Laboratory measurements of the 36 GHz backscattering cross sectionfrom water waves

The backscattering cross section per unit area σ₀ of a roughened water surface at 36 GHz is needed in the development of new remote sensing instrumentation that will operate at this frequency. One instrument, the multimode airborne radar altimeter (MARA), will illuminate the surface at incidence angles ranging from nadir to 12° off-nadir. Laboratory tests were performed at the Wallops Wind-Wave Tank Facility to determine the variability of σ₀ at these angles and as a function of windspeed. The measurement procedures used in a 6-in.-diam aluminum sphere as the calibration source. The results are compared with earlier measurements at this frequency. Little variability with windspeed was found in the cross-section values for 12° off-nadir, while the cross section decreased with windspeed at nadir. The nadir cross section exceeded the off-nadir result by about 7 dB at a windspeed of 10 m/s (at a 10-m effective height above sea level)     

Topographic mapping using a multibeam radar altimeter

An airborne radar altimeter operating at 36 GHz is uniquely capable of measuring the topography of water, land, and ice surfaces. The Multimode Airborne Radar Altimeter (MARA) was designed to combine a narrow transmitted pulsewidth, a high transmitted power level, and a narrow antenna beam to produce a high-precision ranging capability at the nadir of the aircraft platform and at four fixed off-nadir angles out to 12° in the multibeam mode described in this paper, or a single beam scanning between ±22° in the scanning radar altimeter mode. Data collected over water and land surfaces are presented to demonstrate the potential of MARA for topographic mapping with accuracies and precisions of value in dynamic oceanography, geodesy, geology, hydrology, biogeochemistry, biogeography, and glaciology     

The Application of Near-Nadir ?k Radar Techniques to Geodetic Altimetry And Oceanographic Remote Sensing

This paper first examines the extension of two-frequency, or ?k, near-nadir remote sensing techniques to off-nadir radar altimetry. A different approach to sea state sensing is investigated which appears to offer much higher accuracy. The second section examines the ?k method of sensing rough-surface area correlation or ocean wavenumber spectrum. The technique shows promise of directly sensing this parameter in contrast with synthetic-aperture radar methods which are based on complex wave interaction mechanisms and mathematical transformations requiring the acquisition of voluminous data.     

Multibeam radar altimetry: spaceborne feasibility

An analysis of the inherent height and spatial resolution of an off-nadir radar altimeter is presented. For the general case, mean-square height uncertainty is shown to be proportional to the cross-track beamwidth divided by the along-track beamwidth. Thus, the cross-track beamwidth should be minimized and the along-track beamwidth maximized, subject to resolution and other constraints. A pure multibeam system with a 50-km beam offset, an altitude of 800 km, a 13.5-GHz frequency, and a 4.5-m-diameter antenna is found to yield a height random error of less than 5 cm. For the same conditions, errors decrease with increasing frequency: however, rain attenuation becomes a factor     

Satellite radar altimetry

A brief review of the historical development and principles of satellite radar altimetry is presented, with special emphasis on the unique capability of the microwave altimeter to provide valuable information for global geoscientific studies. Altimeter data over the ocean are used to monitor mean sea levels, wave height, wind speeds, and surface topographical features. Over the ice sheets, the altimeter data are used to produce surface elevation maps, while repeated measurements are used to monitor volume changes. The success of earlier altimeter mission has promoted the development of future missions that will provide more accurate data sets     

The average impulse response of a rough surface and its applications

This paper is concerned with the theoretical model for short pulse scattering from a statistically random planar surface with particular application to current state of the art radar altimetry, A short review of the assumptions inherent in the convolational model is presented. Simplified expressions are obtained for both the impulse response and the average backscattered power for near normal incidence under the assumptions common to satellite radar altimetry systems. In particular, it is shown that the conventional two-dimensional surface integration can be reduced to a closed form solution. Two applications of these results are presented relative to radar altimetry, namely, radar antenna pointing angle determination and altitude bias correction for pointing angle and surface roughness effects. It is also shown that these results have direct application to the analysis of the two frequency system proposed by Weissman, and a possible combined long pulse altimeter and two frequency system is suggested.     

