Radar Range Resolution Research Articles

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.

Multi-range-resolution radar using sideband spectrum energy

A radar range resolution is generally determined by its transmitted pulse width and is thus a constant parameter unrelated to the detection range. However, a method has been proposed for greatly improving a radar range resolution by using signal-processing techniques to reduce the received pulse width at the expense of a high target signal-to-noise ratio. In this method, an autocorrelation output is first obtained from a time-series representation of the received signal, and this autocorrelation output is then processed using a least-squares error-shaping filter. This creates a corresponding increase in the bandwidth of the principal frequency spectrum, thereby improving the range resolution. The relationship between this spectral distribution and the range side-lobes in cases where the transmitted signal is a simple pulse or a plain phase-modulated wave is investigated. The interference cancelling effect achieved when a radar based on the proposed scheme receives interference is also investigated.

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.

Comparison of radar and in situ measurements of atmospheric turbulence

We compare measurements of refractive index structure constant Cn2 and energy dissipation rate ε by VHF radar with in situ observations by high‐resolution thermosondes during a campaign near Adelaide, Australia, in August 1998. A total of 17 thermosonde soundings was performed within a distance of 120 km to the radar site. The variance between estimates of 〈Cn2〉 from backscattered radar power and in situ thermosonde observations is found to be comparable to the observed variance of the radar estimates due to temporal and spatial variations of the turbulent parameters (angle brackets denote average over the radar range resolution of 1 km). The high‐resolution thermosonde observations allow us to test the applicability of the turbulent volume fraction model to the estimation of 〈ε〉 from 〈Cn2〉. The data indicate that the turbulent volume fraction should not be modeled as the fraction of unstable layers with gradient Richardson number Ri < 0.25.

High‐resolution atmospheric profiling using combined spaced antenna and range imaging techniques

A novel approach to high‐resolution wind profiling is presented using spaced antenna (SA) and range imaging (RIM) simultaneously. RIM was developed recently to improve the range resolution of pulsed radar by transmitting multiple frequencies. In this work, SA techniques are used to analyze RIM‐synthesized signals and obtain high‐resolution profiles of three‐dimensional wind field. This high‐resolution technique is termed RIM‐SA. Simulation results demonstrate the feasibility of RIM‐SA to measure wind shears embedded within the radar volume. The applications of RIM‐SA in the atmospheric boundary layer and lower free troposphere are demonstrated using the multiple antenna profiler radar (MAPR) of National Center for Atmospheric Research (NCAR). High‐resolution profiles using RIM‐SA are shown to be consistent with profiles measured by a radiosonde and MAPR using a single frequency and short pulse.

Mean Vertical Velocities Measured by Indian MST Radar and Comparison with Indirectly Computed Values

Abstract Mean vertical velocities and their variations observed with Indian mesosphere–stratosphere–troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E), a tropical station in India, are presented. In this study, a comparison has been made between Indian MST radar–measured vertical velocities and those computed by radiosonde data using kinematic and adiabatic methods. From this study, it is observed that the signs of vertical motion estimated by the kinematic method agree well with MST-radar values, although the magnitudes differ, except in a small region where radar vertical velocity changes in sign from negative to positive in the lower troposphere during monsoon months. This upward motion in this season is attributed to horizontal convergence due to change in wind direction that is not observed in radiosonde data when averaged, because of poor height resolution of the radiosonde (500 m or more varying with height) as compared with the radar range resolution (150 m). Profiles of vertical velociti...

Upper mesosphere summer echoes detected with the Antarctic Syowa HF radar

Peculiar HF radar echoes observed at Syowa Station, Antarctica (69.0°S, 39.6°E) are reported. The echoes occurred in summer at slant ranges between 180 (first range gate) and 315 km under very quiet geomagnetic conditions. They are characterized by a duration of less than 80 min, quasi—periodic oscillations of velocity and echo power with periods of 5–20 min, velocities between −40 and +20 ms−1, and narrow spectral widths less than 40 ms−1. From data of June, July, December 1997, and January 1998, we identified such echoes on 4 days in December and January (summer) but not in June and July (winter). Although range resolution of the HF radar (45 km) is very poor, we believe that our echoes are Antarctic Polar Mesosphere Summer Echoes (PMSE), relying on that the echo characteristics are very similar to the Arctic PMSE.

