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At least 1,297 records · Page 72

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NASA NTRS1986

A computational method to model radar return range in a polygonally based, computer-generated-imagery simulation

Described is a method for modeling a ground-mapping radar system for use in simulations where the terrain is in a polygonal form commonly used with computer generated imagery (CGI). The method employs a unique approach for rapidly rejecting polygons not visible to the radar to facilitate the real-time simulation of the radar return. This rapid rejection of the nonvisible polygons requires the precalculation and storage of a set of parameters that do not vary during the simulation. The calculation of a radar range as a function of the radar forward-looking angle to the CGI terrain is carried out only for the visible polygons. This method was used as part of a simulation for terrain-following helicopter operations on the vertical motion simulator at the NASA Ames Research Center. It proved to be an efficient means for returning real-time simulated radar range data.

Moran, F. J.↗

NASA NTRS1986

A Cassegrain reflector system for compact range applications

An integral part of a compact range is the means of providing a uniform plane wave. A Cassegrain reflector system is one alternative for achieving this goal. Theoretically, this system offers better performance than a simple reflector system. The longer pathlengths in the Cassegrain system lead to a more uniform field in the plane of interest. The addition of the subreflector creates several problems, though. System complexity is increased both in terms of construction and performance analysis. The subreflector also leads to aperture blockage and the orientation of the feed now results in spillover illuminating the target areas as well as the rest of the range. Finally, the addition of the subreflector leads to interaction between the two reflectors resulting in undesired field variations in the plane of interest. These difficulties are addressed and through the concept of blending the surfaces, a Cassegrain reflector system is developed that will provide a uniform plane wave that offers superior performance over large target areas for a given size reflector system. Design and analysis is implemented by considering the main reflector and subreflector separately. Then the system may be put together and the final design and system analysis completed.

Rader, Mark D.↗

NASA NTRS1987

Calibration of the EGRET high-energy gamma-ray telescope in the range 20-10,000 MeV with a tunable beam of quasi-monoenergetic gamma rays at SLAC

The EGRET telescope, one of the instruments to be carried on NASA's Gamma Ray Observatory, is designed to explore the high-energy portion (20-24,000 MeV) of the electromagnetic spectrum. A calibration source of gamma-rays over this wide energy range has been used together with an appropriate set of intensity-monitoring instruments, to conduct a calibration of the EGRET telescope over this energy range for the full telescope aperture. The calibration method and an overview of the results are described.

Thompson, D. J.↗

NASA NTRS1986

The coordinate frame of the lunar laser ranging network

The geocentric coordinates for four instruments, which were derived using lunar laser ranging, are compared with the 84L02 coordinates determined from the Lageos satellite. The determination of the geocentric coordinates for the 2.7 m and McDonald Observatory laser ranging system telescopes at McDonald Observatory, the Haleakala site, and the CERGA site near Grasse, France is described. Consideration is given to the McDonald Observatory colocation and station motion due to continential drift. A rms difference of 18 cm is determined for the two sets of geocentric coordinates; however, removing a data anomaly reduces the rms difference to 13 cm.

Williams, J. G.↗

NASA NTRS1987

An improved main reflector design for compact range applications

Compact ranges offer many advantages over other types of ranges, and as a result much effort is being directed toward the improvement of their performance. The use of concave edge contours and blended rolled edge terminations to reduce the unwanted energy diffracted into the target zone from the termination of the main reflector is discussed. The proposed shaping of the edge contour minimizes the diffracted fields by virtue of reducing the spread factor; whereas, the blended rolled edge terminations reduce the diffracted fields by creating a smooth transition in the reflected field. Two design examples are treated in order to illustrate these concepts.

Pistorius, Carl W. I.↗

NASA NTRS1987

Evaluation of a scale-model experiment to investigate long-range acoustic propagation

Tests were conducted to evaluate the feasibility of using a scale-model experiment situated in an anechoic facility to investigate long-range sound propagation over ground terrain. For a nominal scale factor of 100:1, attenuations along a linear array of six microphones colinear with a continuous-wave type of sound source were measured over a wavelength range from 10 to 160 for a nominal test frequency of 10 kHz. Most tests were made for a hard model surface (plywood), but limited tests were also made for a soft model surface (plywood with felt). For grazing-incidence propagation over the hard surface, measured and predicted attenuation trends were consistent for microphone locations out to between 40 and 80 wavelengths. Beyond 80 wavelengths, significant variability was observed that was caused by disturbances in the propagation medium. Also, there was evidence of extraneous propagation-path contributions to data irregularities at more remote microphones. Sensitivity studies for the hard-surface and microphone indicated a 2.5 dB change in the relative excess attenuation for a systematic error in source and microphone elevations on the order of 1 mm. For the soft-surface model, no comparable sensitivity was found.