A C-band scatterometer for remote sensing the air-sea interface

An airborne C-band scatterometer system (C-Scat) has been developed to remotely sense ocean surface winds and improve upon the present understanding of the relationship between normalized radar cross section (NRCS) and ocean surface roughness influences such as wind speed and direction, wave height and slope, and the air-sea temperature difference. The scatterometer utilizes a unique frequency-steered microstrip array antenna that is installed beneath the fuselage of an airplane. The antenna is electronically scanned in elevation, from 20° to 50° off-nadir, and mechanically spins in azimuth. The system is capable of measuring ocean surface NRCS from altitudes as high as 25000 ft. The system is divided into four subsystems: the transmitter and receiver, the spinning antenna, the computer control and data acquisition subsystem, and the digital and analog interface electronics     

无线电／激光高度表复合测高技术研究

无线电高度表和激光高度表是巡航导弹上用于探测地形高度的2种主要传感器,激光高度表具有探测精度高、抗电磁干扰能力强的优点;而无线电高度表不受天气和环境的影响,可全天候使用。因而,将无线电高度表与激光高度表进行复合探测,有利于提高地形探测精度,并增强系统抗干扰能力。在高度表测量方程的基础上,基于卡尔曼滤波公式和简单融合算法,给出了无线电／激光多传感器的滤波及融合模型;并利用M atlab对多传感器融合效果进行了仿真。仿真结果表明：基于多传感器融合的高度测量系统的精度比单个传感器的测量精度要高,且系统的稳定性和可靠性更强,所设计的基于无线电／激光高度表多传感器数据融合的高度测量算法是有效的。     

Radar altimeter mean return waveforms from near-normal-incidence ocean surface scattering

Under assumptions common in radar altimetry, the mean backscattered return power for a short-pulse radar and near-normal-incidence scattering from a rough ocean surface is given by the convolution of several terms. For a nearly Gaussian transmitted pulse shape scattered from a nearly Gaussian distributed sea surface, a small-argument series expansion of one of the terms within the convolution leads to a several-term power series expansion for the mean return waveform. Specific expressions are given for the first four terms. These results, which require much less computer time than would the otherwise necessary numerical convolution, are useful for data analysis from current or past radar altimeters and for design studies of future systems. Several representative results are presented for an idealized SEASAT radar altimeter.     

Precipitation detection by the TOPEX/Poseidon dual frequency radaraltimeter, TOPEX microwave radiometer, Special Sensor Microwave/Imagerand climatological shipboard reports

The athors evaluate the ability of a dual-frequency radar (C and Ku band) altimeter to detect rain events. A TOPEX/Poseidon (T/P) altimeter rain flag for the year 1994 is compared to collocated rain rate (RR) from the Defense Meteorological Satellite Program's Special Sensor Microwave Imager (DMSP SSM/I), as processed to the TOPEX/Poseidon passive radiometer's (TMR) liquid-water content, and to a 34-year climatology of shipboard present-weather reports compiled by G. W. Petty (1995). The altimeter-SSM/I analysis is couched in terms of the tradeoff between the probability of a false positive and the probability of a failure to detect rain. The authors show that the ability of the SSM/I and TMR datasets to detect precipitation are closer to each other than to either the altimeter or the shipboard climatology, and this difference is accentuated at latitudes poleward of 45°. They argue that the different footprint sizes explain only part of this discrepancy. They propose that the difference at high latitudes is caused by the altimeter data's sensitivity to snow. In order to detect precipitation (as opposed to detecting bad altimetric values or out-of-range altimetric corrections), a TMR-only flag with liquid-water content of 600 μm recovers too few rain events, 400 μm is close to climatological moderate-to heavy intensity rains, and 200 μm is close to rain of any intensity. For the same purpose, a combined altimeter and TMR flag, with a TMR threshold of 100 μm and with the Ku radar cross section 1.5 standard deviations below an average Ku/C curve, gives the best match for climatological precipitation of any intensity class     

