Auto Pilot

[ScaleRobotics]

The Amicus platform is very nice. For a gps, a shield might be overkill for just 3 or 4 connections. These are a bit smaller, have WAAS capability, don't require a separate antenna, and are cheaper. They are 3.6v. http://www.sparkfun.com/products/10702

[kellyseye]

WAAS not withstanding you're still looking at an 'error' greater than the width (or length) of the vessel - the difference between docking and missing altogether. GPS will get you in the general area but you would need to go 'manual' or use a more 'local' reference system to get the docking capability.

[mackrackit]

An external antenna is nice for when the unit is inside a structure such as a cabin. Not sure about the cost difference either when one is a "raw" component and the other is ready to use. But it was just a suggestion.

[timmers]

Given the complexity of driving steering servos, pid control filter setups and all the other tweaks required with the potential cost of an ooops moment... buy a marine autopilot that has a warranty / come back!
Raymarine do a good system which can track a gps route from a standard nmea interface. Most gps systems (not all) have a breadcrumb trail to return you to the start.

As a skipper, I can confirm that they do a very good job, providing you programme the gps correctly!