Showing papers in "IEEE Transactions on Geoscience and Remote Sensing in 1982"

Location of Spaceborne Sar Imagery

Abstract: A method has been developed to determine the location of a pixel in a digital SAR image. This technique utilizes the spacecraft ephemeris data and the characteristics of the SAR data collection system to produce an estimate of the latitude and longitude of an arbitrary pixel. This approach has an advantage over previous techniques in that it requires no reference points and is independent of spacecraft attitude knowledge or control. Tests were conducted using SEASAT SAR imagery, comparing predicted feature location with the location as determined by high precision area maps. Results indicate an accuracy of 200 m is attainable with this method. Error sources are analyzed and recommendations are given to improve image location accuracy in future spaceborne SAR's.

Snow-Cover Parameters Retrieved from Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) Data

Abstract: The Nimbus-7 satellite launched on October 24, 1978, carries a multifrequency, dual-polarized microwave imager. The instrument is designed to sense the ocean surface, the atmosphere, and land surfaces remotely. From previous ground-based and satellite-based microwave experiments, it is well known, that snow cover over land has a very distinct effect on the microwave signatures of the earth surface. It was the goal of this study to show that the three snow-cover parameters: extent, snow water equivalent, and onset of snow melt can be determined using scanning multichannel microwave radiometer (SMMR) data. Our analysis has shown, that the three snow parameters mentioned above are retrievable with sufficient accuracy to be of great value in climatology, meteorology, and hydrology. Snow extent is determined for dry snow cover with depth ?5 cm, snow water equivalent can be determined on a regional basis with ?2 g/cm2 rms accuracy, and the onset of snow melt is clearly visible by the detection of melt and refreeze cycles prior to snow runoff. The algorithms derived are simple enough to be incorporated in fully automated operational data analysis schemes.

Estimation of Regional Scale Evapotranspiration Through Analysis of Satellite Thermal-infrared Data

Abstract: Since the mid 70's the LANDSAT series of satelites has acquired visible and near-infrared observations of the earth at a frequency and spatial resolution suitable for agriculture assessment purposes. More recently satellite systems have acquired high precision thermalinfrared data relating to surface thermal properties and moisture status. A data set from the Heat Capacity Mapping Mission [1] illustrates the potential applications of such data for inferring evapotranspiration on a regional scale. Methods described previously [2] are utilized to estimate evapotranspiration rates, yielding results which are consistent with surface measurements of pan evaporation.

Effects of Vegetation Cover on the Radar Sensitivity to Soil Moisture

Abstract: Measurements of the backscattering coefficient ?°, made for bare and vegetation-covered fields, are used in conjunction with a simple backscattering model to evaluate the effects of vegetation cover on the estimation accuracy of soil moisture when derived from radar observations. The results indicate that for soil moisture values below 50 percent of field capacity, the backscatter contribution of the vegetation cover limits the radar's ability to predict soil moisture with an acceptable degree of accuracy. However, for moisture values in the range between 50 and 150 percent of field capacity, the measured ?° is dominated by the soil contribution and the effects of vegetation cover become secondary in importance. It is estimated that in this upper soil moisture range, which is the primary range of interest in hydrology and agriculture, a radar soil moisture prediction algorithm would predict soil moisture with an error of less than ±15 percent of field capacity in 90 percent of the cases.

Towards the Definition of Optimum Sensor Specifications for Microwave Remote Sensing of Snow

Abstract: The interaction of nicrowaves with snow strongly depends on parameters such as snow wetness and the size and structure of snow grains. Therefore microwave radiometry and scatterometry are excellent tools for remote sensing of the snowcover. Multifrequency radiometry can be used to classify snow as was shown with ground-based measurements of the period April-June 1977 at a high altitude Alpine test site. The continuation of the measurement program yielded data of 3 additional snow seasons with widely varying snow conditions, therefore the present information has become representative for alpine regions. Relationships between the brightness temperature and the water equivalents show a similar variation with snow type as in other snow regions, so that the range of validity of our data set is not restricted to the Alps. The problem of discriminating regions of wet snow from snow-free land is found to be solvable with microwave scatterometry. Two cluster analyses in factorial spaces of both the ground truth and the microwave data sets demonstrate the potential of microwave sensors to classify snow which is a prerequisite for snow algorithms retrieving hydrologic parameters. The results are used to define sensor specifications with optimum sensitivity for microwave remote sensing of snow.

