M A G N E T O M E T E R T R A C D A T A S E T
This data set contains TRAC-produced full magnetic field field vectors
at 2.5-sec resolution (ZAP4 memory mode only), in a spacecraft-based
coordinate system (Z along s/c symmetry axis, near-radial from Earth; X
near direction of spacecraft motion). It also contains the non-TRAC
8-bit data and the once-every-43-sec TRAC-produced 12-bit data. In
addition it contains components of the IGRF/1980 field in a coordinate
system fixed by the true radial to the Earth's center and the direction
of spacecraft motion (the orbital system), and a difference vector (TRAC-
produced field minus the IGRF model field) as transformed to an
IGRF/1980-based geomagnetic coordinate system. This difference vector
ignores the difference between the spacecraft coordinate system and the
orbital system which can be up to 15 deg, although ancillary information
includes data needed to transform between spacecraft and orbital systems.
This ASCII data set, also contains needed spacecraft position data.
Time span of data set: 81-11-08 - 83-01-31
Time span for every file is given in accompany "MAGNET.TAB" file
1. MAGNETOMETER TRAC EXPERIMENT
The combined Soviet-French TRAC experiment was aimed to measure
magnetic field variations due to auroral field-aligned currents.
It consisted of a "zoom" which was applied to the signals from
the onboard magnetic field detector for attitude restitution.
These signals were read and treated by the TRAC device in a way
of a "zoom". Each of the three magnetic components measured, once
in 43 seconds, was digitized to ~ 13 nT/bit, and these values,
together with the difference analog signal (currently measured
value minus the last digitized) was given to telemetry. (For a
full description see Berthelier et al., Annales de Geophysique,
v.38, N 5, p.635, 1982).
One of the main difficulties in the treatment of the vector
magnetic measurements from a satellite is the acount of the
satellite attitude. The gravity-gradient oriented satellite of
about 1200 kG mass slowly oscillated around the nominal ORBITAL
PLANE ORIENTATION (one axis along the local vertical, another
axis along the velocity vector, the third axis to form a right-
hand frame). The angular amplitude from the nominal orbital plane
can reach sometimes 15 degrees. Another attitude problem with
this satellite is that after a crossing of the solar terminator,
the satellite oscillated during several minutes with a period of
about 40-45 seconds due to thermal shock on the 17 meters boom
carrying a floating magnet for damping of the satellite's
oscillations.Besides that, during intervals where at least one
of the magnetic field components changed rather fast, the damping
magnet rotated non-homogeneously inducing additional oscillations.
In such cases this component could change more than 1320 nT during
43 seconds driving the TRAC signal off scale, and a constant scale
limit was given to the output. After the measured vector rotation
from the magnetometer axis frame to the SATELLITE, or S/C, FRAME
these off scale constant values lead to false deviations in all
three components ending just before the end of 43-seconds interval.
These motions around the satellite's center of mass, and off-scalings,
when they occur, make quite difficult the problem of automated attitude
restitution, and thus the Magnetometer-TRAC experiment automated
data treatment not feasible. In view of this, in the data
presentation below, a simplifying assumption about the satellite
attitude was adopted for the mass data treatment, as described below,
which made it possible to present these data. However, the auxiliary
data are provided in the data file which allow an experienced user
to determine, and then use, a more precise attitude from the same
magnetometer, or from more precise TRAC data.
2. SHORT DESCRIPTION OF THE INSTRUMENT
The onboard standard magnetometer was mounted on a short boom,
with its axes inclined to the S/C FRAME. Tts output was
digitized to 8 bit words giving about 300 nT/bit. This original
magnetic field vector is rotated to the S/C FRAME and its
resulting components are given in the data files under the titles,
respectively, MAGNX, MAGNY, MAGNZ in units nT ( in the S/C frame)
These standard data were originally aimed to only technological
purposes of the satellite's attitude restitution. When looked through
the TRAC "zoom", this magnetometer had a noise up to about 13-26 nT, a
magnetic deviation and zero shifts. These factors became
noticeable when the magnetometer outputs were digitized to 12 bits
by the "zoom" provided by the TRAC device. Thus these magnetic data
are obviously not of a quality of a typical scientific satellite
magnetometer. But still, after some averaging, they may be used
for evaluation of medium-scale (~ 50 - 500 km) field-aligned
current direction and approximate magnitude.
