J.M.G. Merayo, P.S. Jørgensen, T. Risbo, P. Brauer, F. Primdahl and J. Cain
submitted to Ann. Geophysicae, 2000
Abstract.-The Swedish micro-satellite Astrid-2 was
successfully launched into a near polar orbit in December 1998. Despite the fact that its primary mission
was the research of Auroral phenomena, the magnetic instrumentation has been designed to accomplish high
resolution vector field magnetic measurements, and therefore mapping of the Earth's magnetic field is possible.
The spinning of the spacecraft about a certain axis makes stabilisation in space possible. This fact and the
well distributed data over the globe makes the magnetic data well suited for the estimation of the magnetic
field model at the spacecraft altitude (circa 1000km).
This paper describes the initial analysis of the Astrid-2 magnetic data. As a result of the study of a single
day (February 7, 1999) magnetically fairly quiet, it was possible to calibrate the magnetometer and find a
magnetic field model that fitted the scalar component of the measurements to less than 5nTRMS (for latitudes
equatorward of 50º).
Several methods for field modelling are presented in this paper with the assumption that the direction of the
spin axis is nearly constant. The orientation of the magnetometer might be determined simultaneously with the
instrument calibration and the main field model coefficients. Hence, apart from the scientific use of the
magnetic data, the attitude of the spacecraft may be estimated with high precision.
J.M.G. Merayo, F. Primdahl, P. Brauer, T. Risbo, N. Olsen and T. Sabaka
submitted to Sensors and Actuators A: Physical, 2000
Abstract.- The construction
of an orthogonal reference frame based on a set of three skew axes for a magnetic
coil system is discussed. For a skew system, it is possible to define the coil axes
and the magnetic axes as a dual set of axes that are linked to the system. Therefore,
one orthogonal reference frame can be identified from the coil axes and another from
the magnetic axes. But there are also other possibilities. The parametrizations for
the magnetic sensors CSC (Ørsted satellite) and CDC (Astrid-2 satellite) are analysed.
In principle, only the transformation matrix that orthogonalizes the sensor is needed,
but it is customary to express this matrix as function of some non-orthogonal angles.
Therefore, most relevant conventions from the literature are presented for comparison.
In the case of small angle approximation that is valid for most sensors, they all
agree.
P.S. Jørgensen, P. Brauer, J.M.G. Merayo, N. Olsen and F.Primdahl
Worshop on Calibration of Space-Borne Magnetometers, Braunschweig, 9 March 1999
submitted to ESA-SP on Calibration of Magnetometers, 2000
Abstract.-
The vector data from the magnetometer onboard the Auroral Turbulence I experiment, flown in March 1994,
have been in-flight calibrated. Using information about the rocket attitude it is possible to determine
the offsets for the two axes in the plane perpendicular to the spin axis of the rocket. The remaining
calibration parameters have been determined using the scalar calibration. The field aligned currents present
during the experiment show up in the misalignment parameter between the two axes in the spin plane.
P. Brauer, J.M.G. Merayo, T. Risbo and F. Primdahl
Worshop on Calibration of Space-Borne Magnetometers, Braunschweig, 9 March 1999
submitted to ESA-SP on Calibration of Space-Borne Magnetometers, 2000
Abstract.-
The two fluxgate-sensors for the German satellite "CHAMP" have been calibrated in the high precision
coil-facility "Magnestrode" in Braunschweig. The aim of this calibration was mainly to verify the
linearity and stability of the instruments and to find the variation of the calibration coefficients
as a function of temperature. A linear calibration method is used to find the linear coefficients
(scale factors, offsets and non-orthogonalities) as well as the inline non-linearities of the instrument.
A thin shell calibration exceeding 8 hours and with more than 680 different coil settings show a long-time
stability of the instrument/coil-facility in the order of 0.15 nTrms. This result is obtained in spite of
an insufficient adjustment of the ADC's, giving rise to a relatively large non-linearity. Using the thermal
chamber of the "Magnestrode" facility, a set of thin shell calibrations was performed while the temperature
of the sensor was changed. The sensor was cooled down to -40°C with dry ice placed above the sensor.
The sensor was then passively heated to room temperature and two heat-blankets heated the sensor further
up to +30°C. Applying processes known from digital signal analysis, the calibration method is extended
to resolve evolution of the calibration coefficients. Determining the coefficients vs. temperature,
the importance of correcting for thermal gradients is addressed. From the result of the thermal analysis
the non-orthogonality angles of the magnetometer is found to vary less than 5 arcsec in a span of 70
degrees and the offset stability is better than 0.5 nT. The scale factors of the sensor change with
approximately -30 ppm/°C and the deviation from this linear trend is less than 10 ppm when carefully
correcting for the thermal gradient in the sensor. Slightly different slopes in scale factor vs.
temperature reflect the three different thermal setups (cooling, passive heating and active heating).