Application of a Combined Rough Surface And Volume Scattering Theory to Sea Ice And Snow Backscatter

Abstract: A radiative transfer theory which combines rough surface and volume scattering effects is applied to interpret backscatter measurements from snow and sea ice. The surface scattering effect is accounted for by the Kirchhoff model evaluated either with or without the deep phase modulation assumption. Hence, the major restriction on the surface model is that the horizontal roughness scale must be large enough to satisfy the large radius of curvature requirement. The inhomogeneous layer for simulating snow or sea ice is modeled by either the Rayleigh phase matrix or a continuous random medium with a cylindrically symmetric correlation function for its permittivity function. It is assumed that for the continuous random medium the Born approximation is applicable for computing the scattering phase functions of this inhomogeneous medium. For simplicity only the top boundary of the inhomogeneous layer is assumed rough. Its bottom interface is a plane separating the layer from a homogeneous semiinfinite medium. Comparisons with snow measurements using Polder and Van Santen's mixing formula for the permittivity model show satisfactory agreements at 7.6, 13, and 17 GHz and for sea ice at 9 and 13 GHz. For the cases considered for sea ice, it appears that the Rayleigh phase matrix is an adequate description for volume scattering.

Microwave Radiometry of Lands under Natural and Artificial Moistening

Abstract: This paper is divided into two parts. In the first part the fundamentals of microwave radiation of moistened ground and the main relationships between the characteristics of radiation and the physical and chemical parameters of soils and grounds are given. In the second part applications of microwave radiometry in solving the practical problems are presented.

σ ° Signature of the Amazon Rain Forest Obtained from the Seasat Scatterometer

Abstract: The radar response from the Amazon rain forest has been studied to determine the suitability of this region for use as a standard target to calibrate spaceborne scatterometers Backscattering observations made by the Seasat-1 scatterometer system (SASS) show the Amazon rain forest to be a homogeneous, azimuthally isotropic radar target which is insensitive to polarization The variation with angle of incidence may be adequately modeled as σ°(dB) = aθ + b Typical values for the incidence-angle coefficient a are 007-015 dB/degree No change in the relations was observed over the 99 days of the Seasat mission A small diurnal effect occurs, with measurements at sunrise being 05-1 dB higher than the rest of the day For a fixed incidence angle and time of day the standard deviations ranged from 06 dB at the extremes to 01 dB at the antenna pointing angle of 44° Because of its consistent response the Amazon forest appears to be a suitable target for calibrating spaceborne scatterometers Further research is needed to check for seasonal effects at other times of year

Mixture Formulas Applied in Estimation of Dielectric and Radiative Characteristics of Soils and Grounds at Microwave Frequencies

Abstract: Thirteen well-known formulas for dielectric permittivity of mixtures are examined in order to select the best model for the description of dielectric properties of moistened soil. The selection of the best model is made by comparing theoretical results and experimental data.

Multifrequency Microwave Radiometer Measurements of Soil Moisture

Abstract: Ground-based microwave radiometer experiments were performed to investigate the effects of moisture, temperature, and roughness on microwave emission from bare soils. Measurements were made at frequencies of 0.6-0.9, 1.4, and 10.7 GHz using van-mounted radiometers to observe prepared soil sites in Kern County, CA. The sites were instrumented for monitoring soil characteristics and surface meteorological conditions. Brightness temperature variations of approximately 15 K at 1.4 GHz and 25 K at 10.7 GHz were observed as a result of diurnal changes in the soil temperature. Increasing the soil moisture content from 2 to 15 percent by volume resulted in brightness temperature decreases of approximately 70 K at 0.775 and 1.4 GHz, and 40 K at 10.7 GHz, depending, to a lesser extent, on polarization and viewing angle. The results show the significance of soil temperature in deriving soil moisture from microwave radiometer measurements. Comparisons of the microwave measurements with theoretical predictions using a smooth surface model show reasonable agreement and support previous results of this nature obtained with other soil types. Approximately equal sensitivity to soil moisture was observed at 0.775 and 1.4 GHz, although the sampling depth is greater at the lower frequency.