3. TRAC DATA - TELEMETRY OUTPUT
The TRAC device digitized the magnetometer detector outputs (3
components of the magnetic field) once in 43 seconds to 12 bit
words (about 13 nT/bit). These data were readout through an
additional telemetry channel. Then for each magnetic component
the output signal (currently measured minus the last digitized)
was zeroed, and the difference signal was given to the telemetry
during the next 43 seconds, till the new 12 bit digitization, and
so on. It produced for each magnetic component an analog
saw-tooth shape output signal, and periodic 12 bit digital base
values, which are the basis for the further data treatment.
4. TRAC DATA - TREATMENT
First of all, the times of 12 bit digitisation/zeroing are
determined for each component, spurios points removed, the base
values are smoothed. The resulting base values (in nT) measured
once in 43 seconds, are given in the columns with the names
BXSAT, BYSAT, BZSAT in the S/C FRAME.
These base values are qualified by the NOISE CODE for each
component (a column in a data file, see below). The coding is:
0 is an unchanged analog measured value; 1 - a digital zeroing
point (unchanged); 2 - an interpolated value; 3 - smoothe jump
for more than ~ 50 - 100 nT between sequential points.
Using these data (the base values and the saw-tooth TRAC signal
between them in the S/C frame), a continuous sequence of
the measured values. It is given in the columns in the
BXSATF, BYSATF and BZSATF (S/C FRAME ). After these
procedures, the magnetic field vector is considered as final
(though with its components given in a not convenient frame
of reference). They are given to the output data in the
physical units (in nT).
An indirect quality control of the measured magnetic field vector
is provided by comparisons of the module of the measured
magnetic field with the module of the IGRF80 model field
calculated for the measurement date, time and location. The
difference between the modules MEASURED - MODEL (BMAG) is given
in the column BMODIGRF. (Such a check is justified because the
local field-aligned current rotates the magnetic field, but
nearly does not change its module, while effects of other
currents are small and may be neglected. The difference BMODIGRF
is often less than 500 nT, but occasionally may be larger). If
the difference is smooth in time/space, this usually corresponds
to a zero shift in one or more components. These zero shifts are
not too important for the determination of characteristics of the
field-aligned currents because the time/space derivative of a
magnetic signal carries the most important information. But if
the difference between measured and model modules jumps, this
most probably means a spurious noise in one or more components,
and such a measurement interval must be considred with care.
To reduce the quantity of such jumps, a special filtering
procedures were applied to the signals, so that when the
difference between two sequential points was more than
~ 50 - 100 nT, an interpolation between neighbouring points was
applied. If real, such jumps would imply abnormally large
field-aligned current densities (which, however, cannot be
absolutely excuded). Such a smoothing technique is somewhat
similar to a sliding average procedure but applied locally.
The large-scale signal trends are present in the these data
due to large-scale geomagnetic field gradients
along the satellite trajectory. To remove these trends by
subtracting the model IGRF80 magnetic field, the rotation must
be performed of the magnetic vector from the S/C FRAME, to
the ORBITAL FRAME. It needs the known satellite attitude in
respect to its ORBITAL FRAME.
However, while for parts of some orbits this time-dependent
matrix was well determined, its availability and precision are
inadequate for the majority of the data given here. In view of
this, it was supposed, for simplicity, that the S/C FRAME
of reference is adopted to be coincident with the ORBITAL
FRAME.
(ORBITAL FRAME of reference is the frame with the Z-axis
vertically upward, X-axis along the satellite's velocity vector,
and Y-axis forming the right-hand frame). The IGRF80 model
magnetic field vector is rotated to the ORBITAL FRAME. Its
components are given in the data files in the
columns BXIGRF, BYIGRF, BZIGRF in the ORBITAL FRAME.
These values then were subtracted from the respective full field
measured components BXSATF, BYSATF, BZSATF to get the components
of the disturbance magnetic field vector, again in the ORBITAL
FRAME. Now, this vector is rotated to the GEOMAGNETIC FRAME,
The OUTPUT MAGNETIC DISTURBANCE VECTOR with the components DBXGM,
DBYGM, and DBZGM in the GEOMAGNETIC FRAME is given in the first
data columns just after the UT time.
These results of the TRAC magnetic field data treatment
allow to analyse short-term variations of the magnetic field due to
field-aligned currents. These signal variations are superimposed
with the variations due to the satellite's motions around its
center of mass which were described above, to zero shifts of the
magnetometer-TRAC system, and to not-precise attitude. Due to
the simplification adopted concering the satellite's attitude
(see above), a given X- and/or Y-component's value can reach
10 000 nT or sometimes even more. Besides that, the effects of the
neglected satellite's motions around the center of mass are still
present in the output signals as parasites.
It may be noted that only the time/space derivative of a component
value is significant for evaluation of the field-aligned current
direction. Differentiation of the signal with respect to time/space
removes the rest of the slow trend and zero shifts from the data.
