Ship Detection with DVB-T Software Defined Passive Radar
Amerigo Capria*, Michele Conti#, Dario Petri#, Marco Martorella#, Fabrizio Berizzi#, Enzo Dalle Mese# Rocco Soleti†, Vincenzo Carulli†
*RaSS Center – CNIT, via Moruzzi 1, 56124 Pisa, Italy #Dept. of Information Engineering – University of Pisa, via Caruso 16, 56122 Pisa, Italy †Italian Navy – CSSN ITE G. Vallauri, viale Italia 72, 57127 Livorno, Italy
Abstract— Passive radar systems exploit non-cooperative for implementing telecommunication systems, of realizing a transmitter to detect targets in areas of interest. Some of the passive radar demonstrator. Specifically, in this paper, a low- main advantages of such systems with respect to conventional cost DVB-T software defined passive radar system for costal radars include low cost architectures, low energy requirements ship detection is presented. Firstly, an overview of the DVB-T and potentially null probability of intercept. Digital waveforms standard and a feasibility study has been done in order to like DVB-T and UMTS signals offer relatively wide bandwidth channels which allow achieving good spatial resolution. define the passive radar system architecture and to estimate Moreover, they have spectral properties which are nearly the coverage capabilities. Afterwards, a low cost software independent of the signal content. In this paper a software defined experimental radar system is introduced and finally defined passive radar system based on the use of DVB-T signals some live data detection results are presented and discussed in is proposes and live data detection results are presented in order order to demonstrate the capability of this passive radar to demonstrate the detection capabilities. system.
I. INTRODUCTION II. DVB-T SIGNAL Passive radar systems [1], also referred to as Passive Coherent The DVB-T signal is organized in OFDM frame Location systems (PCL), exploit reflection from illuminators (Orthogonal Frequency Division Multiplexing). Each frame of opportunity (IOs) in order to detect and track objects. A consists of 68 OFDM symbols. PCL receiver generally presents two receiving channels OFDM transmission technique is robust against multipath denoted as reference channel and target channel. The propagation, therefore the usage of OFDM should imply more reference channel is used to capture the direct signal from the robustness of the radar system with respect to multipath transmitter and provides a reference signal to be compared propagation. with the target return. It should be noted that the detection Each symbol is formed by a set of data subcarriers and a set performance of passive radars based on analogue signals are of pilots subcarriers useful for receiver synchronization and strongly dependent on the signal content. On the contrary, transmission parameters estimation. The modulation power of digital waveforms [2], thanks to specific signal coding, have scattered pilots have an average power higher than data sub spectral properties which are nearly independent of the signal carriers. The Italian DVB-T transmission standard adopts content. Among the digital IOs, DVB-T signals have a wide 6817 frequency sub-carriers T 896 s and channel bandwidth that allows achieving good spatial resolution s together with a very good spatial coverage. This characteristic bandwidth isBDVB T 8 MHz . is very attractive for system implementation through the use In PCLs, the range and Doppler resolution are determined of Commercial of the Shelf (COTS) Software Defined Radio by the ambiguity function (AF) of transmitted waveform: (SDR) hardware. In particular, Universal Software Radio Peripherals (USRPs) seems to be one of the most promising j2 f t (,)()() f s t s* t ed dt (1) COTS solutions for the realization of a passive radar operating d with DVB-T. The DVB-T software defined passive radar architecture guarantees a continuous surveillance (24h/7days), without employing transmitters, therefore minimizing costs As shown in [3] the AF of a DVB-T signal shows a main and power consumption. The mentioned advantages of this peak and side peaks due to the presence of pilot carriers. The kind of systems suggest the possibility of employing passive side peaks could generate range Doppler ambiguities therefore radars as early warning sensors for coastal surveillance. The masking targets. Fig 1 shows a 3D view of the DVB-T goal of this paper is to show the capability of the Universal ambiguity function obtained from real data acquired by the Software Radio Peripheral (USRP) technology, typically used USRP. Serra” in Pisa (around 32 km far from the receiver) and the surveillance antenna was directed towards an area of sea in front of the receiver site. Considering a bistatic configuration, the SNR for an integration time Tint can be evaluated by means of [1]:
