## 3 The Receiver Acquisition Chain and the Ad Hoc Back-End of the BIRALET Radar
### Receiver Architecture of the SRT Dedicated Back-End Based on the USRP-2954R Board  
As mentioned above, the same National Instrument USRP-2954R board has been also utilized in the receiver antenna of the BIRALET radar: the SRT system. This new back-end replaced the former based on the Red Pitaya board, that did not include, due to its limited performances [23], an ad-hoc software for measuring also slant-range and therefore it performed only Doppler shift, with an accuracy of few tens of Hertz. A dedicated acquisition chain is necessary in order to carry the RF signal from the SRT feed to the USRP-2954R board, as shown in Figure 7. Analyzing this acquisition chain, it is clear that the RF signal (the UHF radar echo at 410-415 MHz, in space debris observations) is detected by the SRT antenna thanks to its P-band receiver, described in Section 2.2. After the P-band receiver block, the RF signal comes into the focus selector block, which enables to set the antenna focus necessary for the observation (the primary focus in the case of space debris observations with the P-band receiver) and provides an overall gain of about 20 dB. From the top of the primary focus, the UHF radar echo is then transported to a shielded room at ground level, due to a 500 m optical link. After that, the two polarization channels of the P-band receiver are combined using a Power Combiner (ZFSC-2-2500-S+ model from Mini-Circuits (Mini-Circuits, Brooklyn, NY, USA) [33]) and subsequently the combined UHF signal at 410-415 MHz is processed by the USRP-2954R board. Figure 6: Four chirps pulse train. The receiver architecture is shown in Figure 8, where the receiving chain depicted in Figure 7 is summarized with the SRT logo. In particular, the UHF signal enters in the USRP-2954R board and it is down converted to the board baseband (10 MHz to 6 GHz, as reported in [28]) by an I/Q down converter chip and finally digitalized by two 14-bit ADCs. The I/Q signals are then sent to the DELL Precision 7920 Rack (see Figure 7) Operator workstation for processing. The workstation is equipped with the ETTUS drivers of the USRP-2954R board, a LabView Radar processing program and a dedicated user-friendly GUI. The processing task can be done both in real-time mode or in offline mode, by using the real time stored I/Q samples. This feature is extremely useful for testing purposes and for analyzing further improvements of the radar processing chain. The LabView Radar processing program is developed in order to perform the pulse compression technique and, consequently, to extract slant-range and Doppler shift measurements of the detected target. As mentioned in Section 3.1, a matched filter is implemented for correlating the well-known transmitted signal, or template, with the unknown received signal to detect the presence of the template in the unknown signal. The matched filter is the optimal linear filter for maximizing the SNR in the presence of additive stochastic noise. A screenshot of the Operator workstation GUI is shown in Figure 9. The GUI is composed of four main blocks (see Figure 9): the control panel and status display (red block), the 2D plots (green block), the slant-range and Doppler shift extractor (yellow block) and the spectrogram plot (blue block). Figure 8: Receiver architecture of the SRT dedicated back-end based on the USRP-2954R board. Figure 7: Receiving chain for the National Instrument USRP 2954R board as backend. In detail, the control panel and status display (red block) allows two operating modes: USRP mode (data elaboration in real-time) and playback mode (data elaboration in post-processing). However, both operating modes save data in real-time. In addition, the epoch stamp and four diagnostic leds are available in the red block. These diagnostic leds indicate when there are system errors, the state of the data processing, the state of the 1 PPS synchronization and the update state of the GUI plots.