Voyager camera pointing information Options

[Brian Burns]

Sorry if this has been discussed anywhere but I can't seem to find much information about it - why is it that Voyager's pointing is so haphazard, as in this video of all the RAW Jupiter images - https://www.youtube.com/watch?v=bf5QJ8iFxUs?

I came across this link which says that pointing information for the images exists - http://pds-rings.seti.org/voyager/ck.html, but have also read on this forum that it's not very accurate. Would the information be useful in automatically aligning composite images and mosaics, or is it too coarse? Would it at least be useful in getting general alignments that could be refined by hand?

And does anyone know why the cameras could not be pointed more accurately, or why accurate information could not be returned with the images? I assume it was some technical limitation, but just curious what it might be.

[Brian Burns]

I've been having trouble getting camera pointing information from SPICE - I've tried both the older C-kernels from NAIF and the newer versions from PDS, Voyager 1 and 2 data, run through all available image times, tried the scan platform vs cameras, set the time tolerance to increasingly larger values, but it still comes back with pointing information not found. I must be doing something obviously wrong - does anyone know what it is?

Here's the Python code using SpiceyPy (I also tried it in C to make sure there wasn't something wrong with the Python interface but got the same results) -

Thanks for any pointers!

CODE

"""
SPICE C-kernel (camera pointing) test

Kernels from
http://naif.jpl.nasa.gov/pub/naif/VOYAGER/kernels/ck/vgr1_super.bc
http://naif.jpl.nasa.gov/pub/naif/VOYAGER/kernels/sclk/vg100019.tsc
http://pds-rings.seti.org/voyager/ck/vg1_jup_version1_type1_iss_sedr.bc
"""

import spiceypy as spice

# load SPICE kernels
spice.furnsh('kernels/vgr1_super.bc') # voyager 1 pointing data (11mb)
# spice.furnsh('kernels/vg1_jup_version1_type1_iss_sedr.bc') # pds version jupiter (700kb)
spice.furnsh('kernels/vg100019.tsc') # voyager 1 clock data (76kb)
spice.furnsh('kernels/naif0012.tls') # leap second data (5kb)

# settings
spacecraft = -31 # voyager 1
instrument = -31001 # narrow angle camera

utcTime = "1979-01-05T15:14:10" # time of first jupiter image
ephemerisTime = spice.str2et(utcTime) # seconds since J2000
sclkch = spice.sce2s(spacecraft, ephemerisTime) # spacecraft clock string
sclkdp = spice.sce2c(spacecraft, ephemerisTime) # spacecraft clock double

tolerance = spice.sctiks(spacecraft, "0:00:400") # time tolerance
# tolerance = 1e12 # nowork

frame = 'ECLIPB1950' # coordinate frame
# frame = 'J2000' # coordinate frame

# get camera pointing information
cmat, clkout, found = spice.ckgp(instrument, sclkdp, tolerance, frame)

# clean up the kernels
spice.kclear()

# print results
print 'utc',utcTime
print 'et',ephemerisTime
print 'sclkch',sclkch
print 'sclkdp',sclkdp
print 'tolerance',tolerance
print 'frame',frame
print
print 'cmat',cmat
print 'clkout',clkout
print 'found',found

"""
=>
utc 1979-01-05T15:14:10
et -662330699.816
sclkch 2/14623:20:785
sclkdp 701344767.168
tolerance 399.0
frame ECLIPB1950

cmat [[ 0.  0.  0.]
[ 0.  0.  0.]
[ 0.  0.  0.]]
clkout 0.0
found False
"""

and the example C code from https://naif.jpl.nasa.gov/pub/naif/toolkit_...ice/ckgp_c.html, slightly adapted

CODE

#include <stdio.h>
#include "SpiceUsr.h"

int main ()
{
  /*
    Constants for this program:

    -- The code for the Voyager 2 spacecraft clock is -32

    -- The code for the narrow angle camera on the Voyager 2
    spacecraft is -32001.

    --  Spacecraft clock times for successive Voyager images always
    differ by more than 0:0:400.  This is an acceptable
    tolerance, and must be converted to "ticks" (units of
    encoded SCLK) for input to ckgp_c.

    -- The reference frame we want is FK4.

