Passive Radar Applications Research Articles

Recovery of missing samples in Orthogonal Frequency Division Multiplexing signals with optimisation using data carriers

AbstractA method is proposed for reconstructing an Orthogonal Frequency Division Multiplexing (OFDM) signal that contains data gaps, with the aim to improve demodulation. The main objective is to use the method in a passive radar application with missing data samples and to improve target detection. The OFDM signal is assumed to comply with the Digital Video Broadcasting Terrestrial standard. The proposed recovery method is based on optimisation of a novel objective function, which consists of two parts. The first part is a function of the energy in the out‐of‐band frequencies, whereas the second, and novel part, uses the location of data carriers in the constellation diagram. The method is evaluated using both simulations and real data. The authors show that the proposed method significantly improves the OFDM signal in just a few iteration steps. The proposed method improved the condition number more than a factor ten thousand millions compared to using the least square method on the out‐of‐band frequencies only. The authors also decode the symbols with the Viterbi decoding algorithm and show how the required number of iterations with the proposed algorithm depends on the amount of missing samples and on the Signal‐to‐Noise Ratio in order to achieve a Bit Error Rate of less than one in one hundred thousand millions.

Capabilities and challenges of passive radar systems based on broadband low‐Earth orbit communication satellites

AbstractRecently, several constellations of broadband low‐Earth orbit communication satellites, such as Starlink or OneWeb, have started to be deployed to provide global internet access. Based on performed analyses, these emerging constellations are considered promising candidates as space‐based transmitters of opportunity for passive radar applications due to their advantageous characteristics in terms of global coverage, high bandwidth (i.e. better resolution), high received power on Earth surface (i.e. increased maximum range), predictable trajectory, and network density and robustness. However, they also pose certain challenges for the development of passive radar systems due to the necessity to track the satellites with a narrow beam reference antenna, their fast‐relative dynamics which induce range and Doppler migration, and the use of non‐standardised signals with little publicly available information, which in addition are not optimised for radar usage. In this article, these capabilities and challenges are analysed, putting a special focus on the estimation of the achievable performance and on the experimental characterisation of the transmitted signals. To this end, the proposed passive radar architectures and the first developed prototypes are also presented. Showing the potential capabilities of these emerging illuminators of opportunity, the authors aim to promote further development of this technology for multiple applications.

HRP UWB 통신의 채널 특성 변화를 이용한 실내 사람 수 파악

HRP UWB technology has recently attracted attention owing to its advantages of enabling precise distance measurements as well as communication. In addition to communication and distance measurement, passive radar applications that can recognize the surrounding situation are also being considered. Among these radar applications, the ability to detect the number of people indoors is of increasing interest when social distancing is important. In this study, we implemented an HRP UWB passive radar application system that detects the number of people in an indoor environment using the change in channel characteristics between HRP UWB communication devices. Using the implemented system, changes in the channel impulse response (CIR) characteristics of channels 5 and 9 were observed, and the number of people was identified using the singular vector decomposition (SVD) algorithm. From actual measurements, it was possible to confirm that the number of people within five people in an indoor environment could be accurately identified using a pair of HRP UWB wireless communication devices in an indoor environment.

A ultra wideband dual-polarized antenna with high isolation degree for passive radar application

ABSTRACT This paper presents an ultra wideband (UWB) dual polarization antenna with high polarization isolation for passive radar system The antenna is composed of a modified Vivaldi antenna and a folded dipole log periodic antenna. Based on the respective structural characteristics of the printed Vivaldi antenna and the folded dipole log periodic antenna, they are placed orthogonally without touching each other, which form two orthogonal polarization channels and can achieve high polarization isolation between the channels. The radiator shape of traditional Vivaldi antenna is modified and simplified to further reduce the electromagnetic coupling between two polarized radiators. The planar folded dipole log periodic antenna is fed by UWB Microstrip tapered line Balun, and a vertical through hole is introduced to connect the parallel double line of Balun and the coplanar stripline (CPS) at the input terminal of the folded dipole log periodic antenna. The two polarization ports of the antenna are welded by coaxial line and microstrip line. The dual polarization antenna is designed, optimized, fabricated and tested within the range of 3 GHz ~ 8 GHz. In the investigated frequency range, the polarization isolation of the simulated dual polarization antenna is greater than 30 dB, and the average polarization isolation is about –35 dB.The gain of Vivaldi antenna polarization port increases from 2.82dBi to 9.49 dBi, and the gain of folded dipole log periodic antenna polarization port increases from 5.21 dBi to 8.56dBi. A large number of experimental tests and analysis are carried out on the fabricated dual polarization antenna, and the test results are close to the simulation results, which prove the effectiveness of the proposed dual polarization antenna. In view of the printed circuit implementation method and structure characteristics of the dual polarization antenna designed in this paper, the scheme has the advantages of low cost, simple design and good polarization characteristics, so it is suitable for the application of anti radiation missile guidance radar and electronic reconnaissance.

