Meindert,
Not necessarly true for all gps and autopilots. My Garmin GPS48 stores
waypoints and routes. It outputs, among other sentences, RMB and RMC.
These two sentences provide the following information.
XTE,
Direction to steer (L or R),
Orgin Waypoint
Destination Waypoint
Destination Waypoint Latitude
Destination Waypoint Longitude
Range to Destination Waypoint(nm),
Bearing to Destination Waypoint (T),
Velocity Towards Destination Waypoint(kn)
Time of fix
Receiver warning (A=valid, V=warning)
Fix Latitude
Fix Longitude
Speed Over Ground (SOG)
Course Made Good
Date of fix
Magnetic Variation

My Raymarine ST4000+ autopilot can receive and decode both sentences
when the GPS NMEA output is connected to the autopilot and put in the
auto/track mode. It displays Destination Waypoint number (name), XTE,
Bearing to Waypoint (BTW), Distance to waypoint, Course over Ground
(COG), SOG, Heading, UTC, and Average Speed. For automatic track
acquisition, the autopilot needs both XTE and BTW. The autopilot will
steer with just XTE provided but you must be within 0.1 nm of desired
track and 5 degrees of the bearing to next waypoint when engaging auto
mode. I use the COG, SOG and Heading to determine set and drift. Even
though I have two GPS receivers and a laptop with Visual Navigation
Suite, I plot everything on paper charts and keep my DR figures handy.
krj

Meindert Sprang wrote:
"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