Plotting an intercept course

[SeanPatrick]

I was just messing around and found that you can do this on a scientific calculator, as well. Here's how you would solve the example I gave above:

1. Take the cosine of the angle between the other ship's true course and the desired relative course. 320° - 280° = 40°; cos(40°) = 0.766...
2. Multiply that by the other ship's speed. 0.766... · 5 = 3.830...
3. Multiply that by the tangent of the angle in step 1. tan(40°) · 3.830... = 3.213...
4. Divide that by your desired speed and take the arc sine of that. asin(3.213... / 5) = 40°
5. Add that to the desired relative course. Subtract from 180° to get the course to steer. 40° + 320° -180° = 180°
6. Divide the result of step 3 by the tangent of the angle found in step 4. 3.213... / tan(40°) = 3.830...
7. Add that to the value found in step 2 to get the relative speed. 3.830... + 3.830... = 7.660...

Simples.

But seriously. There's an algorithm here. It's simple trig - I just need to work out the rules for different quadrants. Then maybe I can develop something. We'll see. Another project is really the last thing I need right now.

[requiem]

I think it's very cool (and yet unfortunate) that the SubSim community is one of the only places to find such an app. As I run an Apple ecosystem that leaves me with (Radar Plotting) as what appears to be the only option. I've tried to find others for some time without much luck. Screenshot:



Note: I have managed to get the interface to lock up on occasion, but apart from that it works rather well.

[nohal]

Well, in navigation (that is what we develop OpenCPN for), the standard scenario is to avoid a collision, not to look for it And for avoiding a collision with AIS targets OpenCPN's CPA calculation and display already is quite a good tool AFAICT.
Anyway, implementing a tool for pirate interception planning and torpedo firing would be a cool little project so I'm looking forward to see the math....

[Alan Mighty]

Nautical Alamanac has for free download an Excel spreadsheet that models a manoeuvring board at: https://www.nauticalalmanac.it/en/na...ing-board.html

[sy_gilana]

Originally Posted by nohal

Well, in navigation (that is what we develop OpenCPN for), the standard scenario is to avoid a collision, not to look for it And for avoiding a collision with AIS targets OpenCPN's CPA calculation and display already is quite a good tool AFAICT.
Anyway, implementing a tool for pirate interception planning and torpedo firing would be a cool little project so I'm looking forward to see the math....

Actually Nohal, there is a very useful reason to have this.
When crossing a very busy TSS it would be good to aim for a point just past the stern of the last vessel of a group. (or even at him) and then bear off to pass safely. This gives max time to cross before next vessel. (I do this with AIS)


By the way, OpenCPN CPA calculation and handling, especially "Show Target CPA" function is the best implementation I have seen (Even in ECDIS systems) The depiction of that information seems self evident, sitting in a chair in a house somewhere, but, to have that at 03h00 after several rough nights of little sleep is an absolute life saver. Fatigue is a brain-killer.




(in fact its exactly the calculation we make crossing a street)

[SeanPatrick]

Originally Posted by sy_gilana

When crossing a very busy TSS it would be good to aim for a point just past the stern of the last vessel of a group. (or even at him) and then bear off to pass safely. This gives max time to cross before next vessel. (I do this with AIS)

This is what I was thinking. It's not too hard on paper to modify the solution so that you pass a certain distance from another vessel. You simply change the relative motion course to one that passes your own ship at that distance, then solve as usual.


Seems like it would be a handy feature. But I fear there would be a danger of over-reliance on the "solution".

[SeanPatrick]

Originally Posted by Alan Mighty

Nautical Alamanac has for free download an Excel spreadsheet that models a manoeuvring board at: https://www.nauticalalmanac.it/en/na...ing-board.html

Wow, missed this earlier. Thank you. I'm going to look into this.

[Dsanduril]

For some code along with the derivation: https://www.codeproject.com/Articles...cts-in-D-Space

And then of course: https://www.researchgate.net/publica...yapunov_Theory

[requiem]

Originally Posted by nohal

Well, in navigation (that is what we develop OpenCPN for), the standard scenario is to avoid a collision, not to look for it And for avoiding a collision with AIS targets OpenCPN's CPA calculation and display already is quite a good tool AFAICT.

Some might argue calculating the CPA is merely the first step; determining how to best open that CPA to a safe distance is simply a more general case of calculating an intercept.

Instead of adjusting the relative motion vector to pass through own ship, you'd move it to be tangent to the desired safe range.

[SeanPatrick]

Originally Posted by SeanPatrick

There's an algorithm here. It's simple trig - I just need to work out the rules for different quadrants. Then maybe I can develop something. We'll see. Another project is really the last thing I need right now.

...so far, so good:

[rgleason]

Ocpn_Draw has some features that might be useful. As others said O does this in AIS cpa a lot.

[rooiedirk]

In general this done/needed to plot a course to an other vessel that does need help/assistance.
You can do the same in OpenCPN as you would do on paper.

1. Make a route from own ship to target. (blue)
2. Make a route from ownship in the opposite direction as the targets course with a distance of targets speed. (green)
3. Change the last waypoints props by adding a circle with a radius of your own speed/
4. make a route from last waypoint to the crossing of circle and the first route. (yellow)
5. The course of this last route is the course to steer for interception. 286 (if you are doing 5kn)

[chasmains]

If you see the other vessel, then all you need to do is generate a zero bearing rate to apply the “Constant Bearing, Decreasing Range” concept. Any other contact method, I.e. radar, can be exploited the same way. Maneuvering Board plots work but must be performed by a practiced plotter. Most of us aren’t.

[Colin Stone]

Battenberg plotter - eBay??

[reflectionsv37]

Originally Posted by ldomustang

Does OpenCPN have a feature that will allow me to plot an intercept course to another vessel?

The easiest way to do this is install an AIS receiver, link it to Opencpn and then steer a course until OpenCPN shows you are on a collision course. Works every time! Of course, any change of speed or direction of the target vessel will change the results, but that would be true no matter what method you used.