Passive Radar Signal Processing in the Presence of Sample Clock Offset

Passive radar signal processing is hindered by the need to sample a reference signal from the chosen illuminator, which is subject to sensor noise and interference from clutter returns. The reference signal may be demodulated and reconstructed to remove artifacts. However, without accounting for the transmitter's clock offset, the remodulated reference signal will be a poor match for the surveillance signal. This article evaluates clock offset estimation methods in terms of the radar ambiguity surface, thereby appraising them specifically for passive radar applications. Existing offset estimation methods are shown to meet variance bounds; however, these bounds worsen with increased clock offset. Ambiguity surfaces constructed from signals reconstructed with poor clock offsets exhibit power leakage. A new clock offset estimator is proposed, which refines coarse estimates made by an existing method. The new estimator is demonstrated to achieve accurate estimation of clock offset. Reference signals reconstructed with these estimates produce ambiguity surfaces with no significant power leakage.

A Cramér-Rao Lower Bound Derivation for Passive Sonar Track-Before-Detect Algorithms

Track-before-detect (TkBD) algorithms have been shown to greatly abate measurement-to-track association (MTA) challenges. These simplifications are aptly relevant for reducing operator workload in deployed sonar systems that require a human “in-the-loop.” In a prior manuscript a case study of a passive bearings-only target motion analysis TkBD algorithm was demonstrated in complex sonar scenarios relevant to advanced fielded sonar systems. In this manuscript, a Cramer-Rao Lower Bound (CRLB) is derived for the algorithm previously developed. The approximations used in developing the CRLB are validated with a real data set. The CRLB itself, as a predictor of state estimation error performance, is validated with single- and multi-contact simulated data scenarios. The prior algorithm and CRLB derived herein is applicable to passive sonar, active sonar, radar, and optical applications through a change of point spread functions and Jacobians. The CRLB derived is simple to implement, requires minimal statistical assumptions, and is applicable to similarly implemented TkBD algorithms.

LTE‐based multistatic passive radar system for UAV detection

Using the frequency-division duplex long-term evolution (LTE)-based multistatic passive radar system for unmanned aerial vehicle (UAV) detection is considered. The typical outdoor terrestrial LTE eNodeBs and the broadband direct air-to-ground communications network eNodeBs are used to cover the UAVs at the low altitude and at the medium-high altitude, respectively. The bistatic range measurements are obtained from multiple eNodeBs, and the hyperbolic positioning technique is used for target localisation. Since the existing LTE downlink signals are more suitable for the transmission of the communication data rather than target detection, a new LTE downlink signal feature termed as target localisation reference signals (TLRS) is designed exclusively for the passive radar application using the Costas frequency coding and the P4 phase coding. Compared with the existing LTE downlink signal features, the TLRS have better waveform properties and could potentially provide much higher target detection accuracy. Meanwhile, the TLRS occupy only 0.266% LTE downlink time–frequency resources within a time frame of 1 s when the bandwidth is 10 MHz and have no negative impact on the normal LTE downlink data transmission.

Feasibility analysis of non‐orthogonal waveforms for multiple input multiple output passive radar application

New non-orthogonal multiple access technology is proposed with the development of 5G, which is expected to support higher spectrum efficiency and massive terminal access, it enables the communication system to achieve better bit error rate performance and alleviate the spectrum congestion problem effectively under limited resources. With the design of spectrum sharing for radar and communication getting more attention, 5G non-orthogonal signal has become potential passive radar illumination source. In this article, an orthogonal multiple input multiple output (MIMO) radar waveform design metric by minimising the integration sidelobe level, is introduced firstly. Then based on sparse code multiple access (SCMA) technology, the non-orthogonal waveform is designed as the passive radar illumination sources. The distance and Doppler resolution of non-orthogonal waveform are evaluated by a more generalised MIMO ambiguity function. Simulation results demonstrate that non-orthogonal SCMA signals have competitive performance with orthogonal code division multiple access signals.

