| DATA_SET_DESCRIPTION |
Data Set Overview
=================
NEAR MAG RDR volume sets contain a single data set, from one instrument
and one mission phase (defined in the phase table in /AAREADME.TXT).
This data set contains vector magnetic field data acquired by the
fluxgate magnetometer (MAG) on the NEAR spacecraft during the
EARTH phase. The data set begins on 1998-01-23T00:00:00.000
and ends 1998-01-26T23:59:59.999.
The data are processed science data provided in physical units (nT)
in two coordinate systems Near Sun Orbital (NSO) and Eros Body Fixed
(EBF). Instrumental and spacecraft effects have been removed from
the data during processing. The data are provided in ASCII tables
located in the DATA directory. Plots of the data are also provided
for browsing purposes in the BROWSE directory. Spacecraft position
is also included in the data tables and plot files.
Data/Parameters
===============
The UTC time tags assigned to each sample are the spacecraft event
time of the magnetometer output of the A/D converter. The sample
time has not been shifted to account for the time lag in the
anti-aliasing filter. The UTC time tags have the format of
year-mo-dmThr:mn:ss.ss, where year(I4) is year, mo (I2)is month, dm
(I2)is day of month, hr(I2) is UTC hour, mn(I2) is UTC minute, and
ss.ss(F5.2) is UTC second. DMET is the difference between
magnetometer and housekeeping data samples. The difference is kept
as small as possible for accurate evaluation of the time-dependent
magnetic fields generated by currents flowing in the spacecraft.
ASCII tables labeled NSOyyddd.TAB (yyddd indicates year and day of
year) contain the following Level 2 MAG data.
Column Name Description Unit
------ ----- ----------- ----
1. UTC time, UTC see above
2. MET_DAY time, mission day
3. MET_HOUR time, mission hour
4. MET_MINUTE time, mission minute
5. MET_SECOND time, mission second
6. DMET housekeeping data time rel. to mag. second
7. B_X magnetic field NSO X component nT
8. B_Y magnetic field NSO Y component nT
9. B_Z magnetic field NSO Z component nT
10. B_TOT magnetic field magnitude nT
11. NSO_X NEAR position NSO X component km
12. NSO_Y NEAR position NSO Y component km
13. NSO_Z NEAR position NSO Z component km
ASCII tables labeled EBFyyddd.TAB (yyddd indicates year and
day)contain the following Level 2 MAG data.
Column Name Description Unit
------ ----- ----------- ----
1. UTC time, UTC see above
2. MET_DAY time, mission day
3. MET_HOUR time, mission hour
4. MET_MINUTE time, mission minute
5. MET_SECOND time, mission second
6. DMET housekeeping data time rel. to mag. second
7. B_X magnetic field EBF X component nT
8. B_Y magnetic field EBF Y component nT
9. B_Z magnetic field EBF Z component nT
10. B_TOT magnetic field magnitude nT
11. EBF_X NEAR position EBF X component km
12. EBF_Y NEAR position EBF Y component km
13. EBF_Z NEAR position EBF Z component km
Files labeled nso10day_yyyy_dd0_dd1.jpg (yyyy indicate year, dd0 is
the first day and dd1 is last day of a 10-day interval) provide
summary plots of MAG data in NSO coordinates averaged down to 10-min
resolution.
Name Description Unit
----- ----------- ----
BX magnetic field NSO X component nT
BY magnetic field NSO Y component nT
BZ magnetic field NSO Z component nT
BTOT magnetic field magnitude nT
THETA mag. field direction from NSO Z axis degree
PHI mag. field azimuth about NSO Z azis degree
Files labeled nso_ebf_yyddd.jpg (yyddd indicates year and day)
provide daily plots of Level 2 MAG data in both NSO and EBF
coordinates.