WITTEX: an innovative three-satellite radar altimeter concept

The Water Inclination Topography and Technology Experiment (WITTEX) consists of three TOPEX-class radar altimeters on individual satellites in the same orbit plane. If built with delay-Doppler radar altimeter technology, the WITTEX satellites will be sufficiently small that all three can be launched simultaneously by one vehicle. Even though the WITTEX satellites will be in a common inertial orbit plane, Earth rotation separates their tracks at nadir. Cross-track separation will be proportional to intersatellite spacing. WITTEX will provide near-simultaneous and accurate measurement of sea surface heights along three parallel tracks, providing oceanographic data not previously available, including the two-dimensional (2-D) surface gradient. WITTEX offers a variety of beneficial solutions to the classic time/space sampling tradeoff that confronts oceanic radar altimetry     

Radar cross section measurements

The progress in radar cross section measurements is strongly related to the progress in radar technology. The recent acceleration in radar technology and processing techniques has generated a corresponding acceleration in interest for radar cross section measurements. Historically, early radar cross section measurements were performed to determine the detection range of radar systems, a fundamental objective that still exists. Later measurements, coupled with analytic techniques and computer codes, were performed to extend our understanding of the radar scattering process. At the present time, the availability of broad-band electronics, signal processing techniques, and digital technology results in radar cross section measurement programs which are directed toward exploring the performance of operational waveforms and processing, target discrimination, target detectability in clutter, and radar scattering control. The fundamentals of radar cross section measurements are reviewed. Measurement facilities, including the present research activities on compact range techniques, are then described. Instrumentation radars have benefited from both wide-bandwidth electronics and digital processing capabilities; Fourier transform techniques, in particular, provide both additional information on target scattering, and increase measurement accuracy by isolating the target from radar returns from the measurement facility. The frequency coverage has also extended to include millimeter-wave frequencies. Achievable accuracy is important in any measurement program, and those factors that limit the accuracy of radar cross section measurements are discussed.     

激光雷达截面在系统设计评价中的应用分析

讨论分析了激光雷达截面在激光测量系统设计评价中的应用.分析了激光雷达截面的物理意义,给出了目标激光雷达截面的计算、测量原理与方法,从激光雷达截面的理论定义出发,推导出普遍适用于各类激光测量系统和各种散射特性目标的基本测量方程,该方程利用激光雷达截面表征目标的激光散射特性.通过典型实例说明了该方程在激光测量系统设计评价中的应用.与其他用于表征目标激光散射特性的指标相比,激光雷达截面更容易通过计算或测量得到,且更适用于复杂散射特性目标,因此,在激光测量系统的设计评价中应采用由目标激光雷达截面表述的激光测量方程.     

低频太赫兹准目标雷达散射截面的实验研究

以研究太赫兹雷达散射截面的特性为目的,选用所搭建低频太赫兹雷达测试系统,并借助于标准目标法开展了有关太赫兹雷达粗糙铝盘散射截面的实验研究工作.实验结果表明:在小角度散射中太赫兹雷达散射截面随散射角的增大变化比较明显,在散射角超过5后太赫兹雷达散射截面随散射角的变化趋向缓慢,但当散射角超过12后探测信号的强度已衰减到无法测量,在太赫兹雷达散射截面的测试中没有出现微波雷达散射截面的大小随散射角的变化而剧烈振荡的现象;将测试结果与同尺寸微波、激光雷达散射截面的结果进行了对比,得到结论:在0附近太赫兹雷达散射截面的数值比同尺寸微波雷达散射截面的数值要小两个数量级,但比同尺寸激光雷达散射截面的数值要高一个数量级.     

Atmospheric multiple scattering effects on GLAS altimetry. I.Calculations of single pulse bias

Estimates of the effect of pulse stretching on satellite laser altimetry, such as planned for the Geoscience Laser Altimeter System (GLAS), by cloud multiple scattering were made from an analytical method and with Monte Carlo simulations. Altimetry is dependent on the time required for a laser pulse to complete the roundtrip to the surface and return to the transmitter. Since a transmitted Gaussian pulse will be stretched by the effects of multiple scattering, the use of the pulse centroid as the receive time will produce a biased measurement or an apparent delay in the receive time. The magnitude of this delay was found to be dependent on several factors including cloud height, cloud optical depth, cloud particle size, particle shape, and receiver field of view. The delay was found to be largest for low-level clouds with particle radii of 3-20 μm, potentially amounting to altimetry biases of tens of cm. Alternate methods for measuring the receive time, such as a simple Gaussian fit of the return pulse peak reduce the path delay estimates for all cloud conditions. Since GLAS is a dual mode instrument that includes an atmospheric lidar channel, altimeter measurements that are likely to be significantly contaminated by multiple scattering can be identified     

