Doppler Band Research Articles

Variable Doppler Starting Point Keystone Transform for Radar Maneuvering Target Detection

The Doppler band compensated by the keystone transform (KT) is limited. Therefore, it needs to be used in conjunction with the Doppler ambiguity compensation function to correct the range migration (RM) caused by maneuvering targets with Doppler ambiguity. However, the KT implemented by sinc interpolation suffers from significant performance loss at boundaries of compensation Doppler bands. Additionally, in a multi-target scenario, KT implementation methods occupy high complexity when the Doppler range of targets spans over two compensation Doppler bands. To address the aforementioned issues, this study presents a variable Doppler starting point keystone transform (VDSPKT) method, where a new form of ambiguity compensation function is constructed, turning the Doppler starting point of the compensation band in KT variable. Firstly, the position of the compensation Doppler band is changed from fixed to adjustable as needed, enhancing the flexibility of KT. Crucially, the connection points of the compensation Doppler bands in sinc interpolation are reset, avoiding performance loss at their boundaries. Also, the compensation band is adjusted to cover the narrow Doppler frequency range caused by targets, significantly improving computational efficiency. Finally, the simulation and real data experiments demonstrate that the proposed approach effectively addresses the performance degradation and high computational complexity of KT in the aforementioned scenarios, resulting in a computational load reduced by approximately 50% compared to traditional methods.

A non-invasive technique for monitoring supply voltage variation to single phase induction motor using doppler UWB radar signal analysis

The induction motors (IMs) are important components of various industries. The condition of IMs monitored continuously using contact and non-contact methods continuously. The contact methods are expensive, complex and difficult to implement. This paper proposes a non-contact method of fault identification in the single-phase induction motor operating in different conditions by analyzing doppler ultra-wide band (UWB) radar signal. The UWB radar generates a high frequency signal, which is transmitted on to the Induction motor form transmitter and software phase locked loop (SPLL) condition received signal from the receiver. The PLL implements as low pass filter, receives reflected signal from an induction motor along with high frequency noise. The received signal filtered to remove the high frequency noise and filtered output is analyzed to identify the different faults such as over voltage faults and under voltage faults when the motor is running with high, medium and low speed. The proposed non-invasive method has advantages compared to other such as the sensor’s sensitivity will not affect with motor temperature and accuracy will not change with position of the sensor and presence of other machines.

Imaging the retinal and choroidal vasculature using Spatio-Temporal Optical Coherence Tomography (STOC-T)

Spatio-Temporal Optical Coherence Tomography (STOC-T) is a novel imaging technique using light with controlled spatial and temporal coherence. Retinal images obtained using the STOC-T system maintain high resolution in all three dimensions, on a sample of about 700 μm, without the need for mechanical scanning. In the present work, we use known data processing algorithms for optical coherence tomography angiography (OCTA) and modify them to improve the rendering of the vasculature in the human retina at different depths by introducing the angio STOC-T method. The algorithms are primarily sensitive to the strong signal phase variance corresponding to the appearance of a wide Doppler band in STOC-T signals obtained for millisecond exposure times. After using STOC-T angiography, we can render high contrast images of the choroid.

Cascaded Least Square Algorithm for Strong Clutter Removal in Airborne Passive Radar

Owing to the continuous random and nonperiodic characteristics of the passive signal, the random range sidelobe coupling (RRSC) effects of the strong clutter (i.e., the direct path signal and the strong multipath signal) will exacerbate the performance of space-time adaptive processing (STAP) algorithm in airborne passive radar (APR). This article analyzes the influence of RRSC caused by strong clutter on detection range cell of interest and illustrates that strong clutter should be suppressed before applying the STAP algorithm. A cascaded least square (CLS) algorithm based on the directional dependence of clutter Doppler frequency is proposed for side-looking APR, where the first least square (LS) algorithm is performed to achieve the amplitude estimation of the Doppler-shift echo signal, and the channel amplitude and phase error correction factor can be obtained by the estimated amplitude information of the direct patch signal and the APR geometric configuration. The second LS is employed to further estimate the amplitude information of the strong clutter, and the resulting weight vector can be utilized for clutter cancelation. To make the CLS algorithm more practical, a batch version of the CLS (CLS-B) algorithm is proposed in this article. The advantage of the method is that it cannot only cancel clutter along the clutter ridge but also effectively prevent the suppression of targets falling in the clutter Doppler band. In addition, the proposed CLS algorithm performs well in the presence of the channel amplitude and phase error. A series of simulations are conducted to test the proposed algorithms.

