An empirical model and an inversion technique for radar scatteringfrom bare soil surfaces

Polarimetric radar measurements were conducted for bare soil surfaces under a variety of roughness and moisture conditions at L -, C-, and X-band frequencies at incidence angles ranging from 10° to 70°. Using a laser profiler and dielectric probes, a complete and accurate set of ground truth data was collected for each surface condition, from which accurate measurements were made of the rms height, correlation length, and dielectric constant. Based on knowledge of the scattering behavior in limiting cases and the experimental observations, an empirical model was developed for σ°_hh, σ°_vv, and σ° _hv in terms of ks (where k=2π/λ is the wave number and s is the rms height) and the relative dielectric constant of the soil surface. The model, which was found to yield very good agreement with the backscattering measurements of the present study as well as with measurements reported in other investigations, was used to develop an inversion technique for predicting the rms height of the surface and its moisture content from multipolarized radar observations     

Springtime C-band SAR backscatter signatures of LabradorSea marginal ice: measurements versus modeling predictions

During the March 1987 Labrador Ice Margin Experiment (LIMEX '87) two independent investigations were conducted to determine the C-band backscattering cross section of the marginal pack ice along the Newfoundland coast. In one experiment, data from a recently calibrated C-band airborne scatterometer were combined with C-band synthetic aperture radar (SAR) data to measure the normalized scattering cross section of the ice at incidence angles from 10° to 74° to within ±2 dB. In the other experiment, detailed measurements of ice surface roughness and surface properties were made and the radar cross sections were predicted from a scattering model. In the present study, measured and model results are combined and shown to be fully compatible. By extension, the results are expected to apply to any rubbled sea-ice surface when surface scattering dominates     

Calibrated imaging radar polarimetry: technique, examples, andapplications

The authors developed a calibration procedure for imaging radar polarimeters and applied it to a set of images acquired by the NASA DC-8 multifrequency radar system. The technique requires the use of ground reflectors of known cross-section for absolute calibration, that is, solution for σ⁰; however, the image data themselves can usually provide all information necessary for phase calibration and for antenna crosstalk correction. The accuracy of the approach, as measured by calculating the cross-section residuals of known targets in each calibrated scene, is on the order of ±1-2 dB at the P- and C-band, but improves to ±0.5 dB at the L-band. The authors present the results of applying this technique to radar scenes of lava flows of varying roughness, temperate and tropical rain forests, and ocean water surfaces. They also present several example applications which are feasible with calibrated data but which would be difficult to implement with uncalibrated data     

Dependence of radar backscatter on coniferous forest biomass

Two independent experimental efforts have examined the dependence of radar backscatter on above-ground biomass of monospecie conifer forests using polarimetric airborne SAR data at P-, L- and C-bands. Plantations of maritime pines near Landes, France, range in age from 8 to 46 years with above-ground biomass between 5 and 105 tons/ha. Loblolly pine stands established on abandoned agricultural fields near Duke, NC, range in age from 4 to 90 years and extend the range of above-ground biomass to 560 tons/ha for the older stands. These two experimental forests are largely complementary with respect to biomass. Radar backscatter is found to increase approximately linearly with increasing biomass until it saturates at a biomass level that depends on the radar frequency. The biomass saturation level is about 200 tons/ha at P-band and 100 tons/ha at L-band, and the C-band backscattering coefficient shows much less sensitivity to total above-ground biomass     

The SIR-C/X-SAR synthetic aperture radar system

The SIR-C/X-SAR synthetic aperture radar, a three-frequency radar to be flown on the Space Shuttle in September 1993, is described. The SIR-C system is a two-frequency radar operating at 1250 MHz (L-band) and 5300 MHz (C-band), and is designed to get four-polarization radar imagery at multiple surface angles. The X-band synthetic aperture radar (X-SAR) system is an X-band imaging radar operating at 9600 MHz. The discussion covers the mission concept; system design; hardware; RF electronics; digital electronics; command, timing, and telemetry, and testing     

Time-delayed reflections in L-band synthetic aperture radar imageryof icebergs

In some L-band synthetic aperture radar imagery of icebergs, there are relatively strong reflections which do not exist on the coincident X-band imagery and which do not correspond to attributable surface roughness or features. As these false images appear on the down-range side of the iceberg, they were originally explained as multiple reflections within the iceberg. It is shown that the majority of the observations correspond to the expected positions of a transit to and from the ice-water interface at the bottom surface of the iceberg. Factors governing observation of the bottom surface reflection and its significance in terms of underwater draught are discussed     

Relating forest biomass to SAR data

The authors present the results of an experiment defined to demonstrate the use of radar to retrieve forest biomass. The SAR data were acquired by the NASA/JPL SAR over the Landes pine forest during the 1989 MAESTRO-1 campaign. The SAR data, after calibration, were analyzed together with ground data collected on forest stands from a young stage (eight years) to a mature stage (46 years). The dynamic range of the radar backscatter intensity from forest was found to be greatest at P-band and decreased with increasing frequencies. Cross-polarized backscatter intensity yielded the best sensitivities to variations of forest biomass. L-band data confirmed past results on good correlation with forest parameters. The most striking observation was the strong correlation of P-band backscatter intensity to forest biomass     

