## 4 Test Record and Replay Approach  
The record and replay approach is based on the SDR technology, which generally refers to an ensemble of hardware and software technologies and design choices that enable reconfigurable radio communication architectures. According to this approach, functional blocks which are normally hardware implemented are realized as software modules either on programmable platforms or on reconfigurable hardware. Implementation of the GNSS receiver according to the SDR paradigm has been widely discussed (e.g., [13]), and it is now open to new implementation strategies and uses thanks to the evolution of processors and programmable hardware (e.g., cloud-GNSS) [14]. Proper standards for the data description and exchange are also being defined [15; 16]. However, in this paper, we focus on the acquisition and front-end part that allows the storage of the data for future replay, rather than on efficient implementations of the full receiver architectures. The design and implementation of a generic GNSS data acquisition system based on GNSS software radio receivers is presented, for example, in [17]. The most common architecture of a GNSS SDR receiver is composed of an antenna, a radio front-end (RFE) and a software processing unit, as shown in Figure 3.  
The analog signal captured by the antenna and amplified by the low noise amplifier (LNA) is filtered to minimize out-of-band contributions and then down-converted to intermediate frequency (IF) or to baseband. An automatic gain control (AGC) can be used to automatically adjust the signal dynamics. At this point, the signal--still in its analog form--is converted into digital samples and quantized by the analog-to-digital converter (ADC). Note that the data collection process must be regulated by a proper stable clock, in order to have consistency of the collected samples. In some cases, it may be necessary to use an external reference clock to steer the clock of the analog-to-digital converter, which may not by itself grant the required stability. The availability in the SDR chain of the signal samples, at the output of the ADC converter, allows for a recording of the GNSS signals that embed the characteristics of the environment. Such digital values are sampled at a sampling frequency \(f_{\mathrm{s}}\) and represented as binary values on a certain number of bits \(n_{b}\) (typically from 1 bit up to 16 bits). This stream of digital samples is commonly denoted as GNSS raw signal or raw IF signal, and is not to be confused with the I and Q postcorrelation samples, which are the \"raw\" outputs of many GNSS commercial receivers. Interested readers can find more details in [18]. A proper setting of the sampling frequency \(f_{\mathrm{s}}\) and the number of bits \(n_{b}\) used by the quantization process is needed to preserve the information on the specific environment, assuring the fidelity of the recorded scenario with respect to the real one. In detail, the choice of \(f_{\mathrm{s}}\) is driven by the bandwidth not only of the GNSS signal but also by other \"out-of GNSS band\" events that might need to be represented in the saved data log. The choice of the number of bits \(n_{b}\) used by the quantization process is driven by the desired dynamic resolution of the recorded signal. However, an optimum working point between IF recording quality and data volume has to be found. If the data have to be transferred to some remote server, the required network bandwidths for the data transfer must also be considered during the design phase. In the case of concatenated I and Q samples, the storage memory requirement for a range of sampling frequencies and different quantization levels is shown in Figure 4. Note that the requirement of a signal sampled by using \(n_{b}\) equal to either 1, 2, or 4 would be the same as that of a signal sampled by using \(n_{b}\) equal to 8, by using 1-byte coding. As an example, a data-grabber acquiring the L1 GNSS bandwidth sampled at \(f_{\mathrm{s}}=10\) MHz and \(n_{b}=16\) requires 40 Mbyte/s.