# Performance Limits of GNSS Code-based Precise Positioning: GPS, Galileo & Meta-Signals
## 8 Conclusions and Outlook  
In this contribution, we propose to exploit a recently derived compact time-delay estimation CRB which depends only on the signal samples. This is especially useful to correctly evaluate different \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(\text{SNR}_{\text{out}}\) (dB) & \(T_{\text{I}}\) & \(\widehat{\varphi}\left(\widehat{\tau}\right)\) & \(\widehat{\tau}\)**L1 C/A** & \(\widehat{\tau}\)**L5** & \(\widehat{\tau}\)**E5** & \(\widehat{\tau}\)**E5b-E6** & \(\widehat{\tau}\)**E5a-E6** \\ \hline
15 & 1 ms & 5 mm & 7.08 m & 2.03 m & - & - & - \\ \hline
18 & 2 ms & 3.6 mm & 5.01 m & 1.44 m & 18 cm & - & - \\ \hline
21 & 4 ms & 2.5 mm & 3.55 m & 1.02 m & 13 cm & 5.62 cm & 3.94 cm \\ \hline
25 & 10 ms & 1.6 mm & 2.24 m & 64 cm & 8 cm & 3.55 cm & 2.51 cm \\ \hline
28 & 20 ms & 1.1 mm & 1.59 m & 46 cm & 6 cm & 2.51 cm & 1.76 cm \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Phase and Time-delay Estimation Standard Deviation for: GPS L1 C/A (\(F_{S}=10\) MHz), L5 (\(F_{S}=10\) MHz), Galileo E5 (\(F_{S}=120\) MHz), E5b-E6 (\(F_{S}=240\) MHz), and E5a-E6 (\(F_{S}=400\) MHz). signals and obtain meaningful results. The CRB and the corresponding MLE were computed for a set of representative GNSS signals: the legacy GPS L1 C/A; the other GPS civil signals L1C and L5; the family of Galileo signals E1 OS, E6, E5a, E5b, and E5; and the new GNSS meta-signal paradigm with the combination of the E6 with both E5a and E5b. The goal of this contribution was to determine: (i) the ultimate achievable performance of GNSS code-based positioning systems; and (ii) whether we could obtain a GNSS code-only precise positioning solution and under which conditions. Regarding the first question, at a representative \(\text{SNR}_{\text{out}}=25\) dB operation point at the output of the MLE, it was found that the time-delay standard deviation using BPSK and BOC-type signals ranges between 2.24 m for GPS L1 C/A and 64 cm for the GPS L5 or the individual E5 components, i.e., E5a or E5b. Using the complete E5 signal, we can reduce the delay standard deviation by a factor 28, 12, and 8 with respect to the GPS L1 C/A, Galileo E6B, and Galileo E5b-I/GPS L5 signals, respectively, that is, a standard deviation of 8 cm. This performance can be further improved by an additional factor of 2 and 3 using the E5b-E6 and E5a-E6 meta-signals, for which we obtained 3.5 and 2.5 cm, respectively. The latter comes at the expense of possible false locks due to high secondary correlation peaks and a huge bandwidth. With respect to the second question, only the AltBOC-type signals can provide decimetric precision, thus only the E5 and GNSS meta-signals can be considered as an option for code-based precise positioning. For instance, using the complete E5 signal, the 3\(\sigma\) time-delay error is below 25 cm. Obviously, to reach such precisions, we still need to be able to correct for external errors such as ionospheric/tropospheric delays, as well as orbital or satellite clock errors as in PPP solutions, but this could also be exploited to avoid carrier phase ambiguity fixing in code-based RTK solutions. The latter may be a promising line to be explored, because such ambiguity fixing is essentially the bottleneck of RTK techniques, and is still an open issue when using a large number of satellites in multi-constellation/multi-frequency architectures. In addition, exploiting long coherent integration times, we may be able to reach code-based sub-cm accuracy. Finally, we also derived a new sample-based joint delay/phase estimation CRB, which was used to characterize the possible performance improvement of phase-based techniques in the AltBOC-type signals context. It was found that the phase CRB do not depend on the signal, and therefore the gain provided by the phase exploitation must not be worth the ambiguity resolution complexity in AltBOC-type architectures. The results presented in this contribution open the door to new precise positioning receiver design. Conceptualization and methodology, J.V.-V., F.V. and E.C.