Cross-calibration between airborne SAR sensors

A comparative study of data acquired by two different airborne synthetic aperture radar (SAR) sensors from the same site is presented. External, ground-target-based calibration has been performed on the NASA/JPL DC-8 SAR C-band data and the DLR E-SAR C-band data collected over the DLR test site in Oberpfaffenhofen. The consistency of both the polarimetric and radiometric calibration parameters derived from different tracks indicates the stability of DC-8 SAR system during the campaign. Radar cross-sections and backscattering coefficients measured from different tracks under different incidence angles emphasize this stability     

C-band microwave scattering from small balsam fir

An experiment that examined the C-band backscattering characteristics of conifer trees was conducted using a truck-mounted scatterometer. Small (1-m tall) balsam fir (Abies balsamea) trees were arranged at various equidistant spacings on a platform to present canopies of varying density to the radar. C-band backscattering measurements of a range of canopy densities were acquired under different polarizations and incidence angles. In addition, physical measurements of the trees were made including leaf area index, biomass, leaf and branch angle distributions, and dielectric constant. A backscatter model was implemented using measured canopy attributes and showed close agreement with scatterometer measurements over the range of canopy densities     

The L-band PBMR measurements of surface soil moisture inFIFE

The NASA Langley Research Center's L-band pushbroom microwave radiometer (PBMR) aboard the NASA C-130 aircraft was used to map surface soil moisture at and around the Konza Prairie Natural Research Area in Kansas during the four intensive field campaigns of FIFE in May-October 1987. A total of 11 measurements were made when soils were known to be saturated. This measurement was used for the calibration of the vegetation effect on the microwave absorption. Based on this calibration, the data from other measurements on other days were inverted to generate soil moisture maps. Good agreement was found when the estimated soil moisture values were compared with those independently measured on the ground at a number of widely separated locations. There was a slight bias between the estimated and measured values, the estimated soil moisture on the average being lower by about 1.8%     

Absolute radiometric calibration of the CCRS SAR

Determining the radar scattering coefficients from SAR (synthetic aperture radar) image data requires absolute radiometric calibration of the SAR system. The authors describe an internal calibration methodology for the airborne Canada Centre for Remote Sensing (CCRS) SAR system, based on radar theory, a detailed model of the radar system, and measurements of system parameters. The methodology is verified by analyzing external calibration data acquired over a six-month period in 1988 by the C-band radar using HH polarization. The results indicate that the overall error is ±0.8 dB (1σ) for incidence angles ±20° from antenna boresight. The dominant error contributions are due to the antenna radome and uncertainties in the elevation angle relative to the antenna boresight     

Polarization utilization in the microwave inversion of leaf angledistributions

The inverse problem of deducting the inclination angle distribution of leafy vegetation has been investigated using L-band multipolarization backscattered data. The modeling procedure replaces canopy leaves with thin circular dielectric disks. The Born approximation is then used to establish a linear relationship between the radar backscattering coefficients and the leaf inclination angle distribution. The inversion of the leaf angle distribution is carried out for horizontal, vertical, and cross-polarized data. It is shown that the results of the inversion using vertical and cross-polarized data are comparable to the inversion results of horizontally polarized data obtained previously (R. Lang and H. Saleh, 1985)     

Microwave systems design for high-performance moving targetindicators in radars

Clutter cancellation of 65 dB and better is directly proportional to good radar stability, and since many hardware areas produce instabilities at various levels, the architecture of a radar requires special design considerations to support this high stability. The noise character and generation methods of these instabilities in the various hardware areas are described, and design solutions are given to eliminate them. Microwave delay line, a reliable, accurate method of measuring radar stability in L- and S-band radars, is described. The longest microwave delay line available for use at L -band and S-band frequencies is a 15-μs sapphire bulk acoustic wave (BAW) delay line. For higher-frequency radars, smaller delays must be used to keep the insertion loss down to a usable level. The question is raised as to the adequacy of this delay time to provide sufficient visibility for stability measurements of the stable noise. For transmitter measurements, it is adequate for the more common pulse widths, which are less than 15 μs. For LO measurements, the analysis shows that this delay does provide sufficient decorrelation for accurate LO noise measurements     

Mobile satellite system fade statistics for shadowing and multipathfrom roadside trees at UHF and L-band

Field tests related to planned mobile satellite systems (MSS) were performed, and results that add to the existing database of propagation measurements at L-band (1.5 GHz) are described. They are considered particularly useful in that propagation effects were studied systematically with repeated and controlled runs pertaining to different path elevation angles, road types, and path geometries defining shadowing and line-of-sight modes. In addition, simultaneous L-band and UHF measurements were performed for the purpose of establishing scaling factors applicable to previous UHF (870 MHz) results. The control of the experimental parameters was made possible by using a helicopter as the source platform and a mobile van which housed the receiver     

