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At least 433 records · Page 24

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NASA NTRS1981

Radar altimeter waveform modeled parameter recovery

Satellite-borne radar altimeters include waveform sampling gates providing point samples of the transmitted radar pulse after its scattering from the ocean's surface. Averages of the waveform sampler data can be fitted by varying parameters in a model mean return waveform. The theoretical waveform model used is described as well as a general iterative nonlinear least squares procedures used to obtain estimates of parameters characterizing the modeled waveform for SEASAT-1 data. The six waveform parameters recovered by the fitting procedure are: (1) amplitude; (2) time origin, or track point; (3) ocean surface rms roughness; (4) noise baseline; (5) ocean surface skewness; and (6) altitude or off-nadir angle. Additional practical processing considerations are addressed and FORTRAN source listing for subroutines used in the waveform fitting are included. While the description is for the Seasat-1 altimeter waveform data analysis, the work can easily be generalized and extended to other radar altimeter systems.

Source record↗

NASA NTRS1981

Design, development, and field demonstration of a remotely deployable water quality monitoring system

A prototype water quality monitoring system is described which offers almost continuous in situ monitoring. The two-man portable system features: (1) a microprocessor controlled central processing unit which allows preprogrammed sampling schedules and reprogramming in situ; (2) a subsurface unit for multiple depth capability and security from vandalism; (3) an acoustic data link for communications between the subsurface unit and the surface control unit; (4) eight water quality parameter sensors; (5) a nonvolatile magnetic bubble memory which prevents data loss in the event of power interruption; (6) a rechargeable power supply sufficient for 2 weeks of unattended operation; (7) a water sampler which can collect samples for laboratory analysis; (8) data output in direct engineering units on printed tape or through a computer compatible link; (9) internal electronic calibration eliminating external sensor adjustment; and (10) acoustic location and recovery systems. Data obtained in Saginaw Bay, Lake Huron are tabulated.

Wallace, J. W.↗

NASA NTRS1981

Comparison of stratospheric aerosol measurements over Poker Flat, Alaska, July, 1979

Stratospheric aerosols were collected at Poker Flat, Alaska, in July, 1979, to determine particle properties, confirm coincident satellite SAGE measurements, and compare similar results obtained with different airborne samplers. Because of the steep slopes in size-distribution curves for larger particles, it is found that properties such as concentrations, aerosol mass, and optical extinction are very sensitive to small errors in radii. It is calculated that the concentration measurements agree with photoelectric particle counter results when a 16% radius change is introduced. An 8% radius change matches our calculated sulfate mass with filter mass measurements. And a 13% radius change results in agreement between the calculated optical extinction and coincident SAGE satellite results. Recognizing that different instruments can produce 10-20% differences in measured sizes, it is believed the results of these comparative measurements of SAGE and in situ instruments are essentially in agreement.

Farlow, N. H.↗

NASA NTRS1980

Simulated response of a multispectral scanner over wheat as a function of wavelength and view/illumination directions

Oblique viewing sensors are to be launched in the late 1980's on the Multispectral Resource Sampler, being developed by the U.S. Since the resulting oblique measurements will need to be better understood, the reflectance response with a view angle of wheat, excluding atmospheric effects but simulating the response of a multispectral scanner, is analyzed. Spectra were taken continuously in the wavelength range of 0.45 to 2.4 microns at more than 1200 view/illumination directions with a 20 C spectral radiometer, and data were acquired six meters above four wheat canopies, each at a different stage of growth. The study shows that the canopy reflective response is a function of the illumination angle, the scanner view angle, and the wavelength; and the variation is greater at low solar variations.

Vanderbilt, V. C.↗

NASA NTRS1981

Penetrating transmission zeros in the design of robust servomechanism systems

In the design of a robust servomechanism system, it is well known that the system cannot track a reference signal whose frequency coincides with the transmission zeros of the system. This paper proposes a new design method for overcoming this difficulty. The controller to be used employs a sampler and holding device with exponential decay. It is shown that the transmission zeros of the discretized system can be shifted by changing the rate of the exponential decay of the holding device. Thus, it is possible to design a robust controller for the discretized system to track any reference signal of given frequency, even if the given frequency coincides with the transmission zeros of the original continuous-time system.

Wang, S. H.↗

NASA NTRS1980

A model of plant canopy polarization response

Sensors to remotely measure the linear polarization of ground scenes have been proposed for the Multispectral Resource Sampler (MRS), a satellite sensor system proposed to complement the Thematic Mapper. At present justification for a sensor on MRS to measure scene polarization is limited. This paper discusses a model for the amount of linearly polarized light reflected by the shiny leaves of such crops as wheat, corn, and sorghum. The theory demonstrates that, potentially, measurements of the linearly polarized light from a crop canopy may be used as an additional feature to discriminate between crops. Examination of the model suggests that, potentially, satellite polarization measurements may be used to monitor crop development stage, leaf water content, leaf area index, hail damage, and certain plant diseases. The model adds to the understanding of the potential information content of scene polarization measurements acquired by future satellite sensor systems such as MRS.

