Dave S wrote:


Tauno Voipio wrote:


The GPS heading information is taken as differences
of position fixes. It is prone to pretty bad errors,
and probably of too low quality for AP reference.


I have flown AP equipped planes that take their steering from the CDI...
which in some cases is driven by a VOR/LOC and in some cases driven by a
GPS. Direct heading info, no.. deviation from desired course... yes.


The radial / GPS track following in an autopilot has
three control loops:

- innermost loop controls roll with ailerons and
taking reference from horizon gyro,

- next loop controls heading taking reference from
heading gyro,

- outermost loop tracks the navaid course difference.

The outermost loop sets the required heading for the
middle loop, and the middle loop sets the required
roll angle for the innermost loop.

The GPS heading is ill suited to the middle loop
due to the inherent noise enhancing property of
a differencing method. The noise may be attenuated
by filtering, but then the heading reference is
too slow for acceptable control loop stability
and speed.

--

Tauno Voipio
tauno voipio (at) iki fi