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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.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Error analysis of image reconstruction by a synthetic aperture interferometric radiometer

An error analysis is presented for the image reconstruction algorithm currently employed by the electronically steered thinned array radiometer aircraft instrument. This instrument samples components of the Fourier transform of the brightness temperature distribution at 1.4 GHz. Sources of error are identified, including systematic offsets and stochastic noise in the measurements, errors in the interference pattern antenna calibration, multipath scattering from the antenna support structure, mutual coupling between nearby array elements, and variations in the antenna patterns of the array elements. These errors are shown to produce artifacts in the image which appear as 'streaks' running along the instrument flight line. Possible hardware corrections are discussed. These include a reduction in the profile of the antenna support structure and a decoupling of adjacent and other nearby array elements using corrugated 'skirts'. Also discussed are several image enhancement techniques which detect and correct for some of the streaking. These techniques include a renormalization of the interference patterns based on the measured element patterns, a perturbation based interpretation of image contrast, and a covariance analysis of noise present in the image.

Ruf, Christopher S.↗
NASA NTRS2006

Dilution of precision-based lunar navigation assessment for dynamic position fixing

The NASA vision of exploration is focused on the return of astronauts to the Moon [1]. While navigation systems have already been proven in the Apollo missions to the moon, the current exploration campaign will involve more extensive and extended missions requiring new concepts for lunar navigation. In contrast to Apollo missions, which were limited to the near-side equatorial region of the moon, missions under the Exploration Systems Initiative will require navigation on the moon’s limb and far-side. As these regions have poor Earth visibility, a navigation system comprised solely of Earth-based tracking stations will not provide adequate navigation solutions in these areas. In this paper, a Dilution of Precision (DoP) based analysis of the performance of a network of Moon orbiting satellites is provided. The analysis extends previous analysis of a Lunar Network (LN) of navigation satellites by providing an assessment of the capability associated with a variety of assumptions regarding the navigation receiver and satellite visibility. The assessment is accomplished by making appropriately formed estimates of DoP, with different adaptations of DoP (i.e. GDoP, PDoP, etc.) being associated with a different set of assumptions regarding augmentations to the navigation receiver or transceiver A significant innovation described in this paper is the “Generalized” Dilution of Precision. In the same sense that the various versions of DoP can be represented as a functional of the observability grammian, Generalized DoP is defined as a functional of the sum of observability grammians associated with a batch of radiometric measurements. Generalized DoP extends the DoP concept to cases in which radiometric range and range-rate measurements are integrated over time to develop an estimate of user position (referred to here as a ‘dynamic’ solution.) Generalized DoP allows for the inclusion of cases in which the receiver location is underdetermined when assessed in the usual ‘kinematic’ sense. The Generalized DoP concept is thereby a method to assess the navigation capability associated with constellations with sparse coverage without the burden of performing a full “covariance analysis” for each point on the surface of the Moon.

Berry, Kevin↗
NASA NTRS2006

Dilution of Precision-Based Lunar Navigation Assessment for Dynamic Position Fixing

The NASA Vision for Space Exploration is focused on the return of astronauts to the Moon. While navigation systems have already been proven in the Apollo missions to the moon, the current exploration campaign will involve more extensive and extended missions requiring new concepts for lunar navigation. In contrast to Apollo missions, which were limited to the near-side equatorial region of the moon, missions under the Exploration Systems Initiative will require navigation on the moon's limb and far-side. As these regions have poor Earth visibility, a navigation system comprised solely of Earth-based tracking stations will not provide adequate navigation solutions in these areas. In this paper, a Dilution of Precision (DoP) based analysis of the performance of a network of Moon orbiting satellites is provided. The analysis extends previous analysis of a Lunar Network (LN) of navigation satellites by providing an assessment of the capability associated with a variety of assumptions. These assumptions are with regard to the navigation receiver and satellite visibility. The assessment is accomplished by making appropriately formed estimates of DoP. Different adaptations of DoP (i.e. GDoP, PDoP, etc.) are associated with a different set of assumptions regarding augmentations to the navigation receiver or transceiver. A significant innovation described in this paper is the "Generalized" Dilution of Precision. In the same sense that the various versions of DoP can be represented as a functional of the observability grammian, Generalized DoP is defined as a functional of the sum of observability grammians associated with a batch of radiometric measurements. Generalized DoP extends the DoP concept to cases in which radiometric range and range-rate measurements are integrated over time to develop an estimate of user position (referred to here as a 'dynamic' solution.) Generalized DoP allows for the inclusion of cases in which the receiver location is underdetermined when assessed in the usual 'kinematic' sense. The Generalized DoP concept is thereby a method to assess the navigation capability associated with constellations with sparse coverage. This alleviates the burden of performing a full "covariance analysis" for each point on the surface of the Moon.