Vanderbilt, V. C.↗

NASA NTRS1980

A model of plant canopy polarization

A model for the amount of linearly polarized light reflected by the shiny leaves of grain crops is based on the morphological and phenological characteristics of the plant canopy and upon the Fresnel equations which describe the light reflection process at the smooth boundary separating two dielectrics. The theory used demonstrates that, potentially, measurements of the linearly polarized light from a crop canopy may be used as an additional feature to discriminate between crops such as wheat and barley, two crops which are so spectrally similar that they are misclassified with unacceptable frequency. Examination of the model suggests that, potentially, satellite polarization measurements may be used to monitor crop development stage, leaf water content, leaf area index, hail damage, and certain plant diseases. The information content of these measurements is needed to evaluate the proposed polarization sensor for the satellite-borne multispectral resource sampler.

Vanderbilt, V. C.↗

NASA NTRS1982

Conic Sector Analysis of Hybrid Control Systems

A hybrid control system contains an analog plant and a hybrid (or sampled-data) compensator. In this thesis a new conic sector is determined which is constructive and can be used to: (1) determine closed loop stability, (2) analyze robustness with respect to modelling uncertainties, (3) analyze steady state response to commands, and (4) select the sample rate. The use of conic sectors allows the designer to treat hybrid control systems as though they were analog control systems. The center of the conic sector can be used as a rigorous linear time invariant approximation of the hybrid control system, and the radius places a bound on the errors of this approximation. The hybrid feedback system can be multivariable, and the sampler is assumed to be synchronous. Algorithms to compute the conic sector are presented. Several examples demonstrate how the conic sector analysis techniques are applied. Extensions to single loop multirate hybrid feedback systems are presented. Further extensions are proposed for multiloop multirate hybrid feedback system and for single rate systems with asynchronous sampling.

Thompson, P. M.↗

NASA NTRS1983

The ISPM unified radio and plasma wave experiment

Hardware for the International Solar Polar Mission (ISPM) Unified Radio and Plasma (URAP) wave experiment is presented. The URAP determines direction and polarization of distant radio sources for remote sensing of the heliosphere, and studies local wave phenomena which determine the transport coefficients of the ambient plasma. Electric and magnetic field antennas and preamplifiers; the electromagnetic compatibility plan and grounding; radio astronomy and plasma frequency receivers; a fast Fourier transformation data processing unit waveform analyzer; dc voltage measurements; a fast envelope sampler for the solar wind, and plasmas near Jupiter; a sounder; and a power converter are described.

Stone, R. G.↗

NASA NTRS1984

Martian dust storms witnessed by Viking Lander 1

Viking Lander 1 observations on Mars were punctuated by a strong local dust storm after two martian years of mild wind conditions. Tens of micrometers of dust settled to the surface during global dust storms of the first two falls and winters; some of this dust was locally removed during the second year. A late winter local dust storm of the first year caused little or no erosion of the surface materials despite wind speeds of 25 to 30 m/s. The strong local dust storm occurred during late winter of the third martian year. Winds of this storm altered and demolished small conical piles of surface materials constructed at the onset the first winter, removed 4 to 5 mm size fragments, displaced centimeter size fragments, destroyed clouds in areas disrupted by the sampler and footpad, eroded impact pits, and darkened the sky. Movement of erosional products and tiny wind tails indicate easterly to northeasterly winds. If the 4 to 5 mm size fragments were entrained and removd by the wind, threshold friction speeds near 3 to 5 m/s would have been required for the atmospheric temperatures and pressures that prevailed during the late winter of the third year.

Moore, H. J.↗

NASA NTRS1983

Three Mars years - Viking Lander 1 imaging observations

The Mutch Memorial Station (Viking Lander 1) on Mars acquired imaging and meteorological data over a period of 2245 martian days (3.3 martian years). This article discusses the deposition and erosion of thin deposits (ten to hundreds of micrometers) of bright red dust associated with global dust storms, and the removal of centimeter amounts of material in selected areas during a dust storm late in the third winter. Atmospheric pressure data acquired during the period of intense erosion imply that baroclinic disturbances and strong diurnal solar tidal heating combined to produce strong winds. Erosion occurred principally in areas where soil cohesion was reduced by earlier surface sampler activities. Except for redistribution of thin layers of materials, the surface appears to be remarkably stable, perhaps because of cohension of the undisturbed surface material.

Arvidson, R. E.↗

NASA NTRS1985

Observed Changes at Viking Lander 1

A local dust storm raged in Chryse Planitia, Mars, in June 1981. The changes wrought in the vicinity of the lander (Mutch Memorial Station) by this storm sometime near Sol 1742 were partly described previously. Here, changes related to the storm are itemized, evidence for wind directions during the peak of the storm are cited, and two observations unrelated to the storm are noted. The observations suggest that the eroding winds of the Sol 1742 storm were more easterly (N. 35 deg to 90 deg E.) than those (N. 5 deg to 11 deg E.) that formed the large wind tails; and fragments in erosional residues are 0.7 cm and larger, but smaller ones may be present. Some fragments 0.4 to 0.5 cm and smaller were somehow removed, at least locally; wind speeds of the 1742 local storm were probably greater than those of a previous local dust storm (25 to 30 m/s) that occurred during the same season on Sol 423 because the earlier storm did not alter the surface; the major, if not entire, amount of erosion by the storm occurred between Sols 1728 and 1757; and erosion chiefly occurred where the surface configuration and material properties were altered by the lander and its sampler.