SOMARE‐99: A demonstrational field campaign for ultrahigh‐resolution VHF atmospheric profiling using frequency diversity

During May of 1999 the sounding system (SOUSY) VHF radar was operated in a demonstrational field campaign specially designed to test the applicability of range imaging (RIM) to radar studies of the atmosphere. The RIM technique utilizes frequency diversity to offer a novel method of improving radar range resolution over that which can be obtained with a conventional pulsed radar with the same bandwidth. During the field campaign, which is being called SOUSY Multifrequency Atmospheric Radar Experiment 1999 (SOMARE‐99), the application of RIM on a VHF radar has been demonstrated. The data from SOMARE‐99 are intended for investigating the dynamics and morphology of fine‐scale vertical structures in the troposphere. This paper gives an overview of SOMARE‐99 and provides some initial scientific results. A central and important result was obtained by comparing RIM‐processed radar observations and data from radiosondes. Profiles of the vertical gradient of the generalized refractive index and the so‐called RIM‐enhanced echo power are found to bear similarities during one particular case, for which the radiosonde was located directly over the radar. Furthermore, a spectral analysis of these two parameters has provided evidence that RIM is successfully identifying multiple structures within the radar sampling volume with scales smaller than the conventional range resolution.

Extended radar observations with the frequency radar domain interferometric imaging (FII) technique

In this paper, we present high-resolution observations obtained with the Middle and Upper Atmosphere (MU) radar (Shigaraki, Japan, 34.85°N, 136.10°E) using the frequency radar domain interferometric imaging (FII) technique. This technique has recently been introduced for improving the range resolution capabilities of the mesosphere–stratosphere–troposphere (MST) radars which are limited by their minimum pulse length. The Fourier-based imaging, the Capon method have been performed with 5 equally spaced frequencies between 46.25 and 46.75 MHz and with an initial range resolution of 300 m . These results have been compared firstly to results obtained using the frequency domain interferometry (FDI) technique with Δf=0.5 MHz and, secondly, to results from a classical Doppler beam swinging (DBS) mode applied with a range resolution of 150 m . Thin echoing structures could be tracked owing to the improved radar range resolution and some complex structures possibly related to Kelvin Helmholtz instabilities have been detected. Indeed, these structures appeared within the core of a wind shear and were associated with intense vertical wind fluctuations. Moreover, a well-defined thin echo layer was found in an altitude range located below the height of the wind shear. The radar observations have not been fully interpreted yet because the radar configuration was not adapted for this kind of study and because of the lack of complementary information provided by other techniques when the interesting echoing phenomena occurred. However, the results confirm the high potentialities of the FII technique for the study of atmospheric dynamics at small scales.

Spectral estimation and resolution of closely spaced radar target scatterers

Substantial research has been performed on radar-range resolution and its implication on range resolution of point targets. This applies to both radar and optical image processing systems. A method for modelling point targets that may constitute scattering centres of an object is considered. In addition, a targetdetection and estimation algorithm to enhance down-range resolution of multiple targets, through efficient processing of the output of the matched filter of a pulsemodulated carrier radar receiver is illustrated. The amplitude and time of arrival associated with a target scatterer are detected and estimated in noise-free and noisy environments. These parameters can be considered as a part of a radar-target feature vector of minimal dimension, that could be utilized in radar target identification processes.

An improved interpretation of VHF oblique radar echoes by a direct balloon C2n estimation using a horizontal pair of sensors

Successful comparisons between VHF oblique (15°) radar echo profiles and reconstructed ones deduced from high‐resolution temperature balloon measurements were presented by Luce et al. [1996]. The method was based on the evaluation of the three‐dimensional isotropic refractive index spectrum at the Bragg wavelength from the available temperature profiles (“spectral method”). However, the isotropic hypothesis is questionable, especially in regions where temperature sheets [Dalaudier et al., 1994] are observed. Indeed, the associated anisotropic temperature fluctuations should contribute to the one‐dimensional vertical temperature spectrum at small scales. In the present paper, another method, less sensitive to anisotropic contaminations, is used. This method is based on an estimation of the temperature structure constant C2T from variances of horizontal differences of temperature measured by two high‐resolution sensors 1 m apart horizontally (“horizontal variance method”). It is shown that the main differences between the C2T estimations obtained from the two methods are mainly observed at high resolution in stable regions where few turbulent fluctuations in the temperature field are observed. However, the two methods give approximately equivalent results at the radar range resolution (600 m) and the quality of the comparisons with the radar observations is slightly improved with the horizontal variance method. In order to demonstrate the full advantages of this technique, the use of radars with more powerful capabilities is suggested for future investigations.

High-range resolution mono-frequency pulsed radar for the identification of approaching targets using subsampling and the MUSIC algorithm

The range resolution of a mono-frequency pulsed radar is limited by the duration of the emitted pulses. For short targets, the different reflectors are closely spaced and their echoes overlap, making them indistinguishable at the receiver. An algorithm based on the MUSIC algorithm combined with a subsampling technique is used to improve the radar's range resolution and thereafter makes it possible to identify the target according to its length and radar signature.

