Improved Range Resolution Research Articles

Time-of-Flight距離画像センサの距離分解能の改善(立体映像技術一般)

Time-of-Flight距離画像センサの距離分解能の改善(立体映像技術一般)

Improving weather radar observations using pulse‐compression techniques

AbstractPulse‐compression techniques have been used for many years to increase sensitivity and range resolution in military and air traffic control radar systems. However, their application to ground‐based weather radar has, so far, been very limited and restricted to research applications. This article describes the practical implementation of pulse compression using non‐linear frequency modulation (NLFM) in a commercial digital processor/receiver and demonstrates its application to operational weather radars.Three case studies are presented, including the first application of pulse compression to ground‐based travelling wave tube (TWT) weather radars. TWT radars use coherent transmitters with relatively low peak power compared to traditional magnetron or klystron systems, so in this case the sensitivity gain is crucial. The cases are from two different operational radars and illustrate the improved range resolution and sensitivity gain obtained using NLFM pulses compared to traditional simple pulses. Observations obtained using NLFM 40 µs pulses were nearly 15 dB more sensitive than those recorded with lower resolution using 1 µs simple pulses. A comparison of concurrent precipitation observations obtained with a TWT radar using pulse compression and low peak power (NLFM 40 µs and 8 kW) and a nearby traditional magnetron weather radar (2 µs and 250 kW) is also presented. The higher power system was more sensitive by less than 4 dB, but the improved resolution in the pulse compression allowed for enhanced Doppler filtering of ground clutter. Copyright © 2007 Royal Meteorological Society

Comparisons of refractive index gradient and stability profiles measured by balloons and the MU radar at a high vertical resolution in the lower stratosphere

Abstract. Many experimental studies have demonstrated that VHF Stratosphere-Troposphere (ST) radar echo power is proportional to the generalized refractive index gradient squared M2 when using a vertically oriented beam. Because humidity is generally negligible above the tropopause, VHF ST radars can thus provide information on the static stability (quantified by the squared Brunt-Väisälä frequency N2) at stratospheric heights and this capability is useful for many scientific applications. Most studies have been performed until now at a vertical resolution of 150 m or more. In the present paper, results of comparisons between radar- and (balloon borne) radiosonde-derived M2 and N2 are shown at a better vertical resolution of 50 m with the MU radar (34.85° N, 136.15° E; Japan) by benefiting from the range resolution improvement provided by the multi-frequency range imaging technique, using the Capon processing method. Owing to favorable winds in the troposphere, the radiosondes did not drift horizontally more than about 30 km from the MU radar site by the time they reached an altitude of 20 km. The measurements were thus simultaneous and almost collocated. Very good agreements have been obtained between both high resolution profiles of M2, as well as profiles of N2. It is also shown that this agreement can still be improved by taking into account a frozen-in advection of the air parcels by a horizontally uniform wind. Therefore, it can be concluded that 1) the range imaging technique with the Capon method really provides substantial range resolution improvement, despite the relatively weak Signal-to-Noise Ratios (SNR) over the analyzed region of the lower stratosphere, 2) the proportionality of the radar echo power to M2 at a vertical scale down to 50 m in the lower stratosphere is experimentally demonstrated, 3) the MU radar can provide stability profiles with a vertical resolution of 50 m at heights where humidity is negligible, 4) stable stratospheric layers as thin as 50 m or less have at least a horizontal extent of a few km to several tens of kilometers and can be considered as frozenly advected over scales of a few tens of minutes.

Improvement of radar range resolution by processing of synthesized measurements in range strobes

The paper describes a new method and algorithm for improvement of radar range resolving capacity. The approach is based on increasing the number of range strobes, their arrangement in a certain order, and on optimal processing of amplitudes of reflection of signals obtained in these strobes.

Range Resolution Improvement of Airborne SAR Images

This letter proposes an algorithm to improve the range resolution in airborne synthetic aperture radar (SAR) data by coherently combining an interferometric image pair, i.e., two images acquired with slightly different viewing angles. This algorithm is based on the wavenumber shift principle. In contrast to other methods, developed for application to spaceborne SAR data, the proposed algorithm takes the nonlinear effects due the strong variations in incidence angle in airborne SAR data into account. The proposed method is applied to SAR data of German Aerospace Center (DLR)'s E-SAR sensor. Quantitative verification results are obtained by measuring the resolution of several corner reflectors placed in the area under study, as well as the resolution of speckle of different areas. It is demonstrated that a resolution improvement of almost a factor of two can be achieved by incorporating a second interferometric image, which can be acquired easily with an airborne sensor.

