## 6 Conclusions and Future Work  
The BIRALET system is a bi-static radar that collects useful data for space debris studies under the EUSST program. In this paper, a new ad-hoc back-end for the BIRALET radar has been developed and presented, with the aim to perform slant-range and Doppler shift measurements, simultaneously. The BIRALET system is composed of the TRF antenna as transmitter, that permits, thanks to its new back-end based on USRP-2954R board, to transmit a mixed signal (CW tone plus FMCW signal) in order to perform range and Doppler shift measurements. The receiver is the SRT antenna, equally equipped with a new dedicated back-end based on the same USRP-2954R board. Validation measurement campaigns were done in beam-parking mode in order to investigate on the system performances. The results, with the detection of 37 objects between 13 December 2019 and 9 October 2020, confirm that the measurements collected by the BIRALET radar, after calibration and bias correction, are accurate and comply with the EUSST requirements (i.e., a slant-range accuracy of 100 m and a Doppler shift error up to 50 Hz). In particular, the system was able to detect resident space objects with various RCSs, from 0.1199 m\({}^{2}\) to 12.1 m\({}^{2}\), at distances between about 1200 km and 2600 km, with an average slant-range error of 100.189 m, with a standard deviation of 54 m, and an average Doppler shift error of 37.95 Hz, with a standard deviation of 33.3 Hz. An improvement of the system could be done by increasing the integration time (increasing the number of integrated pulses of Figure 6) or changing the transmitted waveform. Many tests could be done by tuning the system parameters, and by processing the real time stored I/Q samples offline. Further measurements and processing refinements will be made in the next future, in order to try improving the system performances. The evaluation will be tested on coverage and accuracy, using several targets with different RCSs and orbiting at several altitudes. Furthermore, since BIRALET is tested in beam-parking mode until now, waiting for the objects to cross the field view of the radar, one future work is represented by tracking mode observations, where the radar will follow the target. Thanks to this observation mode, BIRALET will have the possibility to observe the same object during its motion, in the whole time window of its visibility. In this way, a larger amount of data will be collected,  
Figure 12: Measurement errors respect to estimated values: (**a**) slant-range error for each debris (blue points) and slant-range average error for all debris, equal to 100.189 m (red line); (**b**) Doppler shift error for each debris (blue points) and average error for all objects, equal to 37.95 Hz (red line). such as the pointing coordinates (object position in the time window), the measured range and the measured Doppler shift, between the beginning epoch and the end epoch of the observation time period. These data will permit to graph the object trajectory as a function of the time and, consequently, to have also information about its 3D velocity respect to the observation position. One of the physical limitations of the current BIRALET system could be the relatively low mechanical pointing speed of the SRT (0.85 deg/s in azimuth, 0.5 deg/s in elevation, as listed in Table 1) and the availability of only one beam, resulting in a limited FOV (HPBW of 0.8 degrees, as reported in Table 1). Space debris, with angular speeds greater than the maximum antenna angular movement, cannot be tracked. In addition, it proves impossible to measure the object trajectory direction with only one beam, as the case of the actual P-band BIRALET radar. Imaging the sky with a multi-beam receiver would increase the telescope field of view and survey speed, allowing the coverage of a greater portion of the sky in less time [34]. One way to do it, as a future project, would be to develop a C-band Phased Array Feed (PAF) as receiver for the primary focus of the SRT system.