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At least 271 records · Page 15

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NASA NTRS1987

The performance of trellis coded multilevel DPSK on a fading mobile satellite channel

The performance of trellis coded multilevel differential phase-shift-keying (MDPSK) over Rician and Rayleigh fading channels is discussed. For operation at L-Band, this signalling technique leads to a more robust system than the coherent system with dual pilot tone calibration previously proposed for UHF. The results are obtained using a combination of analysis and simulation. The analysis shows that the design criterion for trellis codes to be operated on fading channels with interleaving/deinterleaving is no longer free Euclidean distance. The correct design criterion for optimizing bit error probability of trellis coded MDPSK over fading channels will be presented along with examples illustrating its application.

Simon, Marvin K.↗

NASA NTRS1985

Analysis of airborne antenna systems using geometrical theory of diffraction and moment method computer codes

Computer codes have been developed to analyze antennas on aircraft and in the presence of scatterers. The purpose of this study is to use these codes to develop accurate computer models of various aircraft and antenna systems. The antenna systems analyzed are a P-3B L-Band antenna, an A-7E UHF relay pod antenna, and traffic advisory antenna system installed on a Bell Long Ranger helicopter. Computer results are compared to measured ones with good agreement. These codes can be used in the design stage of an antenna system to determine the optimum antenna location and save valuable time and costly flight hours.

Hartenstein, Richard G., Jr.↗

NASA NTRS1987

Phased array antenna for space shuttle orbiter

The National Aeronautics and Space Administration is developing a distributed phased array antenna at the Lyndon B. Johnson Space Center as a possible upgrade for the Space Shuttle Orbiter S-band phase modulation communications system. The antenna consists of an eight-element transmit section, eight-element receive section, and a single L-band receive element. The antenna design is constrained by the existing Orbiter system and space environment. The solution to the interface design problems led to an antenna system which provides improved link margins and yet supports previous operational configurations. This paper describes the system development, antenna hardware, and the interface consideration which led to the final design.

Davidson, Shayla E.↗

NASA NTRS1987

The Telecommunications and Data Acquisition Report

Developments in programs managed by the Jet Propulsion Laboratory's Office of Telecommunications and Data Acquisition are discussed. Topics discussed include sorption compression/mechanical expanded hybrid refrigeration, calculated 70-meter antenna performance for offset L-band, systolic arrays and stack decoding, and calibrations of Deep Space Network antennas.

Posner, E. C.↗

NASA NTRS1986

Wetland physical and biotic studies using multispectral data

A November 1982 Landsat-4 TM scene and March and September 1984 airborne L-band radar data for a brackish-wetland area of the Blackwater National Wildlife Refuge (near Chesapeake Bay) are analyzed to monitor changes in vegetation and water area. The accuracy of level-I classification of the TM image is found to be 81 percent, but that of the few level-II/III classes for which ground truth was available is only 53 percent. The value of radar images for discriminating water areas obscured by vegetation and estimating plant heights is indicated.

Ormsby, J. P.↗

NASA NTRS1986

Preliminary analyses of SIB-B radar data for recent Hawaii lava flows

The Shuttle Imaging Radar (SIR-B) experiment acquired two L-band (23 cm wavelength) radar images (at about 28 and 48 deg incidence angles) over the Kilauea Volcano area of southeastern Hawaii. Geologic analysis of these data indicates that, although aa lava flows and pyroclastic deposits can be discriminated, pahoehoe lava flows are not readily distinguished from surrounding low return materials. Preliminary analysis of data extracted from isolated flows indicates that flow type (i.e., aa or pahoehoe) and relative age can be determined from their basic statistics and illumination angle.