    -- The narrow angle camera boresight defines the third
    axis of the instrument-fixed reference frame.
    Therefore, the vector ( 0, 0, 1 ) represents
    the boresight direction in the camera-fixed frame.
  */

#define   SC        -32
#define   INST      -32001
#define   REF       "FK4"
#define   TOLVGR    "0:0:400"
#define   NPICS     2
#define   MAXCLK    30
#define   CKCORR    "voyager2_corrected.bc"
#define   SCLK      "voyager2_sclk.tsc"
#define   FOO "naif0012.tls"


  SpiceBoolean            found;

  SpiceChar               sclkch  [NPICS][MAXCLK] =

    { { "2/18473:46:768" },
      { "2/18381:55:768" } };

  SpiceChar               clkch   [MAXCLK];

  SpiceDouble             cmat    [3][3];
  SpiceDouble             clkout;
  SpiceDouble             sclkdp;
  SpiceDouble             toltik;
  SpiceDouble             vinert  [3];

  SpiceInt                i;


  furnsh_c ( CKCORR );

  /*
    Need to load a Voyager 2 SCLK kernel to convert from
    clock string to ticks.  Although not required for
    the Voyager spacecraft clocks, most modern spacecraft
    clocks require a leapseconds kernel to be loaded in
    addition to an SCLK kernel.
  */
  furnsh_c ( SCLK );
  furnsh_c ( FOO );

  /*
    Convert tolerance from VGR formatted character string
    SCLK to ticks, which are units of encoded SCLK.
  */
  /* sctiks_c ( SC, TOLVGR, &toltik ); */
  /* printf("toltik %f\n", toltik); */
  /* toltik = 800.0; */
  toltik = 1e10;

  for ( i = 0;  i < NPICS;  i++ )
    {
      /*
        ckgp_c requires encoded spacecraft clock time.
      */
      scencd_c ( SC, sclkch[ i ], &sclkdp );

      ckgp_c ( INST,  sclkdp,  toltik, REF,
               cmat,  &clkout, &found       );

      if ( found )
        {
          /*
            The boresight vector, relative to inertial coordinates,
            is just the third row of the C-matrix.
          */
          vequ_c   ( cmat[2], vinert );

          scdecd_c ( SC, clkout, MAXCLK, clkch  );


          printf ( "VGR 2 SCLK time: %s\n", clkch  );

          printf ( "VGR 2 NA ISS boresight pointing vector: "
                   "%f %f %f\n",
                   vinert[0],
                   vinert[1],
                   vinert[2]                               );
        }
      else
        {
          printf ( "Pointing not found for time %s\n", sclkch[i] );
        }

    }

  return ( 0 );
}

[mcaplinger]

I've been having trouble getting camera pointing information from SPICE - I've tried both the older C-kernels from NAIF and the newer versions from PDS, Voyager 1 and 2 data, run through all available image times, tried the scan platform vs cameras, set the time tolerance to increasingly larger values, but it still comes back with pointing information not found.

If it's a type 1 kernel with no interpolation, then you will only get values at specific times no matter how you set the tolerance -- I think. Does it work for the exact time of an image as calculated using the clock strings instead of the ISO time?

If it is a type 1 you could convert it to type 3 with ckspanit. http://naif.jpl.nasa.gov/naif/utilities_PC...dows_32bit.html

[Brian Burns]

If it's a type 1 kernel with no interpolation, then you will only get values at specific times no matter how you set the tolerance -- I think. Does it work for the exact time of an image as calculated using the clock strings instead of the ISO time?

If it is a type 1 you could convert it to type 3 with ckspanit. http://naif.jpl.nasa.gov/naif/utilities_PC...dows_32bit.html

Yes, they're type 1 kernels - they just have pointing information for when the images were taken.

The first photo of Jupiter is C1462321 at 1979-01-05T15:14:10, so I tried this with both kernels but no luck -

CODE

sclkch = "2/14623:21"
sclkdp = spice.scencd(spacecraft, sclkch)
tolerance = spice.sctiks(spacecraft, "0:00:800")

=>

sclkdp 701344783.0
found False

also tried for a later picture, C1504804 at 1979-01-19T19:00:35, and tolerance=1600, but same result.

And tried scanning clock ticks from 0 to a billion, but it didn't find anything -

CODE

tmin = 0
tmax = int(1e9)
tstep = int(1e3)
tolerance = int(1e3)
for t in xrange(tmin,tmax,tstep):
    cmat, clkout, found = spice.ckgp(instrument, t, tolerance, frame)
    if found:
        print cmat

And earlier I had tried scanning through the complete set of Voyager images and used their UTC times to look up pointing information but it didn't find anything.

I've been able to use the position kernels without any problem, so there's something about the pointing kernels that isn't set up correctly.

I know there's information in there somewhere!