# Survey on Ranging Sensors and Cooperative Techniques for Relative Positioning of Vehicles
## 4 Cooperative Relative Positioning
### GNSS-Based Relative Localization
#### 4.2.1 Absolute Positioning  
By using multiple constellations in parallel, as for instance the American GPS, the Russian GLONASS or the European Galileo system, more satellites can be tracked and used for localization, velocity and time estimation, and thus, an increase in availability and accuracy can be achieved [57]. Further on, the usage of multi-frequency GNSS receivers can help to estimate the delay of the dispersive parts of the atmosphere. Besides the L1 and L2 frequency bands in early GPS, the new L5/E5 band will enhance this capability [58]. However, dual- or multi-frequency receivers are usually more expensive and are usually not used in mass-market applications. A technique, which has been known since the early years of GPS and is able to reduce atmospheric and other common errors, is differential GNSS (DGPS). In DGPS, a receiver located at a known position (base station) can determine the offset on the range towards each satellite and send this information to other receivers (vehicles) in the vicinity. By correcting this offset prior to position computation, the vehicles are able to obtain a better absolute position. In general, Ground-Based Augmentation Systems (GBAS), as for instance DGPS, use base stations at known locations to transmit real-time correction data for GNSS absolute positioning. The transmission is accomplished using some communication technology, as for instance a dedicated radio communication link or a cellular communication system. When GNSS correction data are transmitted from geostationary satellites, these are referred to as Satellite-Based Augmentation Systems (SBAS). For instance, the American Wide Area Augmentation System (WAAS) or the European European Geostationary Navigation Overlay Service (EGNOS) provide wide area coverage of differential corrections. The general rule applies that the nearer the base station, the more correlated the errors between the base station and receiver are and the larger the improvement of the position solution can be expected. This way, atmospheric errors can be corrected, in general, with base stations up to 20 km [58]. Further on, vehicles also apply sensor fusion techniques to support the GNSS position solution with on-board sensors in GNSS-determined environments (under dense tree canopy or urban canyons) or GNSS impaired scenarios (in tunnels). Commonly-used on-board sensors for absolute positioning in vehicular environments are wheel speed sensors, odometers, steering wheel position, accelerometers, barometers and magnetic compasses [59]. The fusion of GNSS with an Inertial Navigation System (INS), composed of two triads of accelerometers and gyroscopes, is a well-known technique for supporting GNSS in aircraft, missiles and ships [60]. In the last decade, these techniques have been extended to vehicle positioning by moving from high-grade and expensive ring laser and fiber optic gyroscopes to low-cost inertial MEMS sensors [61]. The performance of vehicle positioning in challenging environments with various low-cost in-vehicle sensors is addressed in [62; 63]. Hence, the topic of absolute positioning for vehicles has been extensively researched in the past two decades. Enhancing GNSS to mitigate the influence of multipath has been explored in [64; 65; 66; 67; 68]. Fusion of GNSS with on-board sensors has been addressed by [69; 70]. Augmenting GNSS with opportunistic signals, such as cellular networks or WiFi access points, has been analyzed in [71; 72; 73]. Skog and Handel authored a comprehensive survey on vehicle localization technologies including GNSS and RF-based positioning fused with motion sensors, vehicle models and road maps [59]. Traditional on-board ranging sensors can also be used for absolute vehicle positioning by using a map of the surrounding static objects. The fusion of GNSS with either laser scanners [74; 75], radar sensors [76; 77] or vision-based systems [78; 79; 80] offers an increase in robustness and availability when GNSS signals are impaired.