How does OpenCPN compare to B&G, Raymarine and Garmin ? New boat install ?

[Dockhead]

Originally Posted by goboatingnow

Heading is not computed in a sat compass that way as it’s too slow and subject to GNSS errors.

The system uses Real time kinematics and either a complex single receiver and two antenna of two receivers one slaved to the other

The phasing relationship of the gps signal received at the multiple antenna is then used to compute heading information the actual GNSS signal doesn’t actually need decoding per se to compute relative heading

I have a hemisphere sat compass.

Right, and it was you who taught me about this years ago

"Differential mode", that is DGPS etc., has nothing to do with it, because position calculations are not used (DGPS is not available more than 200 miles offshore anyway). The range to the satellite is calculate with extreme accuracy, a few orders of magnitude greater accuracy than is possible for position calculations, using the phase of the carrier wave. It's a very cool concept. https://en.wikipedia.org/wiki/Real-t...phase_tracking

That's why satellite compasses can work with baselines of only some centimeters, instead of many meters.


You're now using a Hemisphere? I thought you rolled your own, back then.

[Fore and Aft]

Thanks Dockhead I am really interested in what you're saying. Is it easy to interphase AIS with Open CPN or with a dedicated marine unit? Raymarine units seem to be just plug and play?
I don't think comparing a drone and to a yacht is a fair comparison. No drone I know fly's 24 hours a day in constant motion, sometimes violent for extended periods.
Cheers

[AboutTime1]

Originally Posted by Dockhead

The motion of a yacht sailing hard in a heavy sea is an order or two of magnitude more complex, than the motion of a drone pulling g's in a highly uniform medium like the air.

Not as far as computing its location or speed.

False. You are new to sailing I guess? We sail in the sheer between air and water, so true wind for sailors is water-referenced, not ground-referenced.

You said you wanted true wind, ie what you'd see on land. If you want wind referenced to water, that is different thing.

[Franziska]

I guess it's a question of perspective and budget.
Many approaches just work fine.

Boats have successfully and safely navigated the 7 seas with:

*Sextant, chronometer and compass and paper charts.

*Simple PC with OpenCPN & a simple NMEA 0183 sensor package to it.

*Ohers have added lots of more evolved equipment like chart plotters, radars and more to it and equally arrived at their destinations.

If you are more budget oriented you can easily do it with less evolved systems, everybody did that 20-30 years ago.
If a lightning strikes everyone is back to method one :-)

Today you just add redundancy with perhaps adding a second laptop & GPS or waterproof tablet/phone (setup ready to deploy) stored safely away.

People with bigger pockets tend to go for more expensive solutions which may be more reliable but are often more complex too opening up more failure options too.

So, you do not need that expensive package from B&G or others. You may want it for comfort in navigation or increased perceived safety or love of gadgets or just because you can.

That's all fine, but, you can definitely go cruising without it.

Peace & flowers :-)

[AboutTime1]

Originally Posted by goboatingnow

Heading is not computed in a sat compass that way as it’s too slow and subject to GNSS errors.

The system uses Real time kinematics and either a complex single receiver and two antenna of two receivers one slaved to the other

See my previous comment on sample frequency, IMUs and Kaufmann filtering !

The phasing relationship of the gps signal received at the multiple antenna is then used to compute heading information the actual GNSS signal doesn’t actually need decoding per se to compute relative heading

How do you think those vectors get calculated ?

[AboutTime1]

Originally Posted by Dockhead

"Differential mode", that is DGPS etc., has nothing to do with it, because position calculations are not used (DGPS is not available more than 200 miles offshore anyway).

The receivers used in the GPS (GNSS) compass will use differential mode between them, not between them and a land based reference.

The receivers in the GPS based compass don't care if they are meters out from their actual position, because they'll both be meters out. But the relative accuracy between them will very good.

[AboutTime1]

Originally Posted by Franziska

I guess it's a question of perspective and budget.
Many approaches just work fine.

Boats have successfully and safely navigated the 7 seas with:

*Sextant, chronometer and compass and paper charts.

*Simple PC with OpenCPN & a simple NMEA 0183 sensor package to it.

*Ohers have added lots of more evolved equipment like chart plotters, radars and more to it and equally arrived at their destinations.

