Simrad AP 26 autopilot interfaced with new Garmin GPS

We are beating a dead horse and perhaps arguing semantics. But I think it is important to acknowledge the distinction between "heading" via a compass and COG, via the GPS. They are not the same and not interchangeable although in most cases they should be close. I have an extensive Garmin system and am very familiar with how it operates. First, an MFD/plotter does not require any device that provides electronic heading information unless you want to use features such as MARPA or Overlay. If you want heading, you absolutely can NOT get "Heading" data from the MFD/plotter/GPS and it will NOT act as a compass. You'll need a actual compass, usually the AP in most set-ups. Otherwise "heading" field will be blank on the Garmin display. This is my issue with the term "Garmin Compass". There isn't one.

We are all speculating on the OP's setup but my reasonably informed opinion is that when he refers to his "heading on GPS" he's likely referring to his COG which roughly matches his helm compass as one would expect. What's not matching is his AP heading display, likely a calibration or interference issue as we all seem to agree. But that mismatch has nothing to do with their ability to interface since they could be completely detached from each other and should still approximately agree.

Agreed.

Agreed. Don't think anybody has mentioned anything different.

Agreed.

I think here is the rub; Just removed the same AP from my boat after using it for 15 years. Not in this case.

How about the term; Compass for use by the Garmin.

Agreed.

Agreed.

Oh, my. Dead horse whippings over nomenclature choices. F'ing headache. The choice of the term "compass" was picked from the very common and routinely present fluxgate appendage that exists in many, many installations. A chart plotter knows the location and the direction you are headed, speed over ground, time and distance to a waypoint. Either an MFD has this built in or uses an appendage unit. But either way it uses a "compass" for heading information. The AP needs the same information. It uses a similar source of data. In this case those sources are different and not aligned. That's the whole **** point. Select the same unit for all and/or calibrate all if they can't share the data due to vintage.

Holy smokes...

Sorry you seem exasperated but, I take exception to someone defending their position by suggesting I don't understand a simple reference. The reference was not simple. It was wrong. There is no Garmin compass which is why I was baffled. There is a Simrad compass. Their is Garmin COG.

I will leave you the last word. I have said my piece..

Fine that you're finished speaking, and I'm wrong plenty, learn often, but this is a freaking cut and paste from the Garmin website:

For best results, use the autopilot CCU internal compass for the heading source. Using a third-party GPS compass can cause the data to be delivered erratically and may result in excessive delays. The autopilot needs timely information, and therefore cannot often use third-party GPS compass data for GPS location or speed. If a third-party GPS compass is used, the autopilot will likely report loss of navigation data and speed source periodically.

If you have more than one heading source on the network, you can select a your preferred source. The source could be a compatible GPS compass or a magnetic heading sensor.

Garmin apparently agrees that their MFD needs a reliable heading source from the vessel's electronics system. So, either I was accurate in my suggestion, or we can all get hung up on nomenclature interpretation while parsing "heading source" versus "compass". Maybe we can just agree to reference the device as anything other than "banana" which should never be aboard.

Hard to figure an autopilot that doesn't have a compass. What you and I know as a compass - a device that detects the Earth's magnetic field. Simrad calls it a rate compass (additional information in addition to magnetic heading). It appears, rtrafford, you have discovered that the Garmin autopilot has a CCU internal compass. All is well.

One (of several) reason that an autopilot needs a compass, and that the heading information that can be gleaned from a GPS does not suffice is this. When the vessel gets to a waypoint (a position via GPS) and the autopilot needs to alter course to the next waypoint, the autopilot program looks at many rapid immediate changes in heading while directing the helm to move the vessel quickly and accurately to the new course. It cannot wait long enough for the GPS to figure out the new heading by covering a distance, and at 25-35 knots, you would not want it to. Forgive the jargon, but the autopilot control algorithm is actually doing these corrections and making these adjustments in millisecond or better time.

Autopilots using a compass (heading only - not for waypoints) have been working on the water for a long while before GPS.