Engineering topics

Lerch, Francis J.

Publications and source records attributed to Lerch, Francis J..

3 records

NASA NTRS1993

Expected orbit determination performance for the TOPEX/Poseidon mission

Each of the components required for the computation of precise orbits for the TOPEX/Poseidon (T/P) spacecraft - gravity field modeling, nonconservative force modeling, and satellite tracking technologies - is examined. The research conducted in the Space Geodesy Branch at Goddard Space Flight Center in preparation for meeting the 13-cm radial orbit accuracy requirement for the T/P mission is outlined. New developments in modeling the earth's gravitational field and modeling the complex nonconservative forces acting on T/P are highlighted. The T/P error budget is reviewed, and a prelaunch assessment of the predicted orbit determination accuracies is summarized.

Nerem, R. S.↗

NASA NTRS1991

Optimum data weighting and error calibration for estimation of gravitational parameters

A new approach has been developed for determining consistent satellite-tracking data weights in solutions for the satellite-only gravitational models. The method employs subset least-squares solutions of the satellite data contained within the complete solution and requires that the differences of the parameters of subset solutions and the complete solution to be in agreement with their error estimates by adjusting the data weights. GEM-T2 model was recently computed and adjusted through a direct application of this method. The estimated data weights are markedly smaller than the weights implied by the formal uncertainties of the measurements. Orbital arc tests as well as surface gravity comparisons show significant improvements for solutions when more realistic data weighting is achieved.

Lerch, Francis J.↗

NASA NTRS1989

Optimum data weighting and error calibration for estimation of gravitational parameters

A new technique was developed for the weighting of data from satellite tracking systems in order to obtain an optimum least-squares solution and an error calibration for the solution parameters. Data sets from optical, electronic, and laser systems on 17 satellites in GEM-T1 Goddard Earth Model-T1 (GEM-T1) were employed toward application of this technique for gravity field parameters. Also GEM-T2 (31 satellites) was recently computed as a direct application of the method and is summarized. The method employs subset solutions of the data associated with the complete solution to agree with their error estimates. With the adjusted weights the process provides for an automatic calibration of the error estimates for the solution parameters. The data weights derived are generally much smaller than corresponding weights obtained from nominal values of observation accuracy or residuals. Independent tests show significant improvement for solutions with optimal weighting. The technique is general and may be applied to orbit parameters, station coordinates, or other parameters than the gravity model.

Lerch, Francis J.↗