## 4 Experiment Results and Analyses
The satellite signal of PRN 31 is used in this work as an example, because PRN 31 was also located on the window-side of room 5896 when the indoor experiment was conducted, and the PRN 31 signal might be received by the indoor GPS receiver. Similarly, the proposed approach removes the C/A code effect by multiplying it with the locally generated C/A code with a code delay of 6,178 samples. The time-frequency tool is then applied to map the remaining signal onto the time-frequency plane, and the resulting spectrogram is shown in Fig. 11. \begin{table}
\begin{tabular}{|c|c|c|c|} \hline PRN & Code delay & Doppler & CPPE (dl) \\  & samples & frequency & \\  & & (1/2) & \\ \hline
22 & 1,419 & 1,000 & 0.85 \\ \hline
30 & 2,543 & 2,500 & 7.55 \\ \hline
14 & 2,954 & 1,500 & 1.14 \\ \hline
26 & 15,588 & \(-\)1,500 & 1.52 \\ \hline
18 & 10,815 & \(-\)1,500 & 2.20 \\ \hline
12 & 7,445 & 500 & 1.17 \\ \hline
31 & 6,178 & 3,000 & 2.61 \\ \hline \end{tabular}
\end{table} 
Table 4: The acquisition result of the collected indoor signal by using the conventional acquisition method with 1 ms integration time and applying the SEV process. Figure 11: (a) The spectrogram of the proposed approach for PRN 18 satellite signal. (b) The top view of (a), a characteristic sinusoid wave can be observed around the frequency of \(-\)1,500 Hz. Figure 12. Note that a characteristic sinusoid wave is detected around the Doppler frequency of 3,000 Hz. The strength of the characteristic is stronger than that of PRN 18, and this result agrees with the acquired CPPRs in Table 2. Again, the proposed approach verifies that the satellite signal of PRN 31 is present, although the satellite was categorized as signal absent by the conventional acquisition process. Moreover, the same PRN 31 signal, but with the offsets of 4, 8, and 12 samples from the correct code delay of 6,178 samples, were also tested to show the effectiveness of the proposed approach for the low received power satellite signal detection. By the similar procedures of the proposed approach, the C/A code effect is partially reduced by multiplying it with the locally generated C/A code which is miss-aligned with the incoming signal by the above offsets, and the resulting spectrograms are shown in Figure 13. As shown in the results, the strength of the characteristic of the sinusoid wave decreases as the number of offset samples increases, as shown in the red dashed line box of each spectrogram in Figure 13. For the case of the low received power satellite signal, it is important to note that the strength of the characteristic of the sinusoid wave is sufficient for signal detection even if the miss-alignment between the local C/A code replica and that of the incoming satellite signal is half a chip (_i.e.,_ eight samples). However, the characteristic of the sinusoid wave is undetectable on the spectrogram if the miss-alignment is larger than three-quarters of a chip (_i.e.,_ 12 samples), as shown in Figure 12(f). Consequently, the proposed approach is capable of detecting and verifying the satellite signal with low received power even if the estimated code delay information has an offset of half a chip (eight samples) from the correct one. Figure 12: (a) The spectrogram of the proposed approach for PRN 31 satellite signal. (b) The top view of (a), a characteristic sinusoid wave can be observed around the frequency of 3,000 Hz. Figure 13: (a) The spectrogram of the proposed approach for PRN 31 satellite signal with miss-alignments of four samples. (b) The top view of (a). (c) The proposed approach for PRN 31 satellite signal with miss-alignments of eight samples. (d) The top view of (c). (e) The proposed approach for PRN 31 satellite signal with miss-alignments of 12 samples. (f) The top view of (e). In this paper, the computation load of the SEV process is also investigated. Note that the time-frequency representation of the SEV process dominates the computation load. Furthermore, the resolution of the time-frequency plane depends on the number of data samples in the time and frequency bins.