Linear Frequency Modulated Continuous Wave Research Articles

Detection and Estimation of LFMCW Radar Signals

The Wigner-Ville Hough transform (WVHT) is suboptimal in the detection and parameter estimation of linear frequency-modulated (LFM) continuous wave (LFMCW) low probability of intercept (LPI) radar waveforms because they are composed of concatenated LFM pulses. We formulate the detection and estimation problem to take into account the multiple pulses that are available in an observation interval at the intercept receiver. The new algorithm, called the periodic WVHT (PWVHT), significantly outperforms the WVHT for LFMCW signals.

Chaotic frequency-modulating continuous wave for an ultrasonic doppler blood flow velocity measurement system

Although a pulsed-wave (PW) Doppler system can measure blood velocity with range information, measurable velocity is limited by frequency aliasing, and signal-noise-ratio (SNR) is poor. Continuous wave (CW) Doppler system offers a higher SNR in Doppler signal and no measurable velocity limitations but does not provide ranging information. Linear frequency-modulated continuous wave (LFMCW) and sine frequency-modulating continuous wave are able to provide high SNR Doppler signals with ranging information, and low clutter power, but with a certain range ambiguity due to signal periodicity of the Doppler signal spectrum. Various efforts have been made to improve the SNR, range resolution, and velocity limitations with FMCW Doppler system. An ultrasonic doppler blood flow velocity measurement system based on chaotic FMCW is proposed which has a higher SNR with no range ambiguity because of the aperiodicity of the Doppler signal spectrum. Principle analysis and experimental simulation show that the effectiveness of the new system is verified.

太赫兹极高分辨力雷达成像试验研究

The design and realization of a terahertz(THz) radar system with 0.14 THz carrier frequency and 5 GHz bandwidth have been presented. The Ka band signal was frequency-doubled to be used as the local oscillator signal for sub-harmonic mixer in the systems transceiver chain. For the transmitted signal was a linearly frequency-modulated continuous-wave(LFM-CW), the dechirp technique was used in the THz radar receiver front end. By using this THz radar system, 3 cm high resolution range profiles were obtained as well as inverse synthetic aperture radar(ISAR) images, which exactly represent the subtle features of targets.

SIMPLE TRAFFIC SURVEILLANCE SYSTEM BASED ON RANGE-DOPPLER RADAR IMAGES

A simple tra-c surveillance system based on the extraction of features from range-Doppler radar images is addressed. The concept exploits the High-Resolution Radars (HRR) properties. Speciflcally, a procedure is proposed to obtain some features from the HRR non-cooperative targets to enable their classiflcation. These features are the distance, radial velocity, radial longitudinal dimension of the target, its integrated range-Doppler image based on a group of range-Doppler frames from each target, and both the coherent and non-coherent integrated range proflles. Experimental results from real scenarios using a high-resolution Linear-Frequency-Modulated Continuous-Wave (LFMCW) millimetre-wave radar are shown.

Impact of transmitting light's modulation characteristics on LFMCW Lidar systems

The performance of linear frequency-modulated continuous-wave (LFMCW) coherent Lidar system is inevitably influenced by the modulation characteristics of the transmitting light and some issues may arise when the system does not work under the ideally specified conditions. The relationship between the asymmetric and nonlinear modulation of transmitting light with the quality of the resulting beat frequency signal (BFS) for the non-zero velocity cases is mathematically modeled and the effective means for eliminating or alleviating these harmful influences are given. The results that are obtained from both numerical simulations and practical system experiments demonstrate that the errors of range and velocity measurement are proportional to the asymmetric coefficients and its square, respectively, and 30% relative error of range measurement occurs even through the values of normalized 2nd- and 3rd-order polynomial nonlinear-coefficients is as low as 0.1%. The rating of merit experiments of a Lidar system in range and velocity measurement is conducted, and the obtained results are in good agreement with their analytical and numerical simulation counterparts.

