In article .com,
"Michael" wrote:

Correct. The only time there is a datalink between the GPS and AP is
when you have GPSS, and even then XTE is not what drives it.

We've got GPSS in a couple of our planes (not this one). It's an add-on
box that sits between the HSI the GPS, and the A/P (Century-2000 in that
case). You engage the GPSS and put the A/P in HEADING mode. The GPSS
widget then takes XTE (or maybe angular error?) from the GPS and
synthesizes a heading error signal that makes the A/P do the right thing.

Or, at least that's the theory. It's basically just a huge ball of duct
tape interconnecting several systems that were never meant to talk to each
other in this fashion. The one I've flown is hideously under-damped and
just flies big S-turns if you're dumb enough to turn it on. I'm told
they're working on getting it adjusted. A big ball of duct tape with rows
of trim pots and an STC does not a useful tool make.