Hi Dean,

In a nutshell:

The all-sky models produce a tracking rate derived from the best-fit pointing terms across all its data points. However, it is possible that the equipment has some un-modeled pointing characteristics. If so, then the tracking rate will not account for those un-modeled characteristics.

The declination-arc model is designed specifically for calculating tracking rates inclusive of any un-modeled characteristics. It does this by measuring the changes in pointing error along the declination arcs like in your screenshot. The more values in each arc the better the tracking rate modeling will be. Tracking rates are interpolated between declination arcs so the entire region encompassed by the sky data points is covered.

Some notes about this:

1)    Dec-Arc Tracking is only used for tracking rate calculations. The all-sky model, which is also calculated by APCC, still handles pointing.

2)    Tracking rate correction only works within the area in which sky points are measured. When outside that area APCC uses the all-sky tracking rate model. The All-Sky model will also extrapolate outside of the sky data points that were collected.

BTW, when I mention “All-Sky” model above, I mean one of the four model combinations: East/West, Counterweight Up/Down.

-Ray