On the radar cross section of a dipole

Literature references dealing with the radar cross section of a dipole target contain a possible source of confusion in that two definitions of cross section have been used. To clarify this situation, the radar cross section of a randomly oriented half-wave dipole is considered using both definitions.     

External Calibration of SIR-B Imagery with Area-Extended and Point Targets

Data takes on two ascending orbits of the Shuttle Imaging Radar-B (SIR-B) over an agricultural test site in west-central Illinois were used to establish end-to-end transfer functions for conversion of the digital numbers on the 8-bit image to values of the radar backscattering coefficient ?0 (m2/m2) in dB. The transfer function for each data take was defined by the SIR-B response to an array of six calibrated point targets of known radar cross section (transponders) and to a large number of area-extended targets also with known radar cross section as measured by externally calibrated, truck-mounted scatterometers. The radar cross section of each transponder at the SIR-B center frequency was measured on an antenna range as a function of the local angle of incidence. Two truck-mounted scatterometers observed 20-80 agricultural fields daily at 1.6 GHz with HH-polarization and at azimuth viewing angles and incidence angles equivalent to those of the SIR-B. The form of the transfer function is completely defined by the SIR-B receiver and the incoherent averaging procedure incorporated into production of the standard SIR-B image product. Assuming that the processing properly accounts for the antenna gain, all transfer function coefficients are known except for the thermal noise power and a system " constant" that has been shown to vary as a function of uncommanded changes in the effective SIR-B transmit power.     

Calculation of electromagnetic waves scattering by non-homogeneous surface impedance using moment method

In this paper, using the method of moments to calculate the current density and mono-static and bi-static radar cross section of an unlimited strip by a non-homogeneous impedance. Here, incident wave is a plane wave. To authenticate the method, using the iterative method to solve the integral equations is engaged. Simulation results show that the surface with non-homogeneous impedance has reduced or increase in potential for bi- and mono-static radar cross section in certain azimuth.     

Computerized ray optics method of calculating average value radar cross section

In attempting to estimate the radar cross section of airborne vehicles, it is often only necessary to consider average values of the radar returns. A method of providing a quick estimate of the average bistatic radar cross section of the vehicle components would be useful. Ray optics provides a method of predicting the radar cross section of electrically large, perfectly conducting, simply curved, convex bodies such as spheres, ogives, ellipsoids, etc. This paper extends the method of ray optics to the case of an arbitrary body, which may be concave and/or convex, on which doable reflection and depolarization can occur. The incident radiation on the scattering body is represented by a large number(10^{5}-10^{6})of rays. The rays reflected in a given direction with a given polarization are collected at infinity and combined by phasor addition. For the bodies investigated, this method yields results withinpm2dB of measured results except in small angular regions where trapped waves contribute significantly to the radar return.     

一种鲁棒性雷达高度计跟踪算法的研究

过去的星载高度计主要以海洋应用为主,其常用的跟踪算法一般主要适用于特定情况的海洋环境,但现在随着遥感应用的普及,对于陆地、海冰等不同复杂环境的观测日益成为高度计工作的重点,它要求高度计跟踪算法对环境的变化具有很强的鲁棒性,而这是那些以海洋应用为主的跟踪算法所不具备的.为此,本文在分析以往跟踪算法不足的基础上,从建立与分析一般意义上高度计回波模型的角度出发,以OCOG(offset center of gravity)算法为研究重点,通过理论分析与仿真实验,证明了该算法作为鲁棒型跟踪算法的可行性和优越性,同时讨论了它的缺点.这在很大程度上解决了以往应用中对OCOG算法理论性分析不足造成的概念模糊问题,为以OCOG算法为基础的高度计鲁棒性跟踪算法的深入研究与应用打下了坚实基础.