PN Signal as a New Illuminator of Opportunity for Passive Radar Applications

In this letter, we use the known pseudorandom noise (PN) signal in the digital television terrestrial multimedia broadcasting (DTMB) system as a new illuminator of opportunity for the passive radar. We propose a computationally efficient signal processing method for the PN-based passive radar (PNPR). The proposed method takes advantage of the distinctive structure of the PN signal and constructs a locally remapped PN signal. The circular cross-correlation between the remapped PN signal and the original PN signal is an ideal impulse function. Thus, the PNPR even does not need to do the clutter cancellation. The proposed method is validated via both the simulated and experimental data.

Passive bistatic radar using digital video broadcasting-terrestrial receivers as general-purpose software-defined radio receivers.

We investigate the use of low-cost digital video broadcasting-terrestrial (DVB-T) receivers as general-purpose software-defined radio (SDR) receivers for passive bistatic radar (PBR) applications. Two DVB-T receivers are synchronized using a common clock to perform coherent measurements. By exploiting the direct-path signal in the surveillance channel, we use the cross-correlation process to estimate the time offset between the data streams of reference and surveillance channels caused by the universal serial bus communication. We demonstrate the detection of static and moving targets as well as short-range targets, including a landing airplane at 8 km, multiple ships with different velocities, and vehicles within 20 m from the receiver acquiring at a 2 MHz bandwidth, by using Japan's Integrated Services Digital Broadcasting Terrestrial digital TeleVision (TV) signal broadcast. We also propose to improve the range resolution of the designed PBR system by combining multiple SDR receivers tuned to different carrier frequencies. The designed system and proposed method can be used for various applications, such as airplane navigation, harbor protection, and traffic density monitoring.

Design of a Directional Microstrip Antenna at UHF-Band for Passive Radar Application

Passive radar is an electromagnetic system that can receive the frequency signals from transmitted source such as FM radio, TV broadcasting station, or mobile communication base station to detect the presence of an object by utilizing its frequency reflection. One of the components of a passive radar is an antenna. An antenna in a passive radar is to receive an information signal from the reflection of an object and then the data that already received will be processed into the form of signal before it was displayed on the monitor. This paper aims to create a microstrip antenna with directional pattern for a passive radar. Microstrip antenna is a type of antenna that has a light weight, easy to fabricate, has a simple shape and lower cost. The design and simulation of antenna are using CST Studio Suite software. If the simulation results fit with the requirement, we fabricate and measure it and then compare it with the simulation results. The result of antenna has an ultrahigh frequency (UHF), directional pattern, gain 6dBi and range bandwidth minimum 470Mhz – 780Mhz.

A novel monofilar‐Archimedean metamaterial inspired leaky‐wave antenna for scanning application for passive radar systems

AbstractA novel backfire‐to‐endfire leaky‐wave antenna is presented with ability to scan from −25° to +45°. The antenna is based on metamaterial transmission‐lines and is implemented using monofilar Archimedean spiral and rectangular slots, spiral inductors, and metallic via‐holes. The slots act as series left‐handed capacitances, and the spirals with via‐holes provide the shunt left‐handed inductances to realize the metamaterial antenna. A prototype antenna was fabricated prototype on FR4 dielectric substrate, which has an electrical size of 0.0302λ0 × 0.0357λ0 × 0.0008λ0, where λ0 is free space wavelength at 165 MHz. Measured bandwidth of the antenna is 710 MHz (165–875 MHz) corresponding to a fractional bandwidth of 136.5%. The main advantage of the antenna is its ability to scan over a wide angle from −25° to +45° with acceptable gain and radiation efficiency of 1.2 dBi and 50.1%, respectively, measured at 400 MHz. The wide scanning attributes of the antenna make it suitable for passive radar applications to scan across the VHF–UHF bands for FM‐Radio, television, mobile phones, and GPS applications.