J.M.G. Merayo, P. Brauer, F. Primdahl and J.R. Petersen
Worshop on Calibration of Space-Borne Magnetometers, Braunschweig, 9 March 1999
submitted to ESA-SP on Calibration of Magnetometers, 2000
Abstract.- The 12 coefficiens of a vector magnetometer: 3 offsets, 3 scale factors, 3 non-orthogonal angles and 3 euler angles are determined by only exposing the instrument in the Earth's magnetic field.
Initially, a least-squares linearized scalar calibration is performed to estimated the first 9 parameters of the magnetometer in its intrisic reference frame. This is done by relating the square root of the square sum of its components to the output of an absolute scalar proton magnetometer.
Finally, the last 3 parameters are obtained by comparing the output to a reference vector magnetometer. In this case, a star camera which is attached together with the instrument in a common optical bench is used to provide its attitude. Therefore, the relation between the magnetic and the absolute systems is established.
This inexpensive procedure has shown up to be very reliable, robust and allows to obtain the solution without the possibility of local minima in the estimator. It may be used for the pre-flight as well as for the in-flight calibrations, in which the accumulated delays of the instrumentation and a suitable IGRF model have to be taken into account.
F. Primdahl, P. Brauer, J.M.G. Merayo, J.R. Petersen and T. Risbo
Worshop on Calibration of Space-Borne Magnetometers, Braunschweig, 9 March 1999
submitted to ESA-SP on Calibration of Magnetometers, 2000
Abstract.-
The magnetic coordinate axes of a calibrated magnetic vector field sensor are related to the instrument
digital output vector U by the calibration matrix C, and the offset vector O. This
magnetic coordinate system must then in addition be related to (at least) two externally accessible
optical or geometrical axes in order to be able to determine the precise orientation of the magnetic
coordinate axes in an external reference system.
Two methods for determining a reference axis in the sensor magnetic coordinates are discussed:
1) Using a tri-axial coil facility to measure the sensor orientation in two different positions,
rotated about a fixed optical (or mechanical) reference axis. 2) In the steady Earth's field the magnetometer
sensor is rotated about a fixed mechanical axis into a number of (at least 3) positions.
T. Risbo, P. Brauer, J.M.G. Merayo O.V. Nielsen, J.R. Petersen, F. Primdahl and N. Olsen
Worshop on Calibration of Space-Borne Magnetometers, Braunschweig, 9 March 1999
submitted to ESA-SP on Calibration of Magnetometers, 2000
Abstract.-
Harmonic analysis of the response of magnetometers placed on a spinning space probe in an approximately
constant magnetic field vector has proven successful in detecting non-linear response in vector
magnetometers. The thin shell method is a generalization of this procedure into a full 4p-steradian analysis.
A test coil system produces a sequence of magnetic field vectors with uniform field-strength |B| and
with directions evenly distributed on the unit sphere. Spherical harmonic analysis of the magnetometer
output yields a spectral representation of the response of one sensor axis at the given field strength |B|.
The spherical harmonic model (SHM) contains total information on the sensor axis: the low degree terms
carry information on the basic calibration constants and direction of the sensor axis in the coil system,
the higher degree terms contain information about the deviations from a linear sensor response.
The covariant formulation of the relations between the quantities in SHM is of fundamental importance
and allows a coordinate-system-independent definition and a separation of the ideal sensor axis response
and the deviatoric response. The model is general and will allow description of any non-linearities provided
that that the truncation level of the SHM is adjusted to accommodate the angular roughness of the response
function.
N. Olsen, T. Risbo, P. Brauer, J.M.G. Merayo and F. Primdahl
Worshop on Calibration of Space-Borne Magnetometers, Braunschweig, 9 March 1999
submitted to ESA-SP on Calibration of Magnetometers, 2000
Abstract.- Several methods have been developed
for the in-flight calibration of the Ørsted vector magnetometer. They are based on one of the following
principles:
a) estimating of the magnetometer constants by means of a scalar calibration (comparison of the vector
magnetometer (CSC) with the scalar Overhauser magnetometer (OVH);
b) estimating of the magnetometer constants (and of the Euler angles between the CSC and the star imager)
together with a main field model by means of a vector calibration.
We will present the different methods, and discuss the calibration philosophy adopted for the Ørsted mission.
