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Application of the Two-Step Filter to Process Ranging Measurements for Relative Navigation in an Elliptical OrbitThis estimator breaks a nonlinear estimation problem into a set of over determined 'first step' states which are linear in the observations and 'second step' states which are ultimately the states of interest. Linear estimation methods are applied to filter the observations and produce the optimal first step state estimate. The 'second step' states are obtained through iterative nonlinear parameter estimation considering the first step states as observations. It has been shown that this process exactly minimizes the least squares cost function for static problems and provides a better solution than the iterated extended Kalman filter (EKF) for dynamic problems. The two step filter is applied in this paper to process range and range rate measurements between the two spacecraft. Details of the application of the two step estimator to this problem will be given, highlighting the use of a test for ill-conditioned covariance estimates that can result from the first order covariance propagation. A comparison will be made between the performance of the two step filter and the IEKF.
Document ID
Acquisition Source
Langley Research Center
Document Type
Garrison, James L.
(NASA Langley Research Center Hampton, VA United States)
Axelrad, Penina
(Colorado Univ. Boulder, CO United States)
Date Acquired
August 17, 2013
Publication Date
May 1, 1997
Publication Information
Publication: Flight Mechanics Symposium 1997
Subject Category
Accession Number
Distribution Limits
Work of the US Gov. Public Use Permitted.

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