Satellite Compasses?

[Dockhead]

Originally Posted by noelex 77

I cannot see how a satellite compass with just two GPS units could output Roll information.

The SC30 has a 3-axis gyro in addition to GPS units. This would have to be used for the roll information as there is no way to determine this from the GPS signal (if there are 2 GPS units mounted on the longitudinal axis). I would also expect the information from the gyro is also used for pitch since a GPS unit's estimation of elevation is much worse than its horizontal accuracy.

The Trimble would provide exceptional heading and position information, but I don't think it has a gyro so it could not provide roll information.

I see -- yes, it doesn't seem to have a gyro, which is a shame. I think you need the gyro to correct the heading data, too, on a small boat which is thrown around by the sea. Hmmm.

[goboatingnow]

Hi, I have been involved in Sat compasses,

Couple of things

Sat compasses derive their measurements from two receiving antennas, and use carrier phase relationships to determine the relative aspect of each antenna to the other, as opposed to actually decoding the GPS signal ( which they also do) to determine absolute aspects.

They can resolve such information in 3 dimensions , giving roll and yaw as well.

Because they suffer none of the issues around fluxgate compass, ( damping, dip, etc) they can calculate position very fast , with typical rates upto 20Hz.

Specific directional accuracy is improved by the distance the antenna are apart, but the time to first initial solution goes significantly. Im my own systems , 1 metre apart was about the best compromise.


IN my experience, directional accuracy is not actually that important, whats important is directional stability. ( or repeatability) , Sat compasses are very good at this and its this stability that improves the systems like North up or course up radar/chart plotter displays and particularly ARPA acquisition


As for accuracy, with large antenna separation and the acceptance of slow start up, absolutely amazing precision can be achieved

These are the specs from the Hemisphere Cresent Vector 200 OEM board with different antenna spacing ( this is the latest OEM module)

Base RTK accuracy is 10mm ( yes millimetres ) horizontal and 20 mm vertical ( this is a relative accuracy not absolute)

"
0.30 degrees ( RMS) 0.5m antenna sep
0.15 degrees ( RMS) 1m gap
0.08 degrees( RMS) 2M gap
0.04 degrees ( RMS) 5m gap

pitch and roll accuracy < 1 degree RMS
Heave accuracy 30cm
rate of turn 145 degrees/sec

Updates Rates upto 20 Hz


As an input to APs and stabilised plotters they are the DBs

[goboatingnow]

Originally Posted by noelex 77

I cannot see how a satellite compass with just two GPS units could output Roll information.

The SC30 has a 3-axis gyro in addition to GPS units. This would have to be used for the roll information as there is no way to determine this from the GPS signal (if there are 2 GPS units mounted on the longitudinal axis). I would also expect the information from the gyro is also used for pitch since a GPS unit's estimation of elevation is much worse than its horizontal accuracy.

The Trimble would provide exceptional heading and position information, but I don't think it has a gyro so it could not provide roll information.

Have a read around RTK GPS, The decoded GPS position is only used to establish absolute position, the phase relationship of the GPS carrier as it reaches the antennas is used to provide extremely high relative aspect resolution, down to fractions of a degree if required. ( get that from your flux compasses !!!)

Since aspect is determined in three dimensions, roll, yaw and heave can be determined as well.

Most Sat compasses contain a solid state gyro ( since these are dirt cheap now-days ) that is used in two situations, one it helps the time to first computation algorithm and it is also used if the antennas are blocked for a short time , the gyro takes over providing relative 3 axis output until the GPS antennas re-compute, This often happens due to multi path distortion, physical blockage, and less often due to sat position.

[Jd1]

Forgive the stupid question .... other than for doing survey work or holding position over a bore hole, why is that level of accuracy of position and/or heading important?

[David M]

Originally Posted by Jd1

Forgive the stupid question .... other than for doing survey work or holding position over a bore hole, why is that level of accuracy of position and/or heading important?

It's not a stupid question.

For recreational boating, it's not mandatory to be that accurate. For a survey vessel, it is important to have sub one-meter accuracy. For a research or salvage vessel, sometimes it is depending on what is being done.

For holding a floating unanchored drilling platform or drilling ship over a deep water bore hole, it is done with dynamic positioning using a submerged ranging system using sound or with GPS or both.

[Dockhead]

Originally Posted by goboatingnow

Hi, I have been involved in Sat compasses,

Couple of things

Sat compasses derive their measurements from two receiving antennas, and use carrier phase relationships to determine the relative aspect of each antenna to the other, as opposed to actually decoding the GPS signal ( which they also do) to determine absolute aspects.

They can resolve such information in 3 dimensions , giving roll and yaw as well.

Because they suffer none of the issues around fluxgate compass, ( damping, dip, etc) they can calculate position very fast , with typical rates upto 20Hz.

