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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Spacecraft preliminary orbit determination using tracking measurements obtained from the Tracking and Data Relay Satellite System (TDRSS) and the Ground Spaceflight Tracking and Data Network (GSTDN)

In order to validate the operational and computational capabilities of the Preliminary Orbit Determination System (PODS), tests were performed using tracking measurements for several systems including the ERB satellite, the SMM, the STS and Landsat-4. POD procedures are utilized to generate a state vector following an unplanned orbital perturbation or spacecraft maneuver, when an estimation process such as a differential correction orbit determination cannot obtain a solution. Results are presented to demonstrate POD for several situations involving different qualities of a priori target state vectors, data type combinations, data arc lengths, and mixtures of single-TDRS, dual-TDRS, and GSTDN measurements. The system's ability to determine accurately the state vector for the spacecraft and the effectiveness of the solution screening process are discussed. It is shown that PODS is capable of determining a spacecraft vector when differential correction orbit determination processes fail.

Kirschner, S. M.↗
NASA NTRS1990

Automation of orbit determination functions for National Aeronautics and Space Administration (NASA)-supported satellite missions

The Flight Dynamics Facility (FDF) at Goddard Space Flight Center (GSFC) provides spacecraft trajectory determination for a wide variety of National Aeronautics and Space Administration (NASA)-supported satellite missions, using the Tracking Data Relay Satellite System (TDRSS) and Ground Spaceflight and Tracking Data Network (GSTDN). To take advantage of computerized decision making processes that can be used in spacecraft navigation, the Orbit Determination Automation System (ODAS) was designed, developed, and implemented as a prototype system to automate orbit determination (OD) and orbit quality assurance (QA) functions performed by orbit operations. Based on a machine-resident generic schedule and predetermined mission-dependent QA criteria, ODAS autonomously activates an interface with the existing trajectory determination system using a batch least-squares differential correction algorithm to perform the basic OD functions. The computational parameters determined during the OD are processed to make computerized decisions regarding QA, and a controlled recovery process is activated when the criteria are not satisfied. The complete cycle is autonomous and continuous. ODAS was extensively tested for performance under conditions resembling actual operational conditions and found to be effective and reliable for extended autonomous OD. Details of the system structure and function are discussed, and test results are presented.

Mardirossian, H.↗
NASA NTRS1991

Study of geopotential error models used in orbit determination error analysis

The uncertainty in the geopotential model is currently one of the major error sources in the orbit determination of low-altitude Earth-orbiting spacecraft. The results of an investigation of different geopotential error models and modeling approaches currently used for operational orbit error analysis support at the Goddard Space Flight Center (GSFC) are presented, with emphasis placed on sequential orbit error analysis using a Kalman filtering algorithm. Several geopotential models, known as the Goddard Earth Models (GEMs), were developed and used at GSFC for orbit determination. The errors in the geopotential models arise from the truncation errors that result from the omission of higher order terms (omission errors) and the errors in the spherical harmonic coefficients themselves (commission errors). At GSFC, two error modeling approaches were operationally used to analyze the effects of geopotential uncertainties on the accuracy of spacecraft orbit determination - the lumped error modeling and uncorrelated error modeling. The lumped error modeling approach computes the orbit determination errors on the basis of either the calibrated standard deviations of a geopotential model's coefficients or the weighted difference between two independently derived geopotential models. The uncorrelated error modeling approach treats the errors in the individual spherical harmonic components as uncorrelated error sources and computes the aggregate effect using a combination of individual coefficient effects. This study assesses the reasonableness of the two error modeling approaches in terms of global error distribution characteristics and orbit error analysis results. Specifically, this study presents the global distribution of geopotential acceleration errors for several gravity error models and assesses the orbit determination errors resulting from these error models for three types of spacecraft - the Gamma Ray Observatory, the Ocean Topography Experiment, and the Cosmic Background Explorer.

Yee, C.↗
NASA NTRS1992

Carbon and nitrogen abundances determined from transition layer lines

The possibility of determining relative carbon, nitrogen, and silicon abundances from the emission-line fluxes in the lower transition layers between stellar chromospheres and coronae is explored. Observations for main-sequence and luminosity class IV stars with presumably solar element abundances show that for the lower transition layers Em = BT sup -gamma. For a given carbon abundance the constants gamma and B in this relation can be determined from the C II and C IV emission-line fluxes. From the N V and S IV lines, the abundances of these elements relative to carbon can be determined from their surface emission-line fluxes. Ratios of N/C abundances determined in this way for some giants and supergiants agree within the limits of errors with those determined from molecular bands. For giants, an increase in the ratio of N/C at B-V of about 0.8 is found, as expected theoretically.

