SPICE rotation models

[volcanopele]

For the IAU Rotations, I am using ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... merged.tpc , which is the latest such file for Saturn's small satellites.

This file seems to work just fine for most of the rocks, except Janus, Epimetheus, and Anthe. For the first two, I get a result similar to when I was first trying out rotation kernels for Cassini. the only thing that is different about Janus and Epimetheus is that they have nutation in there. For some reason, Anthe doesn't rotate... all the others work fine. The rotation period seems to be fine for all the others, even Atlas, which I think you are right, the offsets were due to some kind of orbital precession not accounted for in the .tpc file rather than non-synchronous rotation resulting from slight differences between the rotation rate in the .tpc file and the orbital period.

[volcanopele]

Okay, I found out why there are problems with Anthe. As far as I know, Anthe rotates synchronously. If that is the case, the rotation rate for Anthe is 347.32447 degrees per day. However, this kernel file has it at 381.994555 degrees per day. Not sure where they got the number for Anthe.

[volcanopele]

Okay, I've managed to get SPICE rotation models to work pretty well, for prograde rotating satellites. For retrograde rotating moons, the poles become flipped:

Triton_Uniform_rota_test.jpg

The above image is how this should look, approximately. You can see that the north poles roughly line up for Neptune and Triton. Longitude values are measured in East Longitude, as you would expect from retrograde satellites.

Triton_NAIF_rota_test.jpg

Here is how it looks when SPICE_ROTATION is used. The north and south poles are flipped.

No error messages come up in ~ log. Here is the code I used for Triton:

Code: Select all

Modify "Triton" "Sol/Neptune"
{
   Texture "voy_triton.png"
   Timeline
   [
   # Before SPICE
   {
      Ending "1950 01 01 00:01"
      EllipticalOrbit
      {
         Epoch      2447763.5  # 1989 Aug 25 00:00UT (Voyager encounter)
         Period      5.8768541  #  mean
         SemiMajorAxis   354765.286 # at epoch
         Eccentricity   0.00002285 # at epoch
         Inclination   156.826240 # at epoch
         #AscendingNode   172.426656 # at epoch
         AscendingNode   147.899288 # at epoch
         ArgOfPericenter   293.092400 # at epoch
         MeanAnomaly   315.726316 # at epoch
      }

      UniformRotation
      {
         Epoch       2447763.5 # 1989 Aug 25 00:00UT (Voyager encounter)
         Inclination      156.8 # at epoch
         AscendingNode   147.9 # at epoch
         MeridianAngle       71.5 # at epoch
      }
   }
   # nep076-long.bsp
   {
      OrbitFrame { EclipticJ2000 { Center "Sol/Neptune_bary"}}
      # UniformRotation
      # {
      #    Period   141.0444984
      #    Epoch       2447763.5 # 1989 Aug 25 00:00UT (Voyager encounter)
      #    Inclination      156.8 # at epoch
      #    AscendingNode   147.9 # at epoch
      #    MeridianAngle       71.5 # at epoch
      # }
      
      BodyFrame { EclipticJ2000 { } }
      SpiceRotation
      {
         Kernel         "pck00008.tpc"
         Frame               "IAU_TRITON"
         BaseFrame           "eclipJ2000"
         Period              141.0444984
      }
      
      Ending    "2053 10 08 00:00"

      SpiceOrbit
      {
         Kernel "nep076-long.bsp"
                     Target "801"
                   Origin "8" 
         Period         5.8768541
         Beginning "1950 01 01 00:01"
         Ending    "2053 10 08 00:00"

         BoundingRadius 1e10
      }
   }

   # After SPICE
   {
      EllipticalOrbit
      {
         Epoch      2447763.5  # 1989 Aug 25 00:00UT (Voyager encounter)
         Period      5.8768541  #  mean
         SemiMajorAxis   354765.286 # at epoch
         Eccentricity   0.00002285 # at epoch
         Inclination   156.826240 # at epoch
         #AscendingNode   172.426656 # at epoch
         AscendingNode   147.899288 # at epoch
         ArgOfPericenter   293.092400 # at epoch
         MeanAnomaly   315.726316 # at epoch
      }

      UniformRotation
      {
         Epoch       2447763.5 # 1989 Aug 25 00:00UT (Voyager encounter)
         Inclination      156.8 # at epoch
         AscendingNode   147.9 # at epoch
         MeridianAngle       71.5 # at epoch
      }
   }
   ]

}

The .tpc file I used is user modified to define the frame used (IAU_TRITON): http://pirlwww.lpl.arizona.edu/~perry/C ... k00008.tpc . The spk file I used is at ftp://naif.jpl.nasa.gov/pub/naif/generi ... 6-long.bsp .

I performed a similar test on Miranda and Ariel and came up with similar issues.

[volcanopele]

Here is a working example with a prograde satellite (Naiad in this example):

Naiad_Uniform_rota_test.jpg

Naiad_NAIF_rota_test.jpg