Worshop on Calibration of Space-Borne Magnetometers, Braunschweig, 9 March 1999
submitted to ESA-SP on Calibration of Magnetometers, 2000
Abstract.- A new method is described which allows
direct deter-mination of the relative attitude between a star tracker system and a vector magnetometer,
without employing secondary references such as mirror cubes and theodo-lites. The strategy used is to
orient the optical bench, housing the two kinds of sensors, in many different di-rections, and take
readings with both systems at each position. The Advanced Stellar Compass (ASC) derives its attitude
by comparing snapshots of real sky star con-stellations with the HIPPARCOS star catalogue. Si-multaneously,
the Fluxgate Magnetometer (FGM) measures the strength and direction of the ambient mag-netic field in the
reference frame of its sensor. Since the geomagnetic field is neither predictable nor constant a nearby
stationary magnetometer is needed to determine the three elements of the field and their temporal
variations. After transforming the ASC readings from the celestial system into the local Earth-fixed
frame which is also used for the representation of the ambient magnetic field, the relative attitude
between ASC and FGM can be computed by ordinary inversion techniques. When carefully adjusting the free
parameters in the equations, the three Euler angles describing the transformation between the two systems
on the optical bench can be obtained to an accuracy of the order of 10 arcsec.
Sensors and Actuators, A Physical, 82, 161-166, 2000
Abstract.-
The current-output fluxgate may be tuned by using a serial capacitor. Such tuning increases the sensor sensitivity
in the situation when the pick-up coil has a low number of turns. We achieved a signal / feedthrough ratio
improvement by a factor of 5. The measured parameters fit the simplified theoretical model within 20% deviation.
.
Abstract.- The calibration parameters of a vector
magnetometer are estimated only by the use of a scalar reference magnetometer. The method presented in this
paper differs from those previously reported in its linearized parametrization. This allows the determination
of three offsets or signals in the absence of a magnetic field, three scale factors for normalization of the axes
and three non-orthogonality angles which build up an orthogonal system intrinsically in the sensor.
The advantage of this method compared with others lies in its linear least squares estimator, which finds
independently and uniquely the parameters for a given data set. Therefore, a magnetometer may be
characterized inexpensively in the Earth's magnetic-field environment. This procedure has been used successfully
in the pre-flight calibration of the state-of-the-art magnetometers on board the magnetic mapping
satellites Ørsted, Astrid-2, CHAMP and SAC-C. By using this method, full-Earth-field-range magnetometers (±65536.0nT)
can be characterized down to an absolute precision of 0.5 nT, non-orthogonality of only 2 arcsec,
and a resolution of 0.2 nT.
J.M.G. Merayo, P.S. Jørgensen, T. Risbo, P. Brauer, F. Primdahl and J.
Cain
Eos Trans. AGU, 80(17), Fall Meet. Suppl., F892, 1999
Abstract.-The Swedish micro-satellite Astrid-2 was
successfully launched into a near polar orbit last December 98. Despite the fact
that its primary mission was the research of Auroral phenomena, the magnetic
instrumentation has been designed to accomplish high resolution vector
field magnetic measurements and therefore the mapping of the Earth's
magnetic field may be possible.
The spinning of the spacecraft about a certain axis makes the
stabilisation in space possible. This fact and the well distributed data
over the globe makes the magnetic data well suited for the estimation of
the magnetic field model at the spacecraft altitude (circa 1000km).
Several methods for field modelling are presented in this paper with the
assumption that the direction of the spin axis is nearly constant. In
any case the orientation of the magnetometer is to be determined
simultaneously with the instrument calibration and main field model
coefficients. Hence, apart from the scientific use of the magnetic data,
the attitude recovery of the spacecraft may be estimated with more
precision.
E.B. Pedersen, F. Primdahl, J.R. Petersen, J.M.G. Merayo, P. Brauer and O.V. Nielsen
Meas. Sci. Technol., 10, 124-129, 1999
Abstract.-The design and performance of the Astrid-2
magnetometer are described. The magnetometer uses mathematical routines implemented by software for
commercially available digital dignal processors to determine the magnetic field from the fluxgate
sensor. The sensor is from the latest generation of amorphous metal sensors developed by the Department of
Automation at the Technical University of Denmark.
M.A. Danielides, A. Ranta, N. Ivchenco, G. Marklund, D. Poetrowski, K.A. Lynch,
R.B. Torbert and F. Primdahl
Proceedings of the XXXIII annual conference of the
Finnish Physical Society, March 4-6. 1999. Turku, Finland
Abstract.-The Auroral Turbulance II sounding rocket
was launched on February 11, 1997 into moderately active nightside aurora from the Poker Flat Research
Range, Alaska, US.
The experiment consisted of three independent, completely instrumented payloads launched by a single
vehicle. The aim of the experiment was to study the fine structure of an active auroral arc. Ground
based observations were done by a chain all-sky cameras, a photometer and a magnetometer at Poker Flat.
The satellite coverage was obtained by POLAR UV imager and GOES 8 and 9 magnetometer.
The three point measurement allows the distinction of spatial and temporal variations. The first results
of the magnetic, [1,2], electric and particle data analysis are compared with optical observations [2]
of auroral structures.