Boehm-Vitense, Erika↗
NASA NTRS1994

Method of determining load in anisotropic non-crystalline materials using energy flux deviation

An ultrasonic wave is applied to an anisotropic sample material in an initial direction and the intensity of the ultrasonic wave is measured on an opposite surface of the sample material by two adjacent receiving points located in an array of receiving points. A ratio is determined between the measured intensities of two adjacent receiving points, the ratio being indicative of an angle of flux deviation from the initial direction caused by an unknown applied load. This determined ratio is then compared to a plurality of ratios of a similarly tested, similar anisotropic reference material under a plurality of respective, known load conditions, whereby the load applied to the particular anisotropic sample material is determined. A related method is disclosed for determining the fiber orientation from known loads and a determined flux shift.

Prosser, William H.↗
NASA NTRS1994

TDRS orbit determination by radio interferometry

In support of a NASA study on the application of radio interferometry to satellite orbit determination, MITRE developed a simulation tool for assessing interferometry tracking accuracy. The Orbit Determination Accuracy Estimator (ODAE) models the general batch maximum likelihood orbit determination algorithms of the Goddard Trajectory Determination System (GTDS) with the group and phase delay measurements from radio interferometry. ODAE models the statistical properties of tracking error sources, including inherent observable imprecision, atmospheric delays, clock offsets, station location uncertainty, and measurement biases, and through Monte Carlo simulation, ODAE calculates the statistical properties of errors in the predicted satellites state vector. This paper presents results from ODAE application to orbit determination of the Tracking and Data Relay Satellite (TDRS) by radio interferometry. Conclusions about optimal ground station locations for interferometric tracking of TDRS are presented, along with a discussion of operational advantages of radio interferometry.

Pavloff, Michael S.↗
NASA NTRS1994

A method to determine the kinematics of the lower limbs of a subject pedaling a bicycle using encoders and accelerometers

The goal of this research was to determine kinematic parameters of the lower limbs of a subject pedaling a bicycle. An existing measurement system was used as the basis to develop the model to determine position and acceleration of the limbs. The system consists of an ergometer instrumented to provide position of the pedal (foot), accelerometers to be attached to the lower limbs to measure accelerations, a recorder used for filtering, and a computer instrumented with an A/D board and a decoder board. The system is designed to read and record data from accelerometers and encoders. Software has been developed for data collection, analysis and presentation. Based on the measurement system, a two dimensional analytical model has been developed to determine configuration (position, orientation) and kinematics (velocities, accelerations). The model has been implemented in software and verified by simulation. An error analysis to determine the system's accuracy shows that the expected error is well within the specifications of practical applications. When the physical hardware is completed, NASA researchers hope to use the system developed to determine forces exerted by muscles and forces at articulations. This data will be useful in the development of countermeasures to minimize bone loss experienced by astronauts in microgravity conditions.

Liu, Shih-Ching↗
NASA NTRS1995

Evaluation and modeling of autonomous attitude thrust control for the Geostation Operational Environmental Satellite (GOES)-8 orbit determination

The Geostationary Operational Environmental Satellite (GOES)-8 experienced a series of orbital perturbations from autonomous attitude control thrusting before perigee raising maneuvers. These perturbations influenced differential correction orbital state solutions determined by the Goddard Space Flight Center (GSFC) Goddard Trajectory Determination System (GTDS). The maneuvers induced significant variations in the converged state vector for solutions using increasingly longer tracking data spans. These solutions were used for planning perigee maneuvers as well as initial estimates for orbit solutions used to evaluate the effectiveness of the perigee raising maneuvers. This paper discusses models for the incorporation of attitude thrust effects into the orbit determination process. Results from definitive attitude solutions are modeled as impulsive thrusts in orbit determination solutions created for GOES-8 mission support. Due to the attitude orientation of GOES-8, analysis results are presented that attempt to absorb the effects of attitude thrusting by including a solution for the coefficient of reflectivity, C(R). Models to represent the attitude maneuvers are tested against orbit determination solutions generated during real-time support of the GOES-8 mission. The modeling techniques discussed in this investigation offer benefits to the remaining missions in the GOES NEXT series. Similar missions with large autonomous attitude control thrusting, such as the Solar and Heliospheric Observatory (SOHO) spacecraft and the INTELSAT series, may also benefit from these results.

