Unambiguous Velocity Research Articles

Least Squares Reconstruction of Doppler Radar Spectra for Irregular PRT

Abstract A least squares method for the reconstruction of Doppler spectra of weather radars with irregular pulse repetition time used to increase the range of unambiguous velocity is presented and evaluated. This method is a robust spectral method that is based on the least squares minimum norm principle and reconstructs both the magnitude and the phase of the discrete Fourier transform of the signal. The phase spectrum is useful in the estimation of the differential phase in dual-polarization radars with staggered sampling schemes, which is a case of irregular sampling. A computationally efficient iterative algorithm for estimating the mean frequency of the signal, which is required for the reconstruction of the spectrum, is described for possible real-time applications. A clutter filter method based on spectral interpolation, which can be applied to echoes with generally nonzero mean velocity, is also described and combined with the spectrum reconstruction method. Using simulated data it is shown that the least squares reconstruction method with or without the presence of clutter gives results with small bias and standard error and can be applied to wide spectra. The application of the method to real X-band radar data with a low signal-to-noise ratio and a high stagger ratio value of ⅚ showed that the least squares method has low sensitivity to the stagger ratio and satisfactorily gives spectral reconstruction for signal-to-noise ratio values as low as 10 dB.

Pattern-dependent response modulations in motion-sensitive interneurons induced by natural environments

Event Abstract Back to Event Pattern-dependent response modulations in motion-sensitive interneurons induced by natural environments Hanno G. Meyer1*, Anne-Kathrin Warzecha1, Jens P. Lindemann1, 2 and Martin Egelhaaf1, 2 1 Bielefeld University, Department of Neurobiology, Germany 2 Bielefeld University, CITEC Center of Excellence Cognitive Interaction Technology, Germany When an animal, such as a fly, moves through a natural environment, images of the surroundings move across its eyes. In the fly this image motion is evaluated by motion-sensitive neurons such as the LPTCs, i.e. wide-field neurons in the third visual neuropile. Even if the fly moves at a constant velocity, the response amplitude of these cells is usually not constant, but modulates over time in a pattern-dependent fashion. Pattern-dependent response modulations have been interpreted as 'pattern-noise', because they deteriorate the neuron's ability to provide unambiguous velocity information. On the other hand, these modulations provide the system with potentially valuable information about the textural properties of the environment. Due to the sensitivity of LPTCs to contrast and spatial structure of local features within moving scenes, LPTC responses may provide information about the depth-structure as well as textural properties of the environment during translational flight. Here we analyse a) the influence of the size and shape of receptive fields of LPTCs on the amplitude of pattern-dependent response modulations. LPTC responses were simulated using models of arrays of elementary motion detectors which have previously been employed to explain LPTC responses. In a subsequent analysis we investigate b) the textural components of natural images that drive the response of LPTCs. In order to identify the textural components of natural scenes, we perform a statistical analysis of a dataset of high dynamic range panoramic images using a neural network approach. This approach allows us to construct a filter bank representing the textural constituents of natural scenes specific for different LPTC receptive field characteristics. This filter bank is subsequently used to assess which of these constituents of natural scenes are predominantly represented in the neuronal responses of a subset of LPTCs. Keywords: calliphora, elementary motion detector, Invertebrates, motion vision, motion-sensitive interneurons, natural image statistics, neural networks, receptive fields Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Poster Presentation (see alternatives below as well) Topic: Sensory: Vision Citation: Meyer HG, Warzecha A, Lindemann JP and Egelhaaf M (2012). Pattern-dependent response modulations in motion-sensitive interneurons induced by natural environments. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00082 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 10 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Mr. Hanno G Meyer, Bielefeld University, Department of Neurobiology, Bielefeld, 33615, Germany, hanno.meyer@uni-bielefeld.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Hanno G Meyer Anne-Kathrin Warzecha Jens P Lindemann Martin Egelhaaf Google Hanno G Meyer Anne-Kathrin Warzecha Jens P Lindemann Martin Egelhaaf Google Scholar Hanno G Meyer Anne-Kathrin Warzecha Jens P Lindemann Martin Egelhaaf PubMed Hanno G Meyer Anne-Kathrin Warzecha Jens P Lindemann Martin Egelhaaf Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Pattern-dependent response modulations in motion-sensitive visual interneurons--a model study.