LEO-Based Satellite Constellation for Moving Target Detection

With the rapid development of cooperative detection technology, target fusion detection with regard of LEO satellites can be realized by means of their diverse observation configurations. However, the existing constant false alarm ratio (CFAR) detection research rarely involves the space-based target fusion detection theory. In this paper, a novel multi-source fusion detection method based on LEO satellites is presented. Firstly, the pre-compensation function is constructed by employing the range and Doppler history of the cell where the antenna beam center is pointed. As a result, not only is the Doppler band broadening problem caused by the high-speed movement of the satellite platform, but the Doppler frequency rate (DFR) offset issue resulted from different observation configurations are alleviated synchronously. Then, the theoretical upper and lower limits of DFR are designed to achieve the effective clutter suppression and the accurate target echo fusion. Finally, the CFAR detection threshold based on the exponential weighted likelihood ratio is derived, which effectively increases the contrast ratio between the target cell and other background cells, and thus to provide an effective multi-source fusion detection method for LEO-based satellite constellation. Simulation results verify the effectiveness of the proposed algorithm.

Focusing Bistatic SAR Data Under Complicated Motion Through Differential Phase Filtering in Variable Doppler Bands

Bistatic SAR (BiSAR) imaging is becoming more and more mature, nevertheless restricted by the significant problem of 2-D space-variation of echo signal. In practical applications, the geometry configurations and platform motion of BiSAR are often complicated, making the problem of space-variation severe and causing it hard to efficiently process BiSAR data of large imaging scene. Focusing on this problem, this article proposes a new imaging method for large scene-size imaging that can adapt to general spotlight BiSAR with complicated platform motion. This method first roughly focuses the echo data in range-Doppler domain through azimuth-dechirp and removes the residual range-azimuth coupling via Keystone transform. Then making use of the tolerance of SAR imaging to slight phase error, the full Doppler axis is down-sampled with a certain ratio to form a much smaller amount of Doppler cells called Doppler bands. Through restricting the value of the down-sampling ratio, the variance of differential phase in each Doppler band after the dechirp is controlled within a given tolerance. As a result, the differential phase can be compensated for in variable Doppler bands for a large imaging scene with high efficiency. The computational complexity of the proposed method is indicated as low as logarithmic and its effectiveness is verified through numerical simulations.

Doppler Shifting Technique for Generating Multi-Frames of Video SAR via Sub-Aperture Signal Processing

The signal processing technologies of synthetic aperture radar (SAR) have been vastly studied as diverse measurement modes for various applications. One of the technologies is Video-SAR (ViSAR). It is a land-imaging mode where a sequence of images is continuously formed on the spotlight mode. We propose a new signal processing method to generate the multi-frames of ViSAR using Doppler shifting technique via sub-aperture processing. The method is based on our designed wide-angle antenna, which is a corrugated horn shape with low side lobe and Gaussian beam pattern. This wide-angle antenna receives a wide Doppler band along the azimuth direction. Based on the characteristics of the antenna, we separate the wide Doppler band into each chirp pulse signal. Each of these separated signals has a different center time. Through each chirp pulse signal in Doppler domain, the proposed method generates each frame of video at a high frame rate. The proposed ViSAR processing can show high efficiency with low computation and without a fixed coordinate system. Moreover, it can also visualize a wider area on stripmap mode than the conventional ViSAR mode, and can derive the video results based on Range Doppler Algorithm (RDA). These advantages can lead to reduce economic costs and simplify the operating ViSAR system. We have performed the practical experiments using X-band chirp pulse SAR system mounted on an airplane to verify the proposed method.

