## 4 Test Record and Replay Approach
As a result, 30 min of raw data amount to about 72 GB. Figure 3: GNSS software-defined radio (SDR) data acquisition and processing system. ADC: analog-to-digital converter; AGC: automatic gain control; LNA: low noise amplifier; RF: radio frequency. The recording system is coupled with a dual system that implements the replay. The whole system, denoted as record and replay, has gained much attention in recent years, and the design and the implementation are available in the literature. In [19], the authors present a detailed description of the design of a system capable of replaying narrowband GNSS IF signals. They also compare the performance of a replayed data set with its live counterpart with regards to position, timing, and signal-to-noise ratio (SNR). In [20], the author focuses on the setup of the hardware components and assesses the performance of a commercial receiver in terms of signal strength and position. In [21], the design challenges of a system able to record and replay GNSS signals for multiple constellations and frequency bands are presented. The replay system reconstructs and modulates the signals from the recorded digital scenario at IF. Basically, the replay system works as an inverted front-end. In fact, starting from the samples, an analog signal must be created by means of a digital-to-analog converter (DAC) and filtering stage. Then, the signal can be modulated to the original RF frequency and band pass-filtered to remove the image signal. Figure 5 shows the functionality of the components within the replay chain. Depending on the power of the generated signal, an attenuation stage may be needed to emulate the power level received at the output of an active antenna. It is recommended to use a high-quality external reference clock to avoid introducing spurious components to the signal. Note that a too-small \(n_{b}\) limits the fidelity of the replayed scenario, sometimes introducing artifacts in the results. Although the storage of raw signal samples requires large storage capabilities and/or large-bandwidth data connections (in case the collected data have to be transferred), there are several advantages offered by this method for performance evaluation. It enables the possibility of recording a specific event or scenario from the real world and playing back the scenario for deeper and refined  
Figure 4: Memory requirement for a range of sampling frequencies and different quantization levels. Figure 5: Software processing unit and GNSS replay system. DAC digital-to-analog converter. analyses, thus granting the principle of repeatability. Some specific events may be rare, and the analysis in real-time might not provide sufficient information. Nevertheless, the performance of different receiver configurations and architectures, or even different receivers, can be evaluated. The data samples can also act as a basis for the creation of synthetic but realistic scenarios by adding impairments in a controlled environment. This approach is useful for assessing the impact of impairments such as radio frequency interference (RFI). Instead of using models that are often over-simple, the RFI can be added to the replayed signal by mixing them in a lab environment. In this case, the parameters of the interfering signals are under the user's control, thus allowing a parametric assessment of the performance with respect to the nature and features of the interfering source that is synthetically created.