Get ground projection coordinates

[cm2227]

Allright Ladies and Gentlemen,

I have an issue regarding surface and world space coordinates.

For predicting my touchdown coordinates I need my current orbital position in the same reference frame.

If you don't know what I mean, ask KerbalEngineer for your longitude and lattitude while in high orbit.

Vessel.longitude gives you the value in the non-rotating reference frame and not the surface position you are currently flying over.

Methods like CelestialBody.GetWorldSurfacePosition(System.Double,System.Double,System.Double) usually give the world coordinates for surface coordinates, not the other way round.

I may find a way to calculate impact point in world coordinates, but that is more complicated in my view.

TL;DR: How do I find out over which ground coordinate I am currently flying in high orbit?

Update:

I hope you don't mind my thinking loud.

What if I monitor the spacecraft's ascent from the surface until FlightGlobals.RefFrameIsRotating becomes false.

The last coordinate in rotating frame, the first coordinate in non-rotating frame and the universal time of the transition are stored.

Then, knowing Kerbin (or any other body) rotates 360° in 21600 seconds, I can calculate the angular difference between rotating and non-rotating frame for any given universal time, for example just right now. Do you think that might work?

Thanks

cm2227

Edited December 10, 2013 by cm2227
Update

[UbioZur]

I can't help in term of where you can get the data, I am interrested in it however.

If KerbalEngineer get the data you are looking for, have a look at their code, and if it's how it works, ask the dev if you can use this part of the code.

[cm2227]

If KerbalEngineer get the data you are looking for, have a look at their code, and if it's how it works, ask the dev if you can use this part of the code.

KerbalEngineer does the same as my plug-in right now: It reads the longitude from vessel.longitude, which is (while in non-rotating reference frame) not the surface longitude.

So this doesn't really help.

Next thing is: I don't want to reverse-engineer or copy other mods, I want to broaden my knowledge of orbit mechanics and KSP by writing it myself.

[Diazo]

I'm not on my development computer at the moment so I can't check, but there are methods in KSP for converting between reference frames.

Looking at the API docs, do the following help?

<member name="P:FlightGlobals.RefFrameIsRotating">

<summary>

Whether KSP is doing its computations in an inertial reference frame, or the reference frame that

rotates with the current dominant celestial body.

</summary>

</member>

<member name="M:CelestialBody.GetSurfaceNVector(System.Double,System.Double)">

<summary>

Returns a unit vector perpendicular to the surface of the body at the given latitude and

longitude (pretending for the moment that the body is a perfect sphere).

</summary>

<param name="lat">Latitude in degrees</param>

<param name="lon">Longitude in degrees</param>

<returns>A unit normal vector to the surface</returns>

</member>

<member name="M:CelestialBody.GetWorldSurfacePosition(System.Double,System.Double,System.Double)">

<summary>

Returns the position, in world coordinates, of a given latitude, longitude, and altitude

</summary>

<param name="lat">Latitude in degrees</param>

<param name="lon">Longitude in degrees</param>

<param name="alt">Altitude above sea level in meters</param>

<returns>Position in world coordinates</returns>

</member>

I have seen methods for converting to/from the world frame also, but my search skills are failing at the moment.

The other thing is you would probably require a Krakensbane check to make sure that is not interjecting more variables into the equation.

Hopefully that works as a point to start.

D.

[cm2227]

Hey Diazo,

thanks for the advice. I know of these API calls, but I don't know about injecting more variables into the equation. If I do that simple correction for the angular difference between rotating and non-rotating frame, how can any more variables become part of my (or KSPs) equations?

I must admit, I havent't implemented that feature yet. I still battle my iterator calculating bull***** while it worked flawless in C++ code.

Edit:

I now understand the problem. Once the iterative solver has started, it must not refer to any values from the game that might have changed since starting the iteration. I have to re-check my code for these references and see which errors persist.

Edited December 12, 2013 by cm2227