A Backscatter Model for a Randomly Perturbed Periodic Surface

Abstract: A backscatter model for a randomly perturbed periodic surface has been developed to explain backscatter measurements from a bare soil surface with row structure It is assumed that the rowdirection effect can be included by integrating the scattering coefficient ?ss (?') due to the random roughness component over the underlying periodic component of the soil surface It is found that the rowdirection effect is strongly dependent upon the rate-of-change of ?ss (?') with the local angle ?' and that for sinusoidal surfaces, the maximum difference between looking parallel and perpendicular to the row direction occurs around an incidence angle whose tangent is usually smaller than the slope of the periodic surface at the inflection point

Sigma/deg/ signature of the Amazon rain forest obtained from the Seasat scatterometer

Abstract: Scatterometer data from Seasat of the Amazon rain forest were examined to determine if the region is suitable to use as a reference for calibration of radars. The consistency of Skylab data viewing the Amazon region prompted the analyses, and the Seasat-A scatterometer system (SASS) gathered data of the same region at varying angles. The instrument employed a 100 W 14.6 GHz signal with the reflected power sampled 61 times during each 1.89 sec measurement period. Doppler filters were used in 15 parallel channels of reception, and represented areas 20 km by 50-70 km. Tests were made of regional and temporal stability of the Amazon area, with five measurement angles averaged at different incidence angles to find the mean deviation, which was found to be less than 0.5 dB. Diurnal effects were confined to early morning, and further tests are recommended to obtain results for an entire year, to develop screening methods for thick clouds and rain, and to determine the deviation more precisely.

Soil Moisture Information And Thermal Microwave Emission

Abstract: This paper presents theoretical and experimental results that demonstrate the depth to which soil moisture can be directly measured using microwave radiometers. The experimental results also document the effect of uniform surface roughness on the response of thermal microwave emission to soil moisture. Experimental measurements were executed in July 1980 at the Texas A&M University Research Farm near College Station, TX. Thermal microwave emission measurements were made at 1.4, 4.9, and 10.7 GHz at both vertical and horizontal polarization at off nadir angles from 0 to 50°. It has been demonstrated that passive microwave measurements at frequencies down to 1.4 GHz can only measure soil moisture directly to very shallow soil depths, approximately 2 cm. This is due to the fact that the soil moisture dependence of the transmission coefficient across the air-soil interface predominates over the soil moisture dependence of the total energy originating within the soil volume. It also has been demonstrated that the combination of low incident angle and measurement frequency in the C-band range does not minimize the effect of surface roughness for passive microwave measurements. This result is significant in view of the fact that this combination of frequency and incident angle has been described as the optimum combination for minimizing the effect of surface roughness on the response of radar-backscatter measurements to soil moisture.

Radar Image Processing for Rock-Type Discrimination

Abstract: Radar images have unique radiometric and geometric characteristics which present unique problems and opportunities for geological application. This paper reviews preprocessing and analytical techniques found useful or promising for applications of radar images to geologic problems such as rock-type discrimination. The use of coherent monochromatic illumination in radar images results in image speckle noise which interferes with characterization of the imaged surface. Median value filtering of the radar images removes speckle with minimal edge effects and resolution degradation. Variations in radar scene illumination due to uncompensated sensor platform motions or antenna pattern effects can be somewhat corrected for by mean and variance equalization in a direction perpendicular to the resulting image gradient. Registration of radar images to a map base and compensation of terrain induced image distortion can be accomplished by registration to digital elevation models and knowledge of imaging geometry. Analysis of SEASAT images with coregistered LANDSAT images indicates that the radar data can make a significant contribution to rock-type discrimination, especially if textural measures are incorporated. The sensitivity of radar backscatter to local slopes makes radar images an excellent medium from which to extract textural measures. Three techniques for extraction of the textural data inherent in the radar images are presented. Computation of image tone variance over various areas can numerically encode image texture. Hue-saturation-intensity split spectrum processing displays low-frequency variations in color while preserving high-frequency detail.