If you are more budget oriented you can easily do it with less evolved systems, everybody did that 20-30 years ago.
If a lightning strikes everyone is back to method one :-)

Today you just add redundancy with perhaps adding a second laptop & GPS or waterproof tablet/phone (setup ready to deploy) stored safely away.

People with bigger pockets tend to go for more expensive solutions which may be more reliable but are often more complex too opening up more failure options too.

So, you do not need that expensive package from B&G or others. You may want it for comfort in navigation or increased perceived safety or love of gadgets or just because you can.

That's all fine, but, you can definitely go cruising without it.

Peace & flowers :-)

Thanks.

[seandepagnier]

Originally Posted by Dockhead

bright daylight visible waterproof screens

add some bright leds to backlight of a screen and you have a daylight visible screen.

, they have waterproof connectors for network and power.

I use waterproof connectors, they are < $1 each connector.

They have integrated controls which you can use even with gloves on.

I dont wear gloves.. but you mean a tactile knob? It is not difficult to wire some of these up.

To recreate this yourself would cost multiples of what the plotter costs, and would never be as good

I disagree. It would cost a fraction and be a lot better to make your own. The most obvious advantage is the ability to reuse individual components. OpenCPN is more powerful than the alternatives if you consider the abilities of the plugins.

Well, give it a try and let us know how it works, but I rather doubt that this is better than any given run of the mill fluxgate, which are all inertia compensated by now.

unlikely fluxgate is compensated. Fluxgate uses what looks like coils of wire inside. This is basically obsolete technology. Modern compass uses magnetoresistive, or hall sensors, not fluxgate. hall sensors are less sensitive (though sensitive enough) but produce more repeatable results which is why I prefer them.

Satellite compasses have a lot of advantages, and the new Furuno one has sub one degree dynamic heading accuracy and outputs high frequency high accuracy 3D motion (all Six Degrees of Freedom) and attitude data. The attitude data likewise has sub one degree dynamic accuracy. The quality of this data is crucially important to autopilot and radar performance. Even the best fluxgate compasses leave something to be desired.

Again, this is not a fluxgate you are comparing it to. Yes differential gps have the ability to produce data the imu cannot. This data could be useful and could improve autopilots as well as things like automatic calibration of the other sensors, but most of us have never used this.

On what basis do you say this? If this is true, then that's interesting, but I'd like to see the facts. I would be quite surprised if the application is more demanding than for a yacht autopilot, which requires very complex calculations as the vehicle is interacting with moving sea. The motion of a yacht sailing in a moving sea is much more complex than the motion of an aircraft.

The drones are moving much faster and turning a lot faster. The calculations needed are of different frequency for different boats. Larger (and/or slower) boats therefore need fewer calculations to hold a course since they turn slower and dont get tossed by waves as much. The same boat in different conditions can take 50 times more energy for an autopilot to hold course.

So in other words, the drones need to recalculate at a higher frequency, and for more axes of freedom, but the calculation needed is less demanding than the one for the boat in waves (in theory anyway) to get the "best" performance.

Speed through water is measured on big ships with doppler speed logs. These have not so far been miniaturized sufficiently to use on our boats. We have paddlewheel transducers which are not very good -- influenced very much by the boundary layer and requiring a lot of compensation to give decent data.

With enough passes in known areas without currents, it's possible to calibrate the paddle wheel.

Originally Posted by AboutTime1

LOL.
That can be easily fixed.

Have you fixed anything in opencpn? You might remove "easily" but yes, anything can be fixed.

A GPS based compass is just 2 GPS receivers placed some distance apart. You can get the heading of the boat by calculating the heading vector between the two receivers. They'll be operating in differential mode, so the distance error between them will be small.

You may know this, but you cannot just take 2 ordinary gps and make this. The data will be unusable. It requires a specialized receiver that can combine inputs from multiple gps antennas.

Drones fly at 60 MPH and pull 10Gs in their 3D maneuvers. The IMU system updates at 100Hz. A boat in waves is standing still compared to this application.

A boat as small as this drone moving those speeds would have similar requirements. So on a typical cruising boat, the IMU system can update at 10hz or 20hz and still be "faster" than the system on the drone considering the dynamics.