Detection and Parameter Estimation of Multi-targets Based on Secondary Mixing Combined DPT Method in LFMCW Radar

To solve the detection problem of high-speed accelerating targets in LFMCW (Linear Frequency- Modulated Continuous Wave) radar, a secondary mixing combined DPT (Discrete Polynomial-phase Transform) method is proposed. Utilizing the modulation symmetry of tri-LFMCW signals, up-sweep beat signal is secondary mixed with down-sweep one to eliminate MTRC (Migration Through Range Cell) induced by the high-speed. And DPT processing in terms of range cell leads to the estimation of acceleration and initial velocity. Furthermore, parameter estimation of multi-targets is emphasized. By analyzing the characteristics of the cross terms, the conditions for first order cross terms forming effective interfere and for second order cross terms emerging are derived, respectively. Resorting to Clean algorithm, multi-targets can be dealt with successfully, which detects and removes the strongest target for the current time. Processing steps of multi-targets cases are given. Simulation results testify the effectiveness of the method and the conclusions.

Interferometric inverse synthetic aperture radar experiment using an interferometric linear frequency modulated continuous wave millimetre-wave radar

An interferometric linear frequency modulated continuous wave (LFMCW) millimetre-wave radar is presented, along with the results of an experiment conducted to study the feasibility of using it in a future millimetre-wave interferometric inverse synthetic aperture radar (InISAR) system. First, a description of the radar is given. Then, the signal processing chain is described, with special attention to the phase unwrapping technique. The interferometric phase is obtained by unwrapping the prominent target's phase in each antenna using a sliding frame processing technique. Cell migration issues in this method are also addressed. Simulations were carried out to illustrate and assess the processing chain and to show the effects of multipath echoes on the height measurement. In the real experiment, the range, speed and height of a moving target were tracked over consecutive inverse synthetic aperture radar (ISAR) image frames, verifying the performance of the whole system.

A 34 to 36 GHz Vector Modulator for Reflected Power Canceller Techniques in LFMCW Radar

In this paper, a 34 to 36 GHz vector modulator for using in low cost and high performance RPC (Reflected Power Canceller) is presented. The key circuit consists of two push-pull (bi-phase) attenuators arranged in phase quadrature and a 3dB quadrature coupler (branch line couplers but not Lange couplers which is different from traditional circuits in Ka band) and a Wilkinson combiner, and then transition from micro-strip to waveguide using antipodal finline. To fully exploit this circuit’s capacity to generate accurate constellations at millimeter-wave frequencies, a generalized theoretical analysis of the (push-pull) vector modulator is presented. Based on the theoretical model and the measured results, the I-Q (push-pull) vector modulator promises to be a vital component for the realization of reflected power canceller in LFMCW (Linear Frequency Modulated Continuous Wave) radar.

330 GHz FMCW Image Sensor for Homeland Security Applications

A single-pixel imaging remote sensor operating at 330 GHz is described. It is based on a frequency modulated continuous wave (FMCW) and aimed at detection of concealed objects for ranges up to 40 m. The system consists of 2 horn-lens antennas integrated with a homodyne transceiver. The synthesized linear FMCW signal at X-band is multiplied by a factor of 32 to generate the transmitted signal. An intermediate frequency (IF) signal obtained in the output port of the 2-nd harmonic mixer is employed for image processing. Distance measurements were made by performing data acquisition unit based on LabView interface and resulting in a range resolution about 1 cm. Examples of 2D and 3D images reconstructed with this sensor are presented.

A 77 GHz 90 nm CMOS Transceiver for FMCW Radar Applications

The first 77 GHz frequency modulated continuous wave (FMCW) radar transceiver IC with an accurate FMCW chirp signal generator using a 90 nm CMOS process is presented. To realize accurate FMCW radar system in CMOS, a PLL synthesizer based FMCW generator with chirp smoothing technique that is able to output linear FMCW frequency chirp using a nonlinear reference chirp signal supplied from a low spec/cost digital-oriented frequency reference is applied. The fabricated IC consists of an LNA, a down-conversion mixer with an output buffer, a driver amplifier, a power amplifier with power combiner, an LO distributer and an FMCW synthesizer. The measured FMCW signal from the proposed FMCW generator achieves 93 kHz frequency error (nonlinearity) at the 77 GHz chip signal. Radar performance of the IC has less than 1% ranging error from 1 m to 8 m distance from the measurement in a laboratory. These results show the transceiver achieves a fundamental function for radar applications with 520 mW power consumptions.

