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NASA NTRS1982

A review of depolarization modeling for earth-space radio paths at frequencies above 10 GHz

A review is presented of models for the depolarization, caused by scattering from raindrops and ice crystals, that limits the performance of dual-polarized satellite communication systems at frequencies above 10 GHz. The physical mechanisms of depolarization as well as theoretical formulations and empirical data are examined. Three theoretical models, the transmission, attenuation-derived, and scaling models, are described and their relative merits are considered.

Bostian, C. W.↗

NASA NTRS1982

Evaluation of SLAR and simulated thematic mapper MSS data for forest cover mapping using computer-aided analysis techniques

Kershaw County, South Carolina was selected as the study site for analyzing simulated thematic mapper MSS data and dual-polarized X-band synthetic aperture radar (SAR) data. The impact of the improved spatial and spectral characteristics of the LANDSAT D thematic mapper data on computer aided analysis for forest cover type mapping was examined as well as the value of synthetic aperture radar data for differentiating forest and other cover types. The utility of pattern recognition techniques for analyzing SAR data was assessed. Topics covered include: (1) collection and of TMS and reference data; (2) reformatting, geometric and radiometric rectification, and spatial resolution degradation of TMS data; (3) development of training statistics and test data sets; (4) evaluation of different numbers and combinations of wavelength bands on classification performance; (5) comparison among three classification algorithms; and (6) the effectiveness of the principal component transformation in data analysis. The collection, digitization, reformatting, and geometric adjustment of SAR data are also discussed. Image interpretation results and classification results are presented.

Hoffer, R. M.↗

NASA NTRS1983

MIZEX-WEST NASA CV-990 flight report

As part of the Bering Sea marginal ice zone winter experiment (MIZEX-WEST), the CV-990 airborne laboratory was flown to assess the potential of using an extended range of wavelengths for improving passive microwave sea ice observations from spacecraft and second to provide an overview of the MIZ for large-scale processes studies. The aircraft was equipped with both imaging and fixed-beam, dual-polarized passive microwave radiometers ranging from 1.5 millimeter to 3 centimeter wavelengths. Visual, photographic, and thermal (10.7 micron) infrared surface observations were also made from the aircraft to complement the microwave measurements. The flight operations and in-flight observations are discussed and each flight is summarized including flight objective and instrument status. Preliminary mosaic images obtained with the ESMR imager, Nimbus-7 orbits over the Bering Sea, ice observations obtained by an ice observer on board, and composite maps of the general ice conditions for the month of February are also presented.

Cavalieri, D. J.↗

NASA NTRS1982

Initial results from the VPI&SU SIRIO diversity experiment

The first year of observations of the precipitation effects on the 11.6-GHz beacon signal from the SIRIO satellite are reported. This experiment is unique in that it uses dual-polarized receivers in a diversity configuration and a low path elevation angle. Rain rate, attenuation, and isolation statistics are presented for both sites. In addition, attenuation diversity gain is plotted and compared to current models. Isolation diversity gain is also discussed. It was found that little improvement in dual-channel isolation is obtained with site diversity at this low elevation angle. The influence of ice clouds was noted in the months of October and November.

Towner, G. C.↗

NASA NTRS1982

Multiple scattering of electromagnetic waves by rain

As the operating frequencies of communications systems move higher into the millimeter wave region, the effects of multiple scattering in precipitation media become more significant. In this paper, general formulations are presented for single, first-order multiple, and complete multiple scattering. Included specifically are distributions of particle size, shape, and orientation angle, as well as variation in the medium density along the direction of wave propagation. Calculations are performed for rain. It is shown that the effects of higher-order scattering are not noticeable in either attenuation or channel isolation on a dual-polarized system until frequencies of about 30 GHz are reached. The complete multiple-scattering formulation presented gives accurate results at high millimeter wave frequencies as well as including realistic medium parameter distributions. Furthermore, it is numerically efficient.

Tsolakis, A.↗

NASA NTRS1982

Observations of oceanic surface-wind fields from the Nimbus-7 microwave radiometer

Brightness temperatures from the five-frequency dual-polarized scanning multichannel microwave radiometer (SMMR) on Nimbus 7 have been used to obtain surface wind fields over the ocean. The satellite-derived wind field for 1200Z, Feb. 19, 1979, in the eastern North Pacific has been compared with an operationally generated surface-wind analysis field. Previous point comparisons at selected locations have indicated that satellite winds are accurate to 3 m/sec. The results, although of a preliminary nature, indicate that SMMR-derived winds may be used to determine large-scale wind fields over the ocean, particularly in areas of strong wind gradients such as found in cyclonic systems.

