PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "S. Joy, Nov 11, 1999;"
RECORD_TYPE = STREAM
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = MGS
INSTRUMENT_ID = MAG
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = MAGNETOMETER
INSTRUMENT_TYPE = "FLUXGATE MAGNETOMETER"
INSTRUMENT_DESC = "
Abstract:
=========
The Mars Global Surveyor magnetic field instrument consists of dual,
triaxial fluxgate magnetometers, capable of measuring fields between
+/- 4 nT and +/- 65536 nT. Automated range switching allows the
instrument to maintain maximum digital resolution over a wide
range of field strengths.
The text of this instrument description has been abstracted from the
instrument paper [ACUNAETAL1992]:
Acuna, M. A., J. E. P. Connerney, P. Wasilewski, R. P. Lin,
K. A. Anderson, C. W. Carlson, J. McFadden, D. W. Curtis, H. Reme,
A. Cros, J. L. Medale, J. A. Sauvaud, C. d'Uston, S. J. Bauer,
P. Cloutier, M. Mayhew, and N. F. Ness, Mars Observer Magnetic
Fields Investigation, J. Geophys. Res., 97, 7799-7814, 1992.
The description and ASCII drawings of the instrument mounting and
frames are derived from the SPICE instrument kernel version 1.2
dated Sept. 16, 1998. Please review the published material for a
complete description of the instrument.
===================================================================
Scientific Objectives:
======================
The primary objective of the Mars Global Surveyor (MGS) magnetic
field experiment is to establish the nature of the magnetic field of
Mars. This includes determining whether or not Mars has a global
field of internal origin indicating either present or past dynamo
field generation. The existence of an internally generated field
would place significant constraints on the composition, thermal
state, and dynamics of the interior of the planet.
Even if dynamo activity ceased as the planet cooled, there is
evidence that dynamo activity existed in the past. Remanent
magnetization has been observed in meteorites that are widely
believed to have originated on Mars [BOGARD&JOHNSON1983]. An
important objective of the magnetic field investigation is to
identify and characterize crustal remanent magnetization. Magnetic
anomaly maps, together with geological data will provide a history
of Martian magnetism and crustal evolution.
Previous missions to Mars have determined that if there is a global
magnetic field at Mars it must be small [LUHMANN1991]. Remanent
magnetic fields are also likely to be small [ACUNAETAL1992]. In
order to accurately measure magnetic fields of Martian origin, the
nature of the solar wind and interplanetary magnetic field
interactions with Mars must be well determined.
===================================================================
Calibration:
============
The flight software incorporated into the on-board data processor
includes diagnostic and self-calibration routines [ACUNAETAL1992].
On-board calibration sequences provide the currents required for
determination of the gain of each axis sensor for the various
dynamic ranges, as well as for determination of electronic offsets
by reversal of the polarity of the signals processed by the
magnetometer electronics. In addition, spacecraft maneuvers will be
performed that will allow the spacecraft field and sensor offsets
to be determined independently.
===================================================================
Operational Considerations:
===========================
The magnetometer power consumption is 375 mW in zero field and
increases with the magnitude of the measured field up to 420 mW. The
typical rms noise level in the sensors is 0.006 nT over a 10 Hz
bandwidth. The zero-level stability is less than 0.15 nT over the
range of -40 to +60 degrees Centigrade and for durations up to a
year. The upper range of 65,536 nT allows the instrument to be
operated in the Earth's magnetic field without special shields or
field cancellation magnets.
===================================================================
Detectors:
==========
The MGS magnetometer experiment consists of two, fully redundant,
fluxgate magnetometers. There are two sensor triads, two sets
of electronics, and two power converter packages. Either sensor
triad can be connected to either electronics package. Only one of
the two systems is powered at any time. The other system is powered
off and maintained in a standby state for redundancy.
The detectors are constructed using the ring core geometry, which
has been shown to have excellent performance characteristics in
terms of long-term zero-level stability and drive power
requirements. The magnetic material used to manufacture the sensors
in an advanced molybdenum-permalloy alloy developed for low-noise,
high-stability applications.
