snip

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

Wrong....

S-tec AP's do not take any info from the AI. They are rate based not
attitude based and get their roll info from the Turn Coordinator.

The GPS Steer function is a true wonder to behold. Both in simplicity
and accuracy. You know not of which you speak... No offense... :-)

Jon Kraus
'79 Mooney 201
4443H @ TYQ



Tauno Voipio wrote:

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.