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Fang, B. T.

Publications and source records attributed to Fang, B. T..

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Navigation study for low-altitude Earth satellites

This document describes several navigation studies for low-altitude Earth satellites. The use of Global Positioning System Navigation Package data for LANDSAT-5 orbit determination is evaluated. In addition, a navigation analysis for the proposed Tracking and Data Aquisition System is presented. This analysis, based on simulations employing one-way Doppler data, is used to determine the agreement between the Research and Development Goddard Trajectory Determination System and the Sequential Error Analysis Program results. Properties of several geopotential error models are studied and an exploratory study of orbit smoother process noise is presented.

Pastor, P. R.↗
NASA NTRS1985

An evaluation of Global Positioning System data for Landsat-4 orbit determination

The Navstar Global Positioning System (GPS) navigation data obtained from an experimental GPS receiver/processor package (GPSPAC) onboard the Landsat-4 spacecraft are evaluated for their accuracy and reliability for use in Landsat-4 orbit determination. Different orbit determination scenarios and methods are considered. The parameters estimated include a subset of three Landsat-4 clock parameters and an atmospheric drag coefficient, in addition to the orbital elements. It is found that Landsat-4 orbit solutions based on delta pseudorange data generally agree with the definitive solutions to the 50-meter level.

Fang, B. T.↗
NASA NTRS1981

Geometric dilution of precision in Global Positioning System navigation

The evaluation of the GPS navigation performance is essentially equivalent to the computation of the diagonal terms of the geometric dilution of precision (GDOP) matrix. Certain theoretical results concerning the general properties of the GDOP matrix are obtained. An efficient algorithm for the computation of the GDOP matrix and the navigation performance index is given; and applications of the results are illustrated by numerical examples.

Fang, B. T.↗
NASA NTRS1980

An analysis of GDOP in global positioning system navigation

The accuracy of user navigation fix based on the NAVSTAR global positioning system is described. The trace of this matrix serves as a convenient navigation performance index and the square root of the trace is called geometric dilution of precision (GDOP). Certain theoretical results concerning the general properties of the navigation performance are derived. An efficient algorithm for the computation of GDOP is given. Applications of the results are illustrated by numerical examples.

Fang, B. T.↗
NASA NTRS1979

Short-term solar pressure effect and GM uncertainty on TDRS orbital accuracy: A study of the interaction of modeling error with tracking and orbit determination

The TDRS was modeled as a combination of a sun-pointing solar panel and earth-pointing plate. Based on this model, explanations are given for the following orbit determination error characteristics: inherent limits in orbital accuracy, the variation of solar pressure induced orbital error with time of the day of epoch, the insensitivity of range-rate orbits to GM error, and optimum bilateration baseline.

Fang, B. T.↗
NASA NTRS1977

ALTKAL: An optimum linear filter for GEOS-3 altimeter data

ALTKAL is a computer program designed to smooth sea surface height data obtained from the GEOS 3 altimeter, and to produce minimum variance estimates of sea surface height and sea surface slopes, along with their standard derivations. The program operates by processing the data through a Kalman filter in both the forward and backward directions, and optimally combining the results. The sea surface height signal is considered to have a geoid signal, modeled by a third order Gauss-Markov process, corrupted by additive white noise. The governing parameters for the signal and noise processes are the signal correlation length and the signal-to-noise ratio. Mathematical derivations of the filtering and smoothing algorithms are presented. The smoother characteristics are illustrated by giving the frequency response, the data weighting sequence and the transfer function of a realistic steady-state smoother example. Based on nominal estimates for geoidal undulation amplitude and correlation length, standard deviations for the estimated sea surface height and slope are 12 cm and 3 arc seconds, respectively.

Fang, B. T.↗
NASA NTRS1977

Time determination for spacecraft users of the Navstar Global Positioning System /GPS/

Global Positioning System (GPS) navigation is performed by time measurements. A description is presented of a two body model of spacecraft motion. Orbit determination is the process of inferring the position, velocity, and clock offset of the user from measurements made of the user motion in the Newtonian coordinate system. To illustrate the effect of clock errors and the accuracy with which the user spacecraft time and orbit may be determined, a low-earth-orbit spacecraft (Seasat) as tracked by six Phase I GPS space vehicles is considered. The obtained results indicate that in the absence of unmodeled dynamic parameter errors clock biases may be determined to the nanosecond level. There is, however, a high correlation between the clock bias and the uncertainty in the gravitational parameter GM, i.e., the product of the universal gravitational constant and the total mass of the earth. It is, therefore, not possible to determine clock bias to better than 25 nanosecond accuracy in the presence of a gravitational error of one part per million.

Grenchik, T. J.↗
NASA NTRS1976

A general geometric theory of attitude determination from directional sensing

A general geometric theory of spacecraft attitude determination from external reference direction sensors was presented. Outputs of different sensors are reduced to two kinds of basic directional measurements. Errors in these measurement equations are studied in detail. The partial derivatives of measurements with respect to the spacecraft orbit, the spacecraft attitude, and the error parameters form the basis for all orbit and attitude determination schemes and error analysis programs and are presented in a series of tables. The question of attitude observability is studied with the introduction of a graphical construction which provides a great deal of physical insight. The result is applied to the attitude observability of the IMP-8 spacecraft.

Fang, B. T.↗
NASA NTRS1975

Geometric theory of horizon scanners

The note presents a general geometrical theory of spacecraft horizon scanners for the purpose of actual attitude determination, as opposed to just attitude stabilization. Analysis is carried out in terms of the scanning angles and three sets of auxiliary axes: the scanner axes, the nonscanning axes, and the orbital axes. Euler angles (yaw, pitch, and roll) transform the orbital axes to the nonscanning axes, and spacecraft attitude is determined directly from the attitude of the nonscanning axes relative to the orbital axes. In most applications the scanning speed is fast, so that it can be assumed that the attitude of the spacecraft does not change during a scan; however, a perturbation analysis is provided for errors committed by neglecting attitude changes. The analysis is valid for all types of scanners; in addition, the case where two scanners with different half-cone angles are used is considered.

Fang, B. T.↗