Specific directional accuracy is improved by the distance the antenna are apart, but the time to first initial solution goes significantly. Im my own systems , 1 metre apart was about the best compromise.


IN my experience, directional accuracy is not actually that important, whats important is directional stability. ( or repeatability) , Sat compasses are very good at this and its this stability that improves the systems like North up or course up radar/chart plotter displays and particularly ARPA acquisition


As for accuracy, with large antenna separation and the acceptance of slow start up, absolutely amazing precision can be achieved

These are the specs from the Hemisphere Cresent Vector 200 OEM board with different antenna spacing ( this is the latest OEM module)

Base RTK accuracy is 10mm ( yes millimetres ) horizontal and 20 mm vertical ( this is a relative accuracy not absolute)

"
0.30 degrees ( RMS) 0.5m antenna sep
0.15 degrees ( RMS) 1m gap
0.08 degrees( RMS) 2M gap
0.04 degrees ( RMS) 5m gap

pitch and roll accuracy < 1 degree RMS
Heave accuracy 30cm
rate of turn 145 degrees/sec

Updates Rates upto 20 Hz


As an input to APs and stabilised plotters they are the DBs

I was wondering when you would wander by!


OK, so do I correctly understand that: 1. satellite compasses do pitch & roll with no problem and no need for gyros? 2. they don't need gyros in any case?

So what would your advice be? The Vector Compact is economical but seems underqualified. Unless the declared 2 degrees RMS is meaningless, if, as you say, it's the repeatability which is really important, and not the accuracy -- what do you think?

The other commercial solutions receive only GPS (no GLONASS, no Galileo, tc.) and are far less accurate

The Trimble looks like the business but fairly expensive at $4k. A bit hard to justify.

Any other solutions I should be looking at? I see that Hemisphere produces a complete compass based on that board, but I can't find a price anywhere.


For non-Brits trying to interpret Dave's last post -- "DB" means really great; the best -- from Dog's B-------s, the dog's dangly bits.

[Dockhead]

Originally Posted by David M

For recreational boating, it's not. For a survey vessel, it is. For a research or salvage vessel, sometimes it is.

Not so, at least where heading is concerned.

Accurate heading data is crucially important to autopilot performance, as well as to a number of other functions.

The autopilot works better and better, the better it understands which way the boat is pointed, and the more frequently that information is updated. The difference in autopilot performance between a crappy unstabilized fluxgate and a gyro stabilized fluxgate is amazing.

MARPA will not work at all, practically, without accurate heading data. That was the case with my former electronics system with plain fluxgate compass. Now that I have a really good fluxgate with three-axis stabilization (Airmar H2183, supposed to be the best), MARPA works somewhat, but still not all that well. It's really valuable that it works at all, but it would be great if it would work really well, and for that you need better heading data than what I get from the H2183.

Radar overlay will not work without accurate heading data. This is a really useful function, useful for a number of different purposes -- navigation (checking charts against reality), and using the radar with the chart display active. And altogether -- the information you get from your radar about bearing to a target is only as good as the heading data which goes in. On this the usefulness of radar in collision avoidance directly depends.

Then knowing something about true wind, leeway, tidal set from your instruments -- also depends on accuracy of heading data.

I would pay a fair amount of money to solve all of these things, maybe not $4k.

A bit of thread drift, but the other data which I really need and don't have is speed through the water. My Airmar CS4500 ultrasonic log transducer doesn't work as well as I expected. Better than a paddlewheel transducer, but not as great as I hoped.

Accurate STW data is important for:

1. Understanding sailing performance
2. Knowing from your instruments tidal set and drift
3. True wind
4. Dead reckoning (if you ever need that)

[goboatingnow]

Originally Posted by Dockhead

I was wondering when you would wander by!


OK, so do I correctly understand that: 1. satellite compasses do pitch & roll with no problem and no need for gyros? 2. they don't need gyros in any case?

So what would your advice be? The Vector Compact is economical but seems underqualified. Unless the declared 2 degrees RMS is meaningless, if, as you say, it's the repeatability which is really important, and not the accuracy -- what do you think?

The other commercial solutions receive only GPS (no GLONASS, no Galileo, tc.) and are far less accurate

The Trimble looks like the business but fairly expensive at $4k. A bit hard to justify.

Any other solutions I should be looking at? I see that Hemisphere produces a complete compass based on that board, but I can't find a price anywhere.


For non-Brits trying to interpret Dave's last post -- "DB" means really great; the best -- from Dog's B-------s, the dog's dangly bits.

At the low cost end , things are a little tricky, combinations of cheap hardware, small antenna separation etc, can lead to issues. Theres no easy answer, going below 0.5 degrees tends to mean big antenna separation etc.

