navball reference direction

[jf0]

I have never really understood how the navball is oriented:

Does its orientation change whenever you change 'sphere of influence'? So it is not fixed to the same orientation as when you launch, like a gyroscope, but always points toward down in terms of gravity? I guess this must be true since the brown half always seems to point toward the ground say whether i am on kerbin or mun also because if you keep the same attitude when in orbit the ball rotates as you go around the planet.

So how does the game decide how to set the orientation of it when you enter a different sphere of infuence? Setting 'down' to the the center of mass of the planet only sets two axis, how is the other determined? This confuses me greatly when trying to takeoff from mun or whatever to go in a specific direction. This also would mean the navball for two different craft in different points in the same orbit would be oriented in different directions right?

I am confused because I thought it was like a gyroscope with orientation set a launch but that cannot be the case...

[Supernovy]

Blue always points up, the red line (heading 000) always points north and heading 090 always points east.

[Xannari Ferrows]

The center of the blue area is radial, relative to the body, orange being anti-radial. Halfway between those depends on your orbit. Your inclination +/- about 1.571 radians is normal, and antinormal. Your inclination is prograde, and your inclination + 3.141 radians is retrograde.

Is the navball calibrated to work based on the coordinate system, or reference points?

[Edax]

It's important to note that the navball is calibrated by what part of the ship is being controlled. If you give a probe control of the vessel, the probe is mounted backwards, then blue will point down, and red will point up.

[Chaos_Klaus]

The navball shows the angles for azimuth (along the horizon) and altitude (above and below the horizon). So it always shows your vessel's orientation in relation to the surface. The blue area is the "sky" and the brown area is the "ground". Both are separated by the artificial horizon. The navball does switch its orientation when you pass into another SoI.

[jf0]

Ok I get it so the navball's reference frame is effectively a point on the surface of a sphere at your altitude above the planet SOI with 0 degrees pointing north. Ok but what I mean is how does the game choose north when it orients the ball? I guess the answer is the axis of the planets rotation, so that it appears to rotate anticlockwise when viewed from the north?

Does anyone know of a mod for an inertial navball? If you are on the surface and want to launch for a specific orbit (otherthan equitorial if you are on the equator), the only way to do it is watch the orbit map and guess when you are lined up as you rotate on the surface. If the navball was in an inertial frame you could launch in the right direction by watching the ball rotate. It just seems like the wrong system to use for spaceflight, because all the information we use anyway with our maneuver nodes (target, prograde retrograde markers etc) are all on an 'inertial' overlay on the ball anyway! And it has always bugged me that there is no way in the game to plan your trajectory before you launch.

[diomedea]

Ok I get it so the navball's reference frame is effectively a point on the surface of a sphere at your altitude above the planet SOI with 0 degrees pointing north. Ok but what I mean is how does the game choose north when it orients the ball? I guess the answer is the axis of the planets rotation, so that it appears to rotate anticlockwise when viewed from the north?

Does anyone know of a mod for an inertial navball? If you are on the surface and want to launch for a specific orbit (otherthan equitorial if you are on the equator), the only way to do it is watch the orbit map and guess when you are lined up as you rotate on the surface. If the navball was in an inertial frame you could launch in the right direction by watching the ball rotate. It just seems like the wrong system to use for spaceflight, because all the information we use anyway with our maneuver nodes (target, prograde retrograde markers etc) are all on an 'inertial' overlay on the ball anyway! And it has always bugged me that there is no way in the game to plan your trajectory before you launch.

A reference frame (RF) is a system of coordinates tied to reference points (like North, Vernal, mainbody center), so the coordinates are uniquely fixed. The origin of the RF can be located with your ship, and that's the case with the navball RF, so angles are given in relation to the ship attitude. Hope that helps, as the idea of the RF as "a point on the surface of a sphere..." implies there is still confusion.

About the "inertial navball", what you wish seems to be a different thing. An inertial navball would be tied to an inertial RF, that implies no acceleration of sorts. Rotating RFs are never inertial, so what you're asking is for a RF that has a fixed orientation in space. The RF used in astronomy, that is oriented by the ecliptic plane and the vernal point, is inertial, and KSP also uses a similar RF to keep the position af everything orbiting Sun.

Now, your intent is to launch to a specific orbit, therefore to match specific orbital parameters. In particular, you'll want to launch at the correct inclination when the ship is aligned with the plane of the future orbit. Such a plane is univocally identified with two directions, such as the vector to the periapsis and the vector to the ascending node. Those vectors are certainly univocally described in the inertial reference frame (centered on the mainbody of the orbit), but to use them you need to see where they are. The inertial navball would provide the fixed direction in space to define those vectors coordinates (for the ascending node, you just need its longitude, as the latitude is 0 by definition; for the periapsis, it would be simpler to use a point at 90° from the ascending node longitude, so its latitude would be = inclination). By those two directions a unique circle can be drawn over the navball, and your ship position in relation to that circle would give how close it is to launch on the correct orbit. Now, on a "local" navball (like the standard one in KSP) you would observe the circle rotate in relation to your ship "fixed" position; on an inertial navball you would observe your ship location to move (with the body rotation) and the circle stay fixed. In either case, you have to meet the alignment of ship location and circle to launch, and that is trivial only when launching from KSC (almost 0° latitude, and 0° axial tilt), as you launch when aligned with the ascending node.

But, when is the ascending node (AN) aligned in the case of KSC? just when the AN longitude (always sidereal) is the same as KSC longitude (sidereal). The longitude at KSC is a linear function of time: 360*(UT/21599.91201454)+(90+74.4) (the latter from the initial rotation at epoch=0 of Kerbin and the geographic longitude of KSC; hope to not have inverted the values as had no time to verify). Ok, so known the time (UT), you can compute the sidereal longitude, and when that matches the AN longitude, that's the correct time. Easier than creating an inertial RF navball and trying to use it.

[Wemb]

Setting 'down' to the the center of mass of the planet only sets two axis, how is the other determined?

Well, imagine you're in orbit - you're travelling forward in (some) direction - that's the pro/retro grade axis determined. The direction between you and the centre of the (local) gravity field is defines the pro/anti radial vector.

The last one is the normal vector - and that's orthogonal to the other two (i.e. at right angles). You might now be asking 'but how do you know which direction it goes in?' - that's a rule of thumb - literally.

It's a convention from mathematics called the right hand rule. - Stick your right hand's thumb, first and second fingers out at right angles to each other - Thumb up, forefinger forward and middle finger pointing from your palm outward - that defines the polar space you're in - so if you know where the source of the gravity is compared to you (middle finger), and which direction you're moving in (forefinger), then you always knew which way the normal vector is (thumb).

Wemb

Edited July 20, 2015 by Wemb