Thus even a large but slowly varying shift of a component due to a
not-precise satellite attitude and magnetometer zero shift is not
very important for the scientific aims of the TRAC experiment.
However, if an experienced user would like to determine the satellite
attitude in respect to the ORBITAL FRAME more precisely, the
auxiliary information given in the data files allows to do that.
Indeed, the same magnetometer signals, or more precise TRAC
signals in the satellite frame, are to be used together with the
information on the IGRF80 field components given in other columns
of the data files.
The following types of spurious/erroneous data are noted within
the files:
1. Oscillations after a terminator crossing with a period
about 40 -45 seconds - a systematic error, may be removed by
fitting of a damped oscillation and its subtraction from the signal.
2. Oscillations during perigee when the magnetic field rapidly
changes in the S/C FRAME, and the damping magnet at the top of the
17-meter boom rotates not smoothly.
3. Erroneous values during the initial (incomplete) 43-second
period, and/or during the final one (incomplete), in each ZAP
interval due to inadequacy of the data treatment programs for such
cases. To be neglected.
4. Off-scaling of the TRAC scale of +-1320 nT within a
43-second period. Due to the frame transformations involved, a
truncated saw-tooth signal appears sometimes during several
43-second periods. The zeroing occurs at a proper place, but
the last part of a 43-second period is false, and to be neglected.
5. Individual spurious points ("jumps") due to telemetry noise.
Most of them are removed by the data treatment procedures, but some
may be left in the signal, especially when it is not clear wheather
a particular point is real or not.
5. DESCRIPTION OF COORDINATE SYSTEMS FOR THE TRAC EXPERIMENT.
The SATELLITE FRAME is: Z-axis along the upward vertical of the Earth's
oriented satellite (within +/- 15 degrees from local vertical in flight),
X-axis along the forward direction ( within +/- 15 degrees from the
velocity vector in flight), Y-axis forming the right-hand frame (to
the left side from the satellite's velocity vector).
The ORBITAL FRAME of reference is the frame with the Z-axis
vertically upward, X-axis along the satellite's velocity vector,
and Y-axis forming the xright-hand frame.
The GEOMAGNETIC FRAME has the Z-axis along the IGRF80 model
magnetic field vector (Northward in both hemispheres), Y-axis
Eastward, perpendicular to the IGRF80 model local meridian plane,
and X-axis forming the right-hand frame.
6. DESCRIPTION OF FILES AND FORMAT
6.1. Name of files
A filename of a particular data file contains the number
of the memory readout telemetry seance, and the instrument
indication: the letters "trX" for the magnetic (TRAC) data. In
these filenames the last digit (after "tr" instead of X) indicate
the number of the ZAP4 recording time interval within the memory
readout seance, for which the data are presented.
Thus 00642tr2.DAT means that in this file the TRAC data are
recorded during the second interval. A particular interval
can be identified by its UT time as written in the header
"PASSPORT..." of the file.
The file extension is .DAT.
Each file has a header and number of data records.
Logical record -212 bytes +Cr/LF
6.2. Record Format description
WORD ASCII MEANING UNITS/COMMENTS
1 I4 Hour (0-23)
2 I3 Minute (0-59)
3 I3 Second (0-59)
4 I4 Millisecond
5 F8.0 DBXGM, X-component of the
Disturbance Magnetic Field in the nT (From the TRAC)
Geomagnetic Frame
6 F8.0 DBYGM, Y-component of the
Disturbance Magnetic Field in the nT (From the TRAC)
Geomagnetic Frame
7 F8.0 DBZGM, Z-component of the
Disturbance Magnetic Field in the nT (From the TRAC)
Geomagnetic Frame
8 F8.0 BXSAT-Time, and Values of Zeroing
Points (Digitalization, Full Field) nT (From the TRAC)
X-axis, Satellite Frame
9 F8.0 BYSAT- Time, and Values of Zeroing
Points (Digitalization, Full Field) nT (From the TRAC)
Y-axis, Satellite Frame
10 F8.0 BZSAT-Time, and Values of Zeroing
Points (Digitalization, Full Field) nT (From the TRAC)
Z-axis, Satellite Frame
11 I2 X- Noise Code (0,1,2,3) for the 0- unchanged measured
DBXGM component value; 1- a zeroing
point (unchanged);
2- an interpolated,
or modified value;
3- smoothed jump by
more than ~50-100nT
between the points.
12 I2 Y- Noise Code (0,1,2,3) for the 0- unchanged measured
DBYGM component value; 1- a zeroing
point (unchanged);
2- an interpolated,
or modified value;
3- smoothed jump by
more than ~50-100nT
between the points.
