Radar System Parameters Research Articles

TRMM precipitation radar helps address problems of ground‐based weather radar systems

The launch of the first spaceborne precipitation radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite provides a unique opportunity to address the ground‐based weather radar system's calibration problem. The PR measures the same variable as ground‐based systems—radar reflectivity— and is proven to be consistent with respect to calibration accuracy. PR calibration tests, conducted by the Space Agency of Japan (NASDA) using an active radar calibration system [Kumagai et al, 1995] located in Japan, have shown calibration stability within ±0.8 dBZ [Kozu et al, in press, 2000].A fundamental error source in the measurement of precipitation by radar is the inaccurate definition of the radar calibration constant, which obviously affects the interpretation of radar measurements. This has become an emerging issue, as weather radar is used worldwide for applications ranging from hydrological to climatic. Typically radar systems use internally generated test signals to monitor variations of the radar system's parameters associated with temperature changes in the radar electronics. However, calibration drifts may occur due to gradual degradation of the system's performance. During these times, external calibration using either metal spheres or active sources of radiation required.

Simulation-based evaluations of HF radar ocean current algorithms

A computer simulation is used to analyze errors in high-frequency (HF) radar ocean surface current measurements. Two pointing algorithms used for current extraction, a direction finding approach using MUltiple SIgnal Characterization (MUSIC) developed by Schmidt (1986), and conventional beam forming, are compared in terms of the effect of variations in sea state parameters on current measurement error. The radar system parameters used in the simulation were taken from the University of Michigan's multi-frequency coastal radar (MCR), which operates on four frequencies from 4.8 to 21.8 MHz and employs an eight-element linear phased array for its receive antenna. Results show MUSIC direction finding to be applicable to phased array systems and to have a better sensitivity to sharp current features, but larger random error than traditional beam forming methods. Also, for cases where beam forming errors are dominated by beam width or low signal to noise ratio, results show MUSIC to be a viable alternative to beam forming.

Effectiveness of MMW aerosols in defeating a battlefield surveillance radar: field demonstration preliminary results

The US Army ERDEC is developing advanced aerosol systems to combat threat surveillance, fire control, and seeker systems operating in the visible, infrared, and millimeter wave portions of the electromagnetic spectrum. One such system is the M56 multispectral smoke generator, which presently operates in the visible and infrared portions of the spectrum; a millimeter wave (MMW) Module is in development to extend the M56 spectral range. This paper documents preliminary results of a field demonstration test of the M56 MMW Module. An MMW instrumentation radar was modified to simulate the scan pattern and radar parameters of a tactical battlefield surveillance radar system. A test grid was populated with both stationary and moving tactical targets, and the radar scanned the grid to simulate a surveillance radar in operation. Once a realistic tactical engagement scenario was developed, MMW aerosols were deployed to demonstrate the impact such aerosols could have on radar detection and classification performance.

Meteor fluxes and visual magnitudes from EISCAT radar event rates: a comparison with cross-section based magnitude estimates and optical data

Abstract. Incoherent scatter radars (ISR) are versatile instruments for continuous monitoring of ionisation processes in the Earth's atmosphere. EISCAT, The European Incoherent Scatter facility has proven effective also in meteor studies. The time resolution of the radar can be reduced to a few milliseconds, sufficient to resolve the passage of individual meteors through the narrow ISR beam. Methods for group and phase velocity determination of the meteoroids and the discrepancy between the results related to the target behaviour are presented. The radar cross sections of echoes associated with moving meteoroids ("meteor head echoes") are very small and increase with decreasing wavelength. The parent meteoroids are found to have visual magnitudes far below the detection limit of most optical observations. The equivalent visual magnitude limit of the smallest objects observed by EISCAT in the current experiments has been estimated by two different methods, both from the cross-section measurements and from the measured event rates. Both methods give a limit value of +10 for the smallest objects while the upper limit is +4. The lower limit of the visual magnitude for the collocated optical measurement system is +4. Thus the two detection systems observe two different meteor size ranges, with the radar almost reaching micrometeorite population. Meteor fluxes estimated from the event rates and the radar system parameters agree well with previous extrapolated values for this size range.Key words. Ionosphere (ionization mechanisms). Radio science (ionospheric physics). Space plasma physics (ionization processes)

Meteor fluxes and visual magnitudes from EISCAT radar event rates: a comparison with cross-section based magnitude estimates and optical data

