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NASA NTRS1975

Satellite-tracking and earth-dynamics research programs

The activities and progress in the satellite tracking and earth dynamics research during the first half of calendar year 1975 are described. Satellite tracking network operations, satellite geodesy and geophysics programs, GEOS 3 project support, and atmospheric research are covered.

Source record↗
NASA NTRS1975

Applications of satellite-to-satellite tracking to orbit determination and geopotential recovery

Recent simulations have demonstrated the applicability of satellite-to-satellite tracking data to the related problems of orbit determination and geopotential recovery. Specifically, satellite-to-satellite tracking between an earth orbiting satellite and a satellite at geosynchronous altitude (36000 km) produces long continuous data arcs which are not available by means of ground-based tracking. This facility, in conjunction with correct estimation techniques, yields exceptional orbit determination accuracy. The data type also has considerable applicability to geopotential determination when the low satellite has a high inclination.

Argentiero, P.↗
NASA NTRS1978

Orbit determination accuracies using satellite-to-satellite tracking

Results of the ATS-6/GEOS-3 and the ATS-6/NIMBUS-6 satellite-to-satellite tracking orbit determination experiments to be applied to the NASA geostationary Tracking and Data Relay Satellite System (TDRSS) starting in 1980, are presented. A satellite radio or laser tracking system performed measurements of such parameters as range, range ratio, angles and direction cosines to a spacecraft relative to a given tracking station. Resolutions of 1 meter in range and .03 cm/sec in range rate for a 1 second averaging were achieved showing that with proper data reduction procedures the tracking data relay satellite system should provide orbit determination capability comparable to what is normally obtainable from ground based systems.

Vonbun, F. O.↗
NASA NTRS1976

Probing the earth's gravity field using Satellite-to-Satellite Tracking (SST)

Satellite-to-Satellite (SST) tests, namely: (a) the ATS-6/GEOS-3 and (b) the ATS-6/Apollo-Soyuz experiment and some of the results obtained are described. The main purpose of these two experiments was first to track via ATS-6 the GEOS-3 as well as the Apollo-Soyuz and to use these tracking data to determine (a) both orbits, that is, ATS-6, GEOS-3 and/or the Apollo-Soyuz orbits at the same time; (b) each of these orbits alone; and (c) test the ATS-6/GEOS-3 and/or Apollo-Soyuz SST link to study local gravity anomalies; and, second, to test communications, command, and data transmission from the ground via ATS-6 to these spacecraft and back again to the ground. The Apollo-Soyuz Geodynamics Experiment is discussed in some detail.

Vonbun, F. O.↗
NASA NTRS1973

Satellite-tracking and earth-dynamics research programs

The following activities in Smithsonian Astrophysical Observatory's (SAO) earth-dynamics programs are covered: (1) satellite-tracking network operations; (2) satellite geodesy and geophysics programs; (3) atmospheric research. Approximately 46,000 successful range measurements were acquired by the SAO laser stations in Peru, South Africa, Brazil, and Arizona. The Peole satellite-tracking campaign conducted in conjunction with the Centre National d'Etudes Spatiales was completed in August 1973. The SAO network obtained 4482 validated returns of 310 arcs of Peole. These data are of particular value for obtaining more accurate gravity-field and zonal-harmonics coefficients.

Weiffenbach, G. C.↗
NASA NTRS1978

Ionospheric range-rate effects in satellite-to-satellite tracking

Ionospheric range and range rate corrections in satellite-to-satellite tracking were investigated and the magnitude of errors that have to be considered for communications between satellites and related experiments was defined. The major causes of the sudden and sometimes large variations apparent in the ionospheric range and range rate corrections along the satellite arc are the geometric effects of the raypath and the curved ionosphere, and the localized perturbations in electron density along the satellite-to-satellite line of sight. Ionospheric tracking errors are greatly effected by the following parameters, whose influence was thoroughly investigated: satellite height, height of maximum electron density, localized ionospheric perturbations and gradients, and electron density variations with diurnal, day-to-day, seasonal, and solar cycle patterns. The results point to the need of using a sophisticated modeling approach incorporating daily solar data, and where possible actual ionospheric measurements as update information. Simulations were performed for satellites at various heights: Apollo, Geos, and Nimbus tracked by ATS-6; and in two different geometric configurations: coplanar and perpendicular orbits.

Bent, R. B.↗
NASA NTRS2021

Expected Collision Rates for Tracked Satellites

This analysis estimates collision risks between tracked satellites based on the statistically expected collision rate _Nc and number of collisions Nc, which are closely related to the collision probability Pc. For isolated encounters, Nc equals Pc. For multi-encounter interactions, Nc can exceed Pc and is significantly easier to approximate semi-analytically. Estimating Nc requires integrating the collision rate over time throughout an interaction, with each required _Nc value calculated as a statistical expectation value based on the uncertainty distributions of initial orbital states. The uncertainty distributions are estimated from orbit determination analyses of satellite trajectory measurements. The formulation accounts for the nonlinear orbital motion of the satellites, and derives Nc and _Nc expressions for Monte Carlo simulations as well as semi-analytical approximations for single- and multi-encounter interactions.