Radar Scattering from a Diffuse Vegetation Layer over a Smooth Surface

Abstract: A simple model is presented for the oblique backscatter and bistatic scatter from a smooth surface overlain by a diffuse layer. Only single scattering in the diffuse layer is taken into account. The model analysis shows that the combination of volume scattering and oblique reflection at the surface may increase appreciably the waves scattering. The scattering strongly depends on the properties of the smooth surface. These results support some of the observations made with the Seasat spaceborne imaging radar over flooded regions with heavy vegetation cover.

Soil Moisture Inferences from Thermal-Infrared Measurements of Vegetation Temperatures

Abstract: Remote sensing methods for the estimation of soil moisture yield direct information only for the topmost layers of soil. Reflected solar, thermal-infrared (IR), and microwave techniques are sensitive to the surface skin, from the surface to about 5 cm, and from the surface to about 10 cm, respectively. When the growth of vegetation is of major interest, soil moisture needs to be infrared at least to the depth of rooting of the plants. Since remote measurement of soil moisture is depth limited, it has been suggested that plant measurements, specifically plant temperatures, may yield information about soil moisture within the root zone. To examine this possibility, three plots of wheat, initially treated similarly, and later irrigated differently, were monitored for vegetation temperature (by infrazed thermometry) and for soil-water content (thrice weekly neutron moisture meter measurements). Vegetation temperatures were converted to a crop water stress index (CWSI). The CWSI was found to be a nonunique function of extractable water. The nonuniqueness was probably caused by inability to adequately specify the root zone and by the fact that plants require a recovery period (five to six days for this experiment) after being stressed before normal water uptake and transpiration proceeds.

Acoustic Tomography

Abstract: High-resolution measurements of the density field in the ocean are prohibitively expensive if traditional ship-borne instruments are used. Tomography uses acoustic remote sensing to infer ocean structure, and avoids many of the limitations of direct measurements. Sound pulses follow distinct trajectories through the water from source to receiver, and the travel time for a given pulse is a known functional of the sound speed field. This functional can be inverted to recover an estimate of the sound speed field through which it passed. The inversion is accomplished with either detenninistic linear matrix inversion or stochastic optimal estimation, and the sound speed field estimate returned can be converted to an estimate of density. A numerical simulation of the pilot tomography experiment is presented to demonstrate that tomography can be effective in reproducing significant ocean features.

A Tutorial Assessment of Atmospheric Height Uncertainties for High-Precision Satellite Altimeter Missions to Monitor Ocean Currents

Abstract: A large body of information from a number of sources is brought together and an error budget is deduced giving the projected overall height uncertainty correction for a suggested next generation high-precision radar altimeter. Uncertainties introduced by the wet and dry troposphere, clouds, and the ionosphere are reviewed. A suggested next-generation precision altimeter is assumed to be dual frequency (13.5 and 6 GHz) designed to correct out the ionospheric error. The altimeter-carrying satellite will include a nadir pointing near coincidentbeam dual-frequency microwave radiometer for mitigating the wet tropospheric uncertainty. Although there are a number of caveats, the combined uncertainty in the height correction due to the atmosphere for the suggested system should be nominally 3 cm rms compared to at least 6 cm associated with the Seasat-A mission. Improvements in height resolution of the kind referred to here are vital for future satellite missions designed to monitor ocean currents (e.g., TOPEX).

Crop Classification Using Airborne Radar and Landsat Data

Abstract: Airborne radar data acquired with a 13.3-GHz scatterometer over a test site near Colby, KS, were used to investigate the statistical properties of the scattering coefficient of three types of vegetation cover and of bare soil. A statistical model for radar data was developed that incorporates signal fading and natural within-field variabilities. Estimates of the within-field and between-field coefficients of variation were obtained for each cover type and compared with similar quantities derived from Landsat images of the same fields. The second phase of this study consisted of evaluating the classification accuracy provided by Landsat alone, radar alone, and both sensors combined. The results indicate that the addition of radar to Landsat improves the classification accuracy by about 10 percentage points when the classification is performed on a pixel basis and by about 15 points when performed on a field-average basis. As with all crop-classification studies, these results pertain to the specific dates, geographic region, and crop categories.