Method for bearing estimation of target for amplitude modulation radio‐based passive radar application

In a passive radar system, the bearing of the target is difficult to measure for the signal-to-noise ratio of the target's echo signal is generally very low in practise. A novel method to estimate the echo signal's angle is proposed. The cross-ambiguity function of direct-path signal and each antenna's receiving signal is calculated to form a virtual array signal, in which the desired echo signal can be enhanced at a certain time delay and Doppler shift, so it is able to estimate its angle. We make the theoretical analysis about how the desired echo signal is enhanced. Some experimental results of an amplitude modulation (AM) radio-based passive radar are presented to verify the effectiveness of the proposed method.

OFDM reference signal reconstruction exploiting subcarrier‐grouping‐based multi‐level Lloyd‐Max algorithm in passive radar systems

This study proposes a low-complexity subcarrier-grouping-based multi-level Lloyd-Max (SG-ML-LM) algorithm to reconstruct orthogonal frequency division multiplexing (OFDM) reference signal in passive radar systems. The authors’ previously proposed multi-level Lloyd-Max (ML-LM) algorithm is originally intended to blindly estimate the narrowband flat fading channel coefficients for non-constant modulus constellations. The authors extend it into broadband wireless environment to blindly estimate the frequency-selective fading channel coefficients by exploiting OFDM signal structure properties. Then a SG-ML-LM algorithm is further proposed to reduce the computational complexity of ML-LM by utilising OFDM subcarrier correlation. Numerical results validate that: (i) ML-LM and SG-ML-LM algorithms are both feasible schemes to reconstruct OFDM reference signals for passive radar applications; (ii) compared with ML-LM algorithm, SG-ML-LM algorithm significantly reduces the computational complexity at the cost of blind channel estimation error, but the OFDM reference signal reconstruction accuracy is nearly maintained, so that the cross-ambiguity function performance is nearly lossless despite of a negligibly slight increase of main lobe Doppler bandwidth.

Quad channel software defined receiver for passive radar application

Abstract In recent times the growing utilization of the electromagnetic environment brings the passive radar researches more and more to the fore. For the utilization of the wide range of illuminators of opportunity the application of wideband radio receivers is required. At the same time the multichannel receiver structure has also critical importance in target direction finding and interference suppression. This paper presents the development of a multichannel software defined receiver specifically for passive radar applications. One of the relevant feature of the developed receiver platform is its up-to-date SoC (System on hip) based structure, which greatly enhance the integration and signal processing capacity of the system, all while keeping the costs low. The software defined operation of the discussed receiver system is demonstrated with using DVB-T (Digital Video Broadcast – Terrestrial) signal as illuminator of opportunity. During this demonstration the multichannel capabilities of the realized system are also tested with real data using direction finding and beamforming algorithms.

RSN Editorial 2017

It is a pleasure to report that the Premium Award for papers published in the journal in 2015 is made to Branka Jokanovic, Moeness Amin, Yiming Zhang and Fauzia Ahmad of Villanova University, USA, for their paper Multi-Window Time and Frequency Signature Reconstruction from Undersampled Continuous-Wave Radar Measurements for Fall Detection’, which appeared in the February 2015 Special Issue on Application of Radar to Remote Patient Monitoring and Eldercare (vol.9. no.2, pp 173-183). This is an example of an exciting new application of passive radar. It is attractive to provide some form of indoor monitoring of elderly people living in sheltered accommodation and particularly to detect falls, which are a significant issue for the elderly. Radar-based techniques have the advantages over video monitoring that they are not invasive of privacy and do not depend on particular lighting conditions. This might be particularly important in monitoring an individual in the bathroom, where the likelihood of slippage or falling might be high. Furthermore, since elderly people tend to well-defined patterns of behaviour, a monitoring system of this kind could potentially ‘learn’ such patterns and detect and flag any anomalies. The paper analyses the radar time-frequency signature of fall events, as well as bending over, sitting and standing, using an approach based on compressive sensing and multi-window analysis. This demonstrates that falls can reliably be detected and classified. We warmly congratulate the authors on this award. The names of the members of the Editorial Board are listed inside the front cover of this journal; the members are distinguished researchers who represent the full scope of the journal, and they come from all over the world. They help with reaching decisions on submitted papers, encouraging authors to submit their work to the journal, and proposing topics for Special Issues and Special Sections. Finally, we express our gratitude to the referees for their time, expertise and hard work in reviewing papers. This is an essential part of the publication process, but necessarily the referees must remain anonymous. This Editorial is one place where we can acknowledge their contribution and record our thanks to them.

