize if a ranging system performs positively, neutrally or negatively in a certain category. On-board ranging sensors, as radar sensors and laser scanners, offer a high accuracy in their range estimation. While radar sensors have poor lateral resolution, laser scanners can calculate the lateral distance with higher accuracy. Both have an acceptable update rate for safety critical applications (above 10 Hz), but only radar sensors offer a direct estimation of the relative velocity. Vision-based systems, based on stereo cameras, are only able to estimate the distance to vehicles in the near range and have to use additional information about objects and context to estimate the distance to more distant vehicles. Relative speed can only be estimated by looking at consequent images. Laser scanners and camera systems have lower availability since they rely on visible light and are thus sensitive to adverse lighting and climatological conditions. Radar sensors, eventually supported by laser scanner or cameras, are a very suited approach for relative positioning for safety-critical advanced driver assistance systems. Radar and vision-based solutions correctly complement each other regarding longitudinal and lateral performance. By moving towards higher frequency bands, increased bandwidth for radar sensors will become available, which leads to a higher degree of detail in the echoed signal. Future research will provide more robust and precise detection and tracking algorithms for radar sensors. Furthermore, image processing algorithms for vision-based vehicle tracking systems will continue evolving to decrease false detection rates and misclassification of road objects. Regarding cost, radar sensors have dropped in price in the last decade. The same is expected to happen to vision-based systems, since camera technology has found its place in the consumer market and the technology has matured for introduction into the automotive segment. Laser scanners, with its mechanical parts, will probably need some more time to be attractive enough to find market introduction. All non-cooperative approaches have line-of sight characteristics and are easily obstructed by obstacles, such as other vehicles, or have a limited range in curvy rural roads or urban environments. Furthermore, cooperative transponder-based approaches using RSS, RTD and TOA measurements exhibit too large errors when in non-line-of-sight conditions. This is considered an important drawback for vehicular safety applications that need to react in a timely manner to dynamic events ahead of the vehicle immediately in front. For this reason, cooperative approaches based on V2V communication, which can cope with occlusions of the line-of-sight up to several hundreds of meters, offer a real advantage. Relative positioning is achieved by the exchange of positioning information between vehicles. Here, different solutions compete with each other to meet the requirements of advanced driver assistance applications. Standalone GNSS solutions do not meet the requirements on accuracy and availability on relative position and relative speed. The fusion of GNSS with on-board kinematic and inertial sensors for absolute position determination increases both the availability and the accuracy. Centimeter-precise relative position can be achieved with GNSS carrier-phase solutions, but has the drawback of high sensitivity to satellite line-of-sight obstruction causing a limited availability and being only reserved to open-sky scenarios.