Plotting Questions; Radar ranges; magnetic compass bearings; positions on Open CPN...

[wholybee]

Originally Posted by Raymond Pincott

Thank you StuM, I am not familiar with OCPN Draw plugin? I will investigate. Can you use ranges in OCPN Draw plugin? The issue is in restricted visibility when you cannot get a visual bearing and the radar can only give ranges, how can you apply the ranges in OpenCPN.

Definitely play with the OCPN draw plugin. What radar doesn't give a bearing and only a range? I have found creating routes to be the fastest and simplest way to draw simple lines for navigation. It's so fast and simple to do basic ranges it practically doesn't need any explanation. Press "ctrl-r" click on the landmark, then move it in the correct direction for the range/bearing (which will be displayed as you move the cursor) and click again. Esc to finish.
If you truly only have a range and want to plot a circle, OCPN draw might help, but I found OCPN draw awkward and confusing. For example, most of the drawing use the boats location as a starting point. If I am trying to work a fix, that isn't what I need to do.

[requiem]

Before getting back to the OpenCPN Draw plugin, a brief divergence....

There have been more than a few threads on the topic of GNSS failure; a general outcome is that failure is 1) exceedingly unlikely, and 2), a situation where you'd have greater things to worry about, and failures at the receiving end can be mitigated by keeping sufficient spare GPS receivers and batteries (or reverting to paper). Jamming is discussed more in the category of "well maybe you shouldn't be there then".

However, I think this is a bit too complacent. While the chances of encountering jamming are low, I think it's not as unlikely as many think However, persistent jamming is also much more likely in places people are likely to avoid, and intermittent jamming may only persist for a brief period (i.e. probably meaningful only in confined waters).

Here is the current MSCI advisory on GPS Interference & AIS Spoofing:
https://www.maritime.dot.gov/msci/20...e-ais-spoofing

Skimming the list of problem reports, I suspect many of the on-land cases are more to do with the receiver being confused rather than a true case of GNSS outage or jamming (and thus mitigated by a second receiver. I've experienced this once myself, with the iPad showing a position a few miles off for perhaps 5-10 minutes.

The maritime ones appear likely to describe a jamming problem (i.e. affecting multiple receivers and nearby vessels) and the cases in the Mediterranean may be relevant to a few cruisers.

[El Pinguino]

Originally Posted by wholybee

Definitely play with the OCPN draw plugin. What radar doesn't give a bearing and only a range? I have found creating routes to be the fastest and simplest way to draw simple lines for navigation. It's so fast and simple to do basic ranges it practically doesn't need any explanation. Press "ctrl-r" click on the landmark, then move it in the correct direction for the range/bearing (which will be displayed as you move the cursor) and click again. Esc to finish.
If you truly only have a range and want to plot a circle, OCPN draw might help, but I found OCPN draw awkward and confusing. For example, most of the drawing use the boats location as a starting point. If I am trying to work a fix, that isn't what I need to do.

In my experience with small boats gyro stabilised radars are on the far side of rare. As a result we are dealing with relative bearings to which we must apply magnetic heading and variation and deviation. A little bit south of not perfect.

Even with a gyro stabilised radar radar bearings - unless of a point source like a buoy - and buoys are not considered a good choice as they may be off station - accuracy is not perfect and depends on the beam width of the radar.

So if using only radar it is best to get and plot three ranges.

[requiem]

With Draw, instead of a simple VRM there appears to be a vessel-centered guard zone. A generic VRM that can be fixed in place or attached to the ship (or other AIS targets) would be nice. The EBLs at least are reasonably flexible, even if you must pre-prep them by creating a few and then detaching them from own-ship. But, there is still a way to get a basic VRM!

For a simple VRM in OpenCPN, you'd drop a single boundary point, then edit its properties to display a single range ring at the desired distance. The type should be "neither" for best display. All of these you can set in the plugin's preferences dialog in advance, so that dropping a boundary point could default to have, say, a single 0.25 mile ring.

The linked documentation for the plugin appears to be missing. (Correction, it's in the wiki; only the github link from the plugin page is empty.) I can make an attempt at a brief primer, but ideally that should be done by someone familiar with it.

[RaymondR]

Originally Posted by Dockhead

The cursor in OpenCPN has a range and bearing indicator which appears at the bottom of the screen. Just put the cursor on the landmark and see if that corresponds to what your radar is telling you.

Even easier - use the radar overlay. OpenCPN has an excellent radar plug-in which is compatible with most radars. With radar overlay, so see at a glance if anything is wrong with your positioning. I run mine all the time.

Used my radar with an overlay to clear out of the Marble Islands at a moonless midnight to make it through Strong Tide Passage at high tide the next morning. It is a great confidence builder knowing that the boat is where both the radar and GPS agrees it is.

[Dockhead]

Originally Posted by El Pinguino

In my experience with small boats gyro stabilised radars are on the far side of rare. As a result we are dealing with relative bearings to which we must apply magnetic heading and variation and deviation. A little bit south of not perfect.

Even with a gyro stabilised radar radar bearings - unless of a point source like a buoy - and buoys are not considered a good choice as they may be off station - accuracy is not perfect and depends on the beam width of the radar.

So if using only radar it is best to get and plot three ranges.

I don't understand. All the boat radars I've used use the network compass for bearings. So if the main compass is stabilized, so is the radar. Am I missing something?


I think most boat compasses nowadays are stabilized. I was using an Airmar H2183 for years which is gyro stabilized. Now i have the new Furuno satellite compass with accuracy in the sub-degree realm and extremely high refresh rate (60hz). This feeds my radar, giving extremely good bearings.



