| DATA_SET_DESCRIPTION |
Data Set Overview:
=================
This archive contains data from the Dynamic Science Experiment
conducted during the Stardust spacecraft encounter with comet
Wild 2 on January 2, 2004. The data in this archive are organized
into two tables in standard PDS format. The first of these tables
provides high rate attitude control system (ACS) data spanning 40
minutes around closest approach while the second table provides
attitude control thruster firing information (aka ``small
forces'') spanning 3 hours around closest approach. This data
provides a significant measure of the spacecraft's performance
and the attitude control system's reaction to particle impacts.
Parameters
==========
The primary parameters contained in the attitude data table are
quaternions providing spacecraft orientation relative to the EME
2000 inertial reference frame (also known as the J2000 frame),
spacecraft angular rates given in the spacecraft-fixed reference
frame, and attitude and angular rate errors. The primary
parameters contained in the small forces data table are thruster
firing period begin and end times, accumulated delta-V values and
numbers and duration of firings for each of the thrusters.
Processing
==========
The source data for both tables were generated on-board the
spacecraft and were sent down within packets in the spacecraft
telemetry stream.
On the ground the attitude parameters were extracted into a text
file containing data in tabular format using standard JPL
Advanced Multimission Operations System (AMMOS) tools for
channelized telemetry access. This text file was then slightly
modified -- commas delimiting columns were inserted, UTC time
tags were re-formatted to be in ISO format, and string column
values were quoted -- to fully comply with PDS requirements.
The small forces parameters were similarly extracted from
telemetry and then saved in a Small Forces File (SFF) using the
MAKSFF process developed and run by NAIF. For inclusion into this
archive the SFF file, which was a variable length line ASCII text
file containing a table with a fixed number of columns, was
augmented with two more columns (begin and end time as UTC) and
re-formatted similarly to the attitude data to comply with PDS
requirements.
Data
====
Both data files in this archive are plain ASCII text files, each
containing a single PDS-style table with fixed-width,
comma-delimited columns. Each file contains a few-line text
header preceding the data. In the attitude data file the header
lines list telemetry channel IDs, names, channel data types, and
formats. The small forces data file header lines provide the
original SFF file header. Each of the two data files is
accompanied by a detached PDS label, described in detail in the
ONLABELS.TXT document included in the archive. The format of the
table columns is defined by an external table format file,
pointed to by the label.
Attitude Data
-------------
The channelized attitude telemetry produced by the ACS was
collected at different rates during encounter. The data before
and after closest approach was recorded at 2-second intervals.
For the closest approach, which occurred at 2 January 2004
19:21:32 UTC, from 19:09:29 through 19:36:28 UTC most of
attitude data was recorded at 0.1-second intervals. One
exception is the body rate error data, which was still recorded
at 2-second intervals and, therefore, appears to be ``stuck''
at one value while all other parameters are changing.
The ACS_MODE channel included in the attitude data file
describes whether the ACS was controlling attitude according to
the ground-commanded reference attitude (ENCOUNTER ABSOLUTE
pointing mode, or ENC_ABS), or to the flight software-updated
trajectory (ENCOUNTER RELATIVE pointing mode, or ENC_REL).
Before and after the roll slews, the ACS_MODE was ENC_ABS. When
the roll was commanded, the mode changed to ENC_REL. After
closest approach, the return roll was commanded and the mode
changed to ENC_ABS. For about 30 seconds after these command
mode changes, the rate and attitude data reflects the slews. At
all other times it reflects the attitude close to the fixed
attitude that the controller was trying to maintain.
Body rates provided in the attitude file are filtered Inertial
Measurement Unit (IMU) data, and attitude errors are computed
from the estimated spacecraft quaternion compared to the
reference quaternion. Since the controller was maintaining a
fixed attitude (except during the slews) the body rate is the
same magnitude but with opposite sign of the rate error. The
measurement noise of about 0.00002 rad/sec is clearly seen in
the rate data during the whole period covered by the file.
Small Forces Data
-----------------
The flight software included an idealized thruster model, and
was configured to produce data summarizing a 1-second interval
of thruster activity and the resulting change in spacecraft
velocity. Although the control system ran at 10Hz, the 1-second
small forces data collection interval was chosen as a
compromise between providing every pulse detail with the risk
of overwriting the data, and not providing enough resolution
for analysis.
