Sat Compass

My understanding; you have to be moving and you get the direction you are traveling.
Not the way you are pointing.
Course over ground, I think is the term that still applies.

For lots of reasons, nothing beats an Ole Mag Compass.

I don't think that's correct, CR.
I'm aware of some Furuno stuff capable of giving the vessel heading (i.e., not the COG, which you can get with any plain vanilla GPS, mobile phones included) within a 1 degree accuracy at zero speed, and 0.4 while under way. Not to mention that they can also provide pitch and roll in real time.
Pretty impressive instruments.

Now, if you ask me if I'm rushing to spend 2k$ or so for replacing my mag compass, my personal answer is of course not.
But it's just small change in the grand scheme of some modern electronic suites...

I'm Game, Less two receivers on board, somebody please explain to us (me) how this mess works.

BTW, My cell phone has a compass in it. But it's still fluxgate, not sat driven.

GPS can only the supply position of the receiver. Well... it’s antenna. Unless you have two receivers with antennas mounted far apart I really don’t see how it can give you a heading while not moving. Antenna would have to be apart enough for the combined error not to become irrelevant.

Someone with a higher pay grade than myself will surely be able to give you more details, but essentially the principle is that there are at least two (or more, I believe 4 in the more sophisticated units) GPS receivers inside the sat compass radome, which must be precisely positioned along the longitudinal axis of the vessel.
And due to the internal distance between them (which is obviously known and very accurately calibrated), there is a very tiny difference in the sat wavelength that each of them receive. On that basis, the internal processor can determine the exact radome (hence vessel) heading.

Ref. cellphones, I was not referring to the sort of fluxgate that most of them have these days, and which can give you a ballpark heading (of the cellphone itself, and good luck in placing it exactly aligned with the boat), but nowhere near as accurate as any sat compass, obviously.
I mentioned the COG because any gps app, as soon as (and only as long as) you are moving, can calculate your COG - with a quite good accuracy, in fact.
And they can do that regardles of whether you are walking, driving, boating, or whatever.

Your "unless" is precisely how a sat compass works, except the "far apart" bit - see my previous post.
In fact, the really impressive bit is that they can do that also with receivers placed inside a pretty small (25" or so) radome, hence with a distance which by gut feeling nobody in his right mind would expect to be enough for a decent accuracy - but it is!

When GPS precision is still one meter on a good day, I don't see 25" or so working well.
But, your description of how it works would be the theory, I assume.

Let me ask an other question, The Merc and VP Dynamic Positioning System uses fluxgate or sat compass?

Sorry, can't answer your last question.
Those gizmos never interested me enough to bother understanding them.

Ref. GPS accuracy, I did see your skepticism coming!
I believe that positioning accuracy doesn't have much to see with the sat compass logic, which is only aimed at identifying the relative, not the absolute position.
I mean, whenever one antenna receives a clear signal from one satellite, obviously also any other antenna nearby gets the very same signal.
And a computation of the difference in the same signal between each antenna is sufficient to determine the heading (as well as pitch and roll, as I said) with extreme accuracy, regardless of whether you are within one or one hundred meters from where your plotter says you are.

Yep.
Thanks for trying to explain it.
Still going to keep and trust my BA Danforth compasses and bearing tools.

Agreed, I can't actually remember to have ever used the mag compass, but it does look nice in the p/house!

Have a few fluxgate compasses on board for the stuff to use. Ah yes, I luv APs and radar over chart displays.
I also have a computer screens or TVs on board that can show real time chart data while cruising or at anchor.
But I luv to just look over the helm compass and pick out a mag bearing. Something about just watching that card hold steady or swing in a turn to help feel a part with the ship.
And when it is rough off shore, that card will remind you how bad and help find more comfortable heading.

I ask my stepson to shoot hand bearing lines sometimes on coastal runs and compare his DR to the displays.
Darn kid has gotten pretty good at it.
My father trained me the same way long before todays stuff & gizmos.

All this compass text has me wondering, Been over 10 years ago since my last compass check up. May be time again.

Do you? Interesting.
The MFDs of my current boat (and of a few others which I helmed) do allow overlay, but I never use it. Just a matter of habit I suppose, but I'm so used to keep the radar head up and charts N up, that I find overlay confusing - as long as you can keep radar and charts in two screens in parallel, of course.
In fact, I even wondered if Marblehead didn't actually mean "plotter and AP", rather than "plotter and radar".
When used head up, obviously the radar doesn't need any heading signal.

I never use radar overlay either. Hate it. Don’t see the point. .

Oh and I always set my plotters North up! I get confused if course up.

Well, great minds think alike, as the old saying goes!
I'm so used to looking at and working with N up oriented maps, that I keep them that way also in my car navsystem.
Actually, I see the point of head up orientation while driving, particularly when approaching complicated junctions.
But I have a small window that pops up automatically for that purpose, so I prefer to keep the map N up, for an overall view.

Running the upper St Johns river with its twist and turns, I find heads up charting much easier on my main screen.
Elsewhere, during the day, most in north up.
I may leave the main screen in heads up then.
At night, all my play toys are off, but one PC screen in night vision mode and very dim. That is when heads up chart with radar and AIS overlay works very well for me.