Coherency limits and synchronisation of a netted radar system using USRPs as nodes

This paper intends to further evaluate the synchronisation capabilities of multiple networked USRPs. We obtained a priori knowledge of the USRP’s coherency limits between daughterboards within a single device and presented such results in our previous work [1]. The final aim of this work is to build a versatile networked radar system which may take advantage of the transceiver capabilities of its different nodes to form a multistatic radar. At the expense of high complexity, multistatic radar systems exploit spatial diversity to increase the probability of detection and tracking of targets. To properly make use of these advantages, the involved nodes must agree on a common time base $({t}_{0})$. A succession of precisely timed events must occur, as to maximise the resulting SNR on the measurement side. In this paper, a customized software network time protocol (NTP) server is added between the control computers and it is combined with crontab as to timely start the transmitting and sampling sequences on the different nodes. Up-chirp and sinusoid waveforms were used for measuring the offset with ${t}_{0}$ and for evaluating the achievable coherency limits. Thus, by evaluating different synchronisation methods (analogue and digital), fractional frequency of $2.8{\cdot }{10}^{-13}$ and phase noise of −118 dBc/Hz at 10 Hz frequency offset were measured.

I. Introduction

The current market trend shows a growing worldwide interest in R&D in the field of unmanned aircraft systems (UAS), commonly referred to as drones [2]. Several countries are preparing to introduce new regulations into their respective airspaces. In the near future, drones in the EU will be integrated in the U-space, which is the lower airspace for commercial operation within urban environment [3]. However, not all drones using a particular airspace are of cooperative nature. There is an increased concern due to their reduced radar cross-section (RCS) posing a challenge for radar systems to detect and track drone targets [4], [5]. The advantages and increased complexity of bi- and multistatic radar systems are acknowledged and agreed on the diverse literature [6]–[9]. Moreover, they are demonstrated in [10], [11]. Thus, the spatial diversity and increased receiver sensitivity provided by the multistatic radar systems are features of interest for robust drone detection and tracking.