Forcey, W.↗
NASA NTRS1995

Accurate orbit determination strategies for the tracking and data relay satellites

The National Aeronautics and Space Administration (NASA) has developed the Tracking and Data Relay Satellite (TDRS) System (TDRSS) for tracking and communications support of low Earth-orbiting satellites. TDRSS has the operational capability of providing 85% coverage for TDRSS-user spacecraft. TDRSS currently consists of five geosynchronous spacecraft and the White Sands Complex (WSC) at White Sands, New Mexico. The Bilateration Ranging Transponder System (BRTS) provides range and Doppler measurements for each TDRS. The ground-based BRTS transponders are tracked as if they were TDRSS-user spacecraft. Since the positions of the BRTS transponders are known, their radiometric tracking measurements can be used to provide a well-determined ephemeris for the TDRS spacecraft. For high-accuracy orbit determination of a TDRSS user, such as the Ocean Topography Experiment (TOPEX)/Poseidon spacecraft, high-accuracy TDRS orbits are required. This paper reports on successive refinements in improved techniques and procedures leading to more accurate TDRS orbit determination strategies using the Goddard Trajectory Determination System (GTDS). These strategies range from the standard operational solution using only the BRTS tracking measurements to a sophisticated iterative process involving several successive simultaneous solutions for multiple TDRSs and a TDRSS-user spacecraft. Results are presented for GTDS-generated TDRS ephemerides produced in simultaneous solutions with the TOPEX/Poseidon spacecraft. Strategies with different user spacecraft, as well as schemes for recovering accurate TDRS orbits following a TDRS maneuver, are also presented. In addition, a comprehensive assessment and evaluation of alternative strategies for TDRS orbit determination, excluding BRTS tracking measurements, are presented.

Oza, D. H.↗
NASA NTRS1994

UVCS/SOHO capability for determining coronal conditions before, during, and after CME's

The ultraviolet coronal spectrometer (UVCS)/SOHO instrument will provide spectroscopic determinations of plasma parameters describing the evolution of coronal conditions at the site of coronal mass ejections (CME's). Prior to a CME event, UVCS can provide an empirical model of the coronal region where the event originates. The model would include values for the proton density, temperature, and outflow velocity; the electron density and temperature; and minor ion densities, temperatures, and outflow velocities. This information would be determined for heliocentric heights from about 1.5 to 3.5 of the solar radius with a subset of the parameters determined up to heights of 12 of the solar radius. During the event, UVCS would repeatedly sample one or more heights between 1.5 and 3.5 of the solar radius with a time resolution of about 1 min. The goal would be to determine the mass and energy content of the CME as it moves outward. After the event, the conditions in the region would be determined as before. Information about the instrument optical specifications and sensitivity is provided.

Strachan, Leonard↗
NASA NTRS1994

Precision orbit determination for TOPEX/POSEIDON

The TOPEX/POSEIDON mission objective requires that the radial position of the spacecraft be determined with an accuracy better than 13 cm RMS (root mean square). This stringent requirement is an order of magnitude below the accuracy achieved for any altimeter mission prior to the definition of the TOPEX/POSEIDON mission. To satislfy this objective, the TOPEX Precision Orbit determination (POD) Team was established as a joint effort between the NASA Goddard Space Flight Center and the University of Texas at Austin, with collaboration from the University of Colorado and the Jet Propulsion Laboratory. During the prelaunch development and the post launch verification phases, the POD team improved, calibrated, and validated the precision orbit determination computer software systems. The accomplishments include (1) increased accuracy of the gravity and surface force models and (2) improved peformance of both laser ranging and Doppler tracking systems. The result of these efforts led to orbit accuracies for TOPEX/POSEIDON which are significantly better than the original mission requirement. Tests based on data fits, covariance analysis, and orbit comparisons indicate that the radial component of the TOPEX/POSEIDON spacecraft is determined, relative to the Earth's mass center, with an root mean square (RMS) error in the range of 3 to 4 cm RMS. This orbit accuracy, together with the near continuous dual-frequency altimetry from this mission, provides the means to determine the ocean's dynamic topography with an unprecedented accuracy.

