Measuring and Predicting Track Induced Pointing Errors for the Green Bank Telescope

Details Measuring and Predicting Track Induced Pointing Errors for the Green Bank Telescope Measuring and Predicting Track Induced Pointing Errors for the Green Bank Telescope

Jul 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007ursi.confe...1c&link_type=abstract

USNC/CNC/URSI North American Radio Science Meeting in Ottawa, Canada, July 2007

Computer Science

Performance

1

Pointing, Inclinometry

Scientific paper

We are in the process of further improving the pointing model for the Green Bank Telescope (GBT). Current pointing performance, using a model that includes geometrical, gravitational distortion, temperature gradient, and an empirical model of azimuth track induced effects (other than track tilt) has performance of about 5" (1-sigma) in elevation and cross elevation over all times of day. For a variety of reasons, including better model performance and the ability to update only the track components of the model after our upcoming track refurbishment is complete, we have constructed a new first-principles model of track pointing effects. The azimuth track causes pointing errors though a basic tilt of the track with respect to local up but also small scale variations of track height that lead to not only additional tilt but also distortions of the alidade due to the statically indeterminate nature of the track-alidade interface. These distortions have static effects in change of the elevation axle pose and rotation of the elevation encoder and dynamical affects due to excitations of the structure's vibrational modes. Using the new model and rudimentary measurements of the tilts of elevation bearing castings as a function of azimuth can improve pointing performance to about 3" (1-sigma), where only one track model parameter (the effective height of the alidade towers) needs to be estimated using astronomical data. We will also discuss more sophisticated approaches to measuring both the static track model parameters and predicting vibrational pointing perturbations. These techniques include development of a dynamical (state-space) model for predicting inclinometer outputs due to inertial effects and structural vibration on the moving telescope, using the innovations to continuously survey the track during routine observations, and using the dynamical model in conjunction with astronomical measurements to predict vibrational effects on pointing in real time.

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