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     

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     

Improved elevation-change measurement of the southern Greenland icesheet from satellite radar altimetry

A new analysis of Seasat and Geosat satellite radar altimeter measurements over the Greenland ice sheet was performed to determine surface elevation change. The new analysis includes twice as many measurements and has 50% greater spatial coverage than the authors' previous study. In addition, a precise global ocean reference network created from four years of Topex/Poseidon altimeter data is used to obtain improved estimates of altimeter orbit errors and measurement system biases. The results show that the average elevation change of the southern Greenland ice sheet above 2000 m from 1978 to 1988 is not significantly different than zero. This contradicts earlier and even more recent studies that reported positive ice sheet growth rates and suggested increased precipitation due to a warmer polar climate     

Elevation change measurement of the East Antarctic Ice Sheet, 1978to 1988, from satellite radar altimetry

Seasat and Geosat satellite radar altimeter measurements over the East Antarctic Ice Sheet (EAIS) are analyzed to determine surface elevation change over the period from 1978 to 1988. The results show that the average elevation change over 1.7 ×10⁶ km² of the EAIS is -0.4±1.8 cm/yr and is therefore not significantly different than zero. This result is in very good agreement with ERS-1/2 radar altimeter estimates of EAIS elevation change from 1992 to 1996. The combined results suggest that the EAIS has been close to a state of balance for two decades. This further reduces the likelihood that recent changes in snow accumulation have masked a long-term mass imbalance in the EAIS     

Elevation change of the Antarctic ice sheet, 1995-2000, from ERS-2 satellite radar altimetry

We analyzed Antarctic ice-sheet elevation change (dH/dt) from 1995 to 2000 using 123 million elevation change measurements from European Remote Sensing 2 ice-mode satellite radar altimeter data covering an area of about 7.2 million km/sup 2/. Almost all drainage basins in east Antarctica had average dH/dt values within /spl plusmn/3.0 cm/year, whereas drainage basins in west Antarctica had substantial spatial variability with average dH/dt values ranging between -11 to +12 cm/year. The east Antarctic ice sheet had a five-year trend of 1/spl plusmn/0.6 cm/year, where 13 out of the 14 basins had either a positive trend or a trend that was not significantly different than zero. The west Antarctic ice sheet had a five-year trend of -3.6/spl plusmn/1.0 cm/year due largely to strong negative trends of around 10 cm/year for basins in Marie Byrd Land along the Pacific sector of the Antarctic coast. The continent as a whole had a five-year dH/dt trend of 0.4/spl plusmn/0.4 cm/year. Finally, time series constructed for the Pine Island, Thwaites, De Vicq, and Land glaciers in west Antarctic showed five-year dH/dt trends from -26 to -135 cm/year that were significantly more negative than the average dH/dt trends in their respective basins. The strongly negative dH/dt values for these coastal glacier outlets are consistent with recently reported results indicating increased basal melting at these glaciers' grounding lines caused by ocean thermal forcing.     

Satellite altimetry using ocean backscatter

A model for the signal received when satellite altimetry pulses are scattered back by the ocean's surface is postulated and analyzed. The second-order statistics of the complex nonstationary random process appearing at the detector input are graphically and analytically displayed as a function of the sea state. The standard deviation of various altitude estimates derived by subsequent operations on the detector output are computed, too. The analysis applies both to ordinary short-pulse altimeters and to systems that employ pulse compression.     

激光测高仪中飞行时间的高精度测量方法的研究

激光测高仪是测量激光脉冲的飞行时间来获得测高仪与探测目标之间的距离,因此激光飞行时间测量的准确性是衡量其系统能力的根本指标.文中阐述了时间间隔高精度测量技术及其最新进展.介绍了传统的测量时间间隔的方法的不足,引入了插入法.并采用粗测和精测相结合的方法来满足大范围、高精度的测量需求.文中详细介绍了模拟插入法、Vernier法、延时线法以及时间数字转换法在FPGA新技术中的应用.     

Multilateration radar

An airborne radar concept is described which would achieve high-resolution mapping of terrain, even with wide-beam antennas, by utilizing a narrow pulse-width. Successive high-resolution range rings, superimposed in a suitable storage medium, cause the terrain image to build up as the aircraft flies by.     