13 I2 Z- Noise Code (0,1,2,3) for the 0- unchanged measured
DBZGM component value; 1- a zeroing
point (unchanged);
2- an interpolated,
or modified value;
3- smoothed jump by
more than ~50-100nT
between the points.
14 F8.0 BXSATF Full magnetic Field
Component in the Satellite nT, (From the TRAC)
Frame
15 F8.0 BYSATF Full magnetic Field
Component in the Satellite nT, (From the TRAC)
Frame
16 F8.0 BZSATF Full magnetic Field
Component in the Satellite nT, (From the TRAC)
Frame
17 F8.0 MAGNX Full magnetic Field
Component in the Satellite nT, (Raw data from the
Frame Magnetometer)
18 F8.0 MAGNY Full magnetic Field
Component in the Satellite nT, (Raw data from the
Frame Magnetometer)
19 F8.0 MAGNZ Full magnetic Field
Component in the Satellite nT, (Raw data from the
Frame Magnetometer )
20 F8.0 BXIGRF - IGRF80 Model nT
Magnetic Field Along the
X-axis of the Orbital
Frame
21 F8.0 BYIGRF - IGRF80 Model nT
Magnetic Field Along the
Y-axis of the Orbital
Frame
22 F8.0 BZIGRF - IGRF80 Model nT
Magnetic Field Along the
Z-axis of the Orbital
Frame
23 I7 BMODIGRF - a quality control nT (Difference Between
the Module of the
Measured Field and
that of the IGRF80
Model Field
24 F10.0 Altitude km
25 F8.2 Geographic Latitude of the Degrees, +/-90
Spacecraft
26 F8.2 Geographic Longitude of the Degrees, 0-360
Spacecraft
27 F8.2 L, McIlwain's Invariant Magnetic Earth's Radii
Drift Shell Parameter
28 F8.2 L0, Invariant Latitude Degrees, +/-90
L0=arc cos (sqrt(1/L))
29 F7.2 MLT, Magnetic Local Time Hours.decimal hours
(for centered dipole
field)
30 F8.2 BMAG, Module of IGRF80 Magnetic milliGauss
Field
31 F8.2 ZSUN, Solar Zenith Angle Degrees, 0-180
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6.3. STRUCTURE OF A HEADER
The header starts with a title:
"PASSPORT FOR THE SEANCE S-XXXX, ARCAD-3". It
gives (in place of XXXX) the number of the memory readout
telemetry seance and the time intervals of the data recording.
The orbit numbers within a seance thus are lower, or equal, to
the seance number.
The first header text paragraph starts with a row "THE NUMBER OF
THE TIME INTERVALS - YY". It gives (instead of YY) the number of
uninterrupted memory recordings, each one in a particular memory
mode ZAP (described in GENERAL.TXT). Each such recording is
described as a line in the "PASSPORT...". Within a data file it
corresponds to a number of lines each describing a "measurement
point" by a line of the data. The number of measurement "points"
(from the start to the end of each recording) is shown in two first
columns of each line, the next columns give the date (dd.mm.yy)
and the UT time (hh.mm.ss.mss) of the start of the interval, then
the same for the end of an interval. The before last column of a
passport row gives the DT - a data time step in milliseconds
(mss), and the last column gives the memory mode ZAP (1-4).
In the second heading paragraph, a reminder is given that
the first rows (up to 7) with the data in each time/ZAP interval
may be false due to telemetry switches, etc., AND THUS MUST BE
DISCARDED.
The third text paragraph in a header gives some relevant
explanations, or a reminder, concerning the data of an instrument
presented further. It describes the GEOMAGNETIC FRAME, in which the
output data DBXGM, DBYGM, and DBZGM are given.
The headings of each data column within a data file are
provided, which contain the meaning of the data in the column
(see Format description above).
As the date of measurements in a data file for each time
interval is given in the "PASSPORT OF THE SEANCE", it is
not repeated in the data rows, and only the UT is given.
The words 14-31 in format description are provided for a
qualified user who would like to improve the accuracy of the
satellite attitude in respect to the ORBITAL FRAME using these
same magnetic field measurements and geomagnetic model calculations,
to check additionally the data by the variations of their module.
Note that an FTPHBrowswr interface to this dataset, providing data
subsetting and and graphical browsing, is available at
"http://ftpbrowser.gsfc.nasa.gov/"
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Please acknowledge NASA's Space Physics Data Facility and
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Authorizing NASA Official: Dr. R.E. McGuire, Head, SPDF, NASA Goddard Space Flight Center
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