Galileo Magnetometer Pre-Jupiter Calibrated Bundle
Galileo Solar System Magnetometer Averaged Cruise RTN Coordinates
Calibrated Data Collection
PDS3 DATA_SET_ID = GO-SS-MAG-4-SUMM-CRUISE-RTN-V1.0
PDS3 DATA_SET_NAME =
GALILEO MAGNETOMETER CRUISE DATA (RTN COORDINATES)
START_TIME = 1989-12-31T00:14:12.369
STOP_TIME = 1994-12-31T03:58:41.145
PDS3 DATA_SET_RELEASE_DATE = 1996-06-01
PRODUCER_FULL_NAME = MARGARET G. KIVELSON
References:
Kivelson, M.G., Khurana, K.K.,
Russell, C.T., Walker, R.J., Joy, S.P.,Green, J., Aiken, W.C.
GALILEO MAGNETOMETER CRUISE DATA (RTN COORDINATES),
GO-SS-MAG-4-SUMM-CRUISE-RTN-V1.0, NASA Planetary Data
System, 1996"
Collection Data Overview
================
This collection contains data acquired by the Galileo Magnetometer
during the Interplanetary Cruise to Jupiter. The data are at varying
resolution depending on the averaging constant applied by the
instrument. These data have been fully processed to remove instrument
response function characteristics. The data are provided in physical
units (nanoTesla) and in 2 coordinate systems. This set of data files
contains data in Inertial Rotor Coordinates (IRC= despun spacecraft).
These files contain some of the data processing parameters from the
AACS system as well as the sensor zero level for the spin aligned
sensor. The data are also being archived in RTN coordinates which are
provided on a separate collection. Trajectory have been
provided in heliographic coordinates as a separate collection.
In order to provide a more complete solar wind collection the high
resolution magnetometer data from the Earth1 encounter, Earth2
encounter and Ida encounter was averaged down to 16 minute resolution
and included with this interplanetary cruise data. The 16 minute
averages are derived from the following collections:
urn:nasa:pds:galileo-mag-prejup-calibrated:data-earth1-rdr-high-res
urn:nasa:pds:galileo-mag-prejup-calibrated:data-earth2-rdr-high-res
Data inside the earth's bowshock was excluded from the 16 min
averages.
Data Columns (IRC):
sc_clk S/C clock counter given in the form rim:mod91:mod10:mod8
time S/C event time (UT) given in the form yyymmddT<time>Z
Br Magnetic field Radial component in RTN coordinates <nT>
Bt Magnetic field tangential component in RTN coordinates <nT>
Bn Magnetic field normal component in RTN coordinates <nT>
Bmag |B| Magnitude of B <nT>
R Radial distance from the Sun[AU]
LAT Solar Latitude (deg)
LON Solar Longitude (deg)
avg_con averaging interval (Rims)
delta Delta
lambda Lambda
Data Acquisition
================
The data for this collection were acquired as part of the normal
instrument calibration activities associated with the cruise to
Jupiter. As such, the instrument was commonly configured in modes
which required calibration even though they may not have been the
optimal mode for science data acquisition. The Galileo magnetometer
has 8 possible acquisition configurations (modes). There are two
sensor triads mounted 7 and 11 meters from the rotor spin axis
(inboard and outboard) along the boom. Each of the sensor triads has
two gain states (high and low). In addition, the sensor triads can be
'flipped' to move the spacecraft spin-axis aligned sensor into the
spin plane and visa versa. Please see the instrument description for
full details on the instrument, sensors, and geometries. The
combinations of sensor, gain state, and flip direction form modes.