The MGS spacecraft does not have a magnetometer boom to mount the
sensors on as the original Mars Observer spacecraft did. Instead,
the two sensor packages are mounted on the solar panel arrays.
The following diagram shows dimensions required for determination
of locations of the MAG sensors relative to the s/c center:
-Y MAG yoke gimbal s/c gimbal yoke +Y MAG
| | | | | | |
| 3.817m | 0.729m|0.669m | 0.669m| 0.729m| 3.817m |
| or | or | or | or | or | or |
| 150.285in | 28.7in|26.33in|26.33in| 28.7in| 150.285in|
|❮----------❯|❮-----❯|❮-----❯|❮-----❯|❮-----❯|❮----------❯|
| | | ___|____ | | |
| | V / | /| V | |
| __________|_ _____ /____|__/ | ____|__ _________|__
V / V// | | | | | / V // V /
/ -Y Solar // | | | | | / // +Y Solar /
@ Array /@ @--| | | ---@ @/ Array @ -----
/ // / | | | | | // / ^
/___________//______/ | | | | |_____//___________/ |0.934m
|____V__|/ |36.77in
/ \ V
/__@__\ -----------
The orientations of the instrument frames of the +Y and -Y MAG
sensors relative to the corresponding solar array frames are shown
below:
-Y Solar Array frame +Y Solar Array frame
+Z +X +Z
| / |
| / |
| / |
+Y _______|/ |_______ +Y
/
/
+X /
-Y MAG Sensor frame +Y MAG Sensor frame
_______ +Y _______ +Y
/| /|
/ | / |
/ | / |
/ | / |
+Z +X +Z +X
This schema shows that +Y MAG is +90 degrees rotated about Y axis
relative to the +Y solar array and -Y MAG sensor is -90 degrees
rotated about Y axis and after that +180 degrees rotated about the
new position of Z axis relative to the -Y solar array.
Frames diagram
--------------
The following diagrams shows the frames defines for the MGS
spacecraft, solar arrays and MAG sensors:
+Z
|
|
*--- +Y
+X /
S/C body FR
+Z (MGS_SPACECRAFT) +Z
| +X | |
|/ | |
+Y ---* | *--- +Y
| +X /
-Y Gimbal FR | +Y Gimbal FR
(MGS_RIGHT_ | (MGS_LEFT_ +Z
+Z SOLAR_ARRAY) | SOLAR_ARRAY) |
| +X | | | |
|/ | | | *--- +Y
+Y ---* | | | +X /
-Y Yoke FR | | | +Y Yoke FR
(MGS_+Y_SOLAR | | | (MGS_+Y_SOLAR
_ARRAY) | | | _ARRAY)
*--- +Y | | | | | *--- +Y
/| | | | | | /|
+Z | | | | | | +Z |
+X | | | | | +X
| | V | |
-Y MAG sensor FR | | ________ | | +Y MAG sensor FR
(MGS_MAG_-Y_SENSOR) | V / /| V | (MGS_MAG_+Y_SENSOR)
| __________|_ _____ /_______/ | ____|__ _________|__
V / V// | | | | / V // V /
/ -Y Solar // | | | | / // +Y Solar /
@ Array /@ @--| | ---@ @/ Array @
/ // / | | | | // /
/___________//_____/ | | | |_____//___________/
|_______|/
'MGS_SPACECRAFT' frame is the frame associated with the MGS
spacecraft main bus. This frame is defined in an MGS SCLK file
created by mgs_scet2sclk program at LMA. Orientation of this frame
is provided in the CK files produced by the ATTREC program at LMA.
'MGS_LEFT_SOLAR_ARRAY' and 'MGS_RIGHT_SOLAR_ARRAY' frames are
associated with the +Y and -Y solar array gimbals respectively.
These frames are defined in an MGS SCLK file created by the
mgs_scet2sclk program at LMA. Orientation of these frames is
provided in the CK files produced by the MGSSCK program at LMA. Note
that there are no separate frames defined for inboard ('elevation')
and outboard ('azimuth') gimbals for each solar array. Instead each
pair of gimbals is considered as a single gimbal having two degrees
of rotation. These frame can be considered as 'nominal' solar array
position frames since they specify gimbal orientation and do not
take into account any additional rotations/transformation that can
(did) occur due to incomplete deployment of an array.