J.M.G. Merayo, P. Brauer, T, Risbo, E.B. Pedersen,
J.R. Petersen and F. Primdahl
Astrid-2 Satellite Project, Report, 1998
Abstract.-
The Swedish micro-satellite Astrid-2 contains a tri-axial fluxgate magnetometer with the sensor co-located with a Technical University of
Denmark (DTU) star camera for absolute attitude, and extended about 0.9 m on a hinged boom. The magnetometer is part of the RIT EMMA
electric and magnetic fields experiment built as a collaboration between the DTU, Department of Automation and the Department of Plasma
Physics, The Alfvenlaboratory, Royal Institute of Technology (RIT), Stockholm. The final magnetic calibration of the Astrid-2 satellite was
done at the Lovoe Magnetic Observatory under the Geological Survey of Sweden near Stockholm on the night of May 15.-16., 1997. The
magnetic calibration and the intercalibration between the star camera and the magnetic sensor was performed by measuring the Earth's magnetic
field and simultaneously observing the star sky with the camera. The rotation matrix between the magnetometer orthogonalized axes and the
star camera optical axes was determined from the observed stellar coordinates related to the Earth magnetic field from the Magnetic
Observatory. The magnetic calibration of the magnetometer integrated into the flight configured satellite was done in the (almost) constant
Earth's magnetic field of about 50,000 nT by the 'Scalar Calibration Method' developed at the DTU. The satellite was positioned in 60 different
attitudes relative to the Earth's magnetic field and magnetometer readings were recorded for about one minute in each position. Averages of the
magnetometer readings in each position were related to the field magnitudes from the Observatory, and a least squares fit for the 9
magnetometer calibration parameters was performed (3 offsets, 3 scale values and 3 inter-axes angles). After corrections for the magnetometer
digital-to-analogue converters' nonlinearities, and after fitting for a magnetic field gradient at the outdoors measurement site, the linear
mathematical model of the magnetometer showed 1.26 nT rms deviation between the magnetic field measured by the observatory and the field
calculated from the 60 magnetometer reading averages using the best fit calibration parameters. Owing to time shortage, we did not evaluate the
temperature coefficients of the flight sensor calibration parameters. However, this was done for an identical flight spare magnetometer sensor at
the magnetic coil facility belonging to the Technical University of Braunschweig over a temperature range from -47°C to +18°C. The scale
values had temperature coefficients of about 11 ppm/K, axes 1 and 2 showed less than 1 nT in total offset change over the full temperature
range, whereas axis 3 had 0.47 nT/K offset change. The angles between the axes changed less than 0.48 arc sec/K. The flight sensor will be
assumed to have similar temperature behaviour, and the coefficients will be fitted for during the planned in-orbit magnetic calibrations.
Abstract.- A survey of existing instrumentation and
developments is presented emphasizing instrumentation for in-flight calibration
of vector magnetometers on magnetic mapping missions. Proton free or forced
precession magnetometers are at the focus as calibration references, because the
proton gyromagnetic ratio is a basic atomic constant for the SI units of magnetic
and electric current. The classical proton free precession, the Overhauser forced
oscillation and a new field cycling Overhauser are presented. Alkali metal vapor
magnetometers, although not absolute in the same sense as the classical proton
magnetometer, offer stability and resolution well suited for the calibration purposes.
Recent developments are discussed. The metastable Helium magnetometer also offers
quasi-absolute scalar measurements, and the use of semiconductor tuned lasers
replacing an RF-excited Helium lamp holds great promise for improved accuracy and
reduced power consumption.
J. Jørgensen, J.M.G. Merayo, M. Blanke, N. Olsen, T. Risbo
and F. Primdahl
NASA/CP-1998-206900 Proccedings of the Tether Technology
Interchange Meeting, (NASA Marshall Space Flight Center, September 9-10, 1997),
239-252, 1998
Abstract.- We propose a mission of two identical tethered
micro satellites for simultaneous two-point mapping of the Earth's magnetic field and
of the first space derivative of the field. Launch in the 2002 time frame into a low
altitude near polar and approximately circular orbit, slowly drifting in local time.
An approximately 3 month's lifetime in the tether mode (similar to METS), continuing
as free flyer(s) for up to 2 years depending on the actual solar cycle atmospheric swelling.
The Danish Ørsted Earth's field mapping mission brought Denmark into the international
front in space magnetometry. A Danish Magnetic Gradient Mission will bring Denmark far
ahead, and will consolidate our position in space magnetometry and advanced space technology.
The proposed 600-400 km altitude polar orbit and the 20 km gravity gradient extended
tether are optimized for simultaneous observations of the field signatures and for the
separation of the field sources below the orbit, from sources traversed by the satellites
and from sources above the orbit.
