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post #1 of 15 Old 09-25-2008 Thread Starter
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Satellite Compass -- GPS Antenna

Why does a satellite compass cost $3,500? My basic Furuno GP32 GPS unit for $250 gives accurate satellite heading info, so why would anyone spring for a sat compass? Just curious.

Also, is there any advantage to mounting a GPS antenna on the mast? Might there be a universal antenna that I can mast-mount that will last for a few years? I don't have a pole or an arch so my antenna is clamped to the rail in the cockpit. It gets in the way. Thank you.
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post #2 of 15 Old 09-25-2008
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I'm pretty sure you don't want to mount a GPS antenna on the top of the mast. It moves around too much.
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post #3 of 15 Old 09-25-2008
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LoTech-

A GPS doesn't actually give you heading information unless the boat is moving, and it only gives you the direction the boat is moving in, not the direction the boat is pointed in.

For instance, if you're pointed due north, 0˚ and moving at five knots, but have a two knot current from the east, a compass or satellite compass will report your heading as 0˚, where the GPS would have your heading be around 348˚, which is not accurate. The GPS will give you the CMG, not the heading of the boat.

As for mast mounting it... I wouldn't recommend mounting the GPS antenna too high, especially on a monohull. The higher up the GPS antenna is mounted, the more radically it is affected by the boat's motion. Imagine what happens if the boat rolls through 30˚ as commonly happens on a tack, the antenna at deck or cabin top level doesn't do much... but at the spreaders, it ends up moving through a pretty big arc.

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post #4 of 15 Old 09-25-2008
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Hi
I wont repeat the information in the two previous posts, just add..

A satellite compass will give true heading not magnetic heading.
It can replace a gyro compass on comersial boats and i think the primary market for the $3,500 satellite compass are professional boats. Gyro compasses are also costly items.
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post #5 of 15 Old 09-25-2008
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I have a KMV AC105 fluxgate, a Raymarine chartplotter and a Ritchie Globemaster. Realistically, it is the fluxgate that is most accurate compass. I would consider a satellite compass for the reasons given above, and because I have a steel boat and it might be quite difficult to have the Ritchie "reswung" for the Southern Hemisphere. It's perfect at the moment in all directions (10.5 degree W variation), but I might not notice fluctuations in other parts of the world.

I freely admit a satellite compas is more than most need, as the error in a compass bearing over a short distance is rarely an issue. An error on a 1,000 mile passage can be dangerous, and more information is good.
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post #6 of 15 Old 09-25-2008
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I'm not sure I follow the logic of not mounting the GPS antenna on the masthead. I agree that it's not the best place, but not for the reasons stated.

Each GPS satellite orbits at an altitude of 20200 kilometers and moves at a speed of about 14000 km/hour. Whether the antenna is at seal level, or 50 feet up, or 100 feet up is of no consequence at all. Yes, the changes in the heel and pitch of the boat are exaggerated at the mast head, but this is still negligible compared to the speed of the sats.

With WAAS and a clear day with 12 birds in view (pretty normal for a sailboat), you can achieve an accuracy maybe as good as 14 feet. If the idea is that having the mast head outside the rail when viewed from the top impacts where the GPS says you are, I would not worry too much.

For these reasons, there is no harm in putting it there, but also there is no benefit for the same reasons.

The only big concern I would have about the mast head mount is the length of the feed wire, which is going to be lossy. If you are mounting an entire receiver up there then this is not an issue as the unit is sending data down the wire. If it is an actual antenna, use a good quality feedline and it should be fine.

Of course, space on the masthead is at a premium. Seems a waste to put a GPS antenna up there. This, and the feedline issue, are why I would not put the GPS antenna on the mast head.
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post #7 of 15 Old 09-25-2008
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Quote:
Originally Posted by sailingdog View Post
LoTech-
As for mast mounting it... I wouldn't recommend mounting the GPS antenna too high, especially on a monohull. The higher up the GPS antenna is mounted, the more radically it is affected by the boat's motion. Imagine what happens if the boat rolls through 30˚ as commonly happens on a tack, the antenna at deck or cabin top level doesn't do much... but at the spreaders, it ends up moving through a pretty big arc.
I think you are on the wrong tack here. What matters for a GPS antenna is not its position, but where in the sky it is pointing. If the angle changes, you may lose some of the satellites. And the angle change is the same for the stern rail and the mast head.

Fluctuat nec mergitur
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post #8 of 15 Old 09-25-2008
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Quote:
Originally Posted by Valiente View Post
I would consider a satellite compass for the reasons given above, and because I have a steel boat and it might be quite difficult to have the Ritchie "reswung" for the Southern Hemisphere. It's perfect at the moment in all directions (10.5 degree W variation), but I might not notice fluctuations in other parts of the world.
The variation is dictated by your geographical location and is accounted for when doing stuff like calculating CTS. Deviation is dependant on the magnetism built into the boat and doesn't change unless something in the boat changes. So a change in geographical location will not require the compass to be re-swung no matter which hemisphere you're in.

What can change from N to S is that the card in the compass has a tiny little weight on the underside that keeps the card level. When you change from one hemisphere to another, it is POSSIBLE that the card will lie at a strange angle brought about by large changes in magnetic variation from one hemisphere to another but the headings will not change in terms of deviation.

If the compass is a good quality one, then this problem shouldn't manifest itself (mine didn't from US to NZ). I had a bulkhead compass (Suunto, so not junk) years ago on which the card was at a daft angle and the compass repairers didn't want to try and fix it - don't know why. I eventually threw it out.

Oh and I know that the fluxgate in my autopilot can be calibrated to match the deviation in the steering compass. Don't know if your fluxgate can or not but worth bearing in mind in case someone has fiddled with it.


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post #9 of 15 Old 09-25-2008
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JArcher/Pegasus-

If that were the case, why would the Garmin antenna mounting instructions say this:

Quote:
On a sailboat, avoid mounthing the GPS 17x antenna high on the mast, to prevent inaccurate speed readings caused by excessive heeling.

The GPS 17x antenna provides more-stable readings when located nearer to water level.

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post #10 of 15 Old 09-26-2008
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Thanks, Andre. It's early days yet...frankly, I have all the necessary bearings for my local sailing memorized in M and T.
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