Navico radar plugin v3.0 released

[merrimac]

Originally Posted by Andreas29

Hi,

a question for the (m)arpa function:

Does the plugin calculate the (m)arpa informations or does it use the (m)arpa functions from the radar dome?

The plugin calculates it completely autonomously.

Originally Posted by Andreas29

AIS works with SOG and COG and GPS. What does the plugin use? The heading and speed t. water and the bearing information? Or what are the basic information for the (m)arpa calculations?

GPS location + time for own boat position.
Heading for own boat heading so it knows how to rotate the radar image relative to the boat.

From this it computes relative distance and heading to a target and from that the true direction and position of all targets.

Originally Posted by Andreas29

What are the results of the (m)arpa calculations? Course and speed as relative speed and course or true speed and course from the target?

True speed and course.

Originally Posted by Andreas29

What is better when using a navico dome headless (without a mfd). Put a heading source at the radar via the Interface box or just connect it directly with the oc-computer to feed it with the heading?

Send heading to the radar via the interface box. This means we get heading in each spoke correctly synchronised with when the radar generated the image. In practice the difference isn't that big.

[Andreas29]

Hi,

merrimac thanks for your explanation

Regards

Andreas

[rgleason]

https://opencpn.org/wiki/dokuwiki/do...br24_radar#faq

[Tupaia]

Originally Posted by merrimac

Send heading to the radar via the interface box. This means we get heading in each spoke correctly synchronised with when the radar generated the image. In practice the difference isn't that big.

My radar is plugged directly into the RJ45 port on the computer and it has not had any problems. Uses the heading from fluxgate compass over NMEA.

When setting up you need to accurately determine the offset angle while the boat is completely stationary and you have multiple identifiable targets.

[Hakan]

Originally Posted by Andreas29

Hi,

a question for the (m)arpa function:

Does the plugin calculate the (m)arpa informations or does it use the (m)arpa functions from the radar dome?

All (M)ARPA calculations are done in the plugin.

AIS works with SOG and COG and GPS. What does the plugin use? The heading and speed t. water and the bearing information? Or what are the basic information for the (m)arpa calculations?

All calculations ends up in a message including position, course, speed, a ID number and target status(Acquiring/Active/Lost) transferred to OCPN as a $RATTM message. In OCPN this is evaluated by the AIS_Decoder function to a ARPA symbol.

What are the results of the (m)arpa calculations? Course and speed as relative speed and course or true speed and course from the target?

See above.

What is better when using a navico dome headless (without a mfd). Put a heading source at the radar via the Interface box or just connect it directly with the oc-computer to feed it with the heading?

Doesn't matter much. Both way are used. And the course update frequency is not that crucial for normal sailing speed. I use my compass unit to feed OCPN at 1 Hz. Others are using a faster transfer via Navico I-box.

Can´t test the (m)arpa function here in the garden at home cause here´s no marine traffic . Have just tested the normal radar functions.

Well, you can set a (M)ARPA on a stationary target for test. Set a guard zone where just one or two blobs are present and check "ARPA On" or right click near a blob and "Acquire ARPA target".

Håkan

[Andreas29]

Hi,

Håkan, thank you for the explanations

Originally Posted by Hakan

Well, you can set a (M)ARPA on a stationary target for test. Set a guard zone where just one or two blobs are present and check "ARPA On" or right click near a blob and "Acquire ARPA target".

Håkan

Well, it´s not only the thing that here´s no marine traffic. There is some of the basic installation stuff around my pi who is on the boat and not here at home. Building up a test installation with a nmea source and the needed connectors I was too lazy for

Regards

Andreas

[rgleason]

I've updated the FAQ

Hakan is there a test file available for this?
Would wireshark have to be used? It seems to me there used to be a test page for radar, but I can't find it.
Closest I can get to it is:
https://opencpn.org/wiki/dokuwiki/do...l:online_tools
or
https://opencpn.org/wiki/dokuwiki/do...adar:debugging

[Andreas29]

Hi,

first I want to say again that you guys did a great job.

So for a better understanding for me when using the plugin and (m)arpa function I want to discuss how I can use the information from the (m)arpa function for collision avoidance.