P. Brauer, J.M.G. Merayo, O.V. Nielsen, F. Primdahl
and J.R. Petersen
Sensors and Actuators, A Physical, A 59, 70-74, 1997
Abstract.- A model of a fluxgate magnetometer
based on the field interactions in the fluxgate core has been derived. The
non-linearity of the ringcore sensors due to large uncompensated fields
transverse to the measuring axis are calculated and compared with measurements.
Measurements of the non-linearity are made with a spectrum analyzer measuring the
higher harmonics of an applied sinusoidal field. For a sensor with a permalloy ringcore
of 1 in. in diameter the deviation from linearity is measured to about
15 nTp-p in the Earth's field and the
measurements are shown to fit well the calculations. Further, the measurements and the
calculations are also compared with a calibration model of the fluxgate sensor
onboard the 'MAGSAT' satellite. The later has a deviation from linearity of about
50 nTp-p but shows basically the same form of
non-linearity as the measurements.
O.V. Nielsen, P. Brauer, F. Primdahl, T. Risbo,
J.L. Jørgensen, C. Boe, M. Deyerler and S. Bauereisen
Sensors and Actuators, A Physical, A 59, 168-176, 1997
Abstract.- The construction of triaxial
fluxgate sensor with very high axis stability and low temperature coefficiens is
described. The axis orthogonalities change less than 2.1 s of arc in the whole
testing temperature range +20 to -10 °C. The temperature coefficiens for
the sensitivities of the three axes are 6.7, 10.1 and 13.3 ppm
K-1, respectively. This high stability is
achieved by using a newly developed ceramic, C-SiC, as the supporting
construction material.
T. Sabaka, J.A. Conrad, J.M.G. Merayo and R.A. Langel
GSFC-NASA contract NAS5-31760, 1997
Abstract.-The primary mission of the Defense Meteorological Satellite
Program 12 and 13 satellites is to support Air Force meteorological interests. However, each
satellite does carry a body-mounted fluxgate magnetometer. It is of interest then to assess
the utility of such measurements in geomagnetic main-field studies. To this end, data were
provided by Dr. F. Rich of the Air Force Geophysical Lab (AFGL) to the magnetics group at
the Goddard Space Flight Center, along with pertinent pre-processing software. This software
basically corrects for field discontinuities and known contaminates. The pre-processed data
were calibrated in order to account for first-order electronic and geometric effects in the
fluxgate magnetometer using two methods: The first method, described by Langel et al. (1996)
and denoted Type I, finds the calibration of the fluxgate magnetometer whose scalar measurements
best fit those predicted from a nominal main-field model in the least-squares sense. The second
method, described by Estes (1983) and denoted Type II, finds the calibration of the fluxgate
magnetometer and main-field combination whose predicted scalar and vector measurements best
fit the actual measurements in the least-squares sense. These calibration parameters included
implied electronic gains and biases, projection angles onto non-orthogonal basis vectors, and
rotations between spacecraft-body and magnetometer coordinate systems. The main-field was
parameterized as a degree and order 10 internal spherical harmonic expansion. The data were
analyzed in groups of quiet days over a 14 month period for DMSP-12 and an overlapping 7
month period for DMSP-13. The resulting magnetometer calibration coefficients show very
poor constancy over the time envelope of the groups for both methods. Furthermore, there
is at best mediocre correlation between the calibration coefficients from the two methods.
Analysis of the derived Type II main-field models shows first-order agreement with the
propagated IGRF95 model. However, major discrepancies do exist between models for a given
satellite through time as well as between cotemporaneous models from both satellites.
These, as well as other indicators, strongly suggest data contamination or inadequate
modeling. For instance, hard and soft (induced) spacecraft magnetic sources may possess
a spatio-temporal variability that has neither been removed via the data preprocessing
nor can be described by the present model parameterization. Also, position and attitude
errors are no doubt mapping into the data space in complex ways that are difficult to
isolate. Therefore, the usefulness of the DMSP data for main-field studies is questionable
at this time. However, if contaminating fields can be minimized by such things as a boom-mounted
magnetometer, and if major position and attitude errors can be rectified, then the DMSP has the
potential of becoming a major provider for the geomagnetics community by virtue of its excellent
long-term spatial and temporal coverage.
H. Thiemann, G. Mayer, A. Piel, C. Steigies, N. Olsen,
F. Primdahl, R. Sridharan, S.P. Gupta, G.K. Rangarajan, D.R.K. Rao and P.B. Rao
Proceedings 13th ESA Symposium on European Rocket and Ballon
Programmes and Related Research, Öland, Sweden, 26-29 May 1997,
ESA SP-397, 349-354, September 1997
Abstract.-The DEOS-project envisages dynamic investigations
of the dynamics of the low-latitude ionosphere over Sriharikota Range SHAR
(6° dip latitude) with three rockets to be launched in September/October 1997
for the determination of characteristic diurnal conditions and spatial variations.