Name Description Unit
----- ----------- ----
X magnetic field X component nT
Y magnetic field Y component nT
Z magnetic field Z component nT
T magnetic field magnitude nT
THETA mag. field direction from NSO Z axis degree
PHI mag. field azimuth about NSO Z axis degree
R NEAR distance from the Sun AU
SElong NEAR longitude in solar ecliptic coord. degree
SElat NEAR latitude from solar ecliptic coord. degree
Sunward Exc arcsin(NSO_X/NSO_R) degree
Clock Angle atan2(NSO_Y,NSO_Z) degree
Eros lat NEAR latitude in EBF coordinates degree
Eros long NEAR longitude in EBF coordinates degree
Eros-NEAR Distance between NEAR and Eros km
Processing
==========
Raw data contained in the Level 1 MAG dataset were processed by
applying a series of corrections known from pre-flight calibrations
and removing spacecraft fields evaluated in-flight. A summary of the
calibration process is provided in the MAG_CALIBRATION document and
more detailed descriptions can be found in [LOHRETAL1997] and
[ANDERSONETAL2001]. Formally, the correction process within the
spacecraft (SC) coordinates can be expressed as
B_final_SC = [T1](B_Obs_SC - B_SC)
Where B_final_SC is the calibrated field, B_Obs_SC is the measured
field (i.e., the raw Level 1 data converted to physical units
incorporating parameters obtained in the pre-flight calibration),
and B_SC is the total satellite field. The matrix [T1] represents
the correction in sensor orientations and gain factors resulting
from the in-flight calibration [ANDERSONETAL2001].
The pre-flight calibration evaluated the linearity, orthogonality
and cross talk of the MAG sensor block for each axis and each
dynamic range. This calibration provided absolute gain calibrations
to 0.1% (0.5%) in the least (most) sensitive range and orientation
to 1 arc minute [LOHRETAL1997].
The major challenge in producing science-quality Level 2 MAG data
was evaluation of spacecraft fields [ANDERSONETAL2001]. There are
several known sources of spacecraft fields and each source was
evaluated using the extensive in-flight data. The following table
summarizes the sources of spacecraft fields together with their
approximate magnitude, characteristic time scale and type of signal,
and mitigation approach.
Source Propulsion valve motion.
Magnitude ~100nT.
Time Scale Propulsion events.
Type of Signal Discrete pairwise jumps - zero net change.
Mitigation Approach Flag Propulsion events. Correcting data.
during propulsion events not necessary.
Source Terminal board: all spacecraft loads.
Magnitude 25 nT.
Time Scale ~10 min (heaters), < 1 s (NLR).
Type of Signal Variable baseline with discrete jumps.
Mitigation Approach Field is directly proportional to load currents
monitored in spacecraft engineering data.
Loops measured during assembly.
Source Digital power system shunts.
Magnitude 5 to 10 nT.
Time Scale Hours to months.
Type of Signal Discrete jumps.
Mitigation Approach Ratio between digital shunt and total solar
array current identifies shunt step. Field
fixed in each level.
Source Analog power system shunts.
Magnitude 5 nT.
Time Scale minutes.
Type of Signal Gradual variations with intermittent discrete
jumps
Mitigation Approach Field is proportional to analog current within
each of segments in analog shunt current
corresponding to the six analog shunt resistors
on the spacecraft.
Source Solar Arrays.
Magnitude 30 nT.
Time Scale hours to months.
Type of Signal Gradual shift with discrete jumps.
Mitigation Approach Field is proportional to total solar array
current. Independent of digital shunt level.
Source Momentum wheels.
Magnitude 1 to 2 nT.
Time Scale 0.2 to 10s Hz. usually > 0.5 Hz.
Type of Signal Superposition of four sinusoids corresponding
to rotation rate of four wheels.
Mitigation Approach Benign - wheel speeds kept above 0.5 Hz &
monitored in engineering data. Speeds < 0.5 Hz
occur during maneuvers. MAG 0.5 Hz filter
elimiates contamination. 1/s sampling allows
additional filtering if needed.
Predictive correction algorithms (see Software below) were developed
to evaluate these fields using the available spacecraft housekeeping
data. The corrections applied in the production of Level 2 MAG data
include:
1. Subtraction of constant offset fields.
2. Subtraction of magnetic field generated by the system terminal
board.
3. Subtraction of magnetic field generated by the analog and
digital power system shunts.
4. Subtraction of magnetic field generated by solar array.
5. Correction of stepwise fields due to on/off of sensor heaters.
The final Level 2 MAG data were generated by rotating the field
vector from spacecraft (SC) to NSO and EBF coordinates using the
appropriate rotation matrix [T2] derived from information available
in the SPICE volume:
B_final = [T2]B_final_SC
Sampling Rate
=============
MAG data were sampled at a rate that varied by command between 0.01
and 20 samples/s. During Eros observations the nominal rate was 1
sample/s. Brief, 10 minute, periods of 20 sample/s data were taken
as part of weekly calibration checkout sequences. During cruise
operations the MAG sample rate was commanded to coarse values of
0.01 samples/s or 0.02 samples/s. During full MAG instrument
checkout the instrument was commanded through the complete range of
sample rates. Full checkouts were performed after launch
activation, after turn-on following the deep space maneuver and
Mathilde fly-by, and prior to Eros orbit insertion.