High range resolution (HRR) improvement using synthetic HRR processing and stepped-frequency polyphase coding

The authors exploit radar waveform coding techniques to improve high range resolution (HRR) performance. Phase-coded, stepped-frequency (PCSF) waveforms are presented and performance characterised relative to a common 13-element biphase-coded PCSF waveform. Analysis includes PCSF waveforms using biphase coding derived from Gold and minimum peak sidelobe level codes. Synthetic HRR results for PCSF waveforms using Frank and P4 polyphase codes are also presented and achieve HRR performance rivalling their biphase counterparts. Individual code performance is analysed and characterised using a single-point target and realistic target scene comprised of 320 closely spaced complex scattering centres, representing a fighter-sized aircraft. The P4 polyphase coded waveforms achieved best overall HRR performance and were least sensitive to Doppler variation for relative velocities up to 250 m/s. The P4 polyphase coded synthetic HRR profiles exhibit ∼ 7.0 dB and 10.0 dB improvement (peak sidelobe level reduction) relative to Frank and biphase coding, respectively.

Nullifying ACF grating lobes in stepped-frequency train of LFM pulses

An effective way to increase the bandwidth of a coherent pulse-train is to add a frequency step /spl Delta/f between consecutive pulses. A large /spl Delta/f implies a large total bandwidth, hence improved range resolution. However, when the product of the frequency step times the pulse-duration t/sub p/, is larger than one (t/sub p/ /spl Delta/f > 1), the autocorrelation function (ACF) of the stepped-frequency pulse-train suffers from ambiguous peaks, known as It is well known that replacing the fixed-frequency pulses with linear FM (LFM) pulses of bandwidth B can reduce those grating lobes. We present a simple analytic expression for the ambiguity function (AF) and ACF of such a signal and derive from it very simple relationships between /spl Delta/f, B, and t/sub p/ that will place nulls exactly where the grating lobes are located, and thus remove them completely.

SOMARE‐99: Observations of tropospheric scattering layers using multiple‐frequency range imaging

Results from an experimental implementation of multiple‐frequency range imaging (RIM) are presented. The technique exploits the benefits of frequency diversity to improve range resolution of atmospheric radar systems. The theory has been described in the literature, and simulations have proven its usefulness. Nevertheless, experimental results have been extremely limited. Over a 5‐day period in May 1999 we conducted experiments using RIM on the sounding system (SOUSY) radar in northern Germany to observe the layered structure in the troposphere. The experiment is referred to as the SOUSY Multifrequency Atmospheric Radar Experiment 1999 (SOMARE‐99). Estimates of range brightness produced by the RIM analysis provide insight about the layered structure of the atmosphere. The RIM results show distinct similarities to previous in situ measurements, which have shown sharp refractive index discontinuities to exist throughout the troposphere and stratosphere. Examples from selected time periods show layers modulated by possible short‐period gravity waves or advection of periodic structures as well as other layers with apparent downward motion possibly caused by the progression of a warm front or large‐scale subsidence.

Range Resolution Improvement of Ultrasonic Chirp Pulse Compression Signals by Means of Time-Frequency Analysis.

Chirp pulse compression technique can improve signal-to noise ratio of ultrasonic flaw signals. But its disadvantage is the presence of sidelobes which might mask nearby small flaws and make it difficult to separate multiple flaws. In this paper, time-frequency analysis (t-f analysis) is applied to ultrasonic chirp pulse compression signals for range resolution improvement. Short Time Fourier Transform (STFT) is adopted for t-f analysis, followed by the application of Minimization and Polarity Thresholding Algorithm to suppress the sidelobe. In order to achieve the low sidelobe level without suppressing the mainlobe, the STFT is calculated by varying the window length with frequency and both pseudo-twofold and pseudo fourfold frequency cos components obtained from the STFT are utilized. The method is applied to both simulated and measured ultrasonic flaw signals of chirp. The results show a good suppression of sidelobes and a further compression of pulse width with preserving both the polarity and the amplitude of signals.