The bigger difference, is the drone is flying in mostly clean air, not waves, where the boat may be dealing with waves that make it "almost" impossible to steer the boat. This makes compensation algorithms more difficult to avoid over correction.

When the waves make it impossible to steer the boat you just spin in circles and wait. Sometimes currents and waves combine in patterns that create this situation though typically it does not last for more than a few minutes.

Sometimes it is only impossible to steer with the speed and range of movement available to the autopilot, as many autopilot drive motors are slower than manual steering and with less range.

Actually, you need speed over ground. True wind = Apparent wind - boat velocity over ground. Velocity needs magnitude and direction.

True wind is not based on velocity over ground. We sometimes compute it this way for lack of water speed instruments. True wind is actually based on wind speed over water.

Boat speed through water only really matters as far as tides are concerned. If there are no tides then speed over ground is the same as speed over water.

There are many times currents that have nothing to do with tide.

Originally Posted by AboutTime1

A GNSS200L updates at 1 KHz and sells for $50.

Link? Is it differential gps?

[goboatingnow]

Originally Posted by Dockhead

Right, and it was you who taught me about this years ago

"Differential mode", that is DGPS etc., has nothing to do with it, because position calculations are not used (DGPS is not available more than 200 miles offshore anyway). The range to the satellite is calculate with extreme accuracy, a few orders of magnitude greater accuracy than is possible for position calculations, using the phase of the carrier wave. It's a very cool concept. https://en.wikipedia.org/wiki/Real-t...phase_tracking

That's why satellite compasses can work with baselines of only some centimeters, instead of many meters.


You're now using a Hemisphere? I thought you rolled your own, back then.

I have my own as well as the hemisphere. I’m planing to build a new one with the latest U-bloc rtk GNSS modules. Looks really powerful.

[goboatingnow]

Originally Posted by AboutTime1

The receivers used in the GPS (GNSS) compass will use differential mode between them, not between them and a land based reference.

The receivers in the GPS based compass don't care if they are meters out from their actual position, because they'll both be meters out. But the relative accuracy between them will very good.

Rtk is used to compute carried phase relationships. The actual position of each receivers is not needed to decode relative heading. Differential receivers used in a base slave mode are similar but the maths is much simpler where one base is not moving , moving base complicates everything

Again heading is not derived by decoding two positions.

[Dockhead]

Originally Posted by Fore and Aft

Thanks Dockhead I am really interested in what you're saying. Is it easy to interphase AIS with Open CPN or with a dedicated marine unit? Raymarine units seem to be just plug and play?
. . . s

OpenCPN speaks NMEA0183, so connecting it to a modern NMEA2000 network can be slightly tricky. There are different ways to do it:


1. Use an NMEA2000 to 0183 converter, which are now available with USB output (e.g. Actisense NGW). This will convert N2K PGNs back and forth to 0183 which OpenCPN can understand.


2. Most plotters have an N2K/0183 converter built in. You can take the 0183 output from the plotter and connect to a serial to USB adapter (RS422, not RS232). I used to do this.


3. If you have Navico (B&G, Simrad, etc.) plotters, they are networked with Ethernet. You can connect your board computer right into this network via the hub (or some Navico plotters have the hub built in). That's what I do, and you get a bonus in that the radar datastream is also transmitted over this network, and OpenCPN has an excellent radar plugin which allows you to control and use the radar from inside O.


O as of recently has an NMEA2000 plugin called "Can Can" or something like that. I haven't used it and don't know what it does, but the guys on the OpenCPN forum can tell you.



One cool thing in OpenCPN is a totally superior AIS implementation, which shows the geometry of crossings. No plotter I've ever seen has anything like this. In hairy traffic situations (Straits of Dover, German Bight, etc.), I like to do collision avoidance from the nav table using AIS on O, radar on the plotter, and a crewman on deck doing visual.

[Dockhead]

Originally Posted by goboatingnow

I have my own as well as the hemisphere. I’m planing to build a new one with the latest U-bloc rtk GNSS modules. Looks really powerful.

Make sure and post about it!


Do you think about commercializing this?


Have you played with one of the new Furuno ones?

[Dockhead]

Originally Posted by AboutTime1

Not as far as computing its location or speed.