A criterion for radar resolution enhancement with Burg algorithm

A criterion is established for determining the limit on radar resolution enhancement with Burg algorithm (maximum entropy method (MEM)) by comparing the radar range and Doppler resolution limits of MEM with those obtained by the Fourier transform (FT). Also examined are errors in range and Doppler estimation due to MEM and in Doppler space as contiguous pulses are independently extrapolated to form a new train of pulses prior to Doppler processing. It is found that, for two identical point-scatterers, the MEM (Burg algorithm) has approximately a factor of 2.6 advantage over the FT. Data extrapolation by a factor of 2.6 could thus result in improved range and Doppler resolution; beyond that, the effect is primarily due to the increased size of the data window, with a cosmetic improvement in the appearance of the plotted data. Selection of the filter order is also discussed. A practical technique is proposed to facilitate selection of the filter order when a large number of radar pulses have to be extrapolated pulse-by-pulse, thus ensuring a near-optimum result. The two point-scatterer model is used to characterize performance even as the complexity of the target increases from two point-scatterers to two spheres and to a circular cone and finally to a range-Doppler image of a circular cone. Errors in range and Doppler are also characterized by the two point-scatterer model. Doppler error can cause distortion or even sign change in some cases, as exemplified by a conical target with a small surface scatterer The amplitude and the phase errors for each of the two scatterers are also evaluated as a function of the signal-to-noise (SNR). Both are sensitive to the initial phase of the interfering sinusoid. Thus, for a SNR of less than 15 dB, the amplitude error becomes too large to be acceptable. In addition, spurious artifacts may occur due to large pulse-to-poise phase errors. Despite the potential for errors, the comparison clearly establishes the intrinsic gain in the resolution of up to 2.6/spl times/2.6 for a typical range-Doppler image. It is recommended that the methodology discussed here be applied in order to quantify the relative merits of other algorithms that may be used for resolution enhancement. >

Neural networks for automatic target recognition

Many applications reported in artificial neural networks are associated with military problems. This paper reviews concepts associated with the processing of military data to find and recognize targets—automatic target recognition (ATR). A general-purpose automatic target recognition system does not exist. The work presented here is demonstrated on military data, but it can only be consideredproof of principle until systems are fielded andproven “under-fire”. ATR data can be in the form of non-imaging one-dimensional sensor returns, such as ultra-high range-resolution radar returns for air-to-air automatic target recognition and vibration signatures from a laser radar for recognition of ground targets. The ATR data can be two-dimensional images. The most common ATR images are infrared, but current systems must also deal with synthetic aperture radar images. Finally, the data can be three-dimensional, such as sequences of multiple exposures taken over time from a nonstationary world. Targets move, as do sensors, and that movement can be exploited by the ATR. Hyperspectral data, which are views of the same piece of the world looking at different spectral bands, is another example of multiple image data; the third dimension is now wavelength and not time. ATR system design usually consists of four stages. The first stage is to select the sensor or sensors to produce the target measurements. The next stage is the preprocessing of the data and the location of regions of interest within the data (segmentation). The human retina is a ruthless preprocessor. Physiology motivated preprocessing and segmentation is demonstrated along with supervised and unsupervised artificial neural segmentation techniques. The third design step is feature extraction and selection: the extraction of a set of numbers which characterize regions of the data. The last step is the processing of the features for decision making (classification). The area of classification is where most ATR related neural network research has been accomplished. The relation of neural classifiers to Bayesian techniques is emphasized along with the more recent use of feature sequences to enhance classification. The principal theme of this paper is that artificial neural networks have proven to be an interesting and useful alternate processing strategy. Artificial neural techniques, however, are not magical solutions with mystical abilities that work without good engineering. Good understanding of the capabilities and limitations of neural techniques is required to apply them productively to ATR problems.

Resolution limits of ultra wideband synthetic aperture radar using a rectangular aperture for FFT processing

Formulas are derived for synthetic aperture radar range and cross-range resolution without restrictions on percentage bandwidth or integration angle. Restriction to rectangular apertures allows efficient FFT processing and weighting for sidelobe control. The optimum square aperture (equal resolution) and rectangular aperture (minimum resolution area) are derived.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of the radio acoustic sounding system using a chirped acoustic wave

We investigated radio acoustic sounding system (RASS) echoes backscattered from refractive index fluctuations produced by an acoustic pulse linearly modulated in frequency (a chirped acoustic pulse). We have numerically simulated errors of the Doppler frequency shift which are caused by the fact that the widths of the acoustic and radar pulse are finite. We analytically showed that a RASS with a chirped acoustic pulse can correctly measure a wide range of Doppler shifts. The error of the Doppler shift measurement is found to be a function of the radar range resolution, the rate at which acoustic frequency is swept, and the width of the acoustic pulse. The error is also shown to be almost independent of the radar frequency and the lapse rates of the atmospheric temperatures. We have also numerically showed that when the chirped acoustic wave frequency is 50 Hz/s, errors are within 0.018°C when the radar frequency is 1357.5 MHz and the range resolution is 75 m. At the chirped wave frequency, errors are within 0.055°C for a 46.5‐MHz radar with a 150‐m range resolution. We also propose a method for designing a chirped acoustic pulse to measure atmospheric temperature.

