Celestia Forums

As of revision 4247 Celestia supports SPICE rotation models. These work very much like SPICE orbits, except that they of course affect the orientation rather rather than the position of Celestia objects. Any SPICE frame can be used to orient a Celestia object. I expect that this will be most useful for spacecraft, though there is high precision orientation data available for the Earth, Moon, and--apparently--the asteroid Eros.

Here's how they work:



The Kernel property specifies one or more SPICE kernel files that must be loaded in order for the frame to be defined over the required range. Any already loaded kernels will be used if they contain information relevant for defining the frame. Frame and base name are strings that give SPICE names for the frames. The orientation of the SpiceRotation is the orientation of the frame relative to the base frame. If no base frame is specified, the default is eclipj2000.

Beginning and Ending specify the valid time range of the SPICE rotation. If the Beginning and Ending are omitted, the rotation model is assumed to be valid at any time. It is an error to specify Beginning without Ending, and vice versa.

Period specifies the principal rotation period; it defaults to 0 indicating that the rotation is aperiodic. It is not essential to provide the rotation period; it is only used by Celestia for displaying object information such as sidereal day length.

Using a SPICE frame rotation for a planet or natural satellite is easier. With spacecraft, the task is complicated by the difficulty of finding all the necessary kernels.

Here's a sample that uses a SPICE text kernel for the rotation of the Earth:



To use SPICE to get the orientation of Cassini, we need multiple SPICE files:



In this example, the text frame (tf) kernel defines the frame, the tsc file is a spacecraft clock kernel, and 08082_08087ra.bc contains the orientation of the Cassini spacecraft between the 82nd and 87th days of 2008.

--Chris

Just to make sure I understand this clearly, since there's some confusion between the 'types' of Spice kernels and their filenames..

.bsp files are SPK kernels, what we're used to for ephemerides
your 'tpc' file is a text PCK kernel, for 'target' parameters and orientation
your 'tf' file is a text FK kernel, to define the reference frame
your 'tsc' file is a text SCLK kernel, for the spacecraft clock
and your 'bc' file is a CK kernel, for spacecraft orientation

That look right?

I finally broke down and decided to incorporate Spice kernel files to show the orientation of Cassini in Celestia. However, I am not finding success.

Here is the code I used for spice kernel orientation:



However, the second two images show the problem. First, Cassini fails to display unless I select to show its body frames. Second, its orientation doesn't actually change. This issue also shows up when I limit the above to a single c-kernel file:



When I use my normal orientation input:



Cassini shows up just fine (as seen in the first image), though obviously the orientation isn't accurate.

Any thoughts on how I can fix this?

A quick way to test this (and one that provides the same results as what I have above) is to try:


de421: ftp://naif.jpl.nasa.gov/pub/naif/generi ... /de421.bsp
080605R_SCPSE_08078_08126.bsp: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... _08126.bsp
cas_v39.tf: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... cas_v39.tf
cas00112.tsc: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... s00112.tsc
08082_08087ra.bc: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... 08087ra.bc

I am using SVN 4604 from http://www.celestialmatters.org/?q=node/132

volcanopele wrote:A quick way to test this (and one that provides the same results as what I have above) is to try:

Code: Select all

ReferencePoint "Saturn_bary" "Sol"
{
    Beginning "1899 07 30 00:00"
    Ending    "2053 10 08 00:00"

    OrbitFrame { EclipticJ2000 { Center "SSB"}}

    SpiceOrbit
    {
        Kernel "de421.bsp"
        Target "6"
        Origin "0"
        BoundingRadius 1e10
    }
}

Modify "Cassini" "Sol"
{
   Beginning "2008 03 23 00:00"
   Ending    "2008 03 27 00:00"
   OrbitFrame { EclipticJ2000 { Center "Sol/Saturn_bary"}}
   SpiceOrbit
   {
      Kernel "080605R_SCPSE_08078_08126.bsp"
            Target "-82"
            Origin "6"
      Period 15
      BoundingRadius 1e10
   }
   
   SpiceRotation
   {
      Kernel [
         "cas_v39.tf"
         "cas00112.tsc"
         "08082_08087ra.bc"
      ]
      Frame               "CASSINI_SC_COORD"
      BaseFrame           "J2000"
   }
}

de421: ftp://naif.jpl.nasa.gov/pub/naif/generi ... /de421.bsp
080605R_SCPSE_08078_08126.bsp: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... _08126.bsp
cas_v39.tf: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... cas_v39.tf
cas00112.tsc: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... s00112.tsc
08082_08087ra.bc: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... 08087ra.bc

I am using SVN 4604 from http://www.celestialmatters.org/?q=node/132

I have been able to get SPICE rotation models working with Cassini. My guess is that some necessary kernels are still missing. I'll try out your example and see whether something is missing. You may want to check Celestia's console log (press the ~ (tilde) key) to see if there are any SPICE error messages appearing.

--Chris

I get several copies of the following error when I got to Cassini on March 25, 2008 at 18:41 UTC:

The following, needed to convert between the uniform time scales, could not be found in the kernel pool: DELTET/DELTA_T_A, DELTET/K, DELTET/EB, DELTET/M.

