# Survey on Ranging Sensors and Cooperative Techniques for Relative Positioning of Vehicles
## 4 Cooperative Relative Positioning
### Transponder-Based Ranging  
One possibility is reusing the idea of radar at lower frequencies. However, due to the larger wavelength, vehicles appear rather small and do not offer a sufficiently big surface to reflect radio waves. Therefore, an antenna is installed on a target vehicle and the signals are amplified and \"reflected\" back to the transmitter in the ego vehicle. The relative distance of the target vehicle can be estimated by measuring the RTD and multiplying it by the speed of light.  
Radio ranging based on UWB technology has been part of the work of the research group around [PERSON] [49]. Here, a UWB signal at 6.35 GHz is used for estimating the range towards other vehicles with decimeter-level precision up to 300 m of distance. The two-way ranging technique used in their work is assessed in [50]. Unlike [PERSON], [PERSON] used 2.4 GHz and 5.9 GHz radios to estimate the range towards a set of road side units in order to perform Time-Difference-Of-Arrival (TDOA) and absolute position [51]. He concluded that ranging performance using V2V technology is highly dependent on relative velocity and distance. In his test setup, he measured ranging errors of up to 0.7 m at distances up to 90 m in line-of-sight situations. Furthermore, [PERSON] and [PERSON] performed RTD measurements at 5.5 GHz with LoS and multipath-rich environments obtaining accuracies between 0.5 and 1 m [52]. [PERSON] et al. performed simulations for absolute positioning fusing GNSS and RTD measurements from several vehicles in [53]. The main problem with RTD is that the delay caused by the target vehicle to reflect the signal back has to be estimated. In [53], the solution was to use physical level timestamping at reception and retransmission. Other limitations to the accuracy are imposed through multipath propagation and, especially, non-line-of-sight propagation.  
Usually, radio ranging systems based on RTD will output an estimate of the range towards the target, but not a complete 3D relative position by an estimate on the bearing angle or the elevation angles. For this, Angle-Of-Arrival (AoA) techniques are required, where, using multiple antenna elements, the direction of arrival of the wave can be estimated [54].  
Other transponder-based solutions rely on measuring the time the signal takes to travel from the transmitter to the receiver; techniques usually known as Time-Of-Arrival (TOA). This, however, has the drawback of requiring precisely synchronized clocks at every vehicle, in order to compare the transmission and reception timestamps [17]. In order to obtain decimeter-level ranging accuracy, a transceiver clock synchronization accuracy of less than 0.3 ns is required in each transceiver. [PERSON] and [PERSON] investigated TOA measurements in UWB and millimeter wave for inter-vehicle ranging achieving decimeter-level ranging accuracy [55; 56]. The problem of clock unsynchronization, however, falls out of the scope of their work.  
Figure 3: The four figures depict situations where a ranging sensor at the red ego vehicle has a limited performance in estimating the relative position of the green target vehicle. Either because of a limited sensing area in terms of range (**a**), or aperture (**b**,**c**), or due to an obstacle that blocks the line-of-sight (**d**).