'MGS_+Y_SOLAR_ARRAY' and 'MGS_-Y_SOLAR_ARRAY' frames are associated
with the +Y and -Y solar array yokes respectively. These frames are
'fixed offset' frames whose orientation is specified by a set of
Euler angles relative to the corresponding frames associated with
gimbals. Defining these frames was required because of -Y Solar
array deployment failure, which introduced an additional rotation in
the yoke for that panel. For the +Y panel this frame is the same as
the gimbal frame.
'MGS_MAG_+Y_SENSOR' and 'MGS_MAG_-Y_SENSOR' frames are associated
with +Y and -Y MAG sensors. These frames are fixed offset frames
whose orientation is specified by a set of Euler angles relative to
the corresponding yoke frames.
===================================================================
Electronics:
============
Signals from the sensors are first processed by the analog
electronics and then by the digital processing unit (DPU). Analog
data are anti-alias filtered and then sent to a twelve bit
(12-bit) successive approximation analog to digital (A/D) converter
that is controlled by a microprocessor. Variable time resolution
data are derived from the basic measurements and the spacecraft
telemetry mode. The microprocessor activates the automatic gain
control logic in the electronics. If the magnitude of the measured
vector component falls within upper or lower guard bands (256 data
numbers), then the range (scale factor) is incremented or
decremented to maintain maximum digital resolution. Range
adjustments change the dynamic range and digital quantization by a
factor of four.
Range Field Strength Quantization
--------------------------------------------
0 +/- 4 nT .002 nT
1 +/- 16 nT .008 nT
2 +/- 64 nT .032 nT
3 +/- 256 nT .128 nT
4 +/- 1024 nT .512 nT
5 +/- 4096 nT 2.048 nT
6 +/- 16384 nT 8.192 nT
7 +/- 65536 nT 32.768 nT
The DPU unit's primary function is to acquire the magnetic field
data and package it with instrument state and housekeeping data in a
form that can be picked up and transmitted to the ground by the
Payload Data System (PDS). The system consists of a master executive
program that is resident in ROM. The DPU uses the 80C86
microprocessor and associated memory and peripheral devices. Default
parameter tables used for data processing are stored in ROM but can
be modified by ground command. Parameters such as sensor zero
levels, alignment matrices, scale factors, etc. are expected to be
updated periodically under normal operating conditions. RAM memory
is used to double buffer data while packets are being created and
accessed by the PDS. Double buffering allows a completed packet to
be read out while a new packet is being created without access
conflicts between the instrument and the PDS. Data collection and
processing routines are interrupt driven by a real time interrupt
(RTI) signal provided eight times per second by the onboard PDS. The
clock is multiplied four times (32 Hz) and is the fundamental timing
signal for all processes in the instrument.
Data compression techniques are used to maximize data return within
the bandwidth allocated to the experiment. Raw magnetometer data are
averaged and then 6-bit differenced between adjacent averages. The
differences are combined with periodic 12-bit 'full-words' and
formatted into data packets. If the differences exceed the dynamic
range of six bits, the system folds the values (modulo 64) over
rather than saturating. This allows the reconstruction of rapidly
varying fields that would otherwise be lost. If the number of
folded differences exceeds a predetermined value, the DPU left
shifts the differencing scheme by the least significant bit doubling
the dynamic range. There is a maximum of two left shifts permitted.
The data return sample rate is linked to the spacecraft data rate.
The instrument has three data rate allocations. Both the rate of
primary (compressed) samples and secondary (full-word) samples
varies with the data rate.
Data Rate Primary Values Secondary Values
(bits/sec) (samples/sec) (samples/sec)
-----------------------------------------------------------
324 8 1/6
648 16 1/3
1296 32 2/3in
===================================================================
"
END_OBJECT = INSTRUMENT_INFORMATION
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "ACUNAETAL1992"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "BOGARD&JOHNSON1983"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "LUHMANN1991"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
END_OBJECT = INSTRUMENT
END
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