Tapley, B. D.↗
NASA NTRS1995

A new method for determining the CO to H2 conversion factor for translucent clouds

In this paper we describe a new technique for obtaining the conversion factor between the molecular hydrogen column density and the CO(J = 1-0) integrated antenna temperature. This factor, typically known as X(sub CO) is often to be of order a few times 10(exp 20)/sq cm/K km/s) for the molecular clouds in the Galaxy and is one of the primary means of determining the molecular cloud mass from CO observations. However, for the low-extinction interstellar clouds known as the translucent molecular clouds, estimates of X(sub CO) vary by up to a factor of 60 depending on the object and techniques employed to calibrate X(sub CO). Since the cloud mass is directly proportional to X(sub CO) uncertainties in mass estimates of translucent clouds can be more than an order of magnitude. We calibrate the H2 content in translucent clouds by using the linear relationship between the CH and H2 column densities. The CH column density is readily determined from observations of the CH ground-state hyperfine main-line transition at 3335 MHz. Using CH as a surrogate tracer for H2 and CO(J = 1-0) observations of a sample of translucent and dark molecular clouds, we find a wide variation in values for X(sub CO). For translucent clouds, X(sub CO) ranges from 0.3 to 6.8 x 10(exp 20) and for dark clouds the values range from 0.8 to 8.6. Although the average values for both types of cloud are similar to the canonical value determined for the Galactic molecular cloud ensemble (2-4 x 10(exp 20)), the scatter in individual X(sub CO) values may indicate that X(sub CO) for a given translucent cloud cannot be determined a priori and must be obtained for each cloud so that a reliable mass determination may be made.

Magnani, L.↗
NASA NTRS1994

Determination of the extragalactic-planetary frame tie from joint analysis of radio interferometric and lunar laser ranging measurements

Very Long Baseline Interferometry (VLBI) observations of extragalactic radio sources provide the basis for defining an accurate non-rotating reference frame in terms of angular positions of the sources. Measurements of the distance from the Earth to the Moon and to the inner planets provide the basis for defining an inertial planetary ephemeris reference frame. The relative orientation, or frame tie, between these two reference frames is of interest for combining Earth orientation measurements, for comparing Earth orientation results with theories referred to the mean equator and equinox, and for determining the positions of the planets with respect to the extragalactic reference frame. This work presents an indirect determination of the extragalactic-planetary frame tie from a combined reduction of VLBI and Lunar Laser Ranging (LLR) observations. For this determination, data acquired by LLR tracking stations since 1969 have been analyzed and combined with 14 years of VLBI data acquired by NASA's Deep Space Network since 1978. The frame tie derived from this joint analysis, with an accuracy of 0.003 sec, is the most accurate determination obtained so far. This result, combined with a determination of the mean ecliptic (defined in the rotating sense), shows that the mean equinox of epoch J2000 is offset from the x-axis of the extragalactic frame adopted by the International Earth Rotation Service for astrometric and geodetic applications by 0.078 sec +/- 0.010 sec along the y-direction and y 0.019 sec +/- 0.001 sec. along the z-direction.

Folkner, W. M.↗
NASA NTRS1995

Determination of tire cross-sectional geometric characteristics from a digitally scanned image

A semi-automated procedure is described for the accurate determination of geometrical characteristics using a scanned image of the tire cross-section. The procedure can be useful for cases when CAD drawings are not available or when a description of the actual cured tire is desired. Curves representing the perimeter of the tire cross-section are determined by an edge tracing scheme, and the plyline and cord-end positions are determined by locations of color intensities. The procedure provides an accurate description of the perimeter of the tire cross-section and the locations of plylines and cord-ends. The position, normals, and curvatures of the cross-sectional surface are included in this description. The locations of the plylines provide the necessary information for determining the ply thicknesses and relative position to a reference surface. Finally, the locations of the cord-ends provide a means to calculate the cord-ends per inch (epi). Menu driven software has been developed to facilitate the procedure using the commercial code, PV-Wave by Visual Numerics, Inc., to display the images. From a single user interface, separate modules are executed for image enhancement, curve fitting the edge trace of the cross-sectional perimeter, and determining the plyline and cord-end locations. The code can run on SUN or SGI workstations and requires the use of a mouse to specify options or identify items on the scanned image.