_____________________________________________________________________
Mode Characteristics
_____________________________________________________________________
Mode Name Acronym range quantization
_____________________________________________________________________
Inboard, left, high range* ILHR +/- 16384 nT 8.0 nT
Inboard, right, high range* IRHR +/- 16384 nT 8.0 nT
Inboard, left, low range* ILLR +/- 512 nT 0.25 nT
Inboard, right, low range* IRLR +/- 512 nT 0.25 nT
Outboard, left, high range* ULHR +/- 512 nT 0.25 nT
Outboard, right, high range* URHR +/- 512 nT 0.25 nT
Outboard, left, low range* ULLR +/- 32 nT 0.008 nT
Outboard, right, low range* URLR +/- 32 nT 0.008 nT
__________________________________________
s/c clock date/time mode
__________________________________________
00104651:00:0 89-353/20:53 ULLR
00120107:00:0 89-364/17:21 URHR
00120117:00:0 89-364/17:31 IRHR
00120150:00:0 89-364/18:04 IRLR
00120178:00:0 89-364/18:33 ILLR
00120199:00:0 89-364/18:54 ILHR
00120209:00:0 89-364/19:04 URHR
00120231:00:0 89-364/19:26 URLR
00120283:00:0 89-364/20:19 ULLR
00120313:00:0 89-364/20:49 ULHR
00120492:00:0 89-363/23:50 ULHR
00572976:00:0 90-040/03:00 ULHR
00572976:00:0 90-316/17:00 ULLR
00578673:00:0 90-320/17:00 URLR
00586204:00:0 90-325/23:55 URHR
00592915:00:0 90-330/17:01 ILLR
00597439:00:0 90-333/21:15 IRLR
00610156:00:0 90-342/19:33 IRHR
00610509:00:0 90-343/01:30 IRLR
00615701:00:0 90-346/17:00 URLR
00618550:00:0 90-348/17:00 URHR
00624261:00:0 90-352/17:15 ULHR
01003052:72:8 90-253/16:36 ULLR
01600269:00:0 92-308/00:48 ULLR
01605562:00:0 92-311/18:10 ILLR
01629549:00:0 92-328/14:14 IRLR
01632087:00:0 92-330/09:00 ULLR
01635479:00:0 92-332/18:23 URHR
01638675:00:0 92-335/00:01 URLR
01639786:00:0 92-335/18:45 IRLR
01639974:00:0 92-335/21:55 URLR
01640958:00:0 92-336/14:30 IRLR
01641285:00:0 92-336/20:40 URHR
01641351:00:0 92-336/21:07 IRHR
01641407:00:0 92-336/22:15 ILHR
01641465:00:0 92-336/23:02 URLR
01642481:00:0 92-337/16:22 ULHR
01644143:00:0 92-338/20:10 IRLR
01646048:00:0 92-340/04:16 ILLR
01649533:00:0 92-342/15:00 ULLR
01650524:00:0 92-343/07:42 ULHR
01650907:00:0 92-343/14:09 ILHR
01651026:00:0 92-343/16:10 ULHR
01656418:00:0 92-347/11:01 ILLR
01659057:00:0 92-349/07:30 ULLR
02271339:18:2 94-048/05:34 URLR
02378309:54:6 94-123/08:13 ULLR
02618987:18:2 94-292/08:04 URLR
02702827:18:2 94-351/04:56 ULLR
* Range is the opposite of gain - high range equals low gain
Coordinate Systems
==================
The data are being archived in two coordinate systems. The first
coordinate system is referred to as inertial rotor coordinates (IRC).
This coordinate system has the Z axis along the rotor spin axis,
positive away from the antenna and the X and Y axes lies in the rotor
spin plane. In a crude sense, when the spacecraft is far from Earth,
+X points south, normal to the ecliptic plane, positive Y lies in the
ecliptic plane in the sense of Jupiter's orbital motion and positive Z
is in the anti-earth direction. The spacecraft antenna (negative Z
direction) is kept earthward pointing to about +/- 10 deg accuracy.
The second coordinate system is RTN coordinates. This system places
Br along the Sun-spacecraft line with the positive direction away
from the sun. Bnormal is along the direction of the Sun spin axis
crossed with the radial axis. Btangential is in the direction of
Bnormal crossed with the radial axis.
Onboard Spacecraft Processing
=============================
The majority of the processing of the interplanetary cruise data is
done onboard the spacecraft. The following procedures take place
onboard.
1) Data is recursively filtered and decimated to 4.5 vectors/second
2) Sensor zero levels are subtracted and gains are applied
3) Data is transformed from a sensor coordinate system to a spacecraft
coordinate system using an orthogonality matrix
4) Data are decimated again to 2/3 second resolution
5) Data are despun into inertial rotor coordinates
6) A recursive filter is applied and data are decimated to a variable
resolution
These steps are explained in further detail below.
1) Recursive filter and decimation
The Galileo magnetometer samples the magnetic field 30 times per
second. These highest rate samples are recursively filtered and
then resampled by the instrument at 4.5 vectors per second using a
7,7,6 decimation pattern.
Recursive Filter
B(t) = 1/4 Bs(t) + 3/4 B(t-1)
B = output field
Bs = input field measured by the sensor
t = sample time
The pattern is generated by doubling the spacecraft clock modulo 10
counter and then applying the decimation scheme. This gives 3
vectors every spacecraft minor frame (about 2/3 second) which are
sampled unevenly. The first vector in a minor frame is sampled
approximately 0.200 seconds after the last vector in the preceding
minor frame. The other two samples are taken approximately 0.233
seconds apart.
