See The GBT Pointing Model (URSI 2007) for best information so far...

Error Contributors

• Nominally Gaussian error in peak position estimation of ~ 0".4 for peak SNR = 100, ~ 10 GHz. See the bootstrap analysis of peak position uncertainty .Note that this does not include vibration: It is based upon LCP/RCP comparisions which will have identical vibration effects since they are taken simultaneously. Kaper, et al. show that under certain assumptions the error should scale as sqrt(FWHM), but the data suggest that this is optimistic, and at W band the uncertainty will be something less than 0".4.
• Azimuth and elevation encoder error of ~ 0".3 uniformly distributed due to encoder quantization. This is the instantaneous estimate- For tracking the quantization will cause a dither in error. Depending upon the details of the control loop the expected value of the error will go to zero and the standard deviation could be less than U{-0".15,0".15}.
• Azimuth and elevation encoder error specified by BEI to be 0".618 (2 bits). Not clear how this is measured, but it is probably dominated by the integral non-linearity of the encoders, and thus is large-scale error. Additional information from BEI suggests typical ~ 1".5 peak-to-peak fine cycle error and ~ 3" large angle error.
• ~U{-1.5,1.5} error (uniform on the interval {-1.5,1.5}) due to structure vibration during peak scan. This is estimated from fitting of high SNR peaks, see presentation on peak fitting. _NB: Could be encoder fine cycle error rather than vibration. Need to confirm with independent instrument, e.g., integrated FA accelerometers, QD.
• ~ N(0,0".8) evident in half power tracks (070410) after correction for track effects. This should be separate from the vibration effect in peaks. See the workup on half power tracks
• Uncertainty in catalogue position of a source, of ~ 0".7.

Composite Error

The composite error depends upon the context. In the case of tracking and integration, the contributors will be the preliminary jack-scan of the nearby calibrator, the accuracy of the offset to the source position, and the tracking error itself.

-- KimConstantikes - 31 May 2007