Danielson, Kent T.↗
NASA NTRS1996

Determination of Elastic Moduli of Fiber-Resin Composites Using an Impulse Excitation Technique

The elastic moduli of graphite/epoxy and graphite/cyanate ester composite specimens with various laminate lay-ups was determined using an impulse excitation/acoustic resonance technique and compared to those determined using traditional strain gauge and extensometer techniques. The stiffness results were also compared to those predicted from laminate theory using uniaxial properties. The specimen stiffnesses interrogated ranged from 12 to 30 Msi. The impulse excitation technique was found to be a relatively quick and accurate method for determining elastic moduli with minimal specimen preparation and no requirement for mechanical loading frames. The results of this investigation showed good correlation between the elastic modulus determined using the impulse excitation technique, strain gauge and extensometer techniques, and modulus predicted from laminate theory. The flexural stiffness determined using the impulse excitation was in good agreement with that predicted from laminate theory. The impulse excitation/acoustic resonance interrogation technique has potential as a quality control test.

Viens, Michael J.↗
NASA NTRS1996

Verification of Experimental Techniques for Flow Surface Determination

The concept of a yield surface is central to the mathematical formulation of a classical plasticity theory. However, at elevated temperatures, material response can be highly time-dependent, which is beyond the realm of classical plasticity. Viscoplastic theories have been developed for just such conditions. In viscoplastic theories, the flow law is given in terms of inelastic strain rate rather than the inelastic strain increment used in time-independent plasticity. Thus, surfaces of constant inelastic strain rate or flow surfaces are to viscoplastic theories what yield surfaces are to classical plasticity. The purpose of the work reported herein was to validate experimental procedures for determining flow surfaces at elevated temperatures. Since experimental procedures for determining yield surfaces in axial/torsional stress space are well established, they were employed -- except inelastic strain rates were used rather than total inelastic strains. In yield-surface determinations, the use of small-offset definitions of yield minimizes the change of material state and allows multiple loadings to be applied to a single specimen. The key to the experiments reported here was precise, decoupled measurement of axial and torsional strain. With this requirement in mind, the performance of a high-temperature multi-axial extensometer was evaluated by comparing its results with strain gauge results at room temperature. Both the extensometer and strain gauges gave nearly identical yield surfaces (both initial and subsequent) for type 316 stainless steel (316 SS). The extensometer also successfully determined flow surfaces for 316 SS at 650 C. Furthermore, to judge the applicability of the technique for composite materials, yield surfaces were determined for unidirectional tungsten/Kanthal (Fe-Cr-Al).

Lissenden, Cliff J.↗
NASA NTRS1995

A Comparison of Three Determinants of an Engagement Index for Use in a Simulated Flight Environment

The following report details a project design that is to be completed by the end of the year. Determining how engaged a person is at a task is rather difficult. There are many different ways to assess engagement. One such method is to use psychophysical measures. The current study focuses on three determinants of an engagement index proposed by researchers at NASA-Langley (Pope, A. T., Bogart, E. H., and Bartolome, D. S., 1995). The index (20 Beta/(Alpha+Theta)) uses EEG power bands to determine a person's level of engagement while performs a compensatory tracking task. The tracking task switches between manual and automatic modes. Participants each experience both positive and negative feedback within each trial of the three trials. The tracking task is altered in terms of difficulty depending on the participants current engagement index. The rationale of this study is to determine the optimal level of engagement to gain peak performance. The three determinants are based on an absolute index which differs from the past research which uses a slope index.

Hitt, James M., II↗
NASA NTRS1998

Lunar Prospector Orbit Determination Results

The orbit support for Lunar Prospector (LP) consists of three main areas: (1) cislunar orbit determination, (2) rapid maneuver assessment using Doppler residuals, and (3) routine mapping orbit determination. The cislunar phase consisted of two trajectory correction maneuvers during the translunar cruise followed by three lunar orbit insertion burns. This paper will detail the cislunar orbit determination accuracy and the real-time assessment of the cislunar trajectory correction and lunar orbit insertion maneuvers. The non-spherical gravity model of the Moon is the primary influence on the mapping orbit determination accuracy. During the first two months of the mission, the GLGM-2 lunar potential model was used. After one month in the mapping orbit, a new potential model was developed that incorporated LP Doppler data. This paper will compare and contrast the mapping orbit determination accuracy using these two models. LP orbit support also includes a new enhancement - a web page to disseminate all definitive and predictive trajectory and mission planning information. The web site provides definitive mapping orbit ephemerides including moon latitude and longitude, and four week predictive products including: ephemeris, moon latitude/longitude, earth shadow, moon shadow, and ground station view periods. This paper will discuss the specifics of this web site.

Beckman, Mark↗