Helicopter Classification with a High Resolution LFMCW Radar

Helicopter classification using a high resolution linear frequency modulated continuous wave (LFMCW) radar is addressed. A blade echo modelling for this radar system is derived, taking into account that the stop-and-go assumption is not longer applicable. The standard classification schemes based on the L/N-quotient and the evenness of the number of blades are easily reproduced in this high resolution context, although two main advantages are added: a high pulse repetition frequency (PRF) to acquire the Doppler bandwidth is no longer necessary, and the time for which we receive returned signal from the target may be maximized. On the other hand, a characteristic critical classification problem, for which the standard approaches do not properly work, is presented. Two classification schemes are proposed for this situation, the first one based on coherent matched masks and the second one based on incoherent masks where a robustness parameter has also been defined. Identification probabilities as a function of the signal-to-noise ratio (SNR) are obtained in order to characterize the performance of both techniques. Real data validate both the derived model and the proposed classification algorithms.

Millimeter-Wave Sensor With FMICW Capabilities for Medium-Range High-Resolution Radars

Frequency-modulated continuous-wave (FMCW) radars offer many advantages such as low peak power, low probability of interception, low interference with other systems, and high-range resolution. However, their major drawback is the isolation required between the transmitter and receiver that typically leads to the use of two separate antennas for transmission and reception. Some schemes have been proposed to enable FMCW radars to operate with a single antenna, such as frequency-modulated interrupted continuous-wave (FMICW) technology. Recently, a stagger procedure has been proposed to overcome the problems associated with the use of the FMICW technique in high-resolution radars. The technique was tested using simulated data. Now, a high-resolution millimeter-wave radar sensor, in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band, has been developed at the Universidad Politecnica de Madrid, Madrid, Spain, to perform an experimental validation of the theoretical approach of the stagger procedure. The sensor transmits a linear frequency-modulated interrupted continuous wave with a maximum bandwidth of 2 GHz and a transmitted power of +29 dBm.

Squint LFMCW SAR Data Processing Using Doppler-Centroid-Dependent Frequency Scaling Algorithm

A linear frequency-modulated continuous-wave (LFMCW) synthetic aperture radar (SAR) is a highly miniaturized SAR that is suitable for use on a small unmanned aerial vehicle. In squint stripmap mode, the LFMCW SAR image quality is degraded by the presence of nonlinearities in the transmitted signal, combined with the large bandwidth introduced by the frequency scaling in the nonlinear frequency scaling (NLFS) algorithm. This paper proposes an extension of the NLFS algorithm called the Doppler-centroid-dependent frequency scaling algorithm. The algorithm introduces a new Doppler-centroid-dependent scaling factor to decrease the introduced bandwidth, effectively removing the range frequency aliasing, without extra computational load. Nonlinearity correction is performed at the same time as range cell migration correction. The validity of the proposed algorithm is demonstrated with point target simulation results.

Theory and Application of Motion Compensation for LFM-CW SAR

Small low-cost high-resolution synthetic aperture radar (SAR) systems are made possible by using a linear frequency-modulated continuous-wave (LFM-CW) signal. SAR processing assumes that the sensor is moving in a straight line at a constant speed, but in actuality, an unmanned aerial vehicle (UAV) or airplane will often significantly deviate from this ideal. This nonideal motion can seriously degrade the SAR image quality. In a continuous-wave system, this motion happens during the radar pulse, which means that existing motion compensation techniques that approximate the position as constant over a pulse are limited for LFM-CW SAR. Small aircraft and UAVs are particularly susceptible to atmospheric turbulence, making the need for motion compensation even greater for SARs operating on these platforms. In this paper, the LFM-CW SAR signal model is presented, and processing algorithms are discussed. The effects of nonideal motion on the SAR signal are derived, and new methods for motion correction are developed, which correct for motion during the pulse. These new motion correction algorithms are verified with simulated data and with actual data collected using the Brigham Young University muSAR system.

SAR System for UAV Operation with Motion Error Compensation beyond the Resolution Cell.