Sands, Obed S.↗
NASA NTRS1990

Ionospheric refraction effects on orbit determination using the orbit determination error analysis system

The influence of ionospheric refraction on orbit determination was studied through the use of the Orbit Determination Error Analysis System (ODEAS). The results of a study of the orbital state estimate errors due to the ionospheric refraction corrections, particularly for measurements involving spacecraft-to-spacecraft tracking links, are presented. In current operational practice at the Goddard Space Flight Center (GSFC) Flight Dynamics Facility (FDF), the ionospheric refraction effects on the tracking measurements are modeled in the Goddard Trajectory Determination System (GTDS) using the Bent ionospheric model. While GTDS has the capability of incorporating the ionospheric refraction effects for measurements involving ground-to-spacecraft tracking links, such as those generated by the Ground Spaceflight Tracking and Data Network (GSTDN), it does not have the capability to incorporate the refraction effects for spacecraft-to-spacecraft tracking links for measurements generated by the Tracking and Data Relay Satellite System (TDRSS). The lack of this particular capability in GTDS raised some concern about the achievable accuracy of the estimated orbit for certain classes of spacecraft missions that require high-precision orbits. Using an enhanced research version of GTDS, some efforts have already been made to assess the importance of the spacecraft-to-spacecraft ionospheric refraction corrections in an orbit determination process. While these studies were performed using simulated data or real tracking data in definitive orbit determination modes, the study results presented here were obtained by means of covariance analysis simulating the weighted least-squares method used in orbit determination.

Yee, C. P.↗
NASA NTRS1974

Strategies for estimating the marine geoid from altimeter data

In processing altimeter data from a spacecraft borne altimeter to estimate the fine structure of the marine geoid, a problem is encountered. In order to describe the geoid fine structure, a large number of parameters must be employed and it is not possible to simultaneously estimate all of them. Unless the parameterization exhibits good orthogonality in the data, serious aliasing results. From simulation studies it has been found that amongst several competing parameterizations, the mean free air gravity anomaly model (i.e., Stokes' formula) exhibited promising geoid recovery characteristics. Using covariance analysis techniques, this report provides quantitative measures of the orthogonality properties associated with the above mentioned parameterization. It has been determined that a 5 deg x 5 deg area mean free air gravity anomaly can be estimated with an uncertainty of 1 mgal (40 cm undulation) provided that all free air gravity anomalies within a spherical radius of 10 arc degrees are simultaneously estimated.

Argentiero, P.↗
NASA NTRS1974

Design/cost tradeoff studies. Appendix A. Supporting analyses and tradeoffs, book 2. Earth Observatory Satellite system definition study (EOS)

Attitude reference systems for use with the Earth Observatory Satellite (EOS) are described. The systems considered are fixed and gimbaled star trackers, star mappers, and digital sun sensors. Covariance analyses were performed to determine performance for the most promising candidate in low altitude and synchronous orbits. The performance of attitude estimators that employ gyroscopes which are periodically updated by a star sensor is established by a single axis covariance analysis. The other systems considered are: (1) the propulsion system design, (2) electric power and electrical integration, (3) thermal control, (4) ground data processing, and (5) the test plan and cost reduction aspects of observatory integration and test.

Source record↗
NASA NTRS1976

Strategies for estimating the marine geoid from altimeter data

Altimeter data from a spacecraft borne altimeter was processed to estimate the fine structure of the marine geoid. Simulation studies show that, among several competing parameterizations, the mean free air gravity anomaly model exhibited promising geoid recovery characteristics. Using covariance analysis techniques, quantitative measures of the orthogonality properties are investigated.

Argentiero, P.↗
NASA NTRS1978

Gravitational experiments on solar probe

A covariance analysis was performed for a solar probe trajectory which encounters the sun at four solar radii. The unknown parameters in the analysis are the six initial cartesian coordinates for the probe, six initial cartesian coordinates for the earth, the astronomical unit, the solar gravitational quadrupole coefficient and two post Newtonian meters (beta, gamma). Errors in the unknown parameters were computed as a function of standard errors on the radio tracking data and on the nongravitational forces which act on the probe. Results were obtained for several tracking geometries and for several orbital inclinations to the ecliptic. The analysis shows that the principal scientific result from the radio tracking of a solar probe would be the determination of the quadrupole moment, which would place a constraint on models of the solar interior.

Anderson, J. D.↗
NASA NTRS1978

Possible measurements of J sub 2 and the sun's angular momentum with the solar probe

The metric tensor is given for describing the dynamical effects of the sun on a solar probe and the gravitational redshift of a probe-based clock in a covariance analysis for the detectability of the solar quadrupole moment and the solar angular momentum. Unknown parameters were determined by least squares fit to the probe tracking data. A polar orbit was assumed with perihelion at 5 solar radii and with an earth-sun-probe angle of 135 deg at perihelion. Tracking was assumed to be Doppler only, with a basic uncertainity of .1 mm/sec in the radial velocity. A drag-free system which could reduce nongravitational forces below the level of 2 times 10 to the minus 12th power was found necessary to preserve this same level of accuracy. Both one way and two way Doppler were supposed in order to determine the gravitational redshift as well as the pure spacecraft dynamics.