Parrott, Tony L.↗

NASA NTRS1987

Evaluation of a satellite laser ranging technique using pseudonoise code modulated laser diodes

Several types of Satellite Laser Ranging systems exist, operating with pulsed, high-energy lasers. The distance between a ground point and an orbiting satellite can be determined to within a few centimeters. A new technique substitutes pseudonoise code modulated laser diodes, which are much more compact, reliable and less costly, for the lasers now used. Since laser diode technology is only now achieving sufficiently powerful lasers, the capabilities of the new technique are investigated. Also examined are the effects of using an avalanche photodiode detector instead of a photomultiplier tube. The influence of noise terms (including background radiation, detector dark and thermal noise and speckle) that limit the system range and performance is evaluated.

Ball, Carolyn Kay↗

NASA NTRS1987

Design and implementation of the ESL compact range underhung bridge crane

As the indoor compact range technology has continued to increase, the need to handle larger and heavier targets has also increased. This need for target lifting and handling prompted the feasibility study of the use of an underhung bridge crane to be installed in the ESL (ElectroScience Laboratory, Ohio State University) compact range. This report documents both the design of the underhung bridge crane that was installed and the implementation of the design in the actual installation of the crane.

Shamansky, H. T.↗

NASA NTRS1987

Estimation of geopotential from satellite-to-satellite range rate data: Numerical results

A technique for high-resolution geopotential field estimation by recovering the harmonic coefficients from satellite-to-satellite range rate data is presented and tested against both a controlled analytical simulation of a one-day satellite mission (maximum degree and order 8) and then against a Cowell method simulation of a 32-day mission (maximum degree and order 180). Innovations include: (1) a new frequency-domain observation equation based on kinetic energy perturbations which avoids much of the complication of the usual Keplerian element perturbation approaches; (2) a new method for computing the normalized inclination functions which unlike previous methods is both efficient and numerically stable even for large harmonic degrees and orders; (3) the application of a mass storage FFT to the entire mission range rate history; (4) the exploitation of newly discovered symmetries in the block diagonal observation matrix which reduce each block to the product of (a) a real diagonal matrix factor, (b) a real trapezoidal factor with half the number of rows as before, and (c) a complex diagonal factor; (5) a block-by-block least-squares solution of the observation equation by means of a custom-designed Givens orthogonal rotation method which is both numerically stable and tailored to the trapezoidal matrix structure for fast execution.

Thobe, Glenn E.↗

NASA NTRS1988

A diagnostic technique used to obtain cross range radiation centers from antenna patterns

A diagnostic technique to obtain cross range radiation centers based on antenna radiation patterns is presented. This method is similar to the synthetic aperture processing of scattered fields in the radar application. Coherent processing of the radiated fields is used to determine the various radiation centers associated with the far-zone pattern of an antenna for a given radiation direction. This technique can be used to identify an unexpected radiation center that creates an undesired effect in a pattern; on the other hand, it can improve a numerical simulation of the pattern by identifying other significant mechanisms. Cross range results for two 8' reflector antennas are presented to illustrate as well as validate that technique.

Lee, T. H.↗

NASA NTRS1988

Development and applications of optical interferometric micrometrology in the angstrom and subangstrom range

The recent development of the scanning electron tunneling microscope and the atomic force microscope requires absolute standards for measurements in the angstrom and subangstrom range. Optical interferometry with lasers and multiple mode laser resonances can provide absolute measurements as the laser wavelengths are very accurately known. A key feature of such measurements is the use of piezoelectric crystals as translators of the highest accuracy for very small disturbances. However, the dimensional changes of these crystals resulting from electrical potential changes depend on many variables, among them the method of mounting, so that accurate calibrations are necessary. Starting from advances in optical metrology made by physicists trying to find gravity waves, advances which led to measurements down to 10 to the -5 A, the author designed and built a much simpler system for the angstrom range. The major limiting factors were mechanical vibrations, air currents, thermal changes and laser instabilities.