Even if a stimulus pattern moves at a constant velocity across the receptive field of motion-sensitive neurons, such as lobula plate tangential cells (LPTCs) of flies, the response amplitude modulates over time. The amplitude of these response modulations is related to local pattern properties of the moving retinal image. On the one hand, pattern-dependent response modulations have previously been interpreted as 'pattern-noise', because they deteriorate the neuron's ability to provide unambiguous velocity information. On the other hand, these modulations might also provide the system with valuable information about the textural properties of the environment. We analyzed the influence of the size and shape of receptive fields by simulations of four versions of LPTC models consisting of arrays of elementary motion detectors of the correlation type (EMDs). These models have previously been suggested to account for many aspects of LPTC response properties. Pattern-dependent response modulations decrease with an increasing number of EMDs included in the receptive field of the LPTC models, since spatial changes within the visual field are smoothed out by the summation of spatially displaced EMD responses. This effect depends on the shape of the receptive field, being the more pronounced - for a given total size - the more elongated the receptive field is along the direction of motion. Large elongated receptive fields improve the quality of velocity signals. However, if motion signals need to be localized the velocity coding is only poor but the signal provides – potentially useful – local pattern information. These modelling results suggest that motion vision by correlation type movement detectors is subject to uncertainty: you cannot obtain both an unambiguous and a localized velocity signal from the output of a single cell. Hence, the size and shape of receptive fields of motion sensitive neurons should be matched to their potential computational task.

Alternating Dual-Pulse, Dual-Frequency Techniques for Range and Velocity Ambiguity Mitigation on Weather Radars

Abstract This paper introduces a family of alternating dual-pulse, dual-frequency (ADPDF) techniques. These are based on frequency diversity and are proposed as a means to mitigate range and velocity ambiguities on Doppler weather radars. ADPDF techniques are analyzed theoretically and through simulated and real weather data collected with a prototype C-band radar. Analogous to single-frequency, multiple-pulse-repetition-time (mPRT) techniques, such as staggered or triple PRT, it is demonstrated that ADPDF techniques can extend the maximum unambiguous velocity beyond what is achievable with uniform sampling. However, unlike mPRT techniques, ADPDF techniques exhibit better statistical performance and, more importantly, may be designed to preserve uniform sampling on one of the frequency channels, thus avoiding some of the difficulties associated with processing nonuniformly sampled data.

Motion Parameter Estimation in the SAR System With Low PRF Sampling

A novel approach to motion parameter estimation with low pulse repetition frequency (PRF) sampling based on compressed sensing (CS) theory is introduced. As is known to us, when PRF is less than the Doppler spectrum bandwidth, moving targets suffer both Doppler centroid frequency ambiguity and Doppler spectrum ambiguity. Under this condition, the traditional parameter estimation method in the Doppler domain is out of action. The key of this letter converts motion parameter estimation in the synthetic aperture radar system with low PRF sampling into solving an optimization equation based on CS theory. Because moving targets in the scene can be regarded as sparse signals after clutter cancellation, an optimization algorithm based on CS theory is proposed to reconstruct sparse signals and meanwhile estimate the along-track velocities and azimuth positions of moving targets. Considering the fact that range cell migration of moving targets is not subject to PRF limitations, Radon transform is adopted to obtain unambiguous across-track velocities and range positions. Results on simulation and real data are provided to show the effectiveness of this method.

A New Slant-Range Velocity Ambiguity Resolving Approach of Fast Moving Targets for SAR System

This paper describes an ambiguity resolving approach for slant-range velocity estimation which utilizes the wideband characteristic of the transmitted signal (multiple wavelengths). Based on the wavelength dual-wavelength radar data. Then, two effective approaches are introduced to focus the moving target no matter the Doppler ambiguity exists or not. The slant-range velocity is estimated by the number of azimuth cell displacements between the two focused images. Both imaging methods have different properties and advantages. A performance analysis is made, and deleterious factors in practice are analyzed in detail. The effectiveness of the unambiguous slant-range velocity estimation approach is demonstrated with the use of simulated and real data.

HPRF pulse Doppler stepped frequency radar

Stepped frequency radar is a well known scheme to generate high range resolution profile (HRRP) of targets. Through appropriate radar parameter design, the radar enables both unambiguous velocity measurement and high resolution ranging within a single dwell in a high pulse repetition frequency (HPRF) mode. This paper analyzes in detail the design principle of the HPRF stepped frequency radar system, the solution to its ambiguity issue, as well as its signal processing method. Both theoretical analysis and simulation results demonstrate that the proposed radar scheme can work independently to solve the problem of motion compensation, and is therefore highly applicable to many new types of radar.

