 
 
MGS Spacecraft CK File for Extended Mission
===========================================================================
 
     Created by Boris Semenov, NAIF/JPL, Thu Jan 11 11:08:21 PST 2007
 
 
Orientation Data in the File
--------------------------------------------------------
 
     This file contains orientation and angular velocity data for the Mars
     Global Surveyor (MGS) spacecraft frame, 'MGS_SPACECRAFT', relative to
     the 'J2000' inertial frame. The NAIF ID code for the 'MGS_SPACECRAFT'
     frame is -94000.
 
 
Status
--------------------------------------------------------
 
     This file was created by merging daily CK files produced by the MGS
     Spacecraft Team at LMA, Denver data in which were partially corrected
     for the on-board filter latency. Details about this correction are
     provided below. There is a chance that future analysis will allow
     further correction to these time tags -- if so, even better CK files
     would be available from NAIF/JPL.
 
 
Time Correction
--------------------------------------------------------
 
     The data in the file were partially corrected for the spacecraft
     orientation quaternion telemetry time bias. This bias was originally
     detected by the MGS MOLA team from the instrument data collected
     during the eight targeting passes in SPO-1 when MOLA was allowed to
     remain on while the spacecraft was slewed at fairly rapid rates.
     Mismatch of the expected and actually measured surface profiles on
     these slew passes exposed the time offset of the spacecraft attitude
     C-kernels independent of other systematic errors, such as boresight
     misalignment, which could mimic a time offset. This offset appeared to
     be almost constant and was estimated at +1.140 seconds +- 0.010 from
     the MOLA shot time. It appeared that orientation data in the CK had
     time tags 1.14 seconds later than they should be, i.e. to obtain
     correct orientation at the MOLA measurement time T, someone had to
     read original CK pointing at the time T+1.14 sec.
 
     After extended investigation of the problem, the origin was identified
     to be a lag introduced by the on-board digital filter. The filter
     worked on the calculated raw error quaternion (also called raw delta
     quaternion), which was then applied to the existing filtered
     quaternion.
 
     Since the second order low pass only filter used during pre-mapping
     mission phases was supposed to be replaced with the fourth order IIR
     containing the notch for mapping operations, an additional series of
     investigation were carried out to estimate time delay which would be
     introduced by the mapping filter.
 
     Investigations confirmed that the second order low pass only filter
     introduces a 1.x seconds variable time bias and showed that the fourth
     order IIR containing the notch introduces a 2.x seconds variable time
     bias. Both biases could not be reconstructed on the ground because
     their magnitude was a function of the input signal which was not
     available in the spacecraft engineering telemetry.
 
     Before start of the transition to mapping, the second order filter was
     in place at all times. During the transition the spacecraft used both
     filters by switching between them during 11 day period from February
     26 through March 9, 1999, when MOC calibrations were carried out. The
     4th order IIR containing the notch filter was in places during NADIR
     pointing periods and the second order low pass only filter was used
     for all Earth pointing and Moc ISH periods. Once the transition to
     mapping was completed, the fourth order filter was put in place for
     all times.
 
     Since the fourth order filter was in place during the whole period
     covered by this file, correction of -2.300000 seconds was applied by
     Spacecraft Team at LMA to the data in the original individual s/c
     orientation CK files. Note that this was a partial fix because as
     mentioned above the timing bias is variable.
 
     No additional correction was applied when the data was merged
     together.
 
     There are also two other known sources of timing error in the CK data
     which weren't addressed by this fix:
 
         --   error of up to 60 milliseconds due to inconsistent time
              conversions in telemetry system (TIS) and attitude quaternion
              formatter program (ATTREC).
 
         --   error on the order of 1-3 milliseconds due to a difference in
              time conversion between SFOC services (SCLKvSCET file based)
              and SPICE toolkit (SCLK Kernel based).
 
 
Pedigree
--------------------------------------------------------
 
     The original CK files used to create this file were officially
     delivered to the MGS project data base by the MGS Spacecraft Team at
     LMA, Denver. These files contained spacecraft frame orientation
     relative to the J2000 inertial frame. The orientation was computed,
     converted to quaternions and filtered on board; filtered quaternions
     were transmitted to the Earth in channelized telemetry. Nominal
     orientation data sampling rate was 0.25 Hz. On the ground quaternions
     were extracted from telemetry, quaternion time tags were adjusted by
     -2.3 seconds, and quaternions were written to daily CK files using the
     ATTREC program. The individual daily files were merged together to
     provide complete coverage on a mission phase basis.
 
