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APR mode sensitizes the A/P for tighter control over the signal. This works best with the Localizer since you wouldn't want one dot to be a half mile off as you approach the runway you are aiming at. GPS approaches also decrease the width of your track per dot as you approach. STEC algorithms don't know GPS (pure left/right) from VOR/LOC (inherent variance) so, under GPS guidance one is likely to wander back and forth across the course and coupling is a joke.
GPSS is a different story - it converts ARINC steering data produced by more sophisticated GPS's (such as Garmin 530/430) into "heading bug" directions just as though you were moving the bug on your DG whilst under HDG A/P guidance. It is beautiful to participate in such a flight since the GPS will issue steering commands based on the planes ground track. GPS approach flight plans are flown precisely as turns are started before the waypoint is reached and completed exactly on the new leg's ground track, regardless of the airplane's speed and winds aloft. If the turn can't be made at 2/3 standard rate, you are warned that a steep turn will be required. I think I spent $800 for the GPSS (more costly now?).
The other option is to scrap to STEC 50 series A/P because it is confused in turbulence (vertical displacements are converted to turns) and install a TRU-TRAK, guided digitally by GPS.
Remember, WAAS is almost operational (June, 2003?) and will be followed by GPS managed vertical guidance for your flight plan - have an auto-pilot that can be used to fly altitude changes.
Scott Krueger
N92EX
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