Iterative Deconvolution Algorithm for Improving Range Resolution of a CO2-Laser Based Lidar System

An iterative deconvolution algorithm for improving range resolution of long-pulse lidars is proposed, and can be applied to the lidar data obtained with the typical pulse of a CO2-laser which consists of a gain-switching peak and a long tail. The lidar signal itself with certain temporal shift is set to be the start profile for the unknown maximally resolved profile in the proposed technique, and then is corrected in proportion to the difference between the lidar return calculated with the assumption and the real one. The same process is repeated until the correction is smaller than tolerance. Simulations are made to test the performance of the proposed algorithm. We investigate the errors in the vicinity of data boundary in the retrieved profile when a part of lidar data is absent. The sensitivity of the iteration algorithm to noise in the lidar signals and the laser pulse profile is also numerically determined.

Novel high range resolution approach in microwave imaging

The authors propose a method of gaining range superresolution in a microwave imaging system. After frequency domain deconvolution of echo to the transmitted signal, the signal model is proved to be of the same form as that of DOA estimation. Hence, as in the case of DOA estimation, a nonlinear spectrum estimation algorithm can be applied to improve range resolution. Numerical simulation and experiments have shown the method to be cost-effective.

Analysis of ladar range resolution enhancement by sinusoidal phase modulation

The ability of a ladar system to resolve two or more separate returns from a combined echo is related to the effective correlation bandwidth of the pulse emitted by the ladar system. Phase modulation of an outgoing pulse introduces additional frequency components, which increases the effective correlation bandwidth of the pulse and thus improves the range resolution of the system. In this paper, we discuss the general theoretical basis for achieving improved range resolution using a modulated waveform and a matched filter receiver. We then demonstrate these concepts by considering the particular case of improved range resolution for a sinusoidally phase modulated carrier with a rectangular amplitude function. We also perform computer simulations with a realistic pulsed ladar envelope possessing the same modulation function. Our calculations indicate that the resolution of a pulsed ladar system may be improved by a factor of 70 with a phase-modulated pulse and a matched-filter receiver.

Carrier-free signal design for look-down radars

The detection of low flying targets with small radar cross section (RCS), known as low observables, such as cruise missiles and stealth airplanes adds a new dimension to radar signal design and radar signal processing. A high resolution look-down radar is very attractive since it takes advantage of target shape to overcome difficulties encountered with small RCS. The look-down geometry, however, imposes three requirements: 1) the radar should detect targets with small relative velocities from almost zero to about the velocity of sound with no blind speeds, 2) it should minimize ground clutter by using short pulses, and 3) the radar signal must have a thumbtack ambiguity function. We investigate a look-down radar that eliminates time side lobes of compressed signal correlation functions to improve range resolution, reduces ground clutter to enhance receiver dynamic range, and uses thumbtack resolution function to resolve moving low observable targets from the surface of the Earth. The side lobe elimination technique transforms the correlation function of a coded waveform, based on complementary codes, to the correlation function of a single pulse. Features of side lobe elimination technique along with clutter cancellation circuits are presented in terms of blind speeds and range-velocity resolution function.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Range Detection of Targets using Hydroacoustics in the Laboratory

Investigations were conducted in the laboratory on the accuracy in terms of range resolution of a commercial sonar system (420 kHz) in detecting underwater targets. Accurate range resolution of fish is necessary for reliable estimates of numbers and for detecting the presence and behaviour of fish near the bottom. Results indicated that the existing sounder was limited in target separation below 25 cm at a transmitted pulse of 0.1 ms under laboratory conditions. Modifications to equipment resulted in an improved range resolution (to approximately 10 cm) of targets in both vertical and horizontal tank experiments.

The Wigner-Ville distribution: Properties, efficient computation, and application to blosonar signal analysis

Use of a spectrogram is the common way to perform time-frequency analysis, but it suffers from inherent trade-off of time resolution versus frequency resolution that does not provide necessary resolution in the time-frequency domain for bat pursuit signals and dolphin signals. The Wigner distribution (WD) appears ideally suited for nonstationary signal analysis. The main limitations of the WD are the interference terms, which are discussed in detail in this paper. A pair of values x(tt), x(tj) at times tt and tj produce interference oscillatory with “frequency” |tj − tt| at the position of (tt + tj)/2. The appropriate time (frequency) window length is the trade-off for the smooth and frequency (time) resolution. An efficient algorithm is proposed. This algorithm employs the fast Fourier transform (FFT) to compute two sequences of smooth pseudo-Wigner distribution; 40% of the number of arithmetic operations are saved. Twenty-one signals emitted by a Chinese Pipistrellus abramus temmink while hunting an insect over a fish pond were recorded. The 2-D time-frequency structures of bat sonar signals for searching, identification, pursuit, and capture are systematically investigated. Using the first-order moment of the WD, the instantaneous frequencies of bat signals are also obtained. As a bat is approaching an insect, the FM components are rather stationary, the CF components are shortened, and the instantaneous frequency tends to be an ideally hyperbolic one, which means the bat is keeping good range resolution, is saving energy, is providing good Doppler tolerance, and is eliminating the coupling error of Doppler delay from search to capture. In terminal phase, very short-duration and harmonic structures are significant techniques to improve range resolution. [Work supported by NSFC 1881155 of China.]

