# Experimental Seaborne Passive Radar
## 2 Materials and Methods
### System Setup and Signal Acquisition
Each RSPduo device was connected to the host PC via a High-Speed USB 2.0 interface (also used as the power supply) and contained two complete RF frontends. Each frontend included a bank of bandpass filters, variable-gain LNA (low-noise amplifiers), and optional notch filters. The ADC (analog-to-digital converter) had 14-bit performance (for a 2 MSPS sample rate), and the sensitivity was around 0.1 \(\upmu\)V [16]. The receiver's performance, combined with the completeness of the RF frontend, the small size of the device, and the low power consumption, determined the choice of the device for our experiments. Two or more RSPduo devices can be synchronized by connecting their dedicated reference OUT/IN ports, so it is possible to obtain the same sampling rates and carrier frequencies in multiple RF tuners. However, providing repeatable, coherent multichannel signal acquisition requires additional hardware and software modules [15], such the training signal generator (see Figure 1). Due to the lack of a common trigger signal, the coherent acquisition from two or more RSPduo devices becomes a challenge [17]. Each of the devices starts signal reception with an unknown delay (because of sequential initialization in the software) and a random phase shift due to the uncontrolled initial conditions of PLLs (phase-locked loops) in the downconverters. To solve this problem, a method for estimating phase shifts and time delays introduced in the receivers has been developed and implemented [15] based on the matched filtering of signals with superimposed pilot sequences from the training signal generator, whose architecture is shown in Figure 5.  
The pilot sequence consisted of three repetitions of a complete PRN (pseudo-random) sequence, generated in an MCU (microcontroller) that modulates the carrier signal obtained from the frequency synthesizer followed by an attenuator. The carrier frequency (Fc) was programmed in accordance with the frequency of the broadcast transmitter used as the illumination source, and it was generated only during the short periods of pilot signal  
Figure 4: RSPduo: dual-tuner SDR (software-defined radio) receiver. Figure 5: Internal structure of the training signal generator. emission to avoid interferences with the received signals. BPSK (bipolar phase-shift keying) modulation was performed in a dual-balanced mixer. The modulated signal was then passed to a simple power divider to obtain its four copies that are finally fed into the directional couplers (see Figure 1.) The pilot signal was injected periodically in every 5 s.  
The acquired signals were stored on an SSD in the host PC, and were later processed offline in the MATLAB environment. During the preprocessing stage, the pilot signals were extracted with matched filters and used to calibrate the phase offsets and time delays between the received signals. The details of the training signal properties, generation, and processing have been disclosed in [15]. The experimental hardware setup that was used during the experiments (without a host machine, which was a typical notebook) is presented in Figure 6.