Passive Microwave Measurements of Temperature And Salinity in Coastal Zones

Abstract: A technique to remotely measure sea-surface temperature and salinity was demonstrated with a dual-frequency microwave radiometer system developed at the NASA Langley Research Center Acuracies in temperature of 1°C and 1 part per thousand in salinity were obtained using state-of-the-art radiometers Several aircraft programs for the measurement of coastal area waters demonstrating the application of the microwave radiometer system are discussed Measurements of coastal zone ocean temperature and salinity are useful for studying the circulation in bay areas and tracing river outflow Flight measurements in 1976 from an aircraft at an altitude of 14 km over the lower Chesapeake Bay and coastal areas of the Atlantic Ocean resulted in contour maps of sea-surface temperature and salinity with a spatial resolution of 05 km Recent measurements (1980) at an altitude of 170 m were obtained over the Chesapeake Bay mouth and southward along the Virginia coast to study the Chesapeake Bay Plume Because the surface area above submarine springs of fresh water exhibit temperatures and salinities lower than the surrounding sea waters, the multifrequency radiometer system was used in 1978 to detect submarine fresh water springs of the coastal areas around the island of Puerto Rico Forty-four submarine fresh water springs were identified In 1979, a quasi-synoptic survey of tidally induced salinity changes off the Georgia coast was performed using the microwave radiometers onboard a NASA aircraft

Estimation of Context for Statistical Classification of Multispectral Image Data

Abstract: Recent investigations have demonstrated the effectiveness of a contextual classifier that combines spatial and spectral information employing a general statistical approach [1], [2]. This statistical classification algorithm exploits the tendency of certain ground-cover classes to occur more frequently in some spatial contexts than in others. Indeed, a key input to this algorithm is a statistical characterization of the context: the context function. Here we discuss an unbiased estimator of the context function which, besides having the advantage of statistical unbiasedness, has the additional advantage over other estimation techniques of being amenable to an adaptive implementation in which the context-function estimate varies according to local contextual information. Results from applying the unbiased estimator to the contextual classification of three real Landsat data sets are presented and contrasted with results from noncontextual classifications and from contextual classifications utilizing other context-function estimation techniques.

Electric Drill Stem Telemetry

Abstract: Use of the drill stem-earth system for electric down-hole-to- surface telemetry at low frequencies is considered in this article. Estimates of signal attenuation for both the perfectly conducting drill stem and a drill stem with finite conductivity are obtained and compared with previously reported work. It appears that a link of several kilometers length can be operated at frequencies in the hertz range in 10-100-? · m earth at power levels of the order of a few watts.

Theoretical and Experimental Studies of Microwave Emission Signatures of Snow

Abstract: The brightness temperature of snow in Finland has been studied theoretically and experimentally at 5,12, and 37 GHz for satellite remote sensing applications A snow model consisting of ice spheres covered by a water shell has been used in theoretical calculations taking into account scattering and absorption The brightness temperature of a natural snow field on the bare ground and on the ground covered with aluminum sheets has been measured from a tower The experimental brightness temperatures are compared with calculated ones and show a reasonably good agreement Experimental results also show that relatively small changes in the snow conditions cause large changes in the brightness temperature Possible methods for using satellite observations in the remote sensing of snow are suggested

The Dutch ROVE Program

Abstract: In the Netherlands the ROVE team (Radar Observation of VEgetation) investigates the possibilities of radar remote sensing in agriculture. It is an interdisciplinary working group in which five institutes collaborate. Each year, from 1975 to 1980, noncoherent backscatter measurements have been made on different types of vegetations, crops, and bare soils using groups of test fields laid out on a test farm of one of the participating institutes. This approach warrants adequate control over botanical, soil, and surface parameters. For the radar backscatter measurements a short-range FM/CW scatterometer is used mounted on a carriage which is moved along the fields. The combination of the FM/CW principle with movement of the system guarantees a sufficient number of independent observations in a measurement where the illuminated patch is large enough to contain an adequate number of scatterers. So the radar return parameter ? (or ?°) is determined with sufficient accuracy as a function of grazing angle and of time through the growing season. An accurate X-band side-looking airborne radar (SLAR) with digital recording is available for airborne verification experiments. The program is a continuation of the experiments described by the authors at the URSI meeting in Berne in 1974.