     A table containing the complete list of original CK files from which
     data were included in this file is provided at the end of these
     comments.
 
 
Approximate Time Coverage
--------------------------------------------------------
 
     This file covers the following period of the Extended phase of
     the mission:
 
        COVERAGE BEGIN TIME (UTC):   2006-OCT-11 00:01:00.726
        COVERAGE END TIME (UTC):     2006-NOV-02 23:24:48.359
 
     This file is a type 3 CK file which provides linear interpolation
     between orientation data points extracted from telemetry. Such
     interpolation is not applied to the whole coverage of the file but
     only inside intervals where enough orientation telemetry data was
     available and orientation data points were close to each other in time
     for such interpolation to make sense.
 
     Given that nominal spacecraft quaternions rate was 0.25 Hz (one
     quaternion every 4 seconds) the duration of the longest interval over
     which the data may be interpolated was set to 8 seconds, i.e.
     interpolation was enabled only over a single point drop-out. For all
     requests for data falling between two adjacent data points separated
     by more than 8 seconds the SPICELIB CK readers will return a logical
     flag stating "POINTING NOT FOUND".
 
     A table containing the complete list of valid interpolation intervals
     in the file is provided at the end of these comments.
 
 
Usage Note
--------------------------------------------------------
 
     In order to use this file an MGS SCLK file containing coefficients
     mapping MGS on-board time to ET must be loaded into a user's program.
 
 
Related Kernels
--------------------------------------------------------
 
     This file was created with LSK file naif0008.tls and MGS SCLK file
     MGS_SCLKSCET.00061.tsc.
 
 
Contacts
--------------------------------------------------------
 
     If you have any question regarding this data contact NAIF at JPL:
 
        Charles H. Acton, Jr
        (818)-354-3869
        chuck.acton@jpl.nasa.gov
 
        Boris V. Semenov
        (818)-354-8136
        boris.semenov@jpl.nasa.gov
 
 
Appendix 1: List of the original CK files
--------------------------------------------------------
 
        mgs_spice_c_kernel_2006-284.bc
        mgs_spice_c_kernel_2006-285.bc
        mgs_spice_c_kernel_2006-286.bc
        mgs_spice_c_kernel_2006-287.bc
        mgs_spice_c_kernel_2006-288.bc
        mgs_spice_c_kernel_2006-289.bc
        mgs_spice_c_kernel_2006-290.bc
        mgs_spice_c_kernel_2006-291.bc
        mgs_spice_c_kernel_2006-292.bc
        mgs_spice_c_kernel_2006-293.bc
        mgs_spice_c_kernel_2006-294.bc
        mgs_spice_c_kernel_2006-298.bc
        mgs_spice_c_kernel_2006-299.bc
        mgs_spice_c_kernel_2006-300.bc
        mgs_spice_c_kernel_2006-301.bc
        mgs_spice_c_kernel_2006-302.bc
        mgs_spice_c_kernel_2006-303.bc
        mgs_spice_c_kernel_2006-304.bc
        mgs_spice_c_kernel_2006-305.bc
        mgs_spice_c_kernel_2006-306.bc
 
 
Appendix 2: Interpolation Intervals in the File
--------------------------------------------------------
 
     The following interpolation interval summary was produced by CKBRIEF
     utility program ran with "-dump -utc" options:
 
 
        CKBRIEF Version: 3.2.0, 2006-11-02. SPICE Toolkit Version: N0061.
 
 
        Summary for: mgs_sc_ext26.bc
 
        Segment No.: 1
 
        Object:  -94000
          Interval Begin UTC       Interval End UTC         AV
          ------------------------ ------------------------ ---
          2006-OCT-11 00:01:00.726 2006-OCT-11 11:59:56.718 Y
          2006-OCT-11 12:00:56.718 2006-OCT-11 23:59:52.710 Y

          (many lines were skipped here to reduce the example size)


          2006-NOV-02 21:26:40.360 2006-NOV-02 22:25:56.360 Y
          2006-NOV-02 23:13:32.359 2006-NOV-02 23:24:48.359 Y
 
 