In-situ measurements of relevant low-latitude plasma parameters are coordinated
with TEC-measurements, where onboard transmitter signals are received at different
ground stations, located within and outside the equatorial electrojet region.
These rocket related data are supported by various ground-based measurements at
different locations in the vicinity of the electrojet, providing information about
magnetic disturbances and ionospheric anomalies.
The primary scientific goals of the DEOS-campaign refer to the prereversal current
enhancement during evening hours, equatorial Spread-F (ESF) conditions during night
hours and the equatorial ionization anomaly (EIA) and Sporadic-E effects during
noon time. These phenomena, identified by ionograms and magnetograms, define the
launch criteria. With the limited prediction possibility of Spread-F conditions only
during late noon/early evening hours the campaign will begin with the evening launch.
Sq- and electrojet current features will be investigated for noontime conditions as
well as for the sunset terminator. The planned launches of the DEOS-rockets allow
to study the F-region dynamo mechanisms at evening and night and the E-region dynamo
as dominant mechanism during daytime.
Proceedings 13th ESA Symposium on European Rocket and Ballon
Programmes and Related Research, Öland, Sweden, 26-29 May 1997,
ESA SP-397, 355-360, September 1997
Abstract.-The dynamic investigation of the low-latitude
ionosphere over SHAR - subject of the DEOS-project - requires the determination of
basic plasma parameters on spatial scales ranging from 1m to several km.
Absolute electron density and electron temperature, obtained from the characteristic
angular radiation patterns, are the primary parameters of the resonance cone
experiment.
Plasma drifts and non-Maxwellian plasma features can be derived from radiation
asymmetries within certain limits.
Magnetic field measurements of the vector magnetometer will provide information
about ionospheric currents and associated magnetic disturbances, This instrument
will further support the resonance cone experiment to identify the magnetic field
conditions and the orientation of characteristic structures of the relevant
radiation pattern.
J. Piil-Henriksen, J.M.G. Merayo, O.V. Nielsen, H. Petersen,
J.Raagaard Petersen and F. Primdahl
Meas. Sci. Technol., 7, 897-903, 1996
Abstract.- A new full Earth's field dynamic feedback
fluxgate magnetometer is described. It is based entirely on digital signal
processing and digital feedback control, thereby replacing the classical second
harmonic tuned analogue electronics by processor algorithms. Discrete mathematical
cross-correlation routines and substantial oversampling reduce the noise to
71 pT root-mean-square in a 0.25-10 Hz bandwidth for a full Earth's field range
instrument.
Abstract.- This report shows the results of the calibration
of the flight and flight spare CSC magnetometers for the Ørsted satellite.
The instrument shows an outstanding behavior as regards of both constant temperature
and temperature dependance.
Neither transverse effects nor non-linear terms have been found. The non-linearity is
less than 4.0 ppm, taking into account that the coil facility where the magnetometer
has been scanned has a very low noise (0.5 nT p-p)
but not sufficient low to precise much more in this parameter. There is no non-linearity
over the entire temperature range.
The sensor offsets are less than 2.0 nT and 10.0 nT for the flight and flight spare
electronics, respectively. They have been determinated by two procedures which give
consistant results. Firstly, as output in the spherical harmonic method, and secondly, by
rotation of the CSC sensor by 180° in two axes. It does not change with temperature.
The sensitivities are different in each axis, due to the fact that any ADC has its own
voltage reference and there is a slight difference between them. They change linearly with
temperature. The temperature coefficient for each sensor is different since the radius of
the feedback coil is bigger for sensor 1 than sensor 2, and bigger for sensor 2 than
sensor 3. They are 13.3 ppm/°C, 10.2 ppm/°C and 6.8 ppm/°C for sensor 1, 2
and 3, respectively in the flight unit. And 36.8 ppm/°C, 34.2 ppm/°C and
31.7 ppm/°C for sensor 1, 2 and 3, respectively in the flight spare unit.
The difference between flight and flight spare units reflects the different constructions.
The sensor offset and sensitivity change, depending on which of the ADC's or which
electronic box is used with the sensor.
The non-orthogonal angles are defined as the error respect to 90°, so
angle1=-90°, angle2=90°- and angle3=90°-. In the flight
unit they are 317.3, 113.3 and -34.3 arc sec, respectively. And 65.8, -195.8 and 406.3
arc sec, respectively, for the flight spare unit. They do not change with temperature
within 0.5 arc sec. of r.m.s. deviation.
The SIM turned out to change the sensitivities of the CSC sensor giving an error
corresponding to 5 to 10 nT in full scale field. It rotates as well the CSC sensor axes. In
principle this is due to the presence of soft magnetic material. No remanent magnetization
has been observed. Hermann Luehr has ratified this fact and recommended a final calibration
in Magnetsrode with the final assembled flight gondola. This could not be done at IABG,
whose absolute accurately is not sufficient for this final test. Final calibration
parameters will be obtained in test with SIM mounted just before delivering for
integration in the satellite.
