## 3 Results and Analysis
### Vehicular Positioning  
A test to assess the GPSMap 66 units in vehicular mode was carried out on a relatively open 43-km road rectangle at speeds of 80 to 100 km/h. Two 15 km legs of the rectangle was oriented east-west while the two others were north-south. The rectangle was driven twice to assess consistency, resulting in a 2-lap, 86 km trajectory. The R10 and GPSMap 66 units were mounted on a car roof as shown in Figure 16, to secure access to line-of-sight signals. The antennas are at the same height but separated horizontally from each other by 35 cm. There were unavoidable obstructions by foliage and passing vehicles. A R10 unit at a base station 6 to 20 km away was used for differential processing. The differential R10 trajectory served as reference for evaluating the GPSMap 66 trajectories in each coordinate component. The R10 RTKLib code/carrier phase solution was in float mode and its accuracy in each coordinate would have been better than 10 cm much of the time except during short periods when carrier phase lock losses caused by obstructions along the road occurred. Signal attenuation occurring along the trajectory is seen when examining the C/N\({}_{0}\) measurements of all units in Figure 17. The two GPSMap 66 were 60 cm but still largely exposed to the same attenuation and multipath. Their C/N\({}_{0}\) patterns are highly correlated, which is also a strong indicator of the high reproducibility of their two helix antenna horizontal and vertical radiation patterns. A code-minus-carrier analysis of the results confirmed that the lower C/N\({}_{0}\) values were caused by multipath. The plots indicate where trees and other obstructions were present. The attenuation is caused by partial line-of-sight blockage and a mix of specular and diffuse multipath. While the average multipath is the same for all units, that of the GPSMap 66 units exhibits deeper attenuations during short time periods, which would be antenna related. The GPSMap 66 code measurements were processed in epoch-by-epoch code single point mode and in differential code/carrier phase mode, the latter over 95% in float carrier phase ambiguity mode. The differences with respect to the R10 reference trajectory are shown in light and dark blue, and light and dark green in the two plots of Figure 18. The single point code position performances of the two units are at the same level, with RMS values of 2.10 m, 1.17 m and 3.39 m in latitude, longitude and height for the blue unit versus 1.75 m, 1.11 m and 3.26 m for the green unit. The mean differences are well within the above RMS values. These differences are due mostly to a combination of GPSMap 66 code multipath and noise. The differential code/carrier phase results are significantly better with RMS values of 0.65 m, 0.27 m and 0.82 m in latitude, longitude and height for the blue unit and 0.46 m, 0.37 m and 1.19 m for the green unit, hence clearly better than the 1 m RMS level. The float ambiguity mode of the GPSMap RTKLib code/carrier phase solution means that phase lock was maintained on a sufficient number of GPS SVs to produce a carrier phase based solution, albeit float, over 95% of the time. The effects of carrier phase ambiguity float mode solutions with the GPSMap 66 units and the R10 unit some of the time are due to signal shading that results in occasional losses of phase lock. This has a significant impact on the magnitude of the mean and RMS differences reported above. In float mode, code measurements are used with varying weights and their noise and multipath enter the solutions. The ionosphere has no effect on the GPSMap 66 L1 measurements in differential mode. In single point mode, it has no significant effect in this case either. The magnitude of the ionospheric effect was measured by successively processing the R10 data at the nearby base station in L1/L2 and L1 mode. The latitude, longitude and height differences (L1/L2 \(-\) L1) were \(-\)3, \(-\)4 and \(-\)4 cm. Figure 16: Vehicular test antenna mounting.