Surface-penetrating radar

Surface-penetrating radar is a nondestructive testing technique which uses electromagnetic waves to investigate the composition of nonconducting materials either when searching for buried objects or when measuring their internal structure. A typical surface-penetrating radar transmits a short pulse of electromagnetic energy of 1 ns (10^-9 s) time duration from a transmit antenna into the material. Energy reflected from discontinuities in impedance is received by means of a receive antenna and is then suitably processed and displayed by a radar receiver and display unit. If the transmit and receive antennas are moved at a constant velocity along a linear path, a cross-sectional image of the material can be generated. Alternatively, if the antennas are scanned in a regular grid pattern, a three-dimensional image of the target can be derived. This paper provides a review of the principles of the technique, discusses the technical requirements for the individual subsystems comprising a surface-penetrating radar and provides examples of typical applications for the method. Continued technical improvements in system performance enable clearer radar images of the internal structure of materials to be obtained, thus advancing the application of the technique     

Compact range radar cross-section measurements using a noise radar

This paper discusses the measurement of radar cross section (RCS) with a very low-cost system that transmits band-limited random noise over the band from 1.0 to 4.0 GHz. The received signal is correlated with a delayed version of the transmitted signal. A variable delay line is used to obtain the response at various delay times. This yields the radar target impulse response as a function of delay. This can be transformed to yield both the amplitude and phase of the scattering matrix in the frequency domain     

Characterizing errors in airborne laser altimetry data to extract soil roughness

Airborne laser altimetry has the potential to make frequent detailed observations that are important for many aspects of studying land surface processes. However, the uncertainties inherent in airborne laser altimetry data have rarely been well measured. Uncertainty is often specified as generally as 20 cm in elevation and 40 cm planimetric. To better constrain these uncertainties, we present an analysis of several datasets acquired specifically to study the temporal consistency of laser altimetry data and, thus, assess its operational value. The error budget has three main components, each with a time regime. For measurements acquired less than 50 ms apart, elevations have a local standard deviation in height of 3.5 cm, enabling the local measurement of surface roughness of the order of 5 cm. Points acquired seconds apart acquire an additional random error due to differential geographic positioning system fluctuation. Measurements made up to an hour apart show an elevation drift of 7 cm over a half hour. Over months, this drift gives rise to a random elevation offset between swathes, with an average of 6.4 cm. The root mean square planimetric error in point location was derived as 37.4 cm. We conclude by considering the consequences of these uncertainties on the principle application of laser altimetry in the U.K. intertidal zone monitoring.     

Multiresolution optimal interpolation and statistical analysis ofTOPEX/POSEIDON satellite altimetry

A recently developed multiresolution estimation framework offers the possibility of highly efficient statistical analysis, interpolation, and smoothing of extremely large data sets in a multiscale fashion. This framework enjoys a number of advantages not shared by other statistically-based methods. In particular, the algorithms resulting from this framework have complexity that scales only linearly with problem size, yielding constant complexity load per grid point independent of problem size. Furthermore these algorithms directly provide interpolated estimates at multiple resolutions, accompanying error variance statistics of use in assessing resolution/accuracy tradeoffs and in detecting statistically significant anomalies, and maximum likelihood estimates of parameters such as spectral power law coefficients. Moreover, the efficiency of these algorithms is completely insensitive to irregularities in the sampling or spatial distribution of measurements and to heterogeneities in measurement errors or model parameters. For these reasons this approach has the potential of being an effective tool in a variety of remote sensing problems. In this paper, the authors demonstrate a realization of this potential by applying the multiresolution framework to a problem of considerable current interest-the interpolation and statistical analysis of ocean surface data from the TOPEX/POSEIDON altimeter     

Development of an active radar calibrator for the TRMMprecipitation radar

An active radar calibrator (ARC) was developed for the calibration of the first spaceborne Precipitation Radar (PR) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The ARC has three functions of the radar receiver, beacon-signal transmitter, and delayed-transponder. A ground-based measurement using a prototype ARC and a bread-board model of the TRMM-PR demonstrated the ARC's capability for the on-orbit TRMM-PR calibration     

Double-polarisation radar measurements

This letter shows some experimental results obtained with a double-polarisation S-band radar using fixed circular polarisation at transmit and two orthogonal circularly polarised channels at receipt. The results refer to the short-time polarisation behaviour of aircraft target and ground clutter. From these results it can be inferred that adaptative polarisation techniques can be effective in improving the signal/clutter ratio.     

Planetary radar astronomy

Measurements of the time delays and Doppler shifts of radar waves have been used to determine the orbits, radii, and rotation vectors of the inner planets and to improve by almost five orders of magnitude the accuracy of relating the Astronomical Unit to a terrestrial unit of length.     

Synthetic aperture radar

A short introduction to the background and theory of synthetic aperture radar (SAR) imaging is given. Some of the key issues in SAR design are discussed and possible future developments involving SAR operation with phased arrays are suggested     

Adaptive radar detection

In the letter a new adaptive procedure for the detection of a radar target in clutter is described. The procedure is based on the development of linear models for the two hypotheses H0 and H1. Under hypothesis H0, there is no target present, and the received signal is modelled as a regressive process. Under hypothesis H1, a target is present and the received signal is modelled as an autoregressive process. Results based on actual radar data are included to confirm the validity of this new approach, and comparison is made with the previously published innovations-based detection algorithm.     

Multifunctional radar systems

The paper contains data about characteristics of several radar systems designed for visualization of radar situation during surveillance of sea areas of economic regions and fishing areas, and for security purpose.