## 3 Prototype Design
### Functional Tests for the Validation of the Sensor  
For the functional validation of the prototype, an intensive test campaign was conducted in the laboratory. Such tests were divided into two categories: first, we validated the sensor in a controlled environment, generating GNSS signals through a hardware signal generator; then, we tested the signal conditioning with live GNSS signals. First, we were able to verify all hardware components, from RF to IF, as well as the software routines implementing the usage modes and the grabbing functionality. The use of a professional GNSS hardware generator [28] allowed for excluding effects due to phenomena related to a real environment, such as multipath and interferers. Several tests were performed on each single RF receiving chain of the two FEs, which were first validated separately. Then, all of the channels were tested simultaneously, with the sensor configured such that the master FE provided the reference clock to the slave one. The same GNSS signal was split and sent to the four channels by means of a four-way power splitter, as reported in the simplified scheme of Figure 5. The signals at the RF input were digitalized, and the samples were stored in the ODROID-X2 eMMC memory, then post-processed by a software receiver. Several test metrics were considered in the analysis and validation process: the power spectral density (PSD) of the digitalized signal, the amplitude of the main peak of the cross ambiguity function (CAF) computed during the acquisition of the GNSS signals, the quality of the tracking loop lock through the mean and variance of the correlators and the estimate of the carrier-to-noise power density ratio (\(C/N_{0}\)). The sensor successfully passed all of the tests and demonstrated the ability to process the signal properly in all cases. The FEs resulted in being well synchronized through the master-slave configuration, whereas the ODROID-X2 was able to handle the two FEs' streams, thanks to the real-time management of two USB ports and the fast memory storage. The post-processing analysis revealed that the receiver is able to successfully acquire and track all generated satellite signals in all performed tests. As an example, Figure 6 shows the estimated \(C/N_{0}\) for two tracked satellites, processing the streams of samples at the output of the four channels. The \(C/N_{0}\) is estimated at the tracking loop stage and provides a valid measure of the quality of the received signal [29]. Looking at Figure 6, it can be noticed that the values of \(C/N_{0}\) measured on samples out of the slave FE after the initial transient (_i.e._, the pink line for the first channel and green for the second) are on average \(1\) dB lower with respect to the \(C/N_{0}\) estimated on the corresponding chains of the master FE, depicted in blue and black, respectively. The reason for this small power loss is explained considering that the slave FE receives the clock from the master, via a logic translator circuit, which introduces noise to the reference signals of the slave board. In Figure 7, another example of the in-lab test results is reported. Here, the estimated PSDs of the digitalized signals entering the FEs are shown: Channel IF signals in Figure 7A and Channel BB signals in Figure 7B. In particular, the black plot represents the master spectrum, while the pink one is the slave spectrum. The GPS signal is strong and well evident in the bandwidth center. From these results, a strong interferer at \(\pm 2.4\) MHz respectively for \(IF=0\) Hz (BB) and \(IF=3.55\) MHz (IF) appears. This is probably due the FE clock, but being far from the GPS main lobe, no performance degradation is produced in the acquisition and tracking loop. Figure 5: In-laboratory test setup with the simulated scenario. Figure 6: \(C/N_{0}\) evaluation for PRN16 (**A**) and PRN 20 (**B**), obtained during a test in a simulated scenario.