2 ERP Gr b SNRT 3 2 2 int (2) (4 ) r1 r 2 kT 0 Fn
where ERP PGt t is the Effective Radiated Power, Pt is the
transmitted power, Gt is the transmission gain, Gr is the
receiver gain, b is the bistatic radar cross section, λ is the carrier wavelength, r1 is the transmitter to target distance, r2 is Fig. 1. Real data DVB-T AF (3D view) the target receiver distance, Fn is the noise figure, T0 =290°K, III. EXPERIMENTAL SETUP k is the Boltzmann constant. The used DVB-T channel carrier frequency is 818 MHz and The equipment that has been used in this experiment is the targets of interest were ships arriving and departing from composed by commercial off-the-shelf low cost TV antennas, the nearby harbour. an USRP equipped with a RF front-end tuneable from In Fig. 3 the expected coverage capability of the proposed 800MHz to 2400MHz. The main technical specification of the system is presented. The colormap is relative to the SNR USRP board (version1) are: levels at the receiver, the blue and white contours are the FPGA Altera EP1C12 Q240C8 “Cyclone” maximum range capability as a result of the number of bits 4 High-Speed Analog to Digital Converters (ADCs) used for the sampling, respectively 8 and 12 bits. operating at 12 bits with a sampling rate of 64 Mega- samples per seconds (64 MS/s) 4 High-Speed Digital to Analog Converters (DACs) operating at 14 bit with a sampling rate at 128MS/s USB 2.0 data port with a limited throughput of 32 MBps (MegaByte-per-second) The antenna used during the experiment for the target channel is a Yagi-Uda antenna with a receiving gain equal to 18 dB and a Half Power Beam Width of 20 degrees in the horizontal plane. On reference channel a Yagi-Uda antenna with a gain of 15 dB has been employed.
IV. EXPERIMENT SCENARIO 40 km The experiment scenario geometry is shown in Fig. 2.
Fig. 3. Expected coverage capability (SNR values)
In a passive radar scenario the Doppler frequency is given by [4]:
15 km 32 km 1 r v r v f 1 2 (3) d r1 r 2
Where v is the target velocity vector, r1 is the transmitter to Fig. 2. Experiment scenario geometry target vector, r2 is the target receiver vector. Specifically the receiver was located at the “CSSN-ITE G. In Fig. 4 the expected Doppler frequencies for ships Vallauri” institute in Livorno, the used illuminator of departing from the nearby harbour are shown. opportunity was a DVB-T transmitter located on “Monte The echo relative to the ship is clearly visible at a range of about 2.1 nm with a negative Doppler frequency equal to -32 Hz (i.e.: about 6.5 kts). It should be noted that the target echo is actually formed by two main peaks with the same Doppler frequency, respectively a strongest one further away from the radar and a closer one that is weaker. Looking at the close up view of the target reported in Fig. 5, it can be noted that the target presents two main scattering structures, a huge one at bow and another big one at stern. The two main peaks visible in the CAF can be directly associated with the two main scattering structures. Another confirmation comes from the target length (around 170 m) that is accordance with the Fig. 4. Expected Doppler frequencies for ships departing from the nearby distance of the two main target peaks in the CAF (around 160 harbour (receding from receiver) m). The target of interest was voluntarily a big ship in order to Within the distance of interest (1.5-3.3 nm) and in the verify the effectiveness of the proposed passive radar system, assumption of ship speed ranging between 5 and 10 kts, the nevertheless more experiments are planned in order to deal expected Doppler frequency absolute value will be comprised with smaller ships located at further distances. between 20 and 60 Hz.
V. EXPERIMENTAL RESULT The surveillance area during the acquisition is shown Fig. 5; the target of interest is clearly shown in the zoomed view (on the bottom).
Fig. 6. DVB-T CAF of the surveillance area
VI. CONCLUSIONS Experimental results obtained with low-cost equipment have proven the feasibility of a DVB-T based passive radar system by using a software defined architecture. An experimental system has been set up and live data acquired. The radar functionality has been tested on moving ship. Further experimental measurements in a costal environment with cooperative targets are planned and the feasibility of multistatic radar architecture will be studied.
REFERENCES
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