SEARCH · Engineering Papers

Results for “Relative accuracy”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 217 records · Page 12

All sources NASA NTRS DOE OSTI

NASA NTRS1979

Timing and frequency considerations in the worldwide testing of a spread spectrum communication system

During 1978 and 1979, an Air Force C-135 test aircraft was flown to various locations in the North and South Atlantic and Pacific Oceans for satellite communications experiments. A part of the equipment tested on the aircraft was the SEACOM spread spectrum modem. The SEACOM modem operated at X band frequency from the aircraft via the DSCS II satellite to a ground station. For data to be phased successfully, it was necessary to maintain independent time and frequency accuracy over relatively long periods of time (up to two weeks) on the aircraft and at the ground station. To achieve this goal, two Efratom atomic frequency standards were used. The performance of these frequency standards as used in the spread spectrum modem is discussed, including the effects of high relative velocity, synchronization and the effects of the frequency standards on data performance is discussed. The aircraft environment, which includes extremes of temperature, as well as long periods of shutdown followed by rapid warmup requirements, is also discussed.

Woodring, D. G.↗

NASA NTRS1980

A modified T-value method for selection of strain gages for measuring loads on a low aspect ratio wing

A technique which may be useful for selecting strain gages for use in load equations is described. The technique is an adaptation of the previously used T-value method and is applied to a multispar structure. The technique, called the modified T-value method, is used to reduce the number of strain gages used in a load equation from twelve to two. A parallel reduction is made by calculating relative equation accuracies from three applied load distributions. The equations developed from the modified T-value method proved to be accurate more consistently than the T-value method.

Tang, M. H.↗

NASA NTRS1981

Relative performance of algorithms for autonomous satellite orbit determination

Limited word size in contemporary microprocessors causes numerical problems in autonomous satellite navigation applications. Numerical error introduced in navigation computations performed on small wordlength machines can cause divergence of sequential estimation algorithms. To insure filter reliability, square root algorithms have been adopted in many applications. The optimal navigation algorithm requires a careful match of the estimation algorithm, dynamic model, and numerical integrator. In this investigation, the relationship of several square root filters and numerical integration methods is evaluated to determine their relative performance for satellite navigation applications. The numerical simulations are conducted using the Phase I GPS constellation to determine the orbit of a LANDSAT-D type satellite. The primary comparison is based on computation time and relative estimation accuracy.

Tapley, B. D.↗

NASA NTRS1981

A new method for the rapid calculation of infrared transmittances of atmospheric gases

A new procedure is outlined for the rapid calculation of transmittances of atmospheric gases in the infrared. A feature of the method is the relatively high accuracy (better than 1%) and the fact that any line shape can be handled. The highest gain in speed over direct line-by-line integration is in the low pressure region (Voigt or Doppler profiles).

Oinas, V.↗

NASA NTRS1982

Fundamentals of absolute pyroheliometry and objective characterization

The radiometric methodology in use with a narrow field of view radiometer for observation of the solar constant is described. The radiation output of the Sun is assumed to be constant, enabling the monitoring of the solar source by an accurately pointed radiometer, and the Sun's output is measured as a function of time. The instrument is described, its angular response considered, and principles for absolute radiometric measurement presented. Active modes of operation are analyzed, taking into consideration instrumental perturbations and sensor efficiency, heating wire effect, cavity sensor efficiency, thermal effects on the surface of the sensitive area, the effect of the field of view limiting system, and the frequency response of the heat flux detector and absolute radiometric system. Performance of absolute measurements with relatively high accuracy is demonstrated.