2) The data was processed onboard the spacecraft using a fixed zero
level which was provided at the start of each data acquisition
interval. These zero levels were determined using various methods
and were periodically updated as significant changes in level were
noted. Knowledge of the two spin plane sensor zero levels is not
critical. The data is generally averaged over a long enough
interval that any error in the zero levels is removed. Knowledge of
the spin aligned sensor zero level is critical. However, this can
be corrected for once the data has been received on the ground
and is discussed below under Ground Data Processing.
3) Orthogonality matrix applied: The determination of a calibration
matrix is too complex to describe here. The method employed has
been well described in [KHURANAETAL1996].
4) The data is decimated to 2/3 second resolution by keeping
only the first minor frame of data.
5) Despinning data to inertial rotor coordinates: The nominal
transformation to IRC from SRC is
(Bx) ( cos(theta) -sin(theta) 0 ) (Bxs)
(By) = ( sin(theta) cos(theta) 0 ) (Bys)
(Bz) ( 0 0 1 ) (Bzs)
Where s denotes spinning coordinates and theta is the rotor spin
angle.
6) Recursive filter and decimation: A recursive filter is applied in
in the following manner.
Recursive Filter
B(t) = 1/C Bs(t) + (1-1/C) B(t-1)
B = output field
Bs = input field measured by the sensor
t = sample time
C = averaging constant
The averaging constant is variable and is given at the start of
each data acquisition period. The data is then decimated. There
is a one to one correspondence between the averaging constant
used and the decimation factor as outlined below.
Time per sample(Rims) Averaging constant Decimation Factor
-------------------------------------------------------------------
1 1024 1
2 512 2
4 256 4
8 128 8
16 64 16
32 32 32
64 16 64
128 8 128
256 4 256
512 2 512
1024 1 1024
Ground Data Processing
======================
Once the data is on the ground there is very little that can be done
to improve the onboard processing because of the heavy filtering
and decimation. The following procedure is done on the ground
1) Data is converted to nanoTesla
2) The zero level of the spin aligned sensor is corrected for.
3) Timing is set to account for a phase delay
4) Rotation is performed to account for phase delay
5) Data are transformed to RTN coordinates to
produce the RTN collection.
These steps are outlined in greater detail below.
1) Conversion to nanoTesla simply requires dividing the instrument
data numbers by a constant scale factor. For the inboard high range
(low gain) mode the scale factor is 2. For the inboard low range
and outboard high range, the scale factor is 64. The outboard low
range data has a scale factor of 1024.
2) The zero level of the spin axis aligned sensor was determined by
several means. One method is to take 25 day averages of the data
from the spin aligned sensor. The basis of this method is that if
the spacecraft is relatively close to the ecliptic plane, over one
solar rotation the average of the suns magnetic field in the
spacecraft z-direction is zero. Therefore whatever remains after
averaging is taken to be an offset. The second method is dependent
on the four quadrant structure of the solar wind. For this method
we look only at points where By(IRC coordinates) crosses from
positive to negative values. Here we assume that based on the
four quadrant structure, as we pass from quadrant to quadrant
By(IRC) will change signs and Bz(IRC) will go to zero. The values
of Bz(IRC) where By(IRC) crosses zero are then averaged over a
fifty day period. For the majority of the cruise data the
instrument was configured in the ULLR mode. This mode puts sensor 3
along the spin axis. After looking at the results of these two
method it became apparent that the spin aligned sensor 3 offset had
a substantial drift that was correlated to the change in
temperature. The offset determinations for sensor 3 were fit to
temperature. It was found that two different fits were necessary.
For temperatures greater than 272K a straight line fit was used.
zoff = -17.60926 + .02608*(Temp)
zoff = Offset for sensor three [nT]
Temp = Temperature at the outboard sensor [K]
For temperatures less than 272K a second fit was determined.
zoff = 22.9016799 + (2511.27490/Temp) - (3176088.25)/(Temp*Temp)
zoff = Offset for sensor three [nT]
Temp = Temperature at the outboard sensor [K]
A second instrument configuration commonly used during the
cruise to Jupiter was the URLR mode. This mode places sensor2
along the spin aligned axis. For unknown reasons this sensor
was not as susceptible to changes in temperature and showed
no strong zero level drifts over time. A fixed offset of
2.75574nT was removed from sensor 2.
3) In order to account for the long recursive filters applied
the timing of the data is corrected to be:
rim=rim + (avg_con - (rim % avg_con)) +1/e*avg_con
rim = starting rim of the averaging interval
avg_con = length of the averaging interval(rims)
4) There is a phase delay associated with the Analog to Digital
converter of the instrument. This means that when the data is
despun onboard the spacecraft, the data and the angle used to
despin it are slightly out of phase. In order to correct this
delay the data are rotated by a phase angle. The delay is
equivalent to the rotation outlined below.
