Engineering topics

Goad, C. C.

Publications and source records attributed to Goad, C. C..

13 records

NASA NTRS1985

A comparison of four precise global positioning system geodetic receivers

Four precise global positioning system (GPS) geodetic receivers were operated simultaneously in January and February 1984 over ten baselines ranging in distance from 13 to 1304 km. Several of the baselines had been previously measured using very long baseline interferometry and, therefore, provide very good standards to which the satellite results can be compared. Results of these experiments are presented along with a brief description of each receiver and the associated analysis techniques.

Goad, C. C.↗

NASA NTRS1980

Determination of the geopotential from satellite-to-satellite tracking data

Ground-based observation of the deviation of artificial satellite trajectories from a reference path is the classical means for determining the parameters of the global geopotential. However, for the short wavelengths (less than 10 deg), tracking coverage from ground stations of sufficient density is impossible to obtain. But one or more satellites can observe another satellite and obtain the needed global data coverage. Results are presented of error analyses of possible satellite-to-satellite tracking missions to determine the geopotential at a resolution of 1 x 1 deg. To achieve an accuracy of a few milligals at this resolution requires a satellite altitude at or near 150 km and measurements of intersatellite speed to 10 to the -6th m/s.

Douglas, B. C.↗

NASA NTRS1979

Probable LAGEOS contributions to a worldwide geodynamics control network

The paper describes simulations performed on the contributions which LAGEOS laser ranging data can make to the establishment of a worldwide geodynamics control network. A distribution of 10 fixed ranging stations was assumed for most of the calculations, and a single 7-day arc was used, measurements assumed to be made every 10 minutes in order to avoid artificial reductions in the uncertainties due to oversampling. Computer simulations were carried out in which the coordinates of the stations and improvements in the gravity field coefficients were solved for simultaneously. It is suggested that good accuracy for station coordinates can be expected, even with the present gravity field model uncertainties, if sufficient measurement accuracy is achieved at a reasonable distribution of stations. Further, it is found that even 2-cm range measurement errors would be likely to be the main source of station coordinate errors in retrospective analyses of LAGEOS ranging results five or six years from now.

Bender, P. L.↗

NASA NTRS1978

Lunar tidal acceleration obtained from satellite-derived ocean tide parameters

One hundred sets of mean elements of GEOS-3 computed at 2-day intervals yielded observation equations for the M sub 2 ocean tide from the long periodic variations of the inclination and node of the orbit. The 2nd degree Love number was given the value k sub 2 = 0.30 and the solid tide phase angle was taken to be zero. Combining obtained equations with results for the satellite 1967-92A gives the M sub 2 ocean tide parameter values. Under the same assumption of zero solid tide phase lag, the lunar tidal acceleration was found mostly due to the C sub 22 term in the expansion of the M sub 2 tide with additional small contributions from the 0 sub 1 and N sub 2 tides. Using Lambeck's (1975) estimates for the latter, the obtained acceleration in lunar longitudal in excellent agreement with the most recent determinations from ancient and modern astronomical data.

Goad, C. C.↗

NASA NTRS1978

Lunar tidal acceleration obtained from satellite-derived ocean tide parameters

Observation equations for the M2 ocean tide are computed from Geos 3 data for the long periodic variations of the inclination and node of the orbit. M2 ocean tide parameter values C22+ = 3.23 + or - 0.25 cm, epsilon 22+ = 331 + or - 6 deg, and epsilon 42+ = 113 + or - 6 deg are determined. With the assumption of zero solid tide phase lag, the lunar tidal acceleration is mostly (85%) due to the C22+ term in the expansion of the M2 tide with additional small contributions from the O1 and N2 tides. The calculated value for the tidal acceleration in lunar longitude is -27.4 + or - 3 arc sec/sq (100 yr) which is similar to values determined from astronomical data. The mean elements of Geos 3 are presented in tabular form.

Goad, C. C.↗

NASA NTRS1977

Effects of tropospheric and ionospheric refraction errors in the utilization of GEOS-C altimeter data

The effects of tropospheric and ionospheric refraction errors are analyzed for the GEOS-C altimeter project in terms of their resultant effects on C-band orbits and the altimeter measurement itself. Operational procedures using surface meteorological measurements at ground stations and monthly means for ocean surface conditions are assumed, with no corrections made for ionospheric effects. Effects on the orbit height due to tropospheric errors are approximately 15 cm for single pass short arcs (such as for calibration) and 10 cm for global orbits of one revolution. Orbit height errors due to neglect of the ionosphere have an amplitude of approximately 40 cm when the orbits are determined from C-band range data with predominantly daylight tracking. Altimeter measurement errors are approximately 10 cm due to residual tropospheric refraction correction errors. Ionospheric effects on the altimeter range measurement are also on the order of 10 cm during the GEOS-C launch and early operation period.

