## 4 Results of Space Objects Measurement Campaigns  
A validation campaign of the new system, based on the USRP-2954R board, was performed with the goal to assess the BIRALET radar performances. Targeted observations were performed, between 13 December 2019 and 9 October 2020, by pointing the system towards calibration objects from the International Laser Ranging Service (ILRS) database, which provides the positional state of the satellites with an accuracy of around a centimeter, and other NORAD target objects, for which the expected range and Doppler shift values were estimated using the available TLEs. The population and related TLEs were retrieved  
Figure 9: Screenshot of the Operator workstation Graphical User Interface (GUI). from the Space-Track website [9; 10; 11]. In these calibration campaigns, the BIRALET radar detected 37 objects. All the observations were made in beam-parking mode, waiting for the objects to cross the Field of View (FOV) of our radar, with a transmission power of about 5 kW. The list of the scheduled passages and measurement results for the debris, complete with the detection time for each object, the estimated RCS, the estimated and measured slant-range and Doppler shift values, is reported in Table 2.  
Before the observations, a forecasting campaign was made in order to obtain the azimuth and elevation pointing coordinates and to predict the slant-range and the Doppler shift frequency projected onto the line of sight of the radar. An algorithm based on the reading of the TLEs provides these pieces of information and consequently it simulates the passages of the objects. In detail, after establishing a temporal window for the observation, for each catalogued object of the list available on the Space-Track web site [10], the software propagates its TLE by means of the SGP4 dynamical model and computes possible passages that could be detected by BIRALET. After that, it is possible to choose the desired passage, by selecting the objects passage epoch and their azimuth and elevation pointing coordinates, in order to perform the validation observation. A passage is considered detectable if the constraints on the receiver and transmitter minimum elevation are satisfied, and the detected SNR is larger than an imposed threshold (typically 6 dB). For each detectable passage, the software provides a list of time epochs and required pointing directions, i.e., SRT and TRF azimuth and elevation, and the estimated values of the Doppler shift and the slant-range. For the considered simulations, the TLE propagated with the SGP4 dynamical model represents the real dynamics of the system. After the measurement campaign, it is possible to compare the measured Doppler shift and slant-range with the estimated values and investigate if the observation was successful. This aspect can be considered true when the object is detected and its measured parameters match the EUSST requirements (distance error lesser than 100 m and Doppler shift error up to 50 Hz). As described in Section 3.2, the Operator workstation GUI provides all observation information, i.e., the measured slant-range and Doppler shift. In addition, several plots are available in order to show the measured values to the user, immediately. Purely by way of example, the spectrogram of the entire observation window for the object ID 37387, detected on December 13, 2019 at 10:47:54 UTC, is shown in Figure 10. Due to the relative close proximity of TRF with the SRT antenna (baseline between the two antennas of about 20 km), the receiving antenna (SRT) of the BIRALET radar was able to receive the carrier frequency (410.085 MHz), which is clearly visible in the image (Figure 10). Additionally, in the spectrogram (Figure 10), it is recognizable the frequency swipe of the debris echo, due to its high velocity, as it moves toward the receiver position (when the echo is located to the left of the carrier, which implies a negative Doppler shift). In this case, the measured Doppler shift is equal to \(-\)1740 Hz.