Andrew Gideon wrote:
I don't understand GPSS. Why is this better than simply setting a GPS to
follow a CDI, and then setting that CDI to take GPS data?

They belong to different layers:

- bottom layer is to keep wings level (or properly banked),
- middle level is to fly a heading,
- top level is to follow a track.

The track following uses heading flying as the tool.
The heading flying uses suitable banking as the tool.

The autopilot can directly control the ailerons
to get the proper bank. For setting a required
banka angle several aileron movements are needed.

The flying of a heading needs several bank angles
to get and keep the heading.

The flying of a track/radial needs several headings
to follow the track.

In control engineering terms, the order of a control
system to directly follow a track with aileron
control will be too high to be realizable.

The CDI difference is what requests heading
changes to follow the track, but to do it
properly, the lower layers (heading control
and roll control) are needed.

HTH

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Tauno Voipio (MSEE, avionics specialist, CFII)
tauno voipio (at) iki fi