Multi-Channel SAR System Design for HRWS and GMTI in Low PRF

High-resolution and wide-swath (HRWS) is an important challenge in synthetic aperture radar (SAR). Combined SAR with array, the azimuth dual-channel SAR configuration achieves high azimuth resolution and HRWS in low Pulse repetition frequency (PRF) by doppler band synthesizing technology. However, low PRF introduces ambiguity to ground moving target indication (GMTI). Interferometry is performed between two look data in the range frequency domain to resolve the ambiguity by decreasing the interferometry phase. As a result, the phase becomes insensitive to the radial velocity of moving targets. This condition brings difficultly in moving target detection. Two channels together with two equivalent channels are exploited fully to solve these problems. The four-channel interferometry is also conducted to improve the GMTI performance. Parameters, including system and motion parameters, are estimated unambiguously to complete HRWS imaging and GMTI. System parameter design is also considered to satisfy the requirements of HRWS and GMTI simultaneously. Simulations demonstrate the validity of the proposed methods.

A New Azimuth Ambiguity Suppression Algorithm for Surface Current Measurement in Coastal Waters and Rivers With Along-track InSAR

We present a new algorithm for suppressing azimuth ambiguities when measuring surface currents with an along-track interferometric (ATI) synthetic aperture radar in heterogeneous scenes, such as coastal waters or rivers. The key of the proposed algorithm involves a careful analysis of a parameter called the eigenvalue spectrum entropy (EVSE), which is defined as the entropy of the eigenvalue spectrum of the ATI covariance matrix computed in the Doppler domain. The physical meaning of EVSE is that it serves as a descriptor for the degree to which an unambiguous signal component and an ambiguous one are mixed. With the help of EVSE, azimuth ambiguities can be suppressed. Simulation results demonstrate that as compared with the conventional ATI method without azimuth ambiguity suppression, the proposed algorithm allows for a pronounced improvement in the current measuring accuracy. Other advantages of the proposed algorithm lie in the fact that it is not only adaptive, due to its ability to automatically capture the useful Doppler band whose width and position may both vary for different radar and scene parameters, but it also needs a minimal number of user inputs, making it a quite attractive algorithm for routine implementation.

COSMO-SkyMed Staring Spotlight SAR Data for Micro-Motion and Inclination Angle Estimation of Ships by Pixel Tracking and Convex Optimization

In past research, the problem of maritime targets detection and motion parameter estimation has been tackled. This new research aims to contribute by estimating the micro-motion of ships while they are anchored in port or stationed at the roadstead for logistic operations. The problem of motion detection of targets is solved using along-track interferometry (ATI) which is observed using two radars spatially distanced by a baseline extended in the azimuth direction. In the case of spaceborne missions, the performing of ATI requests using at least two real-time SAR observations spatially distanced by an along-track baseline. For spotlight spaceborne SAR re-synthesizing two ATI observations from one raw data is a problem because the received electromagnetic bursts are not oversampled for onboard memory space saving and data appears like a white random process. This problem makes appearing interlaced Doppler bands completely disjointed. This phenomenon, after the range-Doppler focusing process, causes decorrelation when considering the ATI interferometric phase information retransmitted by distributed targets. Only small and very coherent targets located within the same radar resolution cell are considered. This paper is proposing a new approach where the micro-motion estimation of ships, occupying thousands of pixels, is measured processing the information given by sub-pixel tracking generated during the coregistration process of two re-synthesized time-domain and partially overlapped sub-apertures generated splitting the raw data observed by a single wide Doppler band staring spotlight (ST) SAR map. The inclination of ships is calculated by low-rank plus sparse decomposition and Radon transform of some region of interest. Experiments are performed processing one set of COSMO-SkyMed ST SAR data.

Clutter Cancellation Along the Clutter Ridge for Airborne Passive Radar

This letter examines the problem of clutter cancellation in airborne passive radar. This problem is exacerbated by the Doppler spread of clutter due to the receiver motion, since targets falling in the clutter Doppler band will be canceled. To address this problem, a least mean square-based algorithm is proposed for the sidelooking airborne passive radar. Making use of the directional dependence of the clutter Doppler frequency, this algorithm cancels clutter along the clutter ridge and thus avoids the cancellation of targets. Simulations with the experimental data show that clutter can be canceled ideally with the proposed algorithm and signal-to-noise ratio losses are slight, for example, the losses are within 1 dB for targets with radial velocity greater than 10 m/s.