This paper presents an experimental Synthetic Aperture Radar (SAR) system that is under development in the Universidad Politécnica de Madrid. The system uses Linear Frequency Modulated Continuous Wave (LFM-CW) radar with a two antenna configuration for transmission and reception. The radar operates in the millimeter-wave band with a maximum transmitted bandwidth of 2 GHz. The proposed system is being developed for Unmanned Aerial Vehicle (UAV) operation. Motion errors in UAV operation can be critical. Therefore, this paper proposes a method for focusing SAR images with movement errors larger than the resolution cell. Typically, this problem is solved using two processing steps: first, coarse motion compensation based on the information provided by an Inertial Measuring Unit (IMU); and second, fine motion compensation for the residual errors within the resolution cell based on the received raw data. The proposed technique tries to focus the image without using data of an IMU. The method is based on a combination of the well known Phase Gradient Autofocus (PGA) for SAR imagery and typical algorithms for translational motion compensation on Inverse SAR (ISAR). This paper shows the first real experiments for obtaining high resolution SAR images using a car as a mobile platform for our radar.

Uniform rotational motion compensation for inverse synthetic aperture radar with non-cooperative targets

In the current scenario of high-range resolution radars, the rotational motion of the target usually generates migration through resolution cells (MTRC) in the inverse synthetic aperture radar (ISAR) images. A technique to correct the MTRC in the case of uniform rotation rate is proposed. An estimation of the rotation vector of the non-cooperative targets is not required. The approach compensates the rotational motion in two steps: the slant-range rotation compensation (SRRC) and the cross-range rotation compensation (CRRC). CRRC is based on an extension of phase difference (PD) and on a robust method to delete outliers. The technique is verified both with simulated and live data from a millimetre-wave linear frequency-modulated continuous wave radar. A comparison with a minimum entropy-based technique is also made.

Linear FMCW Laser Radar for Precision Range and Vector Velocity Measurements

ABSTRACTAn all fiber linear frequency modulated continuous wave (FMCW) coherent laser radar system is under development with a goal to aide NASA's new Space Exploration initiative for manned and robotic missions to the Moon and Mars. Linear FMCW lidar has the capability of high resolution range measurements, and when configured into a multi-channel receiver system it has the capability of obtaining high precision vector velocity measurements. Precision range and vector velocity data are beneficial to navigating planetary landing pods to the pre-selected site and achieving autonomous, safe soft-landing. This paper discusses the design of a second generation prototype system under development at NASA Langley Research Center and presents preliminary performance data obtained from field experiments.

ANALYSIS OF WAVEFORM ERRORS IN MILLIMETER-WAVE LFMCW SYNTHETIC APERTURE RADAR

In remote sensing applications, there is a special interest in the lightweight, cost effective, and high resolution imaging sensors. The combination of linearly frequency modulated continuous wave (LFMCW) technology and synthetic aperture radar (SAR) technique can lead to such a sensor. This paper concentrates on the analysis of waveform errors in millimeter-wave (MMW) LFMCW SAR. The generating scheme of millimeter-wave LFMCW waveforms with phase locked loop (PLL) and direct digital synthesizer (DDS) combined frequency synthesizer is investigated. The impacts of quantization errors, spurs, and frequency nonlinearities are analyzed. Simulation results show that the quality of LFMCW waveforms has a direct influence on the SAR images. Hence a scheme of frequency synthesizer to achieve high performance MMW LFMCW waveform is proposed. This synthesizer driven by a DDS array can adaptive suppress the spurious level without degradation of excellent frequency linearity and fast switching speed.

Noise analysis of digitised FMCW radar waveforms

A staircase phase approximation for generating a linear frequency-modulated continuous wave radar signal is considered and the use of filtering on the samples of the signal has been assumed in both transmission and reception. It is shown that an excellent spectrum of the radar signal can be obtained, provided that the appropriate value of the sample duration is used. The effect of noise due to the discrete representation of the signal is analysed for both transmitted and received signal. It is shown that the level of the noise may be kept below the noise level of a high-quality oscillator even in the absence of any filter smoothing.

A technique for measuring the linearity of a linearly frequency-modulated signal

A technique for measuring the linearity of a linearly frequency-modulated continuous wave (LFM-CW) signal is presented. It uses a delay-line and a mixer to sense the slope of the output of a sweep oscillator, so that the original form of frequency function deviated from idealized linear slope is retrieved by means of spectrum analysis. Consequently, the linearity of the LFM signal is determined. The formulation is performed based on the principle that an angle-modulated signal can be approximated by an amplitude-modulated signal if the modulation coefficient is sufficiently small. To examine the validity of the procedure and to study the effect of each parameter on the accuracy of measurement, a number of computer simulations has been made. The results of simulation show that the error of the measurement is less than 2%.