Estimating splash pine biomass using radar backscatter

L-band HV multiple-incidence-angle aircraft synthetic aperture radar (SAR) data were analyzed in relation to average stand biomass, basal area, and tree height for 55 slash pine plantations located in northern Florida. This information was used to develop a system of equations to predict average stand biomass as a function of L-band (24.5-cm) radar backscatter. The system of equations developed in this study using three-stage least-squares and combinatorial screening accounted for 97% of the variability observed in average stand biomass per hectare. When applied to an independent data set, the biomass equations had an average bias of less than 1% with a standard error of approximately 3%     

FETs and HEMTs at cryogenic temperatures-their properties and usein low-noise amplifiers

Typical DC characteristics and X-band noise parameters are presented and qualitatively correlated wherever possible with other technological or experimental data. While certain general trends can be identified, further work is needed to explain a number of observed phenomena. A design technique for cryogenically cooled amplifiers is briefly discussed, and examples of realization of L-band, C -band, X-band, and K-band amplifiers are described. The noise temperature of amplifiers with HEMTs in input stages is usually less than half of that for all-FET realizations, setting new records of performance for cryogenically cooled, multistage amplifiers     

Radar measurement of L-band signal fluctuations caused bypropagation through trees

Fluctuations of an L-band, horizontally polarized signal that was transmitted from the ground through a coniferous forest canopy to an airborne radar are examined. The azimuth synthetic aperture radar (SAR) impulse response in the presence of the measured magnitude fluctuations shows increased sidelobes over the case with no trees. Statistics of the observed fluctuations are similar to other observations     

An empirical model for interpreting the relationship betweenbackscattering and arid land surface roughness as seen with the SAR

The nature of the surface in a flat arid zone located in the western Sahara desert was studied and its roughness parameters defined. The main geological units were compared with backscattering data from ERS-1 images. Results showed a close correlation between the backscattering coefficient, σ₀, and the maximum height, h_max, of the rocks making up the arid landscape. The simplified relationship σ₀=4.63.1og h_max-15.29 (with h_max in cm and σ₀ in dB) was established from field measurements, giving an r² of about 88%. This empirical relationship is confirmed by another close relation between σ₀ and the statistical parameter s, the height standard deviation. The advantage of h_max is that it can be extracted for any site, whatever its roughness may be. The spatial distribution of the rock facets and the geometric characteristics of the incidence angle of the radar signal seem to explain the results     

Quantitative retrieval of soil moisture content and surface roughness from multipolarized radar observations of bare soil surfaces

A semiempirical polarimetric backscattering model for bare soil surfaces is inverted directly to retrieve both the volumetric soil moisture content M/sub v/ and the rms surface height s from multipolarized radar observations. The rms surface height s and the moisture content M/sub v/ can be read from inversion diagrams using the measurements of the cross-polarized backscattering coefficient /spl sigma//sub vh//sup 0/ and the copolarized ratio p(=/spl sigma//sub hh//sup 0///spl sigma//sub vv//sup 0/). Otherwise, the surface parameters can be estimated simply by solving two equations (/spl sigma//sub vh//sup 0/ and p) in two unknowns (M/sub v/ and s). The inversion technique has been applied to the polarimetric backscattering coefficients measured by ground-based polarimetric scatterometers and the Jet Propulsion Laboratory airborne synthetic aperture radar. A good agreement was observed between the values of surface parameters (the rms height s, roughness parameter ks, and the volumetric soil moisture content M/sub v/) estimated by the inversion technique and those measured in situ.     

Measurement and modeling of land mobile satellite propagation atUHF and L-band

A propagation experiment is described in which a stratospheric balloon served as a transmitter platform at 870 and 1502 MHz in simulation of a land mobile satellite. A vehicle followed the drifting balloon along roads of western Texas and New Mexico, collecting at L -band amplitude and phase, and at UHF amplitude information only for elevation angles between 25° and 45°. The data obtained has been analyzed and is presented along with results from modeling of multipath scattering and roadside tree attenuation. The signal, with variations caused by multipath propagation and tree shadowing, was reduced by 3 dB at L-band and 2 dB at UHF for one percent of all locations. A median ratio of 3.9 was found between peak-to-peak phase (degrees) and power (dB) fluctuations. The ratio between L-band and UHF dB attenuation averages varied from 1.3 to 1.0 at fade levels from 6 to 23 dB. Optical sky brightness was measured and used to predict fade distribution with great accuracy. A single-scatterer multipath model is introduced. It is used to duplicate some of the measured data and to show the dependence of power variations on satellite elevation angle. Using Fresnel diffraction theory, the attenuation caused by a model tree was calculated to be near 10 dB and the maximum fade was found to increase by the logarithm of the number of branches