Location and speed are not useful for an autopilot. We need heading, attitude, and motion. We care little about location in all this; that's for navigation. For managing the pilot and radar, and for compensating the instruments, we need heading, attitude, and motion with a high refresh rate and high accuracy. Satellite compasses are much more capable in this than fluxgate compasses, no matter what kind of filtering or compensation there is. Motion of a boat in a seaway (which affects all three types of data) is extremely complex and is hard to measure with a fluxgate sensor and accelerometers. It's why big ships used gyro compasses until satellite compasses were invented, not fluxgate plus accelerometers like RC aircraft, or cheap electronics on boats. Now big ships use sat compasses. This technology has only recently trickled down to us in an affordable form.

Originally Posted by AboutTime1

You said you wanted true wind, ie what you'd see on land. If you want wind referenced to water, that is different thing.

Sorry, as I wrote, True Wind for sailors is water referenced. NOT indeed what you see on land. If you follow the link I posted, you'll find a good explanation.


What you see on land we call Ground Wind, which is not useful for sailing because water in the ocean is in constant motion. True Wind is a different thing in maritime terminology, from the way it's used by weathermen. Naturally enough since we are at sea and our point of reference is different.

[Dockhead]

Originally Posted by seandepagnier

. . . With enough passes in known areas without currents, it's possible to calibrate the paddle wheel.. . .

Nice to see you in this thread, Sean. For anyone who doesn't know him, Sean is one of those brilliant OpenCPN developers. Not only brilliant, but helpful and patient.

I'd like to know how you compensate your paddlewheel for nonlinearity. I know Expedition will do this; also the B&G H5000 -- are there other ways to do it?

I have just changed from an ultrasonic to an electromagnetic speed log, and I'm hoping (a) it will work better than the previous one; and (b) that I will be able to calibrate it.

In my experience, non-linearity is less of a problem with the non-mechanical logs, but is still a factor. I have bought a B&G H5000 Hercules which does allow this kind of calibration. The problem however is getting the corrections. The water is always in motion. I've used two way runs, then 4 way runs at 90 degrees, at slack tide on a windless day, and was still not happy with the results.

[ranger58sb]

Originally Posted by AboutTime1

I'm looking at price lists on new boats. The "electronics package" on dual helm cruisers is typically $12-15K on a new boat for a pretty standard B&G package. Not including anything at the chart plotter table.

Would you order a new boat without an electronics package and install OpenCPN on it ? What could you build for $5K ?

Originally Posted by AboutTime1

So what does a B&G or RayMarine chart plotter do that OpenCPN doesn't ?

Or better yet, I'm a software developer. What couldn't I make OpenCPN do if I wrote some code ?

If you're a developer and like to fiddle with that sorta thing... my take from reading here is that OpenCPN could work for you. If so, possibly with a lot of work on your part.

The number of queries in the OpenCPN section here suggest to me that it's a science project -- great for folks who like those, maybe not so great for folks who just want to navigate the boat. You can decide where you fit in all that...

I am NOT a developer (although I have deep background managing teams of developers) so in my case it's much simpler to just plan out a commercial package, get it installed, and go boating. Since we bought our Furuno system (an example) on the previous in 2009 we had exactly one "help desk" sort of query... very early on... that Furuno had solved with a software update that I hadn't known about.

So in answer to your question about what a commercial system does for me that OpenCPN can't: it probably works, without requiring my fiddling with it.

FWIW, we use TimeZero on the ship's laptop as one of our backup layers. I don't have insight into their helpdesk log so dunno how "fiddly" it might be for the whole body of users... but my installation has been stable throughout it's lifetime... and all I've had to do is turn it on and use it.

(Usually in dry interior spaces, and not in bright/direct sunlight. I could shift it to a decent toughbook but haven't found a need to do that... given the installed nav suite at our helm.)

My guess, also FWIW, is that the prices you see for "electronics packages" on new boats might not be representative of what you could have if you spec'd out a system yourself, and especially if you do your own installation. I just had a system installed -- Furuno 16" MFD, sonar, ICOM VHF, a VHF antenna, Furuno AIS, an AIS antenna, a loudhailer -- for approx $7K cost of "stuff". Another ~$2500 for installation with me as helper (and that included some significant time dealing with orphan equipment and wiring from previous installations). Radar would probably have added more $$$ but we already had that, with it's own MFD and DST.

-Chris