Any other possible errors flyby before I can see them. All five error messages are the same.

volcanopele wrote:I get several copies of the following error when I got to Cassini on March 25, 2008 at 18:41 UTC:

The following, needed to convert between the uniform time scales, could not be found in the kernel pool: DELTET/DELTA_T_A, DELTET/K, DELTET/EB, DELTET/M.

Any other possible errors flyby before I can see them. All five error messages are the same.

and why don't you redirect the errors into a file??

like

> celestia > log

Fridger

I hadn't thought of that.

Regardless, Chris and I worked through this issue via IM. The leap second kernel file was needed so those variables could be defined. In addition, the base frame also needed to be changed to eclipJ2000 and model orientation adjusted:

ftp://naif.jpl.nasa.gov/pub/naif/generi ... if0009.tls


http://www.dailymotion.com/symaski62/video/13718323

What settings did you use to make that video? I tried doing that for the T50 flyby, and while the video came out looking alright on my computer, the Youtube uploaded version came out way too dark:

http://www.youtube.com/watch?v=eD4L1-5VibM

14 jan 2008 <= mercury




ftp://naif.jpl.nasa.gov/pub/naif/MESSEN ... _od051.bsp
ftp://naif.jpl.nasa.gov/pub/naif/pds/da ... gr_v090.tf
ftp://naif.jpl.nasa.gov/pub/naif/pds/da ... er_478.tsc
ftp://naif.jpl.nasa.gov/pub/naif/pds/da ... 801_v01.bc

http://www.celestiamotherlode.net/creat ... senger.zip 3D

sorry Orientation [180 0 1 0 ] ???? & 01/14/2008



@@+++

On a similar vein, I would like to use SPICE kernels for the rotation of some of the small Saturnian satellites. Here is what I have at the moment:



cas_rocks_v15.tf: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... cks_v15.tf
cpck_rock_02Feb2009.tpc: ftp://naif.jpl.nasa.gov/pub/naif/CASSIN ... eb2009.tpc
sat299.bsp: ftp://naif.jpl.nasa.gov/pub/naif/generi ... sat299.bsp
de421.bsp: ftp://naif.jpl.nasa.gov/pub/naif/generi ... /de421.bsp

The spice rotation portion is commented out, so the code as it stands should work. It works in my version of Celestia. When I use the spice rotation and comment out (or remove) the uniform rotation part, my code is ignored and the version in numberedmoons.ssc is used instead (a quick way to check this is to view the model used, if you see an irregular body, the code is being ignored, if you see a smooth, egg-shaped object, then the code is being used.) I tried this with using only the .tpc file as well as with both the frame kernel (.tf) and the planetary constants kernel (.tpc)

volcanopele,

Quotes are missing around the kernel file names.

Using Celestia v1.6 r4637 under Windows with your SSC file and the SPICE kernel files you list, when I turn on Celestia's "console log" (type a tilde (~) to toggle it), and use the up-arrow key to go up to its top, I discovered an error message:


line 83 is the line containing the [ starting the rotation's Kernel block:


When I add the quotes, the error message goes away and I see an oval body.
I don't know if it's actually rotating correctly, though.

Thanks, that allows the code to load. The rotation model isn't correct, but at least it works...

volcanopele wrote:Thanks, that allows the code to load. The rotation model isn't correct, but at least it works...

You need to set the body frame for the satellites... For backward compatibility with existing add-ons, the default body frame of an object is the equatorial frame of whatever it orbits. For each moon, add this line:



The BaseFrame in the spice rotation needs to match whatever the BodyFrame of the object is. I'm working on documenting some of these new 1.6.0 things in the Celestia Wikibook.

A couple things to note:

• In 1.6.0, it's not necessary to specify the center for a BodyFrame.
• The default BaseFrame for a SpiceRotation is eclipJ2000; you can remove the BaseFrame from your moon definitions unless you want to leave it there for clarity.

--Chris

Thanks, that seems to resolve the issue.

Okay, everything seems to work fine with a code similar to this:



Now the problem (or is it?) is that the rotation rates may be off in the IAU definition. Not sure at the moment. maybe Atlas does rotate slightly non-synchronously (enough to cause a shift of 1 degree per year).

volcanopele wrote:Now the problem (or is it?) is that the rotation rates may be off in the IAU definition. Not sure at the moment. maybe Atlas does rotate slightly non-synchronously (enough to cause a shift of 1 degree per year).

The IAU rotations may well be inadequate... Is SPICE using the same values as specified here?

http://astrogeology.usgs.gov/Projects/W ... able2.html

If so, note that the only deviation from a strictly uniform rotation is a very slow precession of the rotation pole which matches that of Saturn. This keeps the moon's rotation pole perpendicular to Saturn's equatorial plane, but orbital precession unaccounted for. If there is any orbital precession in the SPICE ephemeris, the moon will rotate non-synchronously. I suspect that the IAU rotations of the minor Saturnian satellites have not been updated to account for new knowledge about their orbits.

--Chris

Pole Positions are the same as on the USGS site. Rotation degs/day differ for some of the moons, particularly Atlas and Pan.

volcanopele wrote:Pole Positions are the same as on the USGS site. Rotation degs/day differ for some of the moons, particularly Atlas and Pan.

Can you provide a link to the text kernel that you're using for the IAU rotations?

--Chris