## 3 Acsa-Olta
### Adaptive Correlation Space Adjusted Method  
In view of the analysis above, the optimal code search grid size is different when the received signal power varies. For the purposes to improve the code phase measurement accuracy of open loop tracking in urban area, the adaptive correlation space adjusted open loop tracking approach (ACSA-OLTA) is proposed. In ACSA-OLTA, the correlation space between the batch correlators is adjusted to the optimal code search grid size calculated from the EWPRE according to \(C/N_{0}\) which displays the signal power [20]. The EMLD based measurement calculation method is utilized at the proper \(C/N_{0}\) to further improve the accuracy and reduce the number of correlator needed at the same time. In addition, the search grid size in frequency dimension is set to a fixed value. When the grid size in code dimension varies according to the \(C/N_{0}\), the size in frequency dimension is unchanged. The Doppler frequency measuring result of the satellite signal is that of the local reference signal of the detected grid. Taking the coherent integration of 300 ms as an example, the search grid size in frequency dimension is 5 Hz. The code search grid sizes for different \(C/N_{0}\) are set as those shown in Figure 5. The process of the method is summarized as below  
1. Initialize the code search grid size as 0.2 chip, Doppler frequency grid size as 5 Hz. Then, carry out the coherent integration in the batch correlators. 2. After the coherent integration, detect the signal in the image of the correlator outputs. Estimate the \(C/N_{0}\) of the incoming signal with the help of the correlation result of the detected peak. 3. Adjust the correlation space between adjacent correlators according to the estimated \(C/N_{0}\). When \(C/N_{0}\) is larger than 23 dBHz, it is set to 0.1 chip. Or else, it is set as Figure 5. The signal code phase and Doppler frequency are directly extracted from the grid and fed back to the local signal generators. 4. Correlate the incoming signal with the new local replicas to form the signal image. 5. After removing the multipath peaks and other improper peaks according to the prior information, the peak with the maximum power in the image is selected. 6. The code phase is calculated by EMLD when \(C/N_{0}\) is higher than 23 dBHz. Otherwise, the code phase is that of the grid. The estimated parameters are fed back to the local signal generators. 7. Estimate the \(C/N_{0}\) with the correlation output of the detected peak. Then, adjust the correlation space as Figure 5 if \(C/N_{0}\) is lower than 23 dBHz, or else it is set to 0.1 chip. Return to Step 4. For other proper coherent integration lengths, the process is probably the same. The only differences are the switch point of the measurement calculation methods and the optimal grid sizes. Since all of these parameters can be calculated based on the methods above, they are not detailed for simplicity. In the method, at the beginning, the signal power is unknown and the correlation space has to be set to an empirical value. In order to keep the correctness of peak detection, the code search grid size is initialized as 0.2 chip which is relatively large. In this way, the most accurate peak can be detected easily in which most signal power is contained. The \(C/N_{0}\) will be perfectly estimated with the corresponding correlation outputs. As the code phase of the detected peak is always aligned with the incoming signal when the correlation space is varying, almost all signal power will be involved in the peak all the time andadjusting the space has tiny effects on \(C/N_{0}\) estimation. The details of \(C/N_{0}\) estimation have been given in [21, 27] which can provide accurate \(C/N_{0}\) estimation so that it is not described here for simplicity. With the help of the \(C/N_{0}\) estimation, the correlation space of the batch correlators in open loop tracking is adjusted to the optimal one calculated from EWPRE. The measurement calculation methods are also changed according to the \(C/N_{0}\).