Don't know how this works with other systems.


Little thread drift, but a significant challenge with very accurate compasses is calibration. For the alignment between the radar and the physical compass installation, first of all. I've done this by adjusting the compass to exactly align the radar overlay. It's not as simple as it sounds because of possible inaccuracy of the chart (or position), but I've done it several times successively with different charts and have thrown out the outliers, so it now seems to be within 1 degree.


Any position inaccuracy will throw this off. The Furuno compass (with its 4 GNSS receivers working simultaneously with GPS, GLONASS, Galileo, and BeiDou) also gives extremely accurate GNSS data -- position error of less than 2 meters when it's getting a decent EGNOS differential correction; theoretically in the 10's of centimeters but I haven't seen that so far. However -- the compass antennae are a good 6 meters away from the radar antenna, and this is a problem -- this gives a parallax error. Slight, but if you're trying to produce bearings of better than 1 degree accuracy, it makes a difference. There is no way in my system to compensate for this, so what I've done is align for targets which are far away and directly ahead where there is no parallax, and take into consideration that there will be a slight error with targets which are closer and more to beam.



Alignment with the ship's head is a different question, less important than alignment with the radar but not at all irrelevant. I could align the radar if necessary to correct for any mechanical misalignment between ship's head and radar physical installation. It's hard to check this because the water is always moving, at least a little, but I've done a series of four-way calibration runs over very still water, comparing COG with heading (obviously in True), and I think that the radar mechanical installation is within a degree or two. And this is relatively less important; the key thing is very accurate alignment between radar antenna and compass so that radar overlay is correct and radar bearings are accurate.


Accurate radar bearings are really important if you want to do radar navigation or a radar check of position data like the OP is wanting.

[requiem]

Originally Posted by Dockhead

I think most boat compasses nowadays are stabilized. I was using an Airmar H2183 for years which is gyro stabilized. Now i have the new Furuno satellite compass with accuracy in the sub-degree realm and extremely high refresh rate (60hz).

Does a fluxgate count as stabilized? (I wouldn't think so, even if they tend to be far better than the traditional magnetic ones at the binnacle.) Satellite compasses still seem to be a rare and fancy bit of kit.

[CarCode]

Originally Posted by requiem

Satellite compasses still seem to be a rare and fancy bit of kit.

If you have enough money to buy a Satellite compass you will use also something better than OpenCPN.

[RaymondR]

Originally Posted by CarCode

If you have enough money to buy a Satellite compass you will use also something better than OpenCPN.

You're wandering into dangerous territory here CarCode. From an ease of use and general utility there's little better than OpenCPN. It's the platforms on which to run it and chart availability which causes the problems.

[250224]

Originally Posted by RaymondR

You're wandering into dangerous territory here CarCode. From an ease of use and general utility there's little better than OpenCPN. It's the platforms on which to run it and chart availability which causes the problems.

Agree, and he also mixes up gratis and free. And also that e.g. a Furuno SCX-20 is at a price level which is not completely out of reach for a determined poor cruiser running, by choice, OpenCPN...

It's a bit off-topic but I don't understand what you mean with platform and chart availability? There are o-charts, Win/Mac/Linux, where is the problem?

[El Pinguino]

OK , hands up everyone with a stabilised radar display on their boat?

[El Pinguino]

Originally Posted by blu3534

Agree, and he also mixes up gratis and free. And also that e.g. a Furuno SCX-20 is at a price level which is not completely out of reach for a determined poor cruiser running, by choice, OpenCPN...

It's a bit off-topic but I don't understand what you mean with platform and chart availability? There are o-charts, Win/Mac/Linux, where is the problem?

I've just looked at the Furuno specs https://furunousa.com/en/products/scx20

Is it giving COG or actual true compass heading? The way I read it only COG is given. For navigation you need ship's head in True degrees.

Edit, just read the detailed specs and think I have a grip of how it calculates True heading

[Dockhead]

Originally Posted by El Pinguino

I've just looked at the Furuno specs https://furunousa.com/en/products/scx20



Is it giving COG or actual true compass heading? The way I read it only COG is given. For navigation you need ship's head in True degrees.

It's a compass. Read the specs again.

If you're not familiar with satellite compasses, here's a good article on how they work. https://oceannavigator.com/gps-compass/

[RaymondR]

Originally Posted by blu3534

Agree, and he also mixes up gratis and free. And also that e.g. a Furuno SCX-20 is at a price level which is not completely out of reach for a determined poor cruiser running, by choice, OpenCPN...

It's a bit off-topic but I don't understand what you mean with platform and chart availability? There are o-charts, Win/Mac/Linux, where is the problem?

Since I decided to use up the inheritance and went and bought myself a couple of MFDs, put Navionics on them and upgraded the radar I only use OpenCPN for anchor watch and monitoring below decks. The CM93 charts are fine for these limited purposes and Australia has yet to move into the modern world and the carto folks still charge for electronic charts.

Platform is the gadget you run the program on and whilst I have a few none are suitable to leave on deck. I have a dedicated Win 8.1 10" computer and a RS422/485 to USB converter at the nav station below and my autopilot computer accepts nmea 0183 as backup.

[Raymond Pincott]

A Fluxgate compass is getting its heading by responding to the magnetic field that is situated in, exactly the same as the magnetic compass. If the magnetic field that the fluxgate compass is located in is stronger than the earths magnetic field the fluxgate will respond to the stronger field, the same as the magnetic compass. The advantage of the Fluxgate compass is that it sends out electric signals that can send information to autopilots, radar excreta. The disadvantage is that it requires electric power.
See what happens if you put a magnet near a fluxgate compass.