Signal processing and data analysis in middle atmosphere radar

Use of signal‐processing and data analysis methods in radar studies of middle atmospheric motions is examined. Detection of weak Doppler‐shifted signals from thin atmospheric regions in a noise background requires an optimum receiver. It is noted that the optimum receiver is implemented in successive stages using matched‐filter and correlator structures for locating targets distributed in range and a filter bank for estimating their Doppler shifts. Effectiveness of coherent integration in reducing high data rates, and as an efficient but crude low‐pass filter, is discussed. Use of noiselike binary phase codes and decoding in improving range resolution is outlined. Spectral analysis of radar signals using a time‐averaged periodogram and the autoregressive model is briefly reviewed. Methods of reducing undesirable artifacts in spectral analysis of nonstationary signal components, using windows and trend removal, are delineated. Estimation of signal parameters through spectral moments and least squares estimation is summarized. Finally, simple ad hoc data‐editing methods for suppressing outliers and interference are outlined.

Range resolution improvement by a fast deconvolution method.

Range resolution improvement in ultrasonic echography is considered as an estimation problem which is solved using a new fast minimum variance deconvolution algorithm specially designed for a microprocessor-based on-line processing. This method is used to accurately study the lenses and fundus of the eye and to follow variations of an arterial wall thickness during the cardiac cycle.

An improvement in the range resolution of ultrasonic pulse echo systems by deconvolution.

Ultrasonic pulse echo systems are often limited in range resolution by the bandwidth of the piezoelectric transducer. Significant improvements in the range resolution of such systems can be obtained by minimizing the effects of the transducer's dynamic response on the overall pulse echo process. An approach to minimize the effects of the transducer is developed from linear system and impulse response techniques. In essence, the pulse echo voltage of interest is deconvolved with a pulse echo reference voltage which is obtained from an air/water interface in the nearfield of the transducer . A computer study of the pulse echo process and the deconvolution process is presented to illustrate the nature of the improvement in range resolution for several cases of interest. Finally, experimental results are presented to illustrate the improvement using commercially available transducers.

An improvement in the range resolution of ultrasonic pulse echo systems by deconvolution

Ultrasonic pulse echo systems are often limited in range resolution by the bandwidth of the piezoelectric transducer. Significant improvements in the range resolution of such systems can be obtained by minimizing the effects of the transducer's dynamic response on the overall pulse echo process. An approach to minimize the effects of the transducer is developed from linear system and impulse response techniques. In essence, the pulse echo voltage of interest is deconvolved with a pulse echo reference voltage which is obtained from an air/water interface in the near field of the transducer. A computer study of the pulse echo process and the deconvolution process is presented to illustrate the nature of the improvement in range resolution for several cases of interest. Finally, experimental results are presented to illustrate the improvement using commercially available transducers. [Work supported by NIH.]

Passive ranging with focusing beamformers and shaded arrays

We consider focused beamforming using passive arrays as a means of achieving rapid range solutions and demonstrate that a new class of geometry‐dependent amplitude shading distributions can improve range resolution. The distribution is determined by treating the array as if it were a set of samples distributed in a hypothetical curved wave front originating from the array focus and by projecting the hydrophone coordinates onto the spherical surface along radii emanating from the focus. We define a continuous shading distribution on this surface to assign weights to each hydrophone input and divide the shading distribution by the wave front sample density. This technique is called “wave front sample density compensated (WSDC) shading.” Simulated performance is presented for a large surveillance towed array with a target whose signature contains a characteristic tonal and is located broadside to the array. A comparison among unit shading, a simple element placement scheme, and the WSDC method shows that, although there is some loss in gain, the focused beamforming method with the WSDC shading provides greater resolution and computational speed and lower side‐lobe levels than the other methods. [Work supported by ONR.]