APPLICATION OF CLUTTER SUPPRESSION METHODS TO A GEOSTATIONARY WEATHER RADAR CONCEPT

While a geostationary weather radar would enable unprecedented monitoring of hurricanes and other severe weather, surface clutter could seriously limit its performance. The large incidence angles necessary for wide-area coverage, combined with the footprint size could cause surface clutter to obscure atmospheric return up to several km above the surface. The authors describe a Doppler filtering approach to clutter suppression and show simulation results. They find that Doppler filtering can significantly reduce the surface return, bringing surface clutter to acceptable levels. The authors then consider this approach when a staggered pulse repetition frequency is used to improve the maximum unambiguous velocity. They find that a method previously developed for ground-based weather radars can be successfully applied.

The RMS survey

Context: The Red MSX Source (RMS) survey is an ongoing multi-wavelength observational programme designed to return a large, high-resolution mid-infrared colour-selected sample of massive young stellar objects. Aims: A critical part of our follow-up programme is to conduct 13CO molecular line observations in order to determine kinematic distances to all of our MYSO candidates. These distances will allow us to identify and remove nearby low-mass YSOs and help in identifying evolved stars which are weak CO emitters. Method: We have used the 15 m James Clerk Maxwell Telescope (JCMT), the 13.7 m telescope of the Purple Mountain Observatory (PMO), the 20 m Onsala telescope and the 22m Mopra telescope to conduct molecular line observations towards 508 MYSOs candidates located in the 1st and 2nd Quadrants. Results: We detect 13CO emission towards 780 RMS sources which corresponds to approximately 84% of those observed (911). A total of 2595 emission components are detected above 3sigma level (typically T^*_{\rm{A}} > 0.3K), with multiple components being observed towards the majority of these sources -- 520 sources (~56%) -- with an average of ~4 molecular clouds detected along each line of sight. We have used archival CS (J=2-1) and maser velocities to resolve the component multiplicity towards 175 sources (~20%) and have derived a criterion which is used to identify the most likely component for a further 191 multiple component sources. Combined with the single component detections we have obtained unambiguous kinematic velocities for 638 of the 780 MYSOs candidates towards which CO is detected (~80% of the detections). Using the Galactic rotation curve we calculate kinematic distances for all detected components.

Slant-range velocity estimation based on Small-FM-Rate Chirp

This paper deals with the estimation of the slant-range velocity of a moving target with airborne SAR data. For the signature of a moving target, performing the Beat Frequency (BF) algorithm in the slow time and range frequency domain can generate a Small-FM-Rate Chirp (SFRC), which has some features of a single-frequency signal. The Fourier transform of such SFRC quite approximates a sinc function, and the maximum-value point of such Fourier transform is proportional to the slant-range velocity. Based on these principles, a methodology known as Beat Frequency Coherent Accumulation (BFCA) algorithm is developed to estimate the slant-range velocity. The prominent advantage of the BFCA algorithm is that the unambiguous velocity can be very large. Besides, the proposed methodology can deal with large range cell migration (RCM) and can work with a low signal-to-noise ratio (SNR) scene, requiring a small amount of computation but yields a high accuracy especially for the fast-moving targets. The effectiveness and efficiency of the proposed methodology are validated with both simulated and real data.

Global upper-mantle tomography with the automated multimode inversion of surface andS-wave forms

SUMMARY We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an S vvelocity model of the upper mantle (crust‐660 km). The model is constrained with ∼51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations. Structure of the mantle and crust is constrained by waveform information both from the fundamentalmode Rayleigh waves (periods from 20 to 400 s) and from S and multiple S waves (higher modes). In order to enhance the validity of the path-average approximation, we implement the automated inversion of surface- and S-wave forms with a three-dimensional (3-D) reference model. Linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3-D reference, in this study composed of a 3-D model of the crust and a one-dimensional (1-D), global-average depth profile in the mantle below. Waveform information is related to shear- and compressional-velocity structure within approximate waveform sensitivity areas. We use two global triangular grids of knots with approximately equal interknot spacing within each: a finely spaced grid for integration over sensitivity areas and a rougher-spaced one for the model parametrization. For the tomographic inversion we use LSQR with horizontal and vertical smoothing and norm damping. We invert for isotropic variations in S- and P-wave velocities but also allow for S-wave azimuthal anisotropy—in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred kilometres, varying with data sampling. We validate the imaging technique with a ‘spectral-element’ resolution test: inverting a published global synthetic data set computed with the spectral-element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-S v-velocity anomalies beneath mid-ocean ridges and backarc basins extend down to ∼100 km depth only, shallower than according to some previous tomographic models; this presents a close match to published estimates of primary melt production depth ranges there. In the seismic lithosphere beneath cratons, unambiguous high velocity anomalies extend to ∼200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere.