Crommelynck, D. A.↗

NASA NTRS1983

Coordinate systems

Jovian coordinate systems are different from those employed in the case of the earth. Latitude and longitude coordinates are usually established relative to some solid surface. Because Jupiter does not have a solid surface (at least none which is visible through the clouds), arbitrary, but convenient, coordinate grids have been prescribed. A spin equator is made out from observations of cloud motion, and the direction of the planetary spin axis is, therefore, determined with relatively good accuracy. The problem in establishing a Jupiter longitude system is that the mean rotation period of the clouds is a function of latitude. The solution selected was to define two separate longitude grids. A third longitude system became necessary with the detection of radio signals which gave evidence for a rotating planetary magnetic field. Attention is also given to orbital phase angle and longitude conventions for satellites, and two latitude systems for Jupiter

Dessler, A. J.↗

NASA NTRS1982

Infrared array detectors

Arrays of detectors sensitive to infrared radiation will enable astronomical observations to be made with shorter observing times than with discrete detectors and with good relative spatial accuracy. Systems using such arrays are being developed for astronomy in several regions of the electromagnetic spectrum. An example of an infrared system is given here consisting of a 32x32 element bismuth doped silicon charge injection device array that has been used in an astronomical camera.

Arens, J. F.↗

NASA NTRS1984

RTMPL: A structured programming and documentation utility for real-time multiprocessor simulations

The NASA Lewis Research Center is developing and evaluating experimental hardware and software systems to help meet future needs for real time simulations of air-breathing propulsion systems. The Real Time Multiprocessor Simulator (RTMPS) project is aimed at developing a prototype simulator system that uses multiple microprocessors to achieve the desired computing speed and accuracy at relatively low cost. Software utilities are being developed to provide engineering-level programming and interactive operation of the simulator. Two major software development efforts were undertaken in the RTMPS project. A real time multiprocessor operating system was developed to provide for interactive operation of the simulator. The second effort was aimed at developing a structured, high-level, engineering-oriented programming language and translator that would facilitate the programming of the simulator. The Real Time Multiprocessor Programming Language (RTMPL) allows the user to describe simulation tasks for each processor in a straight-forward, structured manner. The RTMPL utility acts as an assembly language programmer, translating the high-level simulation description into time-efficient assembly language code for the processors. The utility sets up all of the interfaces between the simulator hardware, firmware, and operating system.

Arpasi, D. J.↗

NASA NTRS1984

Calibration of diode laser spectra using a confocal etalon

The dual-beam diode laser spectrometer described by Jennings (1980) is adapted to use a 50-cm confocal etalon for frequency calibration. The collimated radiation from the laser is split at a wedged ZnSe window, and the reference beam is then focused at the midpoint of the etalon length. After the etalon, the reference beam is recollimated and continues its regular path to the monochromator and detectors. An aperture is placed before the etalon in order to limit the entrance beam diameter to approximately 5 mm. Both ends of the etalon are furnished with two-axis adjustments. Initial alignment is achieved using an He-Ne laser, and final optimization involves adjustment of the cavity length as well as the etalon pitch and yaw. The 50-cm confocal etalon produces fringes separated by 150 MHz (0.005/cm). With the aid of a CO2 laser, it is found to have fringe widths (FWHM) of 2 MHz. The confocal etalon makes it possible to improve the accuracy of relative frequency measurements in diode laser spectra and to check the spectral purity and stability of the laser during the recording of spectra.

Jennings, D. E.↗

NASA NTRS1984

Airborne laser topographic mapping results

The results of terrain mapping experiments utilizing the National Aeronautics and Space Administration (NASA) Airborne Oceanographic Lidar (AOL) over forested areas are presented. The flight tests were conducted as part of a joint NASA/U.S. Army Corps of Engineers (CE) investigation aimed at evaluating the potential of an airborne laser ranging system to provide cross-sectional topographic data on flood plains that are difficult and expensive to survey using conventional techniques. The data described in this paper were obtained in the Wolf River Basin located near Memphis, TN. Results from surveys conducted under winter 'leaves off' and summer 'leaves on' conditions, aspects of day and night operation, and data obtained from decidous and coniferous tree types are compared. Data processing techniques are reviewed. Conclusions relative to accuracy and present limitations of the AOL, and airborne lidar systems in general, to terrain mapping over forested areas are discussed.