Incoherent scatter radars (ISR) are versatile instruments for continuous monitoring of ionisation processes in the Earth's atmosphere. EISCAT, The European Incoherent Scatter facility has proven effective also in meteor studies. The time resolution of the radar can be reduced to a few milliseconds, sufficient to resolve the passage of individual meteors through the narrow ISR beam. Methods for group and phase velocity determination of the meteoroids and the discrepancy between the results related to the target behaviour are presented. The radar cross sections of echoes associated with moving meteoroids ("meteor head echoes") are very small and increase with decreasing wavelength. The parent meteoroids are found to have visual magnitudes far below the detection limit of most optical observations. The equivalent visual magnitude limit of the smallest objects observed by EISCAT in the current experiments has been estimated by two different methods, both from the cross-section measurements and from the measured event rates. Both methods give a limit value of +10 for the smallest objects while the upper limit is +4. The lower limit of the visual magnitude for the collocated optical measurement system is +4. Thus the two detection systems observe two different meteor size ranges, with the radar almost reaching micrometeorite population. Meteor fluxes estimated from the event rates and the radar system parameters agree well with previous extrapolated values for this size range.Key words. Ionosphere (ionization mechanisms). Radio science (ionospheric physics). Space plasma physics (ionization processes)

Remote sensing applications to hydrology; imaging radar

Abstract Synthetic Aperture Radar (SAR) data are very useful for hydrological studies because the dielectric properties of materials are generally dependent on the amount of liquid water in the material, and this affects the strength of the radar backscatter, [sgrave]°. Additionally, the ability of microwaves to penetrate the surface of many materials often permits analysis of subsurface properties. Further, SAR sensors can image the Earth through both clouds and darkness which enables dynamic hydrological events, e.g. floods, to be captured. Both radar system parameters and surface characteristics affect[sgrave]°. In the case of snow, [sgrave]° is the sum of the surface scattering at the air/snow interface, volume scattering within the snowpack, scattering at the snow/soil interface and volumetric scattering from the underlying surface (if applicable). For glaciers, [sgrave]° is dominated by scattering from the surface and is dependent upon the roughness of the glacier surface in relationship to the radar wavelength. Imaging radar is proving to be an important tool for measuring flooding beneath a variety of herbaceous and woody vegetation. Use of multi-frequency and multi-polarization sensors is necessary to improve the accuracy of mapping flooded vs non-flooded land in floodplains. SAR data have been shown to have potential for measuring stream discharge in braided rivers and for studying key characteristics of frozen lakes. Work with polarimetric SAR data is becoming increasingly significant in hydrological studies.

An analysis of settlement characterization in central Europe using SIR-B radar imagery

Understanding the consistencies and changes in settlement detection ability related to radar system and environmental parameters is of potential interest to applications-oriented users as well as to those interested in improving our understanding of basic system-target interactions. Previous efforts in radar settlement detection have contributed to an eclectic collection of data, but few studies have been able to employ multiple images of the same area. In this experiment, L-band, like-polarized SIR-B radar imagery of southern Germany and northeastern France are examined. Three data takes at three different incident angles and two different look directions covering some 20,000 square kilometers comprised the study area. Two analyses were performed: one of the entire imaged areas, and one limited to similar terrain conditions to focus on look direction effects. Errors of omission and commission were calculated and accuracies tabulated for both tests. Results indicated that steep incident angles were of minimal utility, generating the highest omission and commission errors. Accuracy improved as incident angle increased, up to a point, and then decreased. The results obtained over mixed terrain were not in accord with those obtained over similar terrain.

On the influence of specular reflections in MF radar wind measurements

Numerical simulations are employed to investigate possible biases in Medium Frequency (MF) wind estimates. To explain apparent biases inferred from Arecibo Initiative in Dynamics of the Atmosphere (AIDA) campaign measurements, Hines et al. (1993) have suggested that MF radar estimates of horizontal winds should be biased toward the phase speed of perturbing gravity waves if the MF scattering process is modulated by the passage of such waves through the scattering volume. One of the proposed scenarios involves modulation of the tilts of anisotropic irregularities by the perturbation winds associated with gravity waves. We use a two‐dimensional propagation model in which anisotropic scattering centers are represented by conducting line segments located at random positions inside the radar scattering volume. The scatterers are advected and tilted by the superposition of a mean wind and the perturbation winds associated with a gravity wave. Simulated complex time series, which would be recorded by a two‐receiver antenna array with a typical MF radar system parameters, are analyzed to obtain spaced antenna (SA) wind estimates for comparison with the specified wind speed at the center of the scattering volume. Numerical simulations are carried out for many different sets of gravity wave parameters. Each simulation is performed twice, once with essentially isotropic scatterers (line segments of length λ/2) and again with anisotropic (“specular”) scatterers of length 10 λ. Numerical results indicate that individual SA wind estimates are accurately predicted by the wave‐induced error model of Kudeki et al. (1993). This is true for both isotropic and specular scatterers as long as the gravity wave has a horizontal wavelength larger than the horizontal dimension of the radar range cell. The results also indicate that estimated winds averaged over one period of the perturbing gravity wave exhibit statistically significant biases when the scattering centers are highly anisotropic, but the bias is not necessarily toward the phase speed of the perturbing gravity wave and, more importantly, the magnitude of the bias exhibited in our simulations was always less than 15 m/s. Implications of the numerical results are discussed in light of observed discrepancies as large as 40 m/s in recent AIDA campaign comparisons between MF and incoherent scatter radar winds. In particular, it is argued that the simulation results support the wave‐induced fluctuations model put forth by Kudeki et al. (1993) as an explanation for the AIDA results.