Among the runs we made, only one has been chosen for each combination. The residual graphs
show typical outputs, any others look in the same way. Nevertheless, anyone is
welcome to ask for any other output or graph. The graphs shown are: sensitivity change of
the three axes, non-orthogonal angles and residuals vs. temperature in 2nd thermal run
of the flight unit. The residuals show some outliers due to the mechanical perturbation
while removing the dry ice. Due to the speed of temperature change the thermodynamic
equilibrium is not well achieved in the cooling phase, and that gives a little bit
dispersion of the parameters.
O.V. Nielsen, J.R. Petersen, F. Primdahl, P. Brauer,
B. Hernando, A. Fernández, J.M.G. Merayo and P. Ripka
Meas. Sci. Technol., 6, 1099-1115, 1995
Abstract.- The experiments and theoretical considerations
leading to the construction of a high-performance three-axis fluxgate magnetometer
are described. The magnetometer will be used (1996) in the Earth's field mapping
satellite named "Ørsted". The fluxgate sensors are based on stress-annealed
metallic glass ribbons as core materials. It is shown that very simple physical models
can be used to explain the fluxgate mode of operation, thereby making it easy to
calculate the overall sensor performance from first principles. Special attention
is drawn to the core excitation current which is analysed on the basis of nonlinear
electrical circuitry. It is furthermore shown that the ring-core demagnetizing field
obeys a simple cosine law which permits the calculation of the sensor sensitivity with
high accuracy. The sensitivity, that is the signal-to-noise ratio, is ultimately
determined by the sensor noise which is about 15 pT RMS (0.06-10 Hz), corresponding
to a noise power density (1/f noise) of 6.2 pT Hz-1/2
at 1 Hz. The actual magnetometer operating range and sensitivity is determined by the
1 bit resolution of the Earth's field represented by the output from the 18 bit
AD converter used in the instrument (±65 536 nT with 0.5 nT resolution). The
maximum attainable bandwidth is half the sensor excitation frequency (½ x 15 kHz)
but the Ørsted magnetometer bandwidth is limited to 250 Hz. The thermal stability
of the sensor has been measured to be better than 1 nT in the temperature range -20 to
+60°C.
P. Ripka, F. Primdahl, O.V. Nielsen, J.R. Petersen and
A. Ranta
Sensors and Actuators, A Physical, A 46-47, 307-311, 1995
Abstract.- Fluxgate sensors are mostly used in closed-loop
d.c. magnetometer systems; they can also measure alternating fields up to several
kilohertz, either in open-loop mode or from an error signal in the slow-feedback
loop as in the Thunderstorm rocket magnetometer, which has 0.1 nT resolution up to
3 kHz. The alternative is to use the direct induction effect in the pick-up or feedback
coil. While the low L/R constant of the pick-up coil causes a high -3 dB
frequency corner, the spherical feedback coil has a narrow frequency characteristics
and low noise up to 10 kHz when used as a search coil. The noise level achieved
is 56 pT r.m.s. from 123 Hz to 10 kHz.
F. Primdahl, B. Hernando, J.R. Petersen and
O.V. Nielsen
Meas. Sci. Technol., 5, 359-362, 1994
Abstract.- This paper describes an experiment where the
flux-gate sensor broad band output signal is digitized with 8-bit resolution at a high
rate, and subsequently numerically analysed in order to extract information on the
external magnetic field. The results show that it is feasible to obtain a noise level
of 1 nT for a data output rate of 100 Hz. The method will allow construction of a
magnetometer based entirely on digital techniques, with only a minimum of analogue
circuits.
Abstract.- AC magnetic fields up to 3 kHz can be
measured directly using a modified high frequency fluxgate magnetometer. The amorphous
metal ring core sensor was excited at 15 kHz and in short circuited output mode has
a -3 dB bandwidth of at least 2 kHz. The total RMS noise level is 93 pT
(64 mHz-3 kHz), and the noise power density as 1/f over the entire frequency band.
P. Ripka, J.R. Petersen, H. Petersen, O.V. Nielsen and
F. Primdahl
Elektrotechn. Cas., 45, no. 8/s, 107-109, 1994
Abstract.- Øersted is a Danish satellite for mapping
the Earth's magnetic field. The main instrument on-board is a three-axial
magnetometer based on amorphous metal fluxgate sensors and spherical compensation coils.
The scalar Overhauser magnetometer with a 0.25 nT resolution will serve for calibration
purposes.