Collision Rates↗
NASA NTRS1976

Lunar farside gravity - An assessment of satellite to satellite tracking techniques and gravity gradiometry

The estimation of local gravity anomalies represented by point masses using gravity gradiometer and satellite to satellite tracking data is discussed. A simulation analysis has been performed to study the recovery of local gravity anomalies from both rotating single axis gravity gradiometer and satellite to satellite tracking measurements. A Lunar Polar Orbiter mission concept is adopted for the orbits and data links. The sensitivity of the gravity determination to data noise, mass point spatial distribution (model errors), unmodelled gravity (gravity anomalies outside the area of interest), and orbit errors is studied. Figure of merit for the comparison is the rms error of radial acceleration.

Ananda, M.↗
NASA NTRS1988

The limits of direct satellite tracking with GPS

Recent advances in high precision differential GPS-based satellite tracking can be usefully applied to the more conventional direct tracking of low earth satellites. To properly evaluate the limiting accuracy of direct GPS-based tracking, it is necessary to take full account of the correlations between the a priori errors in GPS states, Y-bias, and solar pressure parameters. These can be obtained by careful analysis of the GPS orbit determination process. When up-to-date assumptions are used in the analyses of GPS and user orbit solutions, it is found that submeter accuracy can be readily achieved for a user above 1000 km altitude, even when the user solution is obtained with data taken 12 hours after the data used in the GPS orbit solutions.

Bertiger, Willy I.↗
NASA NTRS1975

GEOS-C orbit determination with satellite to satellite tracking

This report studies the feasibility of employing satellite to satellite tracking in lieu of ground based tracking to satisfy the orbit determination requirements of the GEOS-C mission. It is shown that with proper estimation procedures it is possible to obtain from S.S.T. data a GEOS-C orbit whose altitude error averages about 1 meter. The usefulness of this data type for geopotential recovery is also indicated.

Argentiero, P.↗
NASA NTRS1978

On the determination and investigation of the terrestrial ionospheric refractive indices using GEOS-3/ATS-6 satellite-to-satellite tracking data

When the radio link between two satellites (GEOS-3/ATS-6) is intercepted by the earth's ionosphere and neutral atmosphere, a change in the Doppler frequency results. Travel through the atmosphere causes the Doppler phase to be advanced in the ionosphere's portion and retarded in the neutral portion of the atmosphere. Analysis of the shortening and lengthening of the phase of the Satellite-to-Satellite Tracking (SST) data that passed within 40-700 km above the earth's surface during its ATS-6 to GEOS-3 to ATS-6 path, caused by the atmosphere, results in refractivity versus height profiles. The SST Doppler data were used directly to adjust the GEOS-3 orbit. Perturbation from the Moon, Sun and a 15th order/degree earth gravity field were included in the orbit solution. This orbit was continued through the occultation period and a model ionosphere was estimated by a least-square adjustment of the Chapman ionosphere parameters from the SST data residuals. The refractivity profile obtained by this model ionosphere was compared to a refractivity profile obtained by a direct integral inversion of the SST data residuals. Systematic differences between the 2 methods were caused by orbital errors, which propagated into the solution. The SST data yielded refractive index profiles in a novel economical manner because no additional or special on-board equipment were required.

Liu, A. S.↗
NASA NTRS1978

Orbit determination accuracies using satellite-to-satellite tracking

The results are reported of the ATS-6/GEOS-3 and the ATS-6 Nimbus-6 satellite-to-satellite orbit determination experiments. NASA intends to use the tracking data relay satellite system for operational orbit determination of NASA satellites. Hence, in the near future, satellite-to-satellite tracking data will be routinely processed to obtain orbits. The satellite-to-satellite tracking system used in the ATS-6/Nimbus-6 and ATS-6/GEOS-3 experiments performed with a resolution of 1 to 2 m in range and less than 1 mm/s in range rate for a 10-s averaging. A Bayesian least squares estimation technique utilizing independent ranging to the synchronous relay satellite was determined to be the most effective procedure for estimating orbits from satellite-to-satellite tracking data. The use of this technique yields estimates of user satellite orbits which are comparable in accuracy to what is usually obtained from ground based systems.

Vonbun, F. O.↗
NASA NTRS1970

GEOS satellite tracking corrections for refraction in the ionosphere

The analytic formulations at different elevation angles and at a frequency of 2-GHz for the ionospheric refraction corrections used on the GEOS satellite tracking data are compared. The formulas and ray trace results for elevations greater than 10 deg, where most satellite tracking is done, differ in elevation, range, and range rate by less than 0.4 millidegrees (1.4 arc-seconds), 12 meters, and 12 cm/sec, respectively. In comparison to most operational requirements, this is insignificant. However, for the GEOS Observation Systems Intercomparison Investigation, these differences are equivalent in size to observed differences in system biases for some of the best electronic geodetic tracking systems and are probably contributing to the observed biases. The ray trace results and most of the more detailed analytic correction formulas show that the ionospheric refraction correction for range rate on an overhead pass is a maximum for elevation angles between 15 deg and 30 deg and falls off rapidly for both higher and lower elevation angles, contrary to the effect of the troposphere and to some reports in the literature.

Berbert, J. H.↗