## 5 Test Procedures Comparison  
A comparison of the different approaches generally proposed for the GBPT performance assessment is discussed in this section, together with the different performance metrics. Depending on the specific requirements and constraints, they present advantages and drawbacks, as summarized in Table 1. However, they all are valid tools for testing since the characteristics of one device cannot replace the functionality of another.  
Among the advantages offered by the lab tests is that the user has the ability to define different scenarios. Moreover, the tests can be repeated as many times as desired under exactly the same known conditions. However, the major problem associated with such tests is that it is very difficult to model the signal degradation in the case of constrained environment scenarios such as the urban environment, so they ultimately offer low realism. Another drawback is that the hybridization of GNSS and other positioning sensors may be simulated only up to a certain level.  
In contrast, field tests present high realism because they allow the investigation of conditions that are difficult to simulate. Another advantage is that they are suitable for testing hybridized GBPT. However, they present the limitation that the environments are usually time-varying and so exhibit low repeatability.  
Finally, the use of the record and replay approach has the significant advantage of being very close to the real world, so it offers high realism. For this reason, it can be used for performance assessment. However, to have a stable and reliable statistic of the results, long data collections may be necessary. Furthermore, in order to obtain unbiased parameter statistics such as mean and standard deviation of the position, the results have to be averaged over a sufficient number of \"consistent\" scenarios. Although the use of replayed scenarios embeds the limitation that the scenario cannot be changed once the data are collected, the record and replay approach offers high repeatability. The recorded data can be replayed as many times as desired and then the signal can be fed to different devices under test to assess their performance in the desired scenario. However, as in the case of the lab tests, this approach exhibits some limitations when hybridized GPBTs are concerned. It is not straightforward to synchronously replay the GNSS signal and the other signals coming from other sensors in the lab. However, in some cases (e.g., if hybrid GNSS solutions have to be tested), the time series of the measurement of the other sensors do not need to be replayed, and only the GNSS scenario can be modified (e.g., adding interference), thus testing the robustness of the hybrid receiver. Nevertheless, it is important to remark that this approach is valuable if the core structures of the data collection system do not mask or affect the meaningful features of the collected signals, thus preserving the information on the specific environment.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Approach** & **Cost** & **Realism** & **Complexity** & **Repeatability** & **Valid for Hybrid** \\ \hline Lab tests & low & low & medium & high & partially \\ Field tests & high & high & high & low & yes \\ R\&R tests & medium & high & medium & high & yes \\ \hline \hline \end{tabular}
\end{table}
Table 1: Advantages and drawbacks of the approaches for GBPT testing. R&R: record and replay.