[spice] body orientation

[CAP-Team]

I noticed that there's quite a difference in body orientation when using spice kernels for some bodies.

While the location is correct, the body orientation is totally wrong.

Here's the view of Voyager 2 when using kepler elements for Miranda (which is the correct view)

miranda_kepler.jpg

And here's the view when using spice kernels.

miranda_spice.jpg

I've tested the body orientations for other moons of Uranus and Triton as well, but their orientation is also not what it should be

Here's my Miranda data:

Code: Select all

"Miranda" "Sol/Uranus"
{
  Class            "moon"
  Texture          "miranda.*"
  Color            [ 0.45 0.41 0.40 ]
  Radius                 235.8
  Albedo                   0.27

  OrbitFrame { EclipticJ2000 { Center "Sol/Uranus_bary"}}
  BodyFrame { EquatorJ2000 { Center "Sol/Uranus" }}

  Beginning        "1950 01 01 00:00:41.183"
  Ending           "2050 01 02 00:01:05.183"

  SpiceOrbit {
    Kernel         "ura083.bsp"
    Target         "705"
    Origin         "7"
    Period                 1.413
    BoundingRadius      1e10
  }

  UniformRotation {
    Period                33.92350159  # enforce sync rotation
    Inclination            0.0
    MeridianAngle        144.3
  }
}


I've tried changing the bodyframe to Uranus_bary but this doesn't change a bit.
The Uniform rotation is the same as in solarsys.ssc.

[chris]

The problem is that you are setting the BodyFrame of the object to EquatorJ2000. This is not required in order to use the the SPICE kernel for the orbit, and in fact it is not compatible with the UniformRotation from solarsys.ssc (which is defined for the equatorial frame of Uranus.) Simply removing the BodyFrame from the object definition will fix the problem.

Two further notes:

The center of a body frame is irrelevant; only the orientation matters. This fact would be clearer if Celestia allowed you to omit the center specification for a body frame. I plan on adding this in 1.6.0.

You should always be conscious of the reference frame in which an orbit or rotation model is defined. Most of the bodies in solarsys.ssc are defined in the default frame: the equatorial frame of the parent body. However, the new custom rotations being introduced in 1.6.0 must be used with the EquatorJ2000 reference frame. And, with SpiceRotations the choice of SPICE frame determines which Celestia body frame you should use.

--Chris

[CAP-Team]

Thanks! This helps indeed. I removed all bodyframes for bodies that don't have a customrotation, and now it looks fine again!