Miller, J. R.↗

NASA NTRS1983

Sea ice effective microwave emissivities from satellite passive microwave and infrared observations

Near-simultaneous images from the temperature-humidity IR radiometer and dual-polarization scanning multichannel microwave radiometer of the Nimbus 7 satellite are used to investigate microwave sea ice emissivities on a global scale. Emissivities in several Arctic region study areas are found to be approximately constant during a nine-month pereiod covering the fall, winter and spring months, in the cases of both first-year and multiyear ice. During the onset of summer, emissivity increases of about 30 percent are observed at 37 GHz in multiyear ice. A multichannel cluster analysis over very large study areas during winter indicates considerable variability in emissivities of consolidated ice clusters at 37 GHz, and only 1/3 as much variability at 18 GHz. If the variability within each cluster is quantified and taken into account by means of multispectral analysis, data on ice thickness and surface characteristics may also be obtained.

Comiso, J. C.↗

NASA NTRS1984

Passive microwave characteristics of the Bering Sea ice cover during Marginal Ice Zone Experiment (MIZEX) West

Passive microwave measurements of the Bering Sea were made with the NASA CV-990 airborne laboratory during February. Microwave data were obtained with imaging and dual-polarized, fixed-beam radiometers in a range of frequencies from 10 to 183 GHz. The high resolution imagery at 92 GHz provides a particularly good description of the marginal ice zone delineating regions of open water, ice compactness, and ice-edge structure. Analysis of the fixed-beam data shows that spectral differences increase with a decrease in ice thickness. Polarization at 18 and 37 GHz distinguishes among new, young, and first-year sea ice types.

Cavalieri, D. J.↗

NASA NTRS1983

Computer analysis of X-band radar data

The effectiveness of using currently available computer techniques for interpretation of MSS data to interpret SAR imagery for forest monitoring was assessed. Data were gathered with NASA's airborne APQ-102 dual-polarized, X-band SAR in a flight at 60,000 ft. Microdensitometry was employed to digitize the HH- and HV-polarized imagery. A ground spatial resolution of 15 m was obtained, control points were identified, a second order biquadratic transformation was applied to compensate for orientation, and rms errors were calculated. A second data set was taken with 30 m resolution in order to simulate thematic mapper operation. Classification was performed with pixel-by-pixel and textural classification algorithms. A statistical analysis was also carried out to find any significant differences between classifiers in a data set for a given classifier. Each polarization featured an independent distortion which required appropriate preprocessing to correct. Further studies are recommended with multiple frequencies viewing and multiple polarizations and look angles to define the actual forest classifications that can be made with the SAR imagery.

Knowlton, D. J.↗

NASA NTRS1985

Dual polarization VLBI observations of stellar binary systems at 5 GHz

The binary systems Sigma CrB, SZ Psc, HR 1099, UX Ari, HR 5110, Algol, and Cyg X-1 are characterized in terms of marcsec-scale radio structure on the basis of dual-polarization observations at 4.969-4.997 GHz, obtained with an international VLBI array comprising the 100-m Effelsberg, 36-m Haystack, 42-m Green Bank, 26-m Fort Davis, 130-m-equivalent VLA, and 40-m Owens Valley telescopes (system temperatures 40-70 K) at 22-14 h UT on July 26-27, 1983. The data are presented in graphs and tables and discussed in detail. Single Gaussian brightness distributions with theta(FWHM) = 0.5-2.0 marcsec and brightness temperatures (2-200) x 10 to the 8th K are determined for four sources; core-halo structures are found for UX Ari and Algol; and an upper limit of 50 microarcsec is established for the angular separation of the regions of left and right circular polarization. The gross features seen in RS CVn systems are explained in terms of a simple expanding-coronal-loop model of flare events.

Mutel, R. L.↗

NASA NTRS1983

Differential reflectivity (Z-DR) measurements of rainfall compared with ground-based disdrometer measurements

The dual-polarization differential-reflectivity (Z-DR) radar technique of Seliga and Bringi (1976) is used to determine rainfall rates and drop sizes over a site located at a distance of 47.1 km, and the results are analyzed using the empirical calibration relations of Seliga et al. (1983) and compared with electromechanical-disdrometer measurements. The data are presented in graphs and found to be well correlated, demonstrating the validity of the Z-DR estimates and the improvement introduced by applying the empirical relations rather than an a priori drop-size distribution.