Bx'= cos(delay) * Bx - sin(delay) * By
By'= sin(delay) * Bx - cos(delay) * By
delay = -0.045163
To transform the data to RTN coordinates the following step is taken
5) The data are transformed first into Earth Mean Equatorial(equinox
1950) coordinates. This system is directly supported by the SPICE
software provided by the Navigation and Ancillary Information
Facility (NAIF) at JPL as inertial coordinate system 'FK4'. The
angles required for this transformation come directly from the
Galileo Attitude and Articulation Control System (AACS) data. The
transformation matrix for this rotation is:
-- --
| (cosTsinDcosR - sinTsinR) (-sinDsinTcosR - cosTsinR) cosDcosR |
| |
| (cosTsinDsinR + sinTcosR) (-sinDsinTsinR + cosTcosR) cosDsinR |
| |
| -cosDcosT sinTcosD sinD |
-- --
where
R = Rotor-Right Ascension
D = Rotor-Declination
T = Rotor-Twist - Rotor-Spin-angle (despun data)
Once in an inertial coordinate system, SPICE software provides
subroutines which return the transformation matrices to GSE
(G_GSETRN), GSM (G_GSMTRN), or RTN (G_RTNTRN) coordinate systems
for any ephemeris time. These matrices have been used to perform
the coordinate system transformations. The spacecraft/planet (SPK),
leap second (TS), and planetary constants (PCK) kernels required
for these transformations have been archived in the PDS by NAIF.
The SPICE toolkit (software) can be obtained
from the NAIF node of the PDS for many different platforms and
operating systems. At the time of this archive, the SPICE toolkit
was available via an anonymous-ftp site at naif.jpl.nasa.gov"
CONFIDENCE_LEVEL_NOTE = "
Data Quality
============
Data quality assessment is a rather vague concept which we will try to
address in a somewhat qualitative manner. Each aspect of the data
processing sequence can be analyzed to determine its maximum possible
error contribution. In theory, these errors could be summed to provide
estimates of the maximum error for each data point. We have not taken
our error analysis to that level. We believe that our calibrations
(sensor geometry and gains) are good enough that they produce a
negligible source of data error. In addition, we believe that the
coordinate system transformations which are derived from the SPICE
kernels and Toolkit are negligible sources of error in the magnetic
field vectors. The sources of error which we feel are the most
significant are those associated with magnetic sources aboard the
spacecraft, especially those with temporal or spacecraft orientation
variations. The next greatest contributor of error is the data from
the AACS which affects our knowledge of the spacecraft orientation and
hence rotates the magnetic field vector. Lastly, telemetry or software
errors which produce 'spikes' or bit errors in the data are error
sources.
Magnetic sources associated with the spacecraft have been identified.
Over the long averaging intervals of all the interplanetary cruise
data this source of noise is eliminated. There are some time intervals
when the zero levels of the spin plane sensors show large variations
(1-5 nT) on short time scales (minutes to hours). After a while
(hours to days) the offsets return to their nominal levels. The source
of these magnetic fields has not yet been identified. The method of
removing offsets does not remove these rapidly varying effects.
Errors associated with AACS angles have various effects on the data.
The rotor right ascension and declination are crucial to the
understanding of the spacecraft orientation. Fortunately, these angles
are slowly varying and can be interpolated to better than 1 degree of
accuracy for long (many hour) time periods except near major
spacecraft maneuvers.
During some time intervals no AACS data is available. As a result,
these intervals can not be transformed from spacecraft coordinates RTN
coordinates.
During several intervals of the interplanetary cruise the spacecraft
lost it's sector data. Sector data gives the magnetometer the
neccessary information to despin the magnetic field data spin plane
components (Bx_sc and By_sc in spacecraft coordinates). If the
magnetic field spin plane components are not despun they will appear
as a sin wave. The next step in the processing done onboard the
spacecraft is averaging. The data that is not despun will be averaged
to zero as you would expect a sin wave to be averaged over many
periods. The intervals where sector data was lost have not been
removed from the processed IRC collection. The reason for this is that
these intervals can be valuable in helping to determine sensor zero
levels. These intervals have been cut from the processed data in RTN
coordinates. Below is a list of time where sector data was lost.
1990-Jan-26 19:35:48 to 1990-Jan-31 05:17:29
1991-Jan-16 20:24:58 to 1991-Jan-21 17:36:00
1991-Feb-12 18:47:00 to 1991-Feb-25 18:08:00
1993-Jan-27 09:00:40 to 1993-Jan-30 12:30:27"