Moore, H. J.↗

NASA NTRS1985

The Martian dust storm of Sol 1742

After nearly five earth years on Mars, the Mutch Memorial Station (Viking Lander 1) finally witnessed a local dust storm that eroded trenches, conical piles, and other disturbed surfaces in the sample field and near the Lander. The event, called the Dust Storm of Sol 1742, occurred late in the third winter of Lander observations between Sols 1728 and 1757. Analyses of tiny new wind tails and movement of materials indicate that the eroding winds were variable but northeasterly than those that had previously shaped the surface. Pebbly residues and movement of 4-5 mm clods suggest drag velocities or friction speeds of the winds were about 2.2-4.0 m/s. Wind speeds at the height of the meteorology boom (1.6 m) were probably about 40-50 m/s. Much of the observed erosion could have occurred in a few to several tens of seconds, but somewhat longer times are suggested by analogy with the erosion of terrestrial soils. Most of the erosion occurred where preexisting equilibrium conditions of surface configurations and surface material properties had been altered by the Lander during landing and during surface-sampler activities, but thin layers of bright fine-grained dust were also removed and redistributed. Surfaces where preexisting equilibrium conditions were unaltered appeared to be uneroded.

Moore, H. J.↗

NASA NTRS1987

Method and apparatus for telemetry adaptive bandwidth compression

Methods and apparatus are provided for automatic and/or manual adaptive bandwidth compression of telemetry. An adaptive sampler samples a video signal from a scanning sensor and generates a sequence of sampled fields. Each field and range rate information from the sensor are hence sequentially transmitted to and stored in a multiple and adaptive field storage means. The field storage means then, in response to an automatic or manual control signal, transfers the stored sampled field signals to a video monitor in a form for sequential or simultaneous display of a desired number of stored signal fields. The sampling ratio of the adaptive sample, the relative proportion of available communication bandwidth allocated respectively to transmitted data and video information, and the number of fields simultaneously displayed are manually or automatically selectively adjustable in functional relationship to each other and detected range rate. In one embodiment, when relatively little or no scene motion is detected, the control signal maximizes sampling ratio and causes simultaneous display of all stored fields, thus maximizing resolution and bandwidth available for data transmission. When increased scene motion is detected, the control signal is adjusted accordingly to cause display of fewer fields. If greater resolution is desired, the control signal is adjusted to increase the sampling ratio.

Graham, Olin L.↗

NASA NTRS1987

Small-scale physics of the ocean

Observations and theoretical models of small-scale phenomena in the oceans are reviewed, with a focus on progress during the period 1983-1986. Topics examined include surface layers, equatorial turbulence, off-equator mixed layers, the scaling of mixing, turbulence concepts, laboratory results, internal waves and mixing, rings, the nature of the bottom layer, double diffusion and intrusions, salt fingers, and biological interactions. Also discussed are developments in instrumentation (fast sampling profilers with upward-profiling capability, deep profilers, ship-motion correction, horizontal samplers, small submersibles, submarines, towed packages, conductivity sensors, dissolved-oxygen sensors, and acoustic Doppler current profilers) and goals for future research.

Caldwell, Douglas R.↗

NASA NTRS1987

Waveform analysis for Geosat day 96

A detailed model waveform function has been least-squares-fitted to 10-second averages of waveform sampler data from selected over-ocean portions of day 96 of the Geosat mission. The results confirm that the height corrections already applied in the routine Geosat data processing are generally good to well within 10 centimeters. Additional height corrections are provided that can refine height estimates to within the several-centimeter level.

Hayne, George S.↗

NASA NTRS1988

Adaptive Bandwidth Compression For Moving Images

Data compressed in images with fast motion and expanded when motion slows. Proposed data-compression scheme adapts transmission rate for moving image to speed of motion of image. Laser rangefinder measures speed of target. Adaptive sampler determines required resolution of target image on basis of measured speed and provides decommutator at control station with more or fewer partial image fields per unit time. When monitor displays maximum number of fields simultaneously, resolution is highest.

Graham, Olin L.↗

NASA NTRS1987

Elemental composition of the Martian surface

The Viking landers carried X-ray fluorescence spectrometers (XRFS) to the surface of Mars for direct in-situ measurements of the elemental composition of samples that could be acquired by the surface sampler scoops. A total of 21 samples at two landing sites were acquired and analyzed. In this review, the constraints imposed on the design and operation of the XRFS; the accuracy and precision limitations of XRF; and the interpretation of the results are discussed. The latter includes relevance of the investigation to Martian geological processes, SNC meteorites, the formation and history of Mars, and future mission objectives/capabilities.

Clark, Benton C.↗