## A Quick Description of the Pointing Correction Process Flow

This document describes a very high-level view of the pointing correction process used by the GBT antenna controller. It does not describe the acquisition or reduction of pointing data, simply the procedural flow of computations in the antenna controller.

In the process of tracking, a celestial coordinate system is transformed into a time-relative az/el position using functions from the SLALIB Starlink library. Once obtained, the refraction algorithm described in PTCS Memo 35 is used to compute a refracted az/el position.

The traditional model correction is then calculated based upon the refracted az/el position. At the same time a residual correction is calculated based upon the same refracted az/el position.

The residual correction is based upon three numerically represented functions:

where f(), g(), and h() are based on data-sets stored in a database. These values are linearly interpolated to produce a value at any az/el position.

The corrections are summed with the traditional model corrections:

Then the correction is applied:

The next stage processes the LPC's and dynamic corrections:

where az_{mnt} and el_{mnt} are the values sent as commands to the az/el servo.

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JoeBrandt - 20 Oct 2006