Autoregressive modeling for ocean wave — radar modulation transfer function

The standard technique for computing ocean wave — radar modulation transfer functions (MTFs) depends on nonparametric fast Fourier transform (FFT) methods. In this paper we suggest a new approach applying an autoregressive (AR) parametric spectral estimator. The analysis shows that the two‐channel autoregressive model produces smooth estimates of the MTF and permits better frequency resolution than FFT techniques. The feed‐across effect inherent in the methods developed to evaluate the multichannel AR coefficients causes a negligible effect on MTF. The AR‐based MTF depends weakly on the record length; for steady meteorological conditions the AR model of the MTF for a 2‐min data run is comparable with ones calculated using 10 min or more of data. In the same situation the FFT technique is unreliable for records of less than 10 min. The appropriate order of the multichannel AR model can vary without causing significant change in the MTF. In most cases any order between 8 and 16, depending on radar system parameters and meteorological conditions, will give acceptable results.

A statistical model for the error bounds of an active phased array antenna for SAR applications

A model for the theoretical error bounds for the radiometric calibration of SAR imagery is described. The model is then applied utilizing the radar system parameters that will be used in the project PHARUS (Phased Array Universal SAR), a Dutch polarimetric airborne C-band universal SAR, which is currently under construction. An error model for the phased array antenna pattern is presented. This model was applied to a 16*8 phased array antenna to determine the influence of errors in the T/R modules and angle variations of the beam direction. For a wide range of variances the model is in good agreement with the exact solution, but with far less computer effort. Other, more complicated, antennas can be analyzed in a straightforward way. The model can be used as a design tool for digital phased array radar. >

Theory and design of interferometric synthetic aperture radars

A derivation of the signal statistics, an optimal estimator of the interferometric phase, and the expression necessary to calculate the height-error budget are presented. These expressions are used to derive methods of optimizing the parameters of the interferometric synthetic aperture radar system (InSAR), and are then employed in a specific design example for a system to perform high-resolution global topographic mapping with a one-year mission lifetime, subject to current technological constraints. A Monte Carlo simulation of this InSAR system is performed to evaluate its performance for realistic topography. The results indicate that this system has the potential to satisfy the stringent accuracy and resolution requirements for geophysical use of global topographic data.

A statistical approach for determining radiometric precisions and accuracies in the calibration of synthetic aperture radar imagery

A model that estimates a relative error bound for the radiometric calibration of synthetic aperture radar (SAR) imagery is presented. This model is based on a statistical 'Coefficient of Variation of Error Model', which produces a relative error bound by propagating the measured or estimated uncertainties in the radar system parameters utilized to correct digitally processed SAR image intensity values. Using this model, algorithms are generated for absolute and relative radiometric calibration of SAR imagery. These algorithms are parametrically exercised using radar system parameters from an existing airborne SAR system to determine their impact on the relative error bound.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Synthetic aperture radar imaging of ocean-bottom topography via tidal-current interactions: theory and observations

Abstract A numerical model has been developed for the purpose of explaining and quantifying the relationship between the SEASAT synthetic aperture radar (SAR)signatures and the bottom topography of the ocean in the Southern Bight of the North Sea and Nantucket Shoals. The model uses environmental data (wind, current and depth changes)and radar system parameters (frequency, polarization, incidence angle and resolution cell size)as inputs and predicts SAR-observed backscatter changes over topographic changes in the ocean floor. The model results compare favourably with the actual SEASAT SAR-observed backscatter values. The comparisons between the model and the actual data are all within 1·5 dB except for one limiting geometry. The model suggests that for bottom features to be visible on SAR imagery, a moderate to high current velocity (0·4 m/s or greater) and a low to moderate wind (between 1·5 and 7·5 m/s) must be present.

