## 4 Performance Evaluation
### Test in Urban Area  
An experiment was conducted to further evaluate the performance of the proposed ACSA-OLTA in urban area according to the method in [28]. The same as the position accuracy test in Section 4.2, the  
Figure 10: 3-D position error cumulative distribution of open loop tracking. experiment is carried out by off-line processing. The data utilized is GPS L1 CA signal that is collected in Zhongguancun, Beijing, China where is a typical urban scenario. There are a lot of skyscrapers aside the road and the satellite signal is easily sheltered and attenuated so that it is a difficult environment for satellite navigation. The navigation message is gained by AGPS so that coherent integration length is not limited. The total duration of the test is about 30 min and the ephemeris gotten at the beginning is in effect all the time. The vehicular velocity varied from 0-40 km/h with random stops due to the traffic lights and the total distance traveled is about 5 km. The test equipment is composed of a digital intermediate frequency signal collector which can down convert the satellite to intermediate frequency and store it in real time. Besides that, the SPAN-CPT Single Enclosure GNSS/INS Receiver is utilized to obtain the reference solution. It is an integrated system powered by NovAtel's world class OEM6 technology which is comprised of Fiber Optic Gyros (FOG) and Micro Electrical Mechanical Systems (MEMS) accelerometers. It works in double difference mode using phase and Doppler measurements. The baseline separation (relative to a base station located on the Weigongcun) varied between 3-4 km. The reference solution accuracy in these conditions is summarized in Table 3. A power divider is utilized after the antenna and the two outputs are connected to the data collector and the SPAN-CPT receiver respectively to guarantee that the reference solutions and the collected data are correspondence. The equipments and trajectory of the test is showed in Figure 11. \begin{table}
\begin{tabular}{c c} \hline \hline
**Information** & **Accuracy** \\ \hline Position & 0.1 m \\ Velocity & cm/level \\ Attitude & \textless{}1 deg \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The reference solution accuracy. Figure 11: (**a**) Equipments of the test; (**b**) Trajectory of the test (Gained from Google Earth). To facilitate the data analysis, the trajectory is divided into two segments which are marked by the yellow line and red line in Figure 11b. The yellow line is Segment 1 and the red line is Segment 2. They represents different operational scenarios that are considered and examined separately. The satellite number, GDOP and the lowest \(C/N_{0}\) of all the satellite signal received during the test are drawn in Figure 12, where Segment 1 is drawn in blue and Segment 2 is in red. Segment 1 is on a wide road where the satellite visibility is good and the operational conditions can be considered semi-open sky. In this area, the satellite is difficult to be sheltered and attenuated. In most time, the satellite number is larger than 8, the GDOP is smaller than 3 and the \(C/N_{0}\) is above 30 dBHz. Segment 2 represents a very difficult scenario. As shown in the figure, although the sensitivity of open loop tracking is outstanding, there are sudden drops in the satellite number for the reason that the buildings can totally shelter the satellite signals propagating from one direction. The GDOP will fall down as well because of the poor constellation. There is also a long time when the \(C/N_{0}\) is continuous low which is caused by the window plafond in the air. All of them will distort the positioning accuracy. The primary objective of the experiments is to evaluate the code phase measurement accuracy of the proposed ACSA-OLTA and the traditional open loop tracking methods with fixed grid size and measurement calculation approach in urban area. Hence, the positioning algorithm is still LS which has no process of filter and is able to better indicate the measurement accuracy.