The full-time resolution data can be found in the Level 1 MAG
volume. The sampling rate presented in the Level 2 science data
files differs from the original rate since it was necessary to
remove spacecraft fields using housekeeping data that were sampled
differently. The spacecraft housekeeping data were 44 times more
voluminous than the magnetometer data and were recorded at a maximum
rate of one sample/s. In practice, the spacecraft telemetry and
data allocations allowed sampling of housekeeping no more often than
one sample every 10s and typically 50s to 600 s between samples.
The data rate for valid science data therefore actually ranged from
two to four orders of magnitude less than the capabilities of the
magnetometer instrument [ANDERSONETAL2001].
It is important to note that the MAG science data sample rate
varied, often irregularly. This is due to the sequence of the
command and telemetry processor (CTP) - spacecraft housekeeping
data. For the Level 2 science data, MAG samples that were nearest
to CTP samples were selected in order to accurately remove
time-dependent spacecraft fields. There are a variety of reasons
for occasional irregular sampling of CTP data: operations commands,
commands from other instruments, and spacecraft autonomy. All of
the available CTP data were used to maximize the MAG science
records.
Range Setting
=============
The magnetometer had 8 ranges covering full-scale ranges from
plus/minus 4 nT (range 0) to 65536 nT (range 7). A 20-bit A/D
conversion for these ranges resulted in resolutions of 1 pT (range
0, limited by intrinsic instrument noise) to 2.0 nT (range 7).
Range control was done either manually or automatically. Automatic
range control provided transition to less sensitive ranges when any
one axis exceeded 87.5% of full scale for 0.25 s and shifted to a
higher sensitivity when the field on all three axes fell below 17%
of full-scale continuously for 1 min. This ensured that the
instrument follow rapid increases in field strength without rapidly
toggling between ranges. During cruise, the instrument was operated
in range 3 (plus/minus 256 nT full scale, 0.008 nT resolution)
[ANDERSONETAL2001]. The range setting is not provided in the Level
2 data files. All necessary range scale factors have been applied
in Level 1 and Level 2 data files. The range information is
contained in Level 1 MAG files submitted to PDS. The range indicator
ACTUAL_RANGE can be found in the Level 1 MAG data files
MAGyyddd.FIT, where yyddd indicates year and day of year.
The magnetometer operated in range 3 (256 nT full scale) for almost
the entire mission. Exceptions occurred during Earth flyby when the
instrument cycled through all ranges, during instrument calibration
and checkout, and during periods when the spacecraft field was
larger than the maximum for range 3.
Ancillary Data
==============
NEAR attitude and location and EROS location, which are necessary
for MAG coordinate transformation, are included in separate SPICE
volumes. In each of the SPICE volumes that are organized by mission
phase, there are the /DATA/SPK directory for EROS and NEAR position
information and the /DATA/CK directory for the NEAR attitude
information.
Coordinate Systems
==================
Two coordinate systems are adopted for Level 2 MAG data presentation.
NEAR Sun Orbital (NSO) coordinates are Cartesian coordinates defined
with respect to the NEAR-Sun line and the orbit plane of Eros. The
X-axis points from NEAR to the Sun, the Z direction is given by the
Eros velocity vector crossed into X, northward normal to the Eros
orbital plane, and Y completes the right-handed set.
Eros Body Fixed (EBF) coordinate system has its Z-axis along the
Eros spin axis, the X-axis is in the Eros equatorial plane, and lies
in the direction of the prime meridian. The Y-axis completes the
right-handed set.
Software
========
Software for production of science-quality Level 2 data from the raw
magnetometer data contained in the Level 1 data is provided in
MAG_SW document in this volume. The software consists of two
packages, SDC_PROC and SCI_PROC. The packages perform all data
processing needed as the result of sensor calibration and evaluation
of spacecraft fields as described above under the subheading of
Processing.
The SDC_PROC codes were used for reading Hierarchical Data Format
(HDF) data files produced by the Science Data Center (SDC),
extracting the pertinent magnetometer, command telemetry processors
(CTP), and housekeeping data, and producing plots and data files for
the NEAR MAG experiment. The files generated by these codes are
used as input to the second software package SCI_PROC.
The main functions of SCI_PROC are: open and read the input files,
apply corrections to the data, and produce plots and ASCII data
files.
The codes are found in the in the MAG_SW directory of the present
volume.
Media/Format
============
This data set is released as a CDROM set.
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