Goad, C. C.↗

NASA NTRS1975

Orbit and attitude state recoveries from Landmark data

The navigation of earth-referenced satellites with imaging data rather than, or in addition to, conventional radio tracking and attitude sensor telemetry is gaining increased popularity. Driving forces include a trend towards spacecraft autonomy, a need for timely and highly accurate gridding information, and a growing awareness of the presence of high quality navigation information contained in such data. This paper describes the techniques used and the results obtained in an experiment to determine the orbit and attitude state of the geosynchronous SMS-1 spacecraft from Landmark observations extracted from earth images generated by the on-board Visible and Infrared Spin-Scan Radiometer (VISSR).

Fuchs, A. F.↗

NASA NTRS1975

Orbit and attitude state recoveries from Landmark data

The navigation of earth-referenced satellites with imaging data rather than, or in addition to, conventional radio tracking and attitude sensor telemetry is gaining increased popularity. Driving forces include a trend towards spacecraft autonomy, a need for timely and highly accurate griding information, and a growing awareness of the presence of high quality navigation information contained in such data. This paper describes the techniques used and the results obtained in an experiment to determine the orbit and attitude state of the geosynchronous SMS-1 spacecraft from Landmark observations extracted from earth images generated by the on-board Visible and Infrared Spin-Scan Radiometer (VISSR).

Fuchs, A. F.↗

NASA NTRS1975

A study of orbit determination accuracies for future earth observatory missions

It is anticipated that global orbital accuracies of 10 meters will be required for future earth observatory missions. Results are presented of a study of the LANDSAT-1 orbit to determine position accuracies obtainable today so as to assess what future advancements are necessary to meet a 10-meter goal. Accuracies of 50 meters were determined to be achievable now from two-day tracking arcs. One-half-day arcs yield accuracies of about 30 meters. The dominant error source was found to be geopotential model uncertainty. All other systematic errors produced less than 10 meters total position error.

Bryant, W. C., Jr.↗

NASA NTRS1973

Tracking and orbit determination of near earth orbiting satellites using earth synchronous relay satellites

A Tracking Data Relay Satellite System (TDRSS) made up of two earth synchronous data relay satellites is proposed for the late 1970s to aid in the tracking, or take the place of ground tracking, or near-earth orbiters. Theoretical error analysis studies were conducted to evaluate the TDRSS concept of tracking user satellites. All major factors affecting orbit determination accuracy were considered in the analysis, including tracking system and dynamic modeling errors.

Bryant, W. C., Jr.↗

NASA NTRS1972

GEODYN programmers guide, volume 2, part 1

A guide to the GEODYN Program is presented. The program estimates orbit and geodetic parameters. It possesses the capability to estimate that set of orbital elements, station positions, measurement biases, and a set of force model parameters such that the orbital tracking data from multiple arcs of multiple satellites best fit the entire set of estimated parameters. GEODYN consists of 113 different program segments, including the main program, subroutines, functions, and block data routines. All are in G or H level FORTRAN and are currently operational on GSFC's IBM 360/95 and IBM 360/91.

Mullins, N. E.↗

NASA NTRS1972

GEODYN programmer's guide, volume 2, part 2

A computer program for executive control routine for orbit integration of artificial satellites is presented. At the beginning of each arc, the program initiates required constants as well as the variational partials at epoch. If epoch needs to be reset to a previous time, the program negates the stepsize, and calls for integration backward to the desired time. After backward integration is completed, the program resets the stepsize to the proper positive quantity.

Mullins, N. E.↗

NASA NTRS1972

GEODYN system description, volume 1

A computer program for the estimation of orbit and geodetic parameters is presented. The areas in which the program is operational are defined. The specific uses of the program are given as: (1) determination of definitive orbits, (2) tracking instrument calibration, (3) satellite operational predictions, and (4) geodetic parameter estimation. The relationship between the various elements in the solution of the orbit and geodetic parameter estimation problem is analyzed. The solution of the problems corresponds to the orbit generation mode in the first case and to the data reduction mode in the second case.

Chin, M. M.↗