Dispersive distortions of transionospheric broadband VHF signals

The dispersive distortions of broadband signals propagating through the Earth's ionosphere are examined in relation to their influence on the range resolution of radars with frequencies from 100 MHz to 30 GHz. Polarization dispersion is taken into account in addition to phase dispersion, but scattering effects are neglected. Analytical expressions are deduced for the envelope and the frequency modulation of the convolution of the received and transmitted signals. The envelope and the frequency modulation of the convolution depend on the polarization, the Earth's magnetic field, and the total electron content of the ionosphere. The magnetic field does not affect the convolution if the carrier frequency of the broadband signal is more than 600–800 MHz. The distortion of the convolution envelope limits the range resolution of radars. The minimum resolution varies from 270 m to 3 cm when the carrier frequency increases from 100 MHz to 30 GHz.

Signal power for radio acoustic sounding of temperature: The effects of horizontal winds, turbulence, and vertical temperature gradients

A simple expression for the expected average signal power of a radio acoustic sounding system (RASS) comprising a monostatic pulsed Doppler radar and a continuous‐wave broadbeam acoustic source is developed. The effects of horizontal winds, atmospheric turbulence, and vertical temperature gradients are included. Under ideal conditions (i.e., in the absence of winds, turbulence, and gradients) the received signal power is a maximum and, for a broadband acoustic source, is predicted to be proportional to the radar range resolution and inversely proportional to the acoustic bandwidth and square of the range R. Turbulence‐induced distortion of the acoustic wave fronts yields the expected R−26/5 range dependence, provided winds are light. This result is based on the assumption that the turbulence strength does not vary with range. For the more realistic case of turbulence concentrated primarily within the boundary layer (R ≲ 1 km) a much weaker range dependence at longer ranges is predicted. A flattening of the acoustic wave fronts by temperature gradients typical of unstable midday conditions results in an R−6 range dependence, provided winds are light. Turbulence effects are predicted to dominate at shorter ranges (R ≲ 8 km) and gradient effects at longer ranges. The expected compensation of power loss associated with the wind‐induced lateral displacement of the acoustic wave fronts by the turbulence‐ and gradient‐induced broadening of the displaced RASS focal spot is also predicted.

Nonlinear modeling of gated tracker dynamics with application to radar range resolution

Nonlinear dynamic models for gated radar range trackers are developed and applied to the range resolution problem. Two common types of tracking loop dynamics, as well as the automatic gain control (AGC), are accounted for in the models. The null detector is formulated in a general way that encompasses many important error detector laws, including centroid and leading-edge. Both discrete-time dynamic models are presented for each class of tracking loop. The discrete-time models are derived using an analytical description of the pulse-to-pulse dynamics of the tracker. The continuous-time models are approximations of their discrete-time counterparts for sufficiently small values of the pulse repetition interval. Each of the models is analyzed for a deterministic target return condition. General criteria for the asymptotic stability of the equilibrium points of the models are obtained. The most striking of the stability criteria is a sign requirement on the slope of a range error curve. These criteria are used in a two-target example to draw conclusions on a tracker's ability to resolve closely spaced targets as a function of target separation. These conclusions are compared with previously reported conclusions on resolvability obtained using P.M. Woodward's (1955) ambiguity function approach.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On optimization of incoherent scatter measurements

The design of incoherent scatter measurements, as well as other radar measurements is constrained by technical factors, the most important of which are radar power and duty cycle. If these are fixed, it is possible to improve detection accuracy by using different kinds of coding methods. We study a situation where radar backscatter autocorrelation functions (ACF) of a continuously distributed medium are to be measured with a specified radar range and autocorrelation lag resolution. As the measurement is disturbed by thermal noise, it is then shown that the accuracy of the resulting ACF estimates cannot be better than a certain limit, whatever kinds of coding schemes are used. The accuracies of alternating codes are compared to this limit for a range of pulse lengths used. We suppose that the noise power due to thermal fluctuations in the system and background noise is higher than the signal power itself. The proof of the limiting accuracy is based on linear statistical inversion theory.