Modified Propagation Constants for Nonuniform Plane Wave Transmission through Conducting Media

Abstract: A geometrical optics model for propagation in conducting media requires the determination of the propagation constants for the field transmitted through a plane boundary. In general, the transmitted fiJeld produced by a nonuniform plane wave incident upon a plane boundary between two conducting media propagates with constants which are different from the intrinsic constants of the medium. The expressions for these constants are derived, and data are presented to illustrate when these modified propagation constants must be used to yield valid results.

A Simple Relation between Active And Passive Microwave Remote Sensing Measurements of Earth Terrain

Abstract: A simple approximate relation between backscattering coefficient and emissivity measurements is derived. The relation is applicable to active and passive microwave remote sensing of earth terrain where volume scattering plays a dominant role. From the relation, one can obtain a rough estimate of the backscattering coefficient from the emissivity and vice versa. Such estimation is useful in checking experimental measurements and also helps to ascertain the validity of theoretical models. It also safeguards against obtaining arbitrarily high values for both backscattering coefficients and emissivities.

Description of the Dedicated Gravitational Satellite Mission (GRAVSAT)

Abstract: A description of the dedicated gravitational satellite (GRAVSAT) mission is presented. Scientific objectives are to substantially improve our knowledge of solid earth geophysics and oceanography by determining a more accurate geopotential. The most stringent requirement is determination of mean gravity anomalies over regions- 100-km square to an accuracy of 2.5 mgal (1 gal = 1 cm/s2). Fundamental geodetic data will be satellite-to-satellite measurement of relative range rate. Orbital characteristics are discussed. An error budget is presented which indicates that the requisite measurement precision can be achieved by closed-loop RF doppler measurements. Implicit in the discussion of the error budget is a description of the spacecraft and systems components.

Seasat Synthetic-Aperture Radar Data Reduction Using Parallel Programmable Array Processors

Abstract: This paper presents a digital signal processing system that produces the SEASAT synthetic-aperture radar (SAR) imagery. The system consists of a SEL 32/77 host minicomputer and three AP-120B array processors. The partitioning of the SAR processing functions and the design of softwae modules is described. The rationale for selecting the parallel array processor architecture and the methodology for developing the parallel processing scheme on this system is described. This system attains a SEASAT SAR data reduction speed of 2.5 h per 25-m resolution 4-look and 100 km X 100 km image frame. A prelininary performance evaluation of this parallel processing system and potential future applications for remote sensing data reduction are described.

Bistatic LIDAR: A Tool for Characterizing Atmospheric Particulates: Part II---The Inverse Problem

Abstract: The inverse problem of deducing atmospheric particulate properties from bistatic LIDAR angular scattering measurements is investigated, and certain scattering functions are identified as being particularly sensitive to particle size and/or particle refractive index. Methods are presented for retrieving the particle size distribution and refractive index from these scattering functions. Examples are also given of retrievals of particle size distribution and refractive index obtained from actual bistatic LIDAR data.

Effects of Varying Soil Moisture Contents And Vegetation Canopies on Microwave Emissions

Abstract: This paper presents an analysis of radiometric data taken at 21, 2.8, and 1.67 cm during a NASA sponsored flight over agricultural fields in Phoenix, AZ. The objective of the mission was to provide comprehensive information concerning microwave responses due to a broad range of soil moisture contents. Generally, data taken over bare fields agree well with theoretical estimates from a combined multilayer radiative transfer model with simple roughness correction. With the surface moisture content ranging between 35 percent, the emissivity ranges between >0.9 and ~0.7. The response to soil moisture content at 21 cm is more senstive than that at either 2.8 or 1.67 cm. The vegetation model takes into account both the effect of dielectric coefficient and the volume scattering characteristics of the vegetation layer. At the longer wavelengths (e.g., 21 cm) radiation from soil penetrates through vegetation layers of wheat and alfalfa and provides surface moisture information. However, short wavelength radiation from soil cannot penetrate through vegetation canopies; the volume scattering characteristics of vegetation controls the overall microwave signatures.