Super‐resolution Doppler beam sharpening imaging via sparse representation

In Doppler beam sharpening (DBS) imaging, the imaging scene is characterised corresponding to the Doppler band, and the Doppler band occupies only a small part compared with the whole frequency domain. Accordingly, the DBS image is sparse in the frequency domain. Motivated by the sparsity, the authors propose a novel framework of DBS formation via sparse representation to perform super-resolution. In the framework, by exploiting the fact that the ground scene is sparse in frequency domain, they perform the super-resolution formation by incorporating the sparsity constraint with respect to a redundant time–frequency dictionary. The recovered sparse coefficients are utilised to form the final DBS image in frequency domain. Since the dictionary is redundant with more columns than rows, a thinner Doppler frequency resolution and a higher sharpening ratio can be achieved. Experimental results on real measured data verify the effectiveness of the new super-resolution algorithm.

FFT and PLL based GPS signal processing for Software GPS Receiver

This paper presents FFT and PLL based GPS signal acquisition and tracking algorithms for a software GPS receiver. Conventional hardware based acquisition and tracking have some restrictions in processing signal with poor signal to noise ratio. The FFT of digitized local signals of multiple carrier frequencies for a specified Doppler band are pre-computed and are circular correlated with the digitized incoming signal from RF-front-end in an organized computational order. The global maximum of the correlation is associated with the closest estimates of the Doppler shift and the code shift. PLL refines the estimates to track the signal. Doppler information from an external source can readily be integrated to narrow down the frequency band for correlation and is especially useful for tracking in a high dynamic navigation situation. The performance of the proposed algorithms is evaluated through post processing of the IF signals acquired from a commercial hardware GPS receiver.

Clutter Filtering and Spectral Moment Estimation for Doppler Weather Radars Using Staggered Pulse Repetition Time (PRT)

In this paper, a new algorithm for the estimation of spectral parameters from the signal time series, collected using the staggered pulse repetition time (PRT) transmission in a Doppler weather radar, is presented. The algorithm uses the Fourier transform and a magnitude deconvolution procedure to reconstruct the signal spectrum, and then the spectral parameters are estimated from the reconstructed spectrum. There is a significant improvement in the variance of the spectral parameter estimates compared to previously published methods of processing staggered PRT sequences. Further, a novel spectral domain clutter filtering procedure allows 1) accurate velocity estimation even if the clutter-to-signal power ratio is as high as 40 dB and 2) does not incur the loss of velocity information in certain Doppler bands experienced by other clutter filtering techniques. With this algorithm, the staggered PRT technique becomes a practical contender for implementation on Doppler weather radars in the quest to increase the unambiguous velocity and range.

When do obstetricians recommend delivery for a high-risk preterm growth-retarded fetus?

Forty-nine obstetricians from three European countries were asked when they would advise delivery for a preterm fetus failing to thrive in utero, given various gestational ages, and a range of either umbilical artery Doppler flow velocity waveforms or CTG variability measures. Their responses indicated a wide area of disagreement about the correct timing of delivery, and a willingness to randomise patients to clinical trials of management. The area of uncertainty corresponded to the gestational age and Doppler bands at which participants have been entered to the pilot phase of a randomised trial of timed delivery, the Growth Restriction Intervention Trial (GRIT).