Kaupp, V. H.↗

NASA NTRS1987

Relating polarization phase difference of SAR signals to scene properties

This paper examines the statistical behavior of the phase difference Delta-phi between the HH-polarized and VV-polarized backscattered signals recorded by an L-band SAR over an agricultural test site in Illinois. Polarization-phase difference distributions were generated for about 200 agricultural fields for which ground information had been acquired in conjunction with the SAR mission. For the overwhelming majority of cases, the Delta-phi distribution is symmetric and has a single major lobe centered at the mean value of the distribution Delta-phi. Whereas the mean Delta-phi was found to be close to zero degrees for bare soil, cut vegetation, alfalfa, soybeans, and clover, a different pattern was observed for the corn fields; the mean Delta-phi increased with increasing incidence angle Theta = 35 deg. The explanation proposed for this variation is that the corn canopy, most of whose mass is contained in its vertical stalks, acts like a uniaxial crystal characterized by different velocities of propagation for waves with horizontal and vertical polarization. Thus, it is hypothesized that the observed backscatter is contributed by a combination of propagation delay, forward scatter by the soil surface, and specular bistatic reflection by the stalks. Model calculations based on this assumption were found to be in general agreement with the phase observations.

Ulaby, Fawwaz T.↗

NASA NTRS1987

Monsoon flood boundary delineation and damage assessment using space borne imaging radar and Landsat data

Space-borne synthetic aperture radar (SAR) data acquired by the Shuttle Imaging Radar-B (SIR-B) Program and Landsat Multispectral Scanner Subsystem (MSS) Data from Landsat 4 were used to map flood boundaries for the assessment of flood damage in the Peoples Republic of Bangladesh. The cloud penetrating capabilities of the L-band radar provided a clear picture of the hydrologic conditions of the surface during a period of inclement weather at the end of the wet phase of the 1984 monsoon. The radar image data were digitally processed to geometrically rectify the pixel geometry and were filtered to subdue radar image speckle effects. Contrast enhancement techniques and density slicing were used to create discrete land-cover categories corresponding to surface conditions present at the time of the Shuttle overflight. The radar image classification map was digitally registered to a spectral signature classification map of the area derived from Landsat MSS data collected two weeks prior to the SIR-B mission. Classification accuracy comparisons were made between the radar and MSS classification maps, and flood boundary and flood damage assessment measurements were made with the merged data by adding the classifications and inventorying the land-cover classes inundated at the time of flooding.

Imhoff, Marc L.↗

NASA NTRS1987

Nonuniform sampling techniques for antenna applications

A two-dimensional sampling technique, which can employ irregularly spaced samples (amplitude and phase) in order to generate the complete far-field patterns is presented. The technique implements a matrix inversion algorithm, which depends only on the nonuniform sampled data point locations and with no dependence on the actual field values at these points. A powerful simulation algorithm is presented to allow a real-life simulation of many reflector/feed configurations and to determine the usefulness of the nonuniform sampling technique for the copolar and cross-polar patterns. Additionally, an overlapped window concept and a generalized error simulation model are discussed to identify the stability of the technique for recovering the field data among the nonuniform sampled data. Numerical results are tailored for the pattern reconstruction of a 20-m offset reflector antenna operating at L-band. This reflector is planned to be used in a proposed measurement concept of large antenna aboard the Space Shuttle, whereby it would be almost impractical to accurately control the movement of the Shuttle with respect to the RF source in prescribed directions in order to generate uniform sampled points. Also, application of the nonuniform sampling technique to patterns obtained using near-field measured data is demonstrated. Finally, results of an actual far-field measurement are presented for the construction of patterns of a reflector antenna from a set of nonuniformly distributed measured amplitude and phase data.

Rahmat-Samii, Yahya↗

NASA NTRS1987

Retrieval of surface roughness parameters from dual-frequency measurements of radar backscattering coefficients

The use of dual-frequency data of backscattering coefficients at a fixed angle to estimate surface roughness parameters is evaluated. Radar backscattering coefficients at 1.5 and 4.25 GHz are calculated using a model based on Kirchhoff approximation of electromagnetic wave scatttering from a rough soil surface. Plots of the calculated backscattering coefficients for Kansas soil moisture contents at the C- and L-band frequencies and HH polarization are analyzed. The effects of changes in correlation length on the backscattering coefficients are investigated. The calculated backscattering coefficients are compared with scatterometer data collected at 1.5 and 4.25 GHz, and it is detected that the model and field data correlate well. The data reveal that it is possible to retrieve the surface roughness parameters from measured radar data.

