I have finally got the psa algorithm to match the NREL code. Not really sure why I got the errors in the previous post but I suspect it was to do with the inputs I gave the NREL calculator, and not with the simpler psa code. Anyway now I'm getting results that match within ~0.5deg. However I am still having issues with my rig drifting out of alignment over time.

I did a little experiment today and set up a sundial with a thin vertical stick in the centre. I marked the position of the shadow with the current calculated azimuth value and waited for the calculated angle to move 30deg (2hours from 1:30 to 3:30).

The rig tracked nicely around 30deg but had over-rotated and was casting a shadow. When I came to measure the new shadow position on the sundial it was only about 20deg from the old mark... So the results that I'm getting from both NREL and the psa algorithms aren't matching what's happening in the real world.

I have a new piece of code from gabriel over at http://www.cerebralmeltdown.com/, which works from tabulated data. I'll give that a go and hope for the best.