The small forces data included the time the packet was
recorded, the number of commanded thruster pulses for each
thruster, the total on-time for each thruster in milliseconds,
and the computed delta-V in the inertial EME-2000 reference
frame. This data was collected only when necessary; that is, if
no thruster commands occurred over an interval, no small forces
data were generated. The average spacecraft attitude quaternion
for the interval was also included in the packet.
A set of thruster calibration activities undertaken by the
navigation and spacecraft teams over the course of the mission
showed that the timing of the deltaV values computed by the
spacecraft was very accurate while the deltaV magnitude was
within +/- 30 percent of the values estimated from ground
tracking data.
Using Attitude and Small Forces Data Together
---------------------------------------------
Individual thruster firing records from the small forces file
may be correlated to the attitude data by looking for a fast
change in the spacecraft rate immediately following the firing
and slower change in the spacecraft orientation for the period
from this truster firing to the next one. Firing of a
particular thruster or a set of thrusters usually results in
change in a particular rate component. Therefore identifying
the thruster(s) that fired in addition to the time of firing is
essential. Also, the frequency at which the thrusters were
firing at different times during the encounter along with the
information about which of the thrusters were firing may
indicate how much ``effort'' the ACS was spending to compensate
for the effect of the surrounding dust environment while trying
to maintain the commanded attitude.
Ancillary Data
==============
Since neither of the two data files provided in this data set was
supplemented with additional geometry data prior to archiving,
the following SPICE kernels archived in the Stardust SPICE data
set, SDU-C-SPICE-6-V1.0, may be used for computing any derived
geometry parameters that might be needed to analyze the data:
Kernel Type File Name
------------ ------------------------
LSK naif0007.tls
SCLK sdu_sclkscet_00105.tsc
FK sdu_v17.tf
SPKs sdu_l_2003_w2.bsp
sdu_w2_opnav.bsp
------------ ------------------------
The spacecraft orientation data -- both quaternions and angular
rates -- provided in the attitude and small forces data files
were used in making, and are available in, the following SPICE CK
files provided in the Stardust SPICE data set, SDU-C-SPICE-6-V1.0:
Kernel Type File Name
------------ ------------------------
CKs sdu_sc_rec_2003_w2_v2.bc
sdu_sc_rec_w2_opnav.bc
------------ ------------------------
Coordinate System
=================
The two reference frames, with respect to which the data included
in this archive are provided, are:
- Earth Mean Equator and Equinox vector of 2000 JAN
01 12:00 (EME J2000), and
- Stardust spacecraft reference frame shown in this diagram:
|| Dust Collector
Main || Array
Solar Array Shield ||
Shield .-. o HGA ||
.-. +Z ^| | ._____|_____. o
| |==========|====o==\=========/==o===============
`-' || |-------------------. . Solar
|| | |/| Array
|| | .-----. CIDA | |
<-------o| | | ' | | Return
+X +Y | | . __/ | | Capsule
| | `. `. |\|
| |--- `. `.---- -----' o---------
| | `. `. \_______/
`-' `.'`.
.'
The orientation quaternions stored in both tables provide
orientation of the Stardust spacecraft reference frame relative
to the EME J2000 reference frame. These quaternions are of the
popular engineering style: Q = {r1*sin(phi/2), r2*sin(phi/2),
r3*sin(phi/2), cos(phi/2)}, where R = (r1, r2, r3) is a unit
vector specifying the rotation axis defined in the base frame.
(This is not the formation style used within the SPICE system.) A
rotation matrix formed from such a quaternion, when left
multiplied by some vector given in the base frame, will rotate
that vector into the target frame: rt = RM*rb. For help in
understanding quaternion syntax and use refer to the discussion
on quaternions provided in the PDS Standards Reference, also
available from the NAIF node document database as ``A Brief
Discussion of Quaternions as Used in Space Remote Sensing.''
The delta V components provided in the small forces data file are
given with respect to the EME J2000 reference frame.
The angular rates and attitude and rate errors provided in the
attitude data file are given with respect to the Stardust
spacecraft reference frame.
Software
========
The data in this data set are in standard PDS format and,
therefore, can be displayed by a number of PDS-provided tools or
loaded into commercial programs that support comma-delimited
formats. For this reason no special processing software is
included in this data set.
Contact Information
===================
For any questions regarding the data in this archive, contact:
Dr. John D. Anderson, Stardust Radio Science and Dynamic
Science Experiment Lead
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