The Vector OEM board is about $1000 , you need to encase it and add antennas, you could roll you own !. ( I wouldnt recommend that )

sat compasses can resolve relative positions in 3D. since roll , yaw and heave and generally only relative measurements, they can do that without gyros. vertical accuracy is not as good as horizontal accuracy, but more then adequate for marine use.

really without detailed specs, its hard to evaluate sat compasses, its often a suck and see. I do find them more and more common on fishing fleets

[David M]

I have to say that my Furuno ARPA system works fantastic with a fluxgate and a plain old WAAS GPS. I have used shipboard systems that use gyros and GPS heading systems and they are not much more accurate or stable. For recreational use, I don't think it is worth spending gobs of money for a little more accuracy.

Bottom line, you still get a good RML (e to M) which is good enough to create a new heading to get a desired CPA.

I have an unlimited radar endorsement on my license.

[goboatingnow]

Originally Posted by David M

I have to say that my Furuno ARPA system works fantastic with a fluxgate and a plain old WAAS GPS.

How do you know it works fantastic ?

Have you a way of independently confirming vector information , say by AIS or something else.

its often very hard to know, if the info coming back is meaningful

[Dockhead]

Originally Posted by goboatingnow

At the low cost end , things are a little tricky, combinations of cheap hardware, small antenna separation etc, can lead to issues. Theres no easy answer, going below 0.5 degrees tends to mean big antenna separation etc.

The Vector OEM board is about $1000 , you need to encase it and add antennas, you could roll you own !. ( I wouldnt recommend that )

sat compasses can resolve relative positions in 3D. since roll , yaw and heave and generally only relative measurements, they can do that without gyros. vertical accuracy is not as good as horizontal accuracy, but more then adequate for marine use.

really without detailed specs, its hard to evaluate sat compasses, its often a suck and see. I do find them more and more common on fishing fleets

Rolling my own could be an option -- the IT guys in my office are not that busy, and love this kind of stuff. One of them used to design and build missile guidance systems for the Soviet military. I would guess software would be the biggest challenge of that solution, or maybe the board already comes with software?

[Dockhead]

Originally Posted by David M

I have to say that my Furuno ARPA system works fantastic with a fluxgate and a plain old WAAS GPS. I have used shipboard systems that use gyros and GPS heading systems and they are not much more accurate or stable. For recreational use, I don't think it is worth spending gobs of money for a little more accuracy.

Bottom line, you still get a good RML which is good enough to create an accurate CPA to be used for collision avoidance.

I have a unlimited radar endorsement on my license.

Well, you're lucky. Mine works, but not fantastic, despite the high resolution CW radar (Navico 4G). Maybe Furuno has some unique algorithm to average the heading data? On my system, you can clearly see how the solutions are degraded by defects in heading data, because it paints a series of dots on the screen showing the calculated position of the target at every sweep. You can see how the target jerks around due to uncertainty about heading, and the result is that the system cannot calculate an accurate course or speed.

[David M]

I totally agree if the range of the contact and your boats heading is bouncing around that you are going to get less accurate RML of the contact.

I know my fluxgate has some sort of averaging for "smoothness" as Furuno calls it. I do not know exactly how the averaging is done. It's a Furuno PG500R.

[goboatingnow]

Originally Posted by Dockhead

Rolling my own could be an option -- the IT guys in my office are not that busy, and love this kind of stuff. One of them used to design and build missile guidance systems for the Soviet military. I would guess software would be the biggest challenge of that solution, or maybe the board already comes with software?

No the board comes ready to go, its basically encasing it , you can use two conventional GPS antennas ( 5V DC, active but not with integrated GPS obviously ) as the antennas, mounted accurately on a linear horizontal mount.

The dealer for Hemishpere was on the Isle of Man, I brought originally through a dealer in Azriona , its a few years back, I still have the original Vector OEM unit ( 0183 only )

[goboatingnow]

Originally Posted by David M

I know my fluxgate has some sort of averaging or damping. Beyond that I do not know the details.

As far as range goes, that should be pretty consistent.

There are several things that affect APRA performance

(a) Lock persistence
(b) vector computation performance
(c) stability of heading ( in 3D movement )
(d) response to heading change of your boat
(e) radar performance

MAPRA units typically use the pixels on the screen to lock onto. SO as long as the target remains painted reliably, the lock should remain. Its all a function of your radars software algorithm. Some will persist with the lock in the absence of a painted target or a clear response, but of course they will use the old position

heading stability and responsiveness is a combination of the time to recompute and redisplay the vectors and accuracy depends on how close the boats heading is measured by the heading sensor.

The result is most MARPA units give you vectors, what you don't know is if to believe them .?

A clear test is in bumpy conditions, you lock to a sizeable target, if you get consistent reliable course and speed and then best of all compare it to his AIS readings , you should be able to form an opinion as to the veracity of your MARPA system. IN a perfect system, his values should never change irrespective to what your boat is doing