# Survey on Ranging Sensors and Cooperative Techniques for Relative Positioning of Vehicles
## 3 Non-Cooperative Relative Positioning
### Radio Ranging  
One simple technique to estimate the range towards another vehicle consists of using electromagnetic waves and measuring the Received Signal Strength (RSS) of a signal transmitted by another vehicle. This is considered as a non-cooperative approach, since the signals are not transmitted for the primary purpose of localization. Such signals are generally known as signals of opportunity [14]. The signal level is proportional, under ideal conditions, to the distance between the transmitter and receiver antenna. By assuming a certain model for the power decay over distance, given by the path loss exponent, a rough estimate of the range between two cars can be performed. Since V2V communication technology is foreseen to be integrated in future vehicles, several research groups have analyzed the possibility of performing RSS ranging with this technology. [PERSON] et al. use Doppler measurements in [15] to estimate the path loss exponent. Their simulated distance estimation achieves an accuracy of 5.7 m at high relative speeds. In [16], [PERSON] et al. propose a cooperative technique that uses RSS in combination with speed measurements from the vehicles in a cluster. Although they demonstrate theoretically that their approach is accurate, reliable and can be real-time, they do not reveal the measurement uncertainty used for the RSS measurements. This is a key issue in RSS, since real-world experiments show that only a rough estimate of the user position can be acquired in this way, since shadowing and multipath cause large variations in the received signal strength. Additionally, the RSS method is very sensitive to how well the path loss exponent is estimated [17]. In [18], errors above 20 m are reported for short distances between vehicles. A technology based on RSS-ranging is only able to offer a distance towards the target vehicle. To overcome this shortage, [PERSON] et al. propose a multiple antenna relative localization system based on RSS that achieves 98% accuracy in locating another vehicle in the correct lane. Radar is a technology that uses high-frequency electromagnetic waves to measure the distance and relative speed of target objects. Radar sensors can already be found today in middle class vehicles for forward collision warning applications, lane change assistance or automatic cruise control. Two main radar technologies exist in ITS. Impulse radar, on the one hand, measures the time needed for a short pulse to travel from the radar sensor to the object, reflect and travel back to the sensor. On the other hand, Frequency-Modulated Continuous Wave (FMCW) radars transmit a frequency-modulated signal with a constant power envelope. The frequency difference between the outgoing and incoming waves is directly proportional to the relative distance to the target object. Both radar types are capable of measuring the relative speed by exploiting the Doppler effect and looking at the frequency difference between the outgoing and incoming waves. Table 1 lists the characteristics and performance parameters for five commercially available radar sensors taken from the respective manufacturer's data sheet. Radar sensors work typically in two frequencies. On the one hand, the 77 GHz band is used for so-called long-range radars [19]. On the other hand, the 24 GHz UWB band is temporarily allowed in Europe for automotive short-range radar sensors. Also in this band, narrowband radars can be operated using the 100 MHz Industrial, Scientific and Medical (ISM) band. At 24 GHz, radars coexist with radio communication devices and radio astronomy stations that might be impaired if too many automotive radars are operated. Therefore, the European Commission has made available the 79 GHz band for long-term operation of radar sensors seeking worldwide harmonization. This band has the advantage that a common technology for short-range and long-range radars can be developed, along with the decreased dimension and weight and increased Doppler sensitivity [20].