The keyword is seastabilized or ground stabilized.

Maybe I´m wrong or understand the algorithm behind the (m)arpa calculations not right, but the plugin gives out the sog and cog and displays this. With this you can get an cpa and tcpa, right, but it´s not good for decisions for collision avoidance. For this you need the course through water or relative vector, cause all anticollision rules based on the movements relative together, not on ground based movements.

E.g. it´s written down here: https://www.myseatime.com/blog/detai...ich-one-to-use from the point "Speed through water for collision avoidance"
There are a lot of other examples for the difference between sog/cog and stw/ctw for collision avoidance.

The conclusion is that I can´t make a decision for collision avoidance who is based on cog and sog.

So when I understand the plugin and the results of it´s algorithm then it´s very good for trigger an alarm for a cpa / tcpa.
Once an alarm sounds and a potentially collision opponent is identificatet then I use the relative measure instruments and trails for plotting this object and make a decision for collision prevention.

Or uses oc the relative movements for calculating cpa/tcpa and for displaying it on the chart? When oc uses the AIS function for calculate this (both AIS and (m)arpa ) then I think that not, right?

Again, I don´t want to criticize your work. I want to use it right and for this I want understand this tool.

When it was better to start a new thread sorry for hijack this one

Regards

Andreas

[Douwe Fokkema]

You are right, but I do not see a problem. ARPA target movements are calculated based on the differences in GPS target locations over the various sweeps of the radar. This results in true target positions and speed. OpenCPN handles these as AIS targets and calculates CPA and TCPA. From CPA and TCPA the skipper knows if there is a danger or not. When action is needed to avoid the danger he has to rely on relative observations of the danger to decide on what to do. For this he can use the relative radar picture with relative trails. He does not need to know speed through water as he already knows that there is a dangerous situation. Using the relative picture he has to decide on the action to take. Once he has taken an action, he can use the new CPA and TCPA to judge if the action was sufficiently effective.

[Douwe Fokkema]

But on top of this, a radar image with trails relative to the water movement would help of course in the decision making. We don't have these (yet).

[Andreas29]

Hi,

no problem, just want to understand this strong tool and not run into a trap

I agree with you, once the (m)arpa function detects a risk I use the "traditional" radar tools like the relative radardisplay and the relative movements on the ppi.

I thought that the option "Show relative target trails" will show the movements relative to me? So i´ll get the Information about the relative movement on the chartoverlay too (as target trail).

Regards

Andreas

[rgleason]

Very good questions and discussion guys!
I updated the FAQ with this quuestion & douwe's answer. Hope its ok.
https://opencpn.org/wiki/dokuwiki/do...br24_radar#faq

I would like to have a better understanding how Marpa works with with collision avoidance, when the BR24_pi plugin only uuses/displays SOG and COG, when course through water or relative vector, is needed to calculate CPA and TCPA based on movements relative together, not on ground based movements.

[merrimac]

Originally Posted by rgleason

Very good questions and discussion guys!
I updated the FAQ with this quuestion & douwe's answer. Hope its ok.
https://opencpn.org/wiki/dokuwiki/do...br24_radar#faq

I personally think the documentation page is getting very long and is trying to cover a lot of subjects... I personally prefer the style we use on Github, with one page per subject.

But hey, I also prefer it if people don't mix up subjects in CF threads...

[rgleason]

Subject is Navico radar released v3.0
- I think it is on topic, isn't it?
- Will stop adding FAQ
- Manual exists to help users and will be fairly long sometimes.
- Dokuwiki has a good search feature.

[Douwe Fokkema]

Originally Posted by Andreas29

Hi,


I thought that the option "Show relative target trails" will show the movements relative to me? So i´ll get the Information about the relative movement on the chartoverlay too (as target trail).

Right again. Relative trails will show you the movement of targets relative to you. It seems not logical to show relative trails on the chart overlay. Then you map relative trails of a target on a true position, but that is where the target has never been. If you show relative trails on the overlay you should forget about the chart. We will consider to remove the possibility to show relative trails on the chart overlay. Relative trails are better shown and interpreted in the PPI (radar only) window. The overlay may better show only true trails, so that the target has indeed been in every location of the trail.