The Doppler sensitivity of large TW phase modulated waveforms

The narrowband approach to coherent processing of large-TW waveforms in Doppler delay multiplies the data stream by the complex conjugate of the undilated envelope and uses FFT processing to test Doppler. This results in a correlation loss that depends fundamentally on the TW product of the waveform envelope and the amount of Doppler-induced time-base dilation occurring in the data. It is shown that it is possible to reduce the number of dilated envelope replicas required to cover a given Doppler band by dividing the band into subbands. Each narrowband is processed using an envelope replica with a fixed dilation parameter, and the width of each band is a function of a specified correlation loss limit, the TW product, and a Doppler sensitivity parameter that depends on the specific time-frequency allocation of the envelope. Examples are provided for envelopes with both continuous and discrete (FSK) allocations with emphasis on the class of unit allocation (UA) codes that includes all full Costas arrays, for which upper and lower bounds for Doppler sensitivity are derived.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A recursive least-squares algorithm with multiple inputs and outputs, and a cylindrical systolic implementation

A recursive least-squares (RLS) algorithm with multiple inputs as well as multiple outputs is presented. The corresponding processing architecture is a cylindrical systolic array that uses a basic two-input-two-output decorrelation processing element (PE) as its primitive building block. Because of the time recursive property, the implementation only involves scalar computations, so no vector or matrix operations are required in each PE. An efficient algorithm is derived on the basis of various architectural considerations combined with the use of an algebraic interpretation of a geometrical approach used previously. The algorithm presented generates as many output channels as input channels, which is a requirement for many signal processing tasks. One application is to adaptive pulse Doppler processing for radar systems in which a bank of filters is used to recover the entire Doppler band and each Doppler subband is processed in such a way that it is totally decorrelated with the inputs of the remaining subbands.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of band model calculations of upper atmospheric cooling rates for the 15‐micrometer carbon dioxide band

Comparisons are made between cooling rates for the CO2 15‐μm band in the middle to upper stratosphere (30–50 km) that were calculated by three widely used methods for handling infrared emission and absorption: an emissivity parameterization, a band absorptance formulation, and an “exact” line‐by‐line calculation. Cooling rates are presented for two temperature distributions—the U.S. Standard Atmospheric profile and a polar sounding. The band absorptance approach adopts Ramanathan's approximate treatment to account for transition of line shapes to Doppler broadening in the upper stratosphere but employs a new Doppler band model which yields better agreement with more detailed calculations. For both temperature profiles, cooling rates obtained by all three methods show good agreement with one another, differing by less than 0.5 K/day below 50 km. Above 50 km, variations among the cooling rates increase slightly to 0.5–1 K/day. At the stratopause, these differences correspond to a temperature error of 2 to 4 K.

Geos 3 STDN S band Doppler tracking investigation

Geos 3 S band Doppler and laser ranging data were acquired in the spacecraft altimeter calibration area during an extensive tracking campaign from August 1975 to March 1976. These data have been used to support the calibration and evaluation of the Spaceflight Tracking and Data Network S band stations. The Goddard Space Flight Center laser systems are the reference for the precise positioning of the Doppler stations and for evaluating the recovery of Geos 3 arc height derived from Doppler tracking data. The solutions for station coordinates were determined from two‐way and three‐way Doppler data. A comparison of the solutions with survey in a local datum indicates a submeter precision for adjusted coordinate components. Arcs of one revolution were generated for Geos 3 from passes of two‐way and three‐way Doppler data. An evaluation for the calibration area shows arc height being recovered with an uncertainty of about 1 m. The Geos 3 beacon signal has also been tracked in the Doppler difference mode with a precision useful for geodetic purposes. In a solution for Geos 3 arc elements obtained using Doppler difference data, arc height has been recovered in the calibration area with an uncertainty of the order of 2 m.

Band model formulation for inhomogeneous optical paths

The band-model formulation developed by Lindquist and Simmons to handle radiance calculations for highly inhomogeneous and nonisothermal optical paths is discussed and extended. The original formulation was based on the statistical band model for a random array of equal-intensity pressure-broadened Lorentz lines. Here, the formulation is extended to include bands of constant-intensity Doppler lines and bands of Lorentz or Doppler lines with an exponential distribution of line strenghts. Results are presented for the equivalent width derivative functions y( x,ϱ) for the constant-intensity Lorentz, constant-intensity Doppler, and exponential-intensity Doppler cases. Various approximations to these functions for extreme x and ϱ conditions are also presented. A closed-form solution for y( x,ϱ ) is given for the exponentially distributed Lorentz line case.