Nordtvedt, K.↗
NASA NTRS1978

On attitude estimation schemes for fine-pointing control

This paper studies single-axis equations of motion that are applicable to a spacecraft or to a space experiment pointing assembly whose motion has been perfectly isolated from the carrier vehicle. It considers four state estimators for implementation in the control loop for a stellar observation experiment. The first three estimators are very general and do not make use of input torque in their prediction models, while the proposed fourth estimator utilizes this information. It is shown via closed-loop covariance analysis that the best achievable pointing performance with the best of the first three estimators is limited to about 0.125 arc-sec (rms) with the given rate-gyro and star-tracker inaccuracies. It is also shown that the fourth estimator has the capability of achieving a pointing performance far superior to the performance achievable using the first three estimators. The fourth estimator relies on the ability to accurately generate the desired control torque (i.e., low input noise).

Joshi, S. M.↗
NASA NTRS1978

Earth viewing with a shuttleborne experiment pointing mount

The article considers the attitude determination and the control problems of the shuttleborne earth-viewing pointing mounts (EPMs). Per-axis pointing performance requirements are identified for troposphere/stratosphere pollution, tropical storm research, and urban air pollution. Ephemeris error contributions to earth pointing are discussed and candidate attitude reference systems are described. Primary interfaces between the orbiter flight control system, Spacelab, pallet-mounted EPM, and control subsystems are outlined. A block diagram is given of the EPM-mounted stellar-inertial attitude reference system. The system's performance is evaluated on the basis of the inertial sensor and a three-axis covariance analysis.

Iwens, R. P.↗
NASA NTRS1980

Error estimation for ORION baseline vector determination

Effects of error sources on Operational Radio Interferometry Observing Network (ORION) baseline vector determination are studied. Partial derivatives of delay observations with respect to each error source are formulated. Covariance analysis is performed to estimate the contribution of each error source to baseline vector error. System design parameters such as antenna sizes, system temperatures and provision for dual frequency operation are discussed.

Wu, S. C.↗
NASA NTRS1980

On the VLBI-satellite laser ranging Iron Triangle intercomparison experiment

Simulations were performed to determine the suitability of the proposed station locations. The criterion was a comparison among the possible station configuration of the standard deviations of baseline and Earth rotation parameters estimated from a least squares covariance analysis. Only the relative magnitudes of the standard deviations were addressed in the analysis. Only random errors were assumed and no provision was made for systematic effects. The Iron Triangle, consisting of the stations at Westford (Haystack), Massachusetts, Owens Valley, California, and Ft. Davis, Texas, was regarded as the structure of the proposed network with options to incorporate either the Goldstone, California, or Green Bank, West Virginia, station or both. It was decided to include the Richmond, Florida, station in the analysis since it offered more North-South separation and therefore could strengthen the geometry of the network especially in the recovery of Earth rotation parameters.

Bock, Y.↗
NASA NTRS1981

Tracking and Data Relay Satellite System (TDRSS) navigation with DSN radio metric data

The use of DSN radiometric data for enhancing the orbit determination capability for TDRS is examined. Results of a formal covariance analysis are presented which establish the nominal TDRS navigation performance and assess the performance improvement based on augmenting the nominal TDRS data strategy with radiometric data from DSN sites.

Ellis, J.↗
NASA NTRS1981

Application of unconnected phase narrowband Delta VLBI to deep space navigation

Unconnected phase narrowband Delta VLBI data strategies are evaluated for enhancing deep space cruise navigation and for rapidly redetermining the spacecraft trajectory after a midcourse correction maneuver. Results of a covariance analysis study are presented for a Jupiter bound spacecraft based on a simulated data strategy consisting of daily 10 minute narrowband scans from two baselines augmented with conventional two way range and Doppler passes. For post-maneuver recovery, the narrowband strategy achieves a factor of two improvement in orbit determination performance. Cruise OD performance is also significantly enhanced due to the ability of the narrowband data to accurately sense the orientation of the orbital plane.

Ellis, J.↗
NASA NTRS1981

Estimation of solar gravitational harmonics with Starprobe radiometric tracking data

The potential gravitational science return of a proposed spacecraft mission, called Starprobe, is investigated. The current mission plan is to place a spacecraft in a highly eccentric, highly inclined solar orbit with a perihelion distance of four solar radii. A covariance analysis based on Kalman filtering theory is performed to predict the accuracies with which gravitational parameters associated with the nonsphericity of the sun and relativistic effects can be estimated with radiometric tracking data. The accuracies are computed as functions of the quality and quantity of the radiometric data, the orbital parameters, the magnitude and character of the nongravitational accelerations acting on the spacecraft, and the errors in the tracking station locations and the ephemeris of the earth relative to the sun.

Mease, K. D.↗