Direskeneli, H.↗

NASA NTRS1985

Satellite observations of sea ice

An overview is presented of Antarctic and Arctic sea ice studies using data from the Nimbus-5 ESMR and the Nimbus-7 SMMR passive microwave radiometers. Four years (1973-1976) of ESMR data for the Antarctic Ocean define the characteristics of the seasonal cycle including regional contrasts and interannual variations. Major advances include the discovery of the Weddell polynya and the presence of substantial areas of open water in the Antarctic winter pack ice. Regional differences in sea ice extent on time-scales of about a month are shown to be associated with variations in surface-wind fields. In the Arctic, the computation of sea ice concentration is complicated by the presence of multiyear ice, but the amount of multiyear ice becomes an important measurable quantity with dual-polarized, multifrequency passive microwave sensors. Analysis of SMMR data demonstrates its advantage for studying the spatial and temporal variability of the Arctic ice cover. Large observed interannual variations in the distribution of the multiyear pack ice and the presence of significant divergent areas in the central Arctic during winter contrast markedly with the classical view of the Arctic pack ice.

Cavalieri, D. J.↗

NASA NTRS1986

Aircraft and satellite passive microwave observations of the Bering Sea ice cover during MIZEX West

Cavalieri, D. J.↗

NASA NTRS1987

Imaging Radar Polarimeter

Radar measures full polarization tensor of each element in scene in one sweep. New system comprises dual-polarized antenna, single transmitter, and four-channel receiver and digital recorder installed in aircraft, plus digital processor on ground. Produces radar-backscatter images corresponding to 10- by 10-km regions on ground. Signals recorded from orthogonal linearly polarized antennas combined in computer after flight to synthesize any desired combination of transmitted and received polarizations. Data recorded on single flight processed to provide multiple images.

Zebker, Howard A.↗

NASA NTRS1986

Multifrequency passive microwave observations of first-year sea ice grown in a tank

Microwave brightness temperatures of new, young, and optically opaque sea ice grown in a large tank were obtained in the course of a joint microwave experiment at CRREL in Hanover, NH, during the winters of 1983-1984 and 1984-1985. Dual-polarized observations were taken at frequencies of 10, 18, 37, and 90 GHz over a range of incidence angles, and the concurrent temperature and ice thickness were obtained. Emissivity and degree of polarization were observed in detail during the early stages of ice growth and variations were found indicating that the ice became optically opaque at 10 GHz for ice thickness between 30 and 50 mm. Cluster plots of the data show six distinguishable surface types: optically opaque bare ice, thin ice (less than 15 mm), roughened ice, ridged ice, rotting wet ice, and open water.

Grenfell, T. C.↗

NASA NTRS1986

Advances in spaceborne synthetic aperture radar sensor technology

The evolution of space SARs for NASA projects since Seasat (1978) is surveyed, with an emphasis on hardware development. The fundamental principles of SAR are reviewed; the SIR-A and SIR-B instruments flown as Shuttle payloads are characterized; their antennas, transmitters, receivers, and data subsystems are described; the advantages offered by the SIR-C dual-frequency (L and C band) dual-polarization distributed SAR (being developed for a future Shuttle flight and as the basis of an SAR for the Earth Observing System) are explained; and a number of technical challenges are identified (including RF elements, structural fidelity, pointing accuracy, data handling, and dc power). Drawings, diagrams, sample images, photographs, and tables are provided.

Caro, E. R.↗

NASA NTRS1986

Some comments on passive microwave measurement of rain

It is argued that because microwave radiation interacts much more strongly with hydrometeors than with cloud particles, microwave measurements from space offer a significant chance of making global precipitation estimates. Over oceans, passive microwave measurements are essentially attenuation measurements that can be very closely related to the rain rate independently of the details of the drop-size distribution. Over land, scattering of microwave radiation by the hydrometeors, especially in the ice phase, can be used to estimate rainfall. In scattering, the details of the drop-size distribution are very important and it is therefore more difficult to achieve a high degree of accuracy. The SSM/I (Special Sensor Microwave Imager), a passive microwave imaging sensor that will be launched soon, will have dual-polarized channels at 85.5 GHz that will be very sensitive to scattering by frozen hydrometeors. Other sensors being considered for the future space missions would extend the ability to estimate rain rates from space. The ideal spaceborne precipitation-measurement system would use the complementary strengths of passive microwave, radar, and visible/infrared measurements.

Wilheit, Thomas T.↗

NASA NTRSDate not supplied

P-band Antenna Array for NASA's Space Exploration Synthetic Aperture Radar (SESAR) Instrument

A dual-polarized, wideband, lightweight P-band (435 MHz) antenna element and array for spaceborne radar applications has been developed for NASA’s SESAR (Space Exploration Synthetic Aperture Radar) instrument for planetary synthetic aperture radar (SAR) applications. The antenna array is based on lightweight, low-profile antenna elements that form the building blocks of configurable “array panels” that can in turn be used to build larger arrays consisting of multiple panels. The element design combines a novel, mostly planar conductor antenna element geometry with a robust lightweight composite material construction.

Synthetic Aperture Radar↗