Sidelobe Properties of a Complementary Code Used in MST Radar Observations

Sidelobe properties of a complementary code used for the clear atmosphere observations by mesosphere-stratosphere-troposphere (MST) radars are discussed. An analytic expression of sidelobe performance is obtained. Complementary codes generally have larger sidelobe suppression than the Barker codes, but the sidelobelevels depend strongly on the atmospheric fluctuations and the radar system parameters.

A radar image time series

A set of ten side-looking radar images of a mining area in Arizona that were aquired over a period of 14 yr are studied to demonstrate the photogrammetric differential-rectification technique applied to radar images and to examine changes that occurred in the area over time. Five of the images are rectified by using ground control points and a digital height model taken from a map. Residual coordinate errors in ground control are reduced from several hundred meters in all cases to + or - 19 to 70 m. The contents of the radar images are compared with a Landsat image and with aerial photographs. Effects of radar system parameters on radar images are briefly reviewed.

Effects of radar system parameters, population, and environmental modulation on settlement visibility

Abstract. The detectability of settlements and factors influencing their visibility are explored using imagery from two side-looking airborne radar systems. K-band and X-band imagery of diverse areas in the United States are examined to discover the minimum population needed for a settlement to be consistently detected. The percentage of settlements visible by size of population are calculated and omission/commission errors analysed. Particular attention is devoted to the effects of environmental modulation and a near-, mid-, or far-range location, but the factors of scale, resolution, and system are also addressed.

Effects of radar system parameters, population, and environmental modulation on settlement visibility

Abstract. The detectability of settlements and factors influencing their visibility are explored using imagery from two side-looking airborne radar systems. K-band and X-band imagery of diverse areas in the United States are examined to discover the minimum population needed for a settlement to be consistently detected. The percentage of settlements visible by size of population are calculated and omission/commission errors analysed. Particular attention is devoted to the effects of environmental modulation and a near-, mid-, or far-range location, but the factors of scale, resolution, and system are also addressed.

Radar System Design for Track-While-Scan

The problem of selecting search radar system parameters to achieve desired track-while-scan performance is investigated at a fundamental theoretical level. The vehicle being tracked is modeled as an object traveling at a random velocity along a random highway. The radar is modeled as a noisy sampler which occassionally drops samples. The best achievable tracking performance in a least-squares error sense of this vehicle/sampler combination is analyzed in general and for a variety of specific situations.

Microwave Backscatter Dependence on Surface Roughness, Soil Moisture, and Soil Texture: Part I-Bare Soil

This is the first in a series of two papers on the use of active microwave remote sensing for measuring the moisture content of bare (Part I) and vegetation-covered (Part II) soil. An experimental program was conducted to evaluate the response of the backscattering coefficient to soil moisture content as a means to specify radar system parameters for future airborne and/or spaceborne soil moisture mappers. Particular attention was paid to the effects of surface roughness, and a preliminary examination of the role of soil texture was performed. The results of this investigation confirm the findings of a previous experiment [1] which concluded that the effects of surface roughness can be minimized by operating at a frequency in the neighborhood of 5 GHz over the 7-17° angle of incidence range. The precision with which soil moisture in the surface soil layer can be estimated is comparable to the precision of the ground-truthed estimate. Because the moisture in the surface layer is highly correlated to the subsurface moisture, it was not possible to determine experimentally the effective depth of the layer responsible for the observed radar backscatter.

Short pulse radar used to measure sea surface wind speed and SWH

A joint airborne measurement program is being pursued by NRL and NASA Wallops Flight Center to determine the extent to which wind speed and sea surface significant wave height (SWH) can be measured quantitatively and remotely with a short pulse (2 ns), wide-beam ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60\deg</tex> ), nadir-looking 3-cm radar. The concept involves relative power measurements only and does not need a scanning antenna, doppler filters, or absolute power calibration. The slopes of the leading and trailing edges of the averaged received power for the pulse limited altimeter are used to infer SWH and surface wind speed. The interpretation is based on theoretical models of the effects of SWH on the leading edge shape and rms sea-surface slope on the trailing-edge shape. The models include the radar system parameters of antenna beam width and pulsewidth. Preliminary experimental results look promising and indicate that it may be possible to design a relatively compact airborne radar to infer, in real-time, the sea surface SWH and surface wind speed.