Autopilot capabilities

Many small aircraft have a single-axis  autopilot that works with a combination of desired heading (where the bug on the heading indicator is set) and GPS CDI needle deflection.  If HI shows right of course, and the CDI needle shows right of course, the autopilot will input a left roll to try to correct.

This setup allows the aircraft to follow a straight course line, but at the next GPS waypoint with a heading change, the pilot needs to adjust the bug on the HI appropriately for the system to track that next, different, straight course line.  This setup will not allow the aircraft to follow a curved path in the flight plan, at least not without the pilot continually adjusting the HI bug, which is not really practical.

With GPSS, the dependency on the heading indicator bug is removed.  Now, with no action required from the pilot, the autopilot can smoothly fly all the turns in the GPS's flight plan, including course reversals, holding patterns, DME arcs, etc.  Pilot just sits back and watches.

It's a very nice feature.  Just expensive.

I expect the IFD 540 to behave like a GNS 430W in this respect, and the key to determining your GPSS options will be the capabilities and input requirements of the AP…. but one never knows what surprise Avidyne might have up their sleeves.

I have a 430W and Sandel HSI feeding an STEC GPSS which in turn feeds an STEC 30 AP. The GPSS is a stand-alone unit with just a square push-button on the panel that toggles between 2 modes: HDG & GPSS and determines the nav source: AC/DC heading error or GPS roll steering data. The STEC 30 has 4 roll modes: Stabilizer (which is controlled by turn left/right knob), Heading, and Lo/Hi Track. The AP is normally set in HD mode as long as the 430W is producing GPS roll steering data (i.e. following a flight plan or approach procedure), and the GPSS should be in GPSS mode. The GPS roll steering output is wonderful and will make lovely smooth DME arcs, intercepts, turn anticipation, etc, following a GPS overlay of a VOR/LOC proc. If the GPSS is in HDG mode, then the HSI/DG heading bug output is used if the AP is in heading mode or 430W VOR/LOC AC/DC heading error if the AP is in Lo/Hi Track mode (Lo for VOR, Hi for LOC/ILS). I'm not sure, but I suppose that in this case the GPSS is in a sort of pass-through mode of the heading error signal to the AP.

I'm certain that once you have seen GPSS performance, you won't want to be without it due to the accuracy and functionality of the GPS roll steering data. If I recall correctly, the STEC GPSS was about $1000 and really gets the most out of a GPS. I'm not familiar with the capabilities of the Icarus or DAC units. I don't know if the STEC GPSS unit will only mate with a rate-based AP like STEC produces, but perhaps the AC/DC heading error signal is standardised and accepted by both rate and attitude based AP's.

Vince