"Jack Erbes" wrote in message
...
Meindert Sprang wrote:
snip
Are there nav packages that control autopilots? All the autopilots I've
seen had their own processors and processed a magnetic heading input.



If your nav package only receives a position and heading from the GPS
and
this nav package maintains a route, it can calculate the cross track
error
(XTE) and send that info to an autopilot.

That is sort of what I meant or was thinking. The autopilot processes
the NMEA inputs from the heading sensor, GPS, and nav package and then
does the steering.

That is not what I said. In my example, the only thing the autopilot sees is
the XTE.

I thought the implication was that there were nav
packages that actually processed the data and steered the boat.

Precisely. Imagine the following: the GPS only delivers the actual position
to your nav program. The nav program knows the waypoints of your route, they
are entered in the nav program, not in the GPS. So the nav program knows the
straight line between two current waypoints and calculates the error from
this line based on the current position from the GPS. This error is sent as
an XTE sentence to the autopilot which does nothing more that to steer until
the received XTE from the nav program is 0. So in this case the only one
doing the calculations and the processing, is the nav program. The XTE to
the autopilot is just an error signal in a control loop. The autopilot does
it's calculations to mimize the XTE while the nav program does the
calculations to derive the error from the current track.

Meindert