The RMS survey

Context. The Red MSX Source (RMS) survey is an ongoing multi-wavelength observational programme designed to return a large, high-resolution mid-infrared colour-selected sample of massive young stellar objects (MYSOs). We have identified ∼2000 MYSO candidates located within our Galaxy by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. The aim of our follow-up observations is to identify other objects with similar colours such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe) and distinguish between genuine MYSOs and nearby low-mass YSOs. Aims. A critical part of our follow-up programme is to conduct 13CO molecular line observations in order to determine kinematic distances to all of our MYSO candidates. These distances will be used in combination with far-IR and (sub)millimetre fluxes to determine bolometric luminosities which will allow us to identify and remove nearby low-mass YSOs. In addition these molecular line observations will help in identifying evolved stars which are weak CO emitters. Methods. We have used the 15 m James Clerk Maxwell Telescope (JCMT), the 13.7 m telescope of the Purple Mountain Observatory (PMO), the 20 m Onsala telescope and the 22 m Mopra telescope to conduct molecular line observations towards 508 MYSOs candidates located in the 1st and 2nd Quadrants. These observations have been made at the J = 1−0 (Mopra, Onsala and PMO) and J = 2−1 (JCMT) rotational transition frequency of 13CO molecules and have a spatial resolution of ∼20′′−55′′, a sensitivity of T ∗ A 0.1 K and a velocity resolution of ∼0.2 km s−1. We complement these targeted observations with 13CO spectra extracted from the Galactic Ring Survey (GRS), which have a velocity resolution of ∼0.21 km s−1 and sensitivity T ∗ A 0.13−0.2 K, towards a further 403 RMS sources. Results. In this paper we present the results and analysis of the 13CO spectra obtained towards 911 MYSO candidates. We detect 13CO emission towards 780 RMS sources which corresponds to approximately 84% of those observed. A total of 2595 emission components are detected above 3σ level (typically T ∗ A ≥ 0.3 K), with multiple components being observed towards the majority of these sources – 520 sources (∼56%) – with an average of ∼4 molecular clouds detected along each line of sight. These multiple emission features make it difficult to assign a unique kinematic velocity to many of our sample. We have used archival CS (J = 2−1) and maser velocities to resolve the component multiplicity towards 175 sources (∼20%) and have derived a criterion which is used to identify the most likely component for a further 191 multiple component sources. Combined with the single component detections we have obtained unambiguous kinematic velocities for 638 of the 780 MYSOs candidates towards which CO is detected (∼80% of the detections). The 141 sources for which we have not been able to determine the kinematic velocity will require additional line data. Using the rotation curve of Brand and Blitz (1993) and their radial velocities we calculate kinematic distances for all detected components.

A Hybrid Multi-PRI Method to Dealias Doppler Velocities

A robust dealiasing method was developed and evaluated that can extend unambiguous Doppler velocity limits for a pulsed Doppler weather radar with multiple pulse repetition interval (PRI).The dealiasing method can correct a sparsely distributed velocity field with large random noise and/or large velocity gradients. The method includes areal multiple PRI dealiasing and dealiasing with spatial continuity. This method makes the simple assumption that Doppler velocities can be spatially approximated by linear polynomials of the coordinates in each small area.The dealiasing method was evaluated using simulated data that included uniform wind, Rankine vortices, axisymmetric divergence, and wind shifts. The simulated fields also included missing data and random noise. A test with real observations was also conducted. The results show that the method can successfully dealias most folded velocities. The processing time to dealias one PPI data in the evaluation is at most a few seconds using a 2.4-GHz CPU workstation, short enough for real-time radar observations.

Extending the unambiguous velocity range using multiple carrier frequencies

Typically, velocity estimators based on the estimation of the Doppler shift will suffer from a limited unambiguous velocity range. Proposed are two novel multiple carrier based velocity estimators extending the velocity range above the Nyquist velocity limit. Numerical simulations indicate that the proposed estimators offer improved estimation performance compared to other existing techniques.

Combined Doppler and Polarimetric Radar Measurements: Correction for Spectrum Aliasing and Nonsimultaneous Polarimetric Measurements

Combining Doppler and polarimetric information is advantageous for atmospheric studies. On the one hand, Doppler information gives insight into the microphysical and dynamic properties of radar targets, that is, radial velocity and its variability. The polarization diversity, on the other hand, has a strong link to the microphysical properties of targets such as shape and orientation. Polarimetric measurements, however, have an adverse effect on Doppler processing. Measurements of the complete scattering matrix require at least two pulses and result in the reduction of the maximum unambiguous Doppler velocity that can lead to Doppler spectrum aliasing. Moreover, dynamic properties of targets, because of the nonsimultaneity of the measurements performed with different polarizations, affect the accuracy of polarimetric radar measurements. A solution to these two problems is given in this paper. It is shown that by applying a relatively simple processing technique the effect of nonsimultaneous polarimetric measurements can be reduced even in the case of strong Doppler spectrum aliasing. This leads to better estimates of the copolar cross-correlation coefficient. Furthermore, this processing results in the maximum unambiguous Doppler velocity as if no polarimetric measurements were performed, with the added advantage of obtaining the actual Doppler velocities. To illustrate the proposed processing technique, precipitation measurements taken with the Delft Atmospheric Research Radar (DARR) are used.