Modified Cramér‐Rao lower bounds for joint position and velocity estimation of a Rician target in OFDM‐based passive radar networks

AbstractOwing to the increased deployment and the favorable range and Doppler resolutions, orthogonal frequency‐division multiplexing (OFDM)‐based L band digital aeronautical communication system type 1 (LDACS1) stations have become attractive systems for target surveillance in passive radar applications. This paper investigates the problem of joint parameter (position and velocity) estimation of a Rician target in OFDM‐based passive radar network systems with multichannel receivers placed on moving platforms, which are composed of multiple OFDM‐based LDACS1 transmitters of opportunity and multiple radar receivers. The modified Cramér‐Rao lower bounds (MCRLBs) on the Cartesian coordinates of target position and velocity are computed, where the received signal from the target is composed of dominant scatterer (DS) component and weak isotropic scatterers (WIS) component. Simulation results are provided to demonstrate that the target parameter estimation accuracy can be improved by exploiting the DS component. It also shows that the joint MCRLB is not only a function of the transmitted waveform parameters, target radar cross section, and signal‐to‐noise ratio but also a function of the relative geometry between the target and the passive radar networks. The analytical expressions of MCRLB can be utilized as a performance metric to access the target parameter estimation in OFDM‐based passive radar networks in that they enable the selection of optimal transmitter‐receiver pairs for target estimation.

Catching the high altitude invisible by satellite-based forward scatter PCL

This paper presents a feasibility study on using satellite signal for passive radar application. Our focus is on utilizing geostationary Earth orbit satellite signal with suitable properties, like Inmarsat. Then, a new method of detection for passive coherent location using adaptive filter weights variation model is presented. To evaluate the performance, three different scenarios including a low Earth orbit satellite, a space shuttle, and a high-altitude aircraft as the targets of interest are considered. In addition to being covert, it will be shown that using such passive radar system, we can benefit the forward scatter enhancement, which enables us to detect such high-altitude targets that are normally invisible to radar systems. At last, the performance of the proposed method is compared with that of traditional methods. The analysis reveals that the proposed method has higher detection performance compared to the traditional detection methods in passive radars.

Cramér–Rao bound analysis for joint target location and velocity estimation in frequency modulation based passive radar networks

With the wide spread availability and the favourable Doppler resolution, the frequency modulation (FM) commercial radio signals have become attractive for passive radar applications. Passive radar networks using multiple illuminators of opportunity and multichannel receivers have been shown to offer significant performance improvement owing to their advantage of signal and spatial diversities. In this study, the authors compute the joint Cramer–Rao lower bound (CRLB) for the target parameter (delay and Doppler) estimation error utilising FM commercial radio signals as illuminators of opportunity for passive radar network systems, where the non-coherent and coherent processing scenarios are considered. The numerical simulations are provided to show that the joint CRLB is not only a function of the transmitted waveforms but also of the relative geometry between the target and the passive radar networks for both non-coherent and coherent cases. The expressions for joint CRLB are an important performance metric for target parameter estimation in FM-based passive radar networks.

Transmitter Subset Selection in FM-Based Passive Radar Networks for Joint Target Parameter Estimation

Passive radar network systems utilize multiple transmitters of opportunity and multichannel receivers to offer remarkable performance improvement due to the advantage of signal and spatial diversities. The frequency modulation (FM) commercial radio signals have become attractive for passive radar applications owing to their wide-spread availability and the favourable Doppler resolution. In this paper, two transmitter subset selection schemes, balancing the trade-off between target parameter estimation accuracy and infrastructure utilization, are proposed for FM-based passive radar networks. In the first, the subset size of selected transmitters employed in the estimation process is minimized by effectively selecting a subset of transmitters, such that the required target parameter estimation mean-square error (MSE) threshold is attained. In the second, an optimal subset of transmitters of a predetermined size $\kappa $ is selected, such that the estimation MSE is minimized. These problems are formulated as a knapsack problem, where the coherent Cramer–Rao lower bound (CRLB) is used as a performance metric. Both transmitter subset selection schemes are tackled with greedy selection algorithms by successively selecting transmitters so as to minimize the performance gap between the CRLB and a predetermined MSE threshold or a predetermined subset size. Numerical simulations demonstrate that the problem of transmitter subset selection is not only a function of the transmitted waveforms but also of the relative geometry between the target and the passive radar network systems, which leads to reductions in both computational load and signal processing costs.