Mo, Tsan↗

NASA NTRS1987

Preliminary simultaneous L/C-band images from the JPL aircraft synthetic aperture radar

A squint mode processor for the processing of JPL aircraft SAR data collected in L-band and C-band is described which meets the criteria of optimum SNR, polarization isolation, and resolution. A steerable prefilter maximizes SNR while reducing the amount of data for processing efficiency. The resolution of images produced from data recorded at nonzero drift angle is improved by range migration compensation. The standard output products include high resolution images (with a range resolution of 7.5 m and an azimuth resolution of 2.74 m), four-look images (produced by averaging every four pixels in the azimuth direction for speckle reduction), and magnitude/phase-difference images (produced by correlating images of similar polarizations).

Lou, Yunling↗

NASA NTRS1987

Spaceborne Imaging Radar-C instrument

The present discussion of the Spaceborne Imaging Radar-C (SIR-C) hardware design, subsystem functional design, and interfaces with the NASA Space Shuttle, gives attention to antenna characteristics and to instrument performance parameter characteristics in the C- and L-bands. The SIR-C antenna is a dual-frequency, dual-polarization distributed array antenna whose distribution of transmit/receive modules improves the system noise figure and eliminates the need for a single, high-power RF source. Phase shifters for individual subarrays allow electronic beam steering in elevation and azimuth.

Huneycutt, Bryan L.↗

NASA NTRS1988

Mobile Communication Via Satellite

System mixes real-time and delayed-transmission channels. Combination of L-band and SHF links connect fixed and mobile equipment on ground to satellite relay. Software and hardware architecture conforms structure of open-system-interconnection model suggested by International Standards Organization.

Yan, Tsun-Yee↗

NASA NTRS1988

An 8-DPSK TCM modem for MSAT-X

This paper describes the real-time digital implementation of an 8-differentiated phase-shift keying (DPSK) trellis-coded modulation (TCM) modem for operation on an L-band, 5 kHz wide, land mobile satellite (LMS) channel. The modem architecture as well as some of the signal processing techniques employed in the modem to combat the LMS channel impairments are described, and the modem performance over the fading channel is presented.

Jedrey, Thomas C.↗

NASA NTRS1987

Spectrum utilization for the International Space Station communications and tracking systems

The frequency spectrum which will be used over the lifetime of the International Space Station Program is discussed. Primary communications traffic will initially occur in the S-band and Ku-band regions. Tracking will initially consist mostly of L-band satellite links. As the service demand increases, use of millimeter and optical wavelengths will be required. The particular space/ground links, space/space links, and other links that will be used are described.

Novosad, Sydney W.↗

NASA NTRS1987

Potential GPS user architecture for the NASA Space Station based on Landsat 4/5 experience

A Landsat 4/5 GPS system is described which uses an inertial reference attitude control system and precision real-time ephemeris generation to achieve precision earth pointing. The system has application to the validation of the use of GPS for the low earth orbit navigation of the Space Station. The present system consists of a receiver/processor assembly (R/PA), an L-band GPS antenna, a precision oscillator, and the Landsat computer. The R/PA is integrated with a GPS receiver which selects, acquires, tracks, times, and decodes navigation signals from GPS satellites in order to derive ephemerides. Ephemeris estimates were found to be accurate to better than 50 meters.

Korenstein, David A.↗

NASA NTRS1987

The electronically steered thinned array radiometer

An L-band radiometer which allows global imaging of soil moisture from LEO is discussed. The system consists of an array of antennas with amplifiers, a device to cross-correlate the signals from the antennas, and a process to translate the resulting data into an image of brightness temperature over the field of view. The process is based on a Fourier transform relation that exists between the incident electric field strength as a function of incidence angle and electric fields across the aperture plane of the antenna. The system's image reconstruction technique and signal-to-noise performance are examined.

Ruf, C. S.↗

NASA NTRS1988

The design and performance of a modem for land mobile satellite communications

The digital implementation and performance of a multipath fade-tolerant differentially coherent 8-PSK-based modem for a land mobile satellite experiment (MSAT-X) program is described. At the receiver, differential detection using a matched filter arrangement and feedforward Doppler compensation is employed to achieve several operational benefits. For this L-band satellite link, the channel utilization is an efficient 1.0 bps/Hz for raw data and 1.5 bps/Hz for coded data. The experimental data analysis shows that the modem functions over the fading channel and performs within 1 dB of theoretical or simulation estimates.

Jedrey, T. C.↗