GPS Three Dimensional Autopilot Coupled Approaches

[LeadSled]

Folks,
I note in Australian Flying May/June 2007 that there is an article about Dick Smith’s CJ3. It looks like an incredible aircraft. The CJ3 climbs directly to FL450 faster than a Citation X. He’s a lucky man to own one.
The article in relation to the Collins Proline 21 equipment states:
“Dick makes the point that this equipment makes a fully autopilot coupled three dimensional GPS approach perfectly possible on any airfield in Australia right now.”

If this is so, it is obvious that an augmentation system such as WAAS is not required for three dimensional coupled approaches, although not having WAAS probably raises the minima from CAT 1. Safety outcomes would obviously be improved with this system, for any operation, but particularly single pilot operation. The statistics make that more than clear. When the weather is the traditional "dark and stormy night", a proper coupled approached is a great aid in the quest to not become a another statistic.
I am told that GARMIN have no equivalent equipment, unless WAAS is available. That is, GARMIN 400/500 series, and their 1000 series cannot do a coupled (LNAV/VNAV) approach that the Collins Proline 21 equipment can do.
Does anyone know why? Is it just a design difference, a certification issue, or what?
Tootle pip!!

[...still single]

The Proline calculates a glideslope that will keep the aircraft clear of the steps. That is all. All of the normal RNAV standards/restrictions still apply.

[LeadSled]

Stillsingle.

The Proline calculates a glideslope that will keep the aircraft clear of the steps. That is all. All of the normal RNAV standards/restrictions still apply.

If it is as simple as that (and we known the ProLine 21 calculates a 3 degree slope - which may or may not keep you above any steps - something not all that common in US, as we know them in Australia) why can't Garmin and other GPS units do the same thing? Surely this would greatly reduce risk, especially if the approach can then be coupled to the autopilot, with the much reduced workload, and the pilot's job is simply to follow through on the controls and monitor the approach progress, until either disconnecting at some point, if visual at minima and completing the landing, or going around.

The nub of the question is, given the dominance of Garmin in GA, why haven’t GARMIN offered this without WAAS, which is how I read the situation.
There must be somebody out there in ppruneland who has the info.

Tootle pip!!

[Defenestrator]

Leadsled,

Your point of question is sound. It's one for the boffins to answer and I'll be watching with interest.

D