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Journal of Astronomical Telescopes, Instruments, and Systems

Correcting encoder interpolation error on the Green Bank Telescope using an iterative model based identification algorithm
Author(s): Timothy J. Franke; Timothy L. Weadon; John M. Ford; Mario Garcia-Sanz
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Paper Abstract

Various forms of measurement errors limit telescope tracking performance in practice. A new method for identifying the correcting coefficients for encoder interpolation error is developed. The algorithm corrects the encoder measurement by identifying a harmonic model of the system and using that model to compute the necessary correction parameters. The approach improves upon others by explicitly modeling the unknown dynamics of the structure and controller and by not requiring a separate system identification to be performed. Experience gained from pin-pointing the source of encoder error on the Green Bank Radio Telescope (GBT) is presented. Several tell-tale indicators of encoder error are discussed. Experimental data from the telescope, tested with two different encoders, are presented. Demonstration of the identification methodology on the GBT as well as details of its implementation are discussed. A root mean square tracking error reduction from 0.68 arc seconds to 0.21 arc sec was achieved by changing encoders and was further reduced to 0.10 arc sec with the calibration algorithm. In particular, the ubiquity of this error source is shown and how, by careful correction, it is possible to go beyond the advertised accuracy of an encoder.

Paper Details

Date Published: 9 December 2015
PDF: 10 pages
J. Astron. Telesc. Instrum. Syst. 1(4) 044005 doi: 10.1117/1.JATIS.1.4.044005
Published in: Journal of Astronomical Telescopes, Instruments, and Systems Volume 1, Issue 4
Show Author Affiliations
Timothy J. Franke, Case Western Reserve Univ. (United States)
Timothy L. Weadon, National Radio Astronomy Observatory (United States)
John M. Ford, National Radio Astronomy Observatory (United States)
Mario Garcia-Sanz, Case Western Reserve Univ. (United States)

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