Correction of Dual PRF Velocity Errors for Operational Doppler Weather Radars

Abstract Dual pulse repetition frequency (PRF) is a commonly used technique, in operational Doppler weather radar networks, that extends the unambiguous Doppler velocity. The technique requires the measurement of the radial velocity at two different PRFs and it assumes that the data is collected from the same velocity. In practice, the data are collected from adjacent alternating PRF radials as the antenna rotates. However, high azimuthal shear or statistical errors due to uncertainty in the measurements can create dealiasing errors. This paper proposes two algorithms to correct these dealiasing errors and analyzes the results. Simulations and real cases are presented to illustrate the benefits and limitations.

Analysis and Correction of Dual PRF Velocity Data

The dual pulse repetition frequency (dual PRF) technique for extension of the unambiguous velocity interval is available on many operational Doppler weather radars. Radial velocity data obtained from a C-band Doppler radar running in dual PRF mode have been analyzed quantitatively. The standard deviation of the velocity estimates and the fraction of dealiasing errors are extracted and related using a simple model. A postprocessing algorithm for dual PRF velocity data, which removes noise and corrects dealiasing errors, has been developed and tested. It is concluded that the algorithm is very efficient and produces high quality velocity data.

A nonuniform sampling approach for fast ultrasonic flow imaging.

Conventional Pulsed Wave Doppler (PWD) systems acquire an ensemble of N echoes per beam line at a constant pulse repetition frequency fprf, so that the pulse repetition interval equals Tpri = 1/fpn. The total time span determines the velocity resolution, and Tpri the unambiguous velocity range. The ensemble size N is by approximation inversely proportional to the frame rate, assuming that the system performs interleaving. For a given frame rate, a tradeoff can only be made between velocity resolution and velocity range. We propose an approach that allows increasing velocity resolution or range while keeping the frame rate constant. The approach is based on nonuniform sampling, i.e. sampling with varying sampling intervals. Thus, for a given ensemble size N a larger total time span, which would increase velocity resolution, or a shorter minimal Tpn, which would increase the velocity range, may be chosen. The conventional Doppler signal processing techniques are not compatible with nonuniform sampling. We, therefore, developed a velocity estimation algorithm for arbitrary sampling that is based on cross correlation. Furthermore, an adaptive wall filter was implemented that differentiates between tissue motion and blood flow. The new approach was successfully tested with in vitro and in vivo data.

A Dual-Pulse Repetition Frequency Scheme for Mitigating Velocity Ambiguities of the NOAA P-3 Airborne Doppler Radar

Abstract To mitigate some of the deleterious effects of the relatively small unambiguous Doppler velocity range (Nyquist interval) of airborne X-band Doppler radars, a technique has been developed to extend this interval. This technique, termed the batch-mode dual-PRF (pulse repetition frequency) technique, utilizes two batches of pulses that are each averaged to produce two velocity estimates at each range bin, with each batch at a different sampling rate. Comparison of the two velocity estimates produces a difference velocity that is used to dealias the second estimate within an extended velocity range that is larger than the Nyquist velocities of either of the two original samples. In this implementation, the choices of the two PRFs are restricted to ratios of 3/2 or 4/3 of the lowest PRF. Due to the spread of the input Doppler spectrum and spatial displacement from one radial to the next, however, this batch-mode technique can produce processor velocity dealiasing mistakes, particularly in high wind s...

Poleward‐moving HF radar flow bursts in the cusp: Transient changes in flow speed or direction?

Poleward‐moving line‐of‐sight velocity “flow bursts” have been observed in the cusp by two southern hemisphere SuperDARN HF radars with overlapping fields‐of‐view. This has allowed the estimation of unambiguous two‐dimensional velocity vectors in the vicinity of the “flow bursts”. Rather than showing enhancements in the flow magnitude, the velocity vectors suggest that the line‐of‐sight velocity enhancements are a result of a change in the direction of the flow associated with latitudinal motion of the convection reversal boundary. These observations may have important implications for understanding the ionospheric footprint of flux transfer events, and also illustrate that caution is needed when interpreting line‐of‐sight velocity data from single radars.