Lauer, James L.↗

NASA NTRS1988

Laser ranging data analysis

Near real-time Lageos laser ranging data are analyzed in terms of range bias, time bias, and internal precision, and estimates for earth orientation parameters X(sub p), Y(sub p), and UT1 are obtained. The results of these analyses are reported in a variety of formats. Copies of monthly summaries from November, 1986 through November, 1987 are included.

Source record↗

NASA NTRS1988

Long-range exploration by humans on the surface of Mars

A major objective of missions to Mars will be to conduct reconnaissance and to explore the surface. Systems that will allow astronauts to operate freely in the Martian environment are discussed. Special consideration is given to the design of manned rovers and to the safety and science issues involved. To conduct a wide-ranging exploration, a sortie time of at least 7 Martian days and an adequate fuel/power source are needed for at least a 100-km range. The concept of a hybrid rover is proposed, that will fulfill these requirements. In the Hybrid Rover, the driver of the vehicle will transport a space-suit-wearing astronaut to the area of interest, where the 'suitman' will demount and conduct typical field geological exploration, while the driver will continue to reconnoiter the area, and/or conduct independent sampling sorties using manipulator arms. Attention is also given to the design of a life support system. For fulfilling the power requirements of transportation and a power-intensive closed-cycle life supporting system, nuclear power is being considered as one of the possible power sources.

Clark, Benton C.↗

NASA NTRS1988

Reaction rate constants of HO2 + O3 in the temperature range 233-400 K

The reaction rate constants of HO2 + O3 were measured in the temperature range 233-400 K using a discharge flow system with photofragment emission detection. In the range 233-253 K, the constants are approximately a constant value, and then increase with increasing temperature. This result suggests that the reaction may have two different channels. An expression representing the reaction rate constants is presented.

Wang, Xiuyan↗

NASA NTRS1987

Predictability in the extended range

This paper describes the results of extended range predictability experiments using an efficient two-level spherical quasi-geostrophic model. The experiments have an initial rms doubling time of about two days. This growth rate, along with an initial error of about one-half the initial error of present operational models, produces an rms error equal to the climatological rms error and a correlation of 0.5 on about day 12 of the forecast. On the largest scales, this limiting point is reached shortly thereafter. The error continues to grow at a decreasing rate until at about 30 days the forecast skill is extremely small and comparable to the skill of a persistence forecast. Various time averages at various lags are examined for skill in the extended range. Filters that weighted most strongly in the initial forecast days provide increased skill.

Roads, John O.↗

NASA NTRS1988

An improved long range potential for O(1D)+H2

Several dynamics studies have indicated that the product isotopic distributions and vibrational energy distributions in the reaction O(1D) + H2(HD) are very sensitive to the long range interaction of the reactants. In this paper, an improved calculation of the reactant region of the potential energy surface is reported. In agreement with previous work, no barrier is found to edge-on insertion (1 A-prime ground state water surface), but a smaller (less than 0.2 kcal/mol) barrier to collinear addition, 1 Sigma (+) surface, is found than in previous work. The long range potential obtained in the present work most closely resembles the SL3 surface.

Walch, Stephen P.↗

NASA NTRS1989

Range Filtering For Navigation By Satellite

Less processing and storage of data needed. Mathematical basis developed for Kalman filtering of sequential measurements of range from single-channel receiver in Global Positioning System (GPS) to each of several navigation satellites. Range filtering involves separate applications of kinematic Kalman tracking filter to measurements of signal from each satellite in use. State vector consists of pseudorange and time derivatives. Main advantages: simplification of tuning and decreases in required amounts of storage and processing of data for navigation in ships, airplanes, and ground vehicles.

Paielli, Russell↗

NASA NTRS1989

Tau Ranging Revisited

Report reviews history of tau ranging and advocates use of advanced electronic circuitry to revive this composite-code-uplink spacecraft-ranging technique. Very-large-scale integration gives new life to abandoned distance-measuring technique.

Tausworthe, Robert C.↗