## 4 Performance Evaluation
### Test in Urban Area
Since the evaluation focuses on the effects of the code search grid size and measurement extraction method on the positioning accuracy of open loop tracking, other parameters such as coherent integration length and frequency search step are all identical among the methods tested. The main parameters of ACSA-OLTA are listed in Table 4. Besides that, the \(C/N_{0}\) for ACSA-OLTA to adjust the correlation space is accurate to 1 dB. Since the \(C/N_{0}\) of NovAtel receiver is estimated from the correlator whose code phase is best aligned with the incoming signal, which is the same as that estimated in the process of ACSA-OLTA, the test results by using the two sources of \(C/N_{0}\) will be almost the same. Thus, the \(C/N_{0}\) of NovAtel receiver is utilized as reference for simplicity. The parameters of the traditional methods with fixed grid sizes and measurement calculation approaches considered for comparison are also listed in Table 4. The abbreviations of DET and DIS represent the open loop tracking methods with the code phase measurement computed from the EMLD and directly extracted from the grid respectively. Figure 12: Variation of the satellite number, GDOP and \(C/N_{0}\) in the test. Figure 13 illustrates the positioning results of the proposed ACSA-OLTA together with DIS with the grid size of 0.1 chip, DET with the grid sizes of 0.2 chip, 0.1 chip, 0.05 chip and 0.02 chip in Segment 1. The solutions of DET with the grid sizes of 0.1 chip and 0.2 chip shows large errors during the test. Narrowing down grid sizes shows evident improvements relative to those with larger grid sizes. As the \(C/N_{0}\) is high, the detection performance of open loop tracking system is perfect. In this scenario, for DET, the most accuracy peak is easily detected and the measurement will be improved when the grid size is small. Whereas the large uncertainty in wide grid will deteriorate the accuracy. In addition, DIS provides better performance and it is similar to DET with the grid size of 0.02 and 0.05 chip. All of the results are coincide with the analysis in Section 3. Finally, the trajectory obtained by ACSA-OLTA, in which most of the code phase measurements are calculated by the EMLD with the grid size of 0.1 chip, shows perfect positioning accuracy with only small disagreements with the reference trajectory. In order to further clarify the contribution of the proposed ACSA-OPTA, the position error in the segment is evaluated by the position error cumulative distribution expressed in Equation (37). The result is drawn in Figure 14. As can be seen from the figure, the accuracy of DET with the grid size of 0.02 chip and DIS with the grid size of 0.1 chip is close and 90% position results have the error less than 10 m. DIS is preferable due to the low computational costs and reduction of the correlators. When the grid size is 0.05 chip, the accuracy of DET is a little degraded and 80% position errors are smaller than 10 m. However, when the grid size of DET extends to 0.1 chip, 40% positioning results have error larger than 10 m. For DET with the grid size of 0.2 chip, the maximum error reaches about 40 m and only 40% positioning results have error less than 10 m. ACSA-OPTA is better than all the other methods tested because there is still some sudden power reduction during the segment. ACSA-OPTA can adaptively adjust the grid size and measurement extraction method to keep accuracy. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **Grid Size** & **Frequency Step** & **Integration Length** & **Positioning Rate** \\ \hline \multirow{2}{*}{ACSA-OLTA} & Adaptively adjusted & \multirow{2}{*}{5 Hz} & \multirow{2}{*}{300 ms} & \multirow{2}{*}{1 Hz} \\  & based on \(C/N_{0}\) as Figure 5 & & & \\ \hline DIS & 0.1 chip & 5 Hz & 300 ms & 1 Hz \\ \hline \multirow{4}{*}{DET} & 0.2 chip & 5 Hz & 300 ms & 1 Hz \\  & 0.1 chip & 5 Hz & 300 ms & 1 Hz \\  & 0.05 chip & 5 Hz & 300 ms & 1 Hz \\  & 0.02 chip & 5 Hz & 300 ms & 1 Hz \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The parameters of the methods tested.