O.V. Nielsen, T. Johansson, J.M. Knudsen and
F. Primdahl
J. Geophys. Res. 97, no. E1, 1037-1044, 1992
Abstract.- It is proposed that important magnetic
properties of the surface materials on Mars be measured by a simple instrument
comprising a flux gate magnetometer and a magnetizing coil. The paper describes
the basic construction principles and demostrates the instrument's usability on
a few materials that are expected to be similar to those on Mars. Very small traces
of ferromagnetic materials can be detected by the measuring technique. In addition
to material studies, the instrument is suitable for magnetospheric, paleomagnetic,
and magnetotelluric sounding measurements.
J.R. Petersen, F. Primdahl, B. Hernando, A Fernández
and O.V. Nielsen
Meas. Sci. Technol., 3, 1149-1154, 1992
Abstract.- The fluxgate null feed-through signal is
discussed theoretically giving an expression for the excitation current and for
the core flux coupling coefficiens to the secondary coil. Experimental results
show that the two signals cannot be nulled simultaneously by rotating the core
inside the secondary coil, and give the two coupling coefficiens. The excitation
current coupling depends critically on the symmetry of the winding, and it is
shown that the two signals are associated with excess noise adding to the basic
core noise.
Abstract.- The effect of uncompensated transverse fields
on the output of several types of fluxgate sensors has been investigated taking
as a starting point the full 4p steradian
calibration model of the MAGSAT magnetometer, and proceeding with data from
magnetometers in the laboratory and flown on board sounding rockets. Tubular ferrite
core sensors are very sensitive to tranverse fields. Ring-cores are sensitive to
transverse fields but only in the plane of the ring. Linear double-rod sensors
are not affected by tranverse fields. Maintaining the sensor in a homogeneous
null field in all three directions eliminates the tranverse field effect.
F. Primdahl, P. Ripka, J.R. Petersen
and O.V. Nielsen
Meas. Sci. Technol., 2, 1039-1045, 1991
Abstract.- The output current impulses of the
short-circuited fluxgate depend on the input magnetic field, and on a number of core
parameters such as the demagnetization factor, the cross sectional area, the time
spent in saturation, etc. Following a theoretical treatment, experimental test of
the parameter dependences are reported, and additional experimental evidence of
the noise performance is presented.
O.V. Nielsen, J.R. Petersen, A Fernández, B. Hernando,
P. Spisak, F. Primdahl and N. Moser
Meas. Sci. Technol., 2, 435-440, 1991
Abstract.- The sensor offset as a function of the ring
rotation angle with respect to the detector coil has been studied for three
different fluxgate sensors of the ringcore type. The core material consists of
metallic glass ribbons which have been heat treated in different ways so that
different magnetization curves and noise levels are obtained. For the best core
the offsets are rather low (a few nT) and only slightly dependent on the
rotation angle.
O.V. Nielsen, B. Hernando, J.R. Petersen and
F. Primdahl
J. Magn. Mat. 83, 405-406, 1990
Abstract.- Making use of the inverse Wiedemann effect
in a non-magnetostrictive hairpin shaped metallic glass ribbons we have made
flux-gate sensors with small dimensions
(dxl=3 mm2) and RMS noise levels of 2 nT
(0.02-1 Hz). The inverse Wiedemann effect arises from helical anisotropy induced
by torsional stress annealing.
O.V. Nielsen, J. Gutierrez, B. Hernando and
H.T. Savage
IEEE Trans. Magn., vol.26, no.1, 276-280, 1990
Abstract.- The fluxgate sensor presented here consists
of a hairpin shaped nonmagnetostrictive amorphous metal ribbon, which carries the
excitation ac current. Even harmonics are induced in a surrounding coil which may be used
both as a pick-up coil and as a field compensation coil. The principle of operation is
similar to that of a traditional ring-core system except that the core itself carries
the excitation current. The present sensor competes favorably with traditional
fluxgate sensors with respect to simplicity, miniaturization, signal level, and
noise level.
F. Primdahl, B. Hernando, O.V. Nielsen,
and J.R. Petersen
J. Phys. E: Sci. Instrum. 22, 1004-1008, 1989
Abstract.- A method for measuring the demagnetisation
of fluxgate sensors is introduced and used to evaluate the demagnetising factors for
four ring-core sensors having 5, 10, 15 an 20 wraps of magnetic core ribbon.
The demagnetising factor is proportional to the number of wraps, and the noise is
also proportional to the number of wraps except for the more noisy 5-wrap core.
The estimated internal core noise was 1pTRMS taking
into consideration the demagnetising factor and the relative permeability of the
core material.
F. Primdahl, J.R. Petersen, C. Olin and
K.H. Andersen
J. Phys. E: Sci. Instrum. 22, 349-354, 1989
Abstract.- The alternative coupling of short-circuiting
the secondary coil of a fluxgate sensor by connecting it to the inverting input
of