Krabill, W. B.↗

NASA NTRS1985

Operating system for a real-time multiprocessor propulsion system simulator. User's manual

The NASA Lewis Research Center is developing and evaluating experimental hardware and software systems to help meet future needs for real-time, high-fidelity simulations of air-breathing propulsion systems. Specifically, the real-time multiprocessor simulator project focuses on the use of multiple microprocessors to achieve the required computing speed and accuracy at relatively low cost. Operating systems for such hardware configurations are generally not available. A real time multiprocessor operating system (RTMPOS) that supports a variety of multiprocessor configurations was developed at Lewis. With some modification, RTMPOS can also support various microprocessors. RTMPOS, by means of menus and prompts, provides the user with a versatile, user-friendly environment for interactively loading, running, and obtaining results from a multiprocessor-based simulator. The menu functions are described and an example simulation session is included to demonstrate the steps required to go from the simulation loading phase to the execution phase.

Cole, G. L.↗

NASA NTRS1985

Planetary searches using optical astrometric interferometers

The detectability of planets is discussed, and observations made with an astrometric interferometer (AI) at Mount Wilson are presented. Planets are detectable as a result of their periodic perturbation of the position of a central observable star. This detection requires relative astrometric accuracies of about 3 x 10 to the -4th for Jovian planets and about 3 x 10 to the -7th arcsec for terrestrial planets. The principles governing astrometric interferometry are explained, and it is conjectured that a two-color optical AI on earth should be capable of about 0.0001 arcsec rms accuracy for stars brighter than magnitude 7-10; small one-color space systems should achieve at least 0.00001 arcsec. Owing to atmospheric turbulence, two-color systems permit correction every few milliseconds for the random wavefront tilt. A 3.4-meter baseline interferometer on Mount Wilson has demonstrated arcsec rms fluctuations of about 0.02 for 1-sec samples. Space-based optical astrometric interferometry is also discussed.

Staelin, D. H.↗

NASA NTRS1986

Prediction of sound fields in cavities using boundary element methods

Two boundary element formulations of acoustical behavior exist; the Direct Boundary Element Methods (DBEM) based on the Helmholtz Integral Equations and the Indirect Boundary Element Methods (IBEM) based on Huygen's Principle. In this investigation, both methods are implemented utilizing a simple linear superparametric element. In addition the IBEM is studied using a quadratic isoparametric element. The accuracy and relative efficiency of the various techniques are examined. In order to properly model aircraft interior cavities the additional capability to model wall treatments and internal point sources is added to the methods. The procedures are verified for several well-understood cavity problems. The relative merits of each boundary element method and the finite element method are examined.

Bernhard, R. J.↗

NASA NTRS1986

GPS-based geodesy in California, Mexico and the Caribbean

The NASA GPS-based geophysical geodetics system will be capable of 1-3 cm relative position accuracies on regional baselines and GPS ephemerides with submeter accuracies. Simultaneity and mutual visibility allow high differential carrier phase and group delay measurements to be obtained without clock errors. Both mobile GPS terminal sites and fiducial sites whose locations are accurately maintained by independent VLBI and SLR systems are to be used. A system validation and multiyear measurement program is under way.

Melbourne, W. G.↗

NASA NTRS1989

Long range geoid control through the European GPS traverse: Final results

The European north-south Global Positioning System (GPS)-traverse proposed by IAG SSG 3.88, should control and improve the European geoid. This traverse follows first order leveling lines, included in the United European Leveling Network. From May to August 1986 and in July 1987, the central and northern part of this traverse (approx. 3000 km) was observed using up to four TI 4100 receivers, covering Austria, Federal Republic of Germany, Denmark, Sweden and Norway. Both traverse parts contain 71 stations with distances of about 50 km. In addition, 8 stations have been occupied for overlapping connections, and traverse links were established for connecting the fundamental stations Wettzell (VLBI and SLR) and Onsala (VLBI). Final results show a GPS observation precision of a few cm for loops of some 100 km circumference. After transformation of the GPS results to geoid heights using the leveled heights, comparisons with different existing gravimetric geoid determinations including geopotential models were performed. In addition, new geopotential models complete to degree and order 360 tailored to gravity data in Europe, and gravimetric geoid solutions using 6 x 10' mean gravity anomalies were investigated. The comparison with GPS and leveling yields rms discrepancies of + or - 0.1...0.2 m over 1000 km traverse sections for the best solutions, but a strong slope is existing in Sweden and southern Norway in almost all solutions, which is probably caused by systematic errors in the available gravity data for Scandinavia. This is confirmed by a new geoid computation at the Danish Geodetic Institute where the slope has disappeared. If this new solution is taken for the northern traverse section and the best solution for the central part, the rms discrepancy reduces to approximately + or - 0.2 m over 3000 km. Thus, a + or - 10 (exp 7) relative height accuracy seems to be achievable over long distances with the GPS/leveling and the gravimetric geoid calculation techniques, applied in this experiment.

Torge, W.↗

NASA NTRS1990

Precipitable water estimation from high-resolution split window radiance measurements

A technique that uses the spatial variance of image brightness temperature to derive total column precipitable water is applied to high-resolution multispectral aircraft scanner data for the June 19, 1986 COHMEX day. The technique has several advantages over other approaches in that it requires only relative calibration accuracy, is less susceptible to instrument error, and does not directly use a priori information. Results indicate significant horizontal variability of precipitable water at the mesoscale. Precipitable water gradients of 6 mm per 10 km are not uncommon. The results verify well against special rawinsonde measurements and the ensuing cloud field development. While only applied to this specialized aircraft data, the applicability of the technique to operational AVHRR and VAS data is discussed.

Jedlovec, Gary J.↗

NASA NTRS1993

Parametric motion control of robotic arms: A biologically based approach using neural networks

A neural network based system is presented which is able to generate point-to-point movements of robotic manipulators. The foundation of this approach is the use of prototypical control torque signals which are defined by a set of parameters. The parameter set is used for scaling and shaping of these prototypical torque signals to effect a desired outcome of the system. This approach is based on neurophysiological findings that the central nervous system stores generalized cognitive representations of movements called synergies, schemas, or motor programs. It has been proposed that these motor programs may be stored as torque-time functions in central pattern generators which can be scaled with appropriate time and magnitude parameters. The central pattern generators use these parameters to generate stereotypical torque-time profiles, which are then sent to the joint actuators. Hence, only a small number of parameters need to be determined for each point-to-point movement instead of the entire torque-time trajectory. This same principle is implemented for controlling the joint torques of robotic manipulators where a neural network is used to identify the relationship between the task requirements and the torque parameters. Movements are specified by the initial robot position in joint coordinates and the desired final end-effector position in Cartesian coordinates. This information is provided to the neural network which calculates six torque parameters for a two-link system. The prototypical torque profiles (one per joint) are then scaled by those parameters. After appropriate training of the network, our parametric control design allowed the reproduction of a trained set of movements with relatively high accuracy, and the production of previously untrained movements with comparable accuracy. We conclude that our approach was successful in discriminating between trained movements and in generalizing to untrained movements.

Bock, O.↗

NASA NTRS1993

What determines the stellar mass functions in globular clusters?

We analyze the dependence of stellar mass function slopes for a sample of 17 globular clusters on a variety of cluster parameters. The principal novelty of our approach is the use of appropriate multivariate statistical methods to disentangle the complex situation which is present in this problem: the slopes depend simultaneously on more than one variable, and many cluster parameters are mutually correlated. We find that the mass function slopes in the range M/M(solar) = 0.5-0.8 are largely determined by the position in the Galaxy and to a lesser extent by the cluster metallicity. Clusters closer to the Galactic center or plane have shallower mass function slopes. At a given distance to the Galactic center, clusters closer to the Galactic plane have shallower mass function slopes. At a given R(GC) and/or Z(GP), more metal-rich clusters have shallower mass function slopes. Thus, the monovariate correlations with the position or metallicity are both correct, but only partial, and in terms of slopes, biased descriptions of the situation. We present trivariate least-squares solutions where the mass function slopes can be predicted within the measurement accuracy. This relation can serve as a powerful observational constraint for theories of globular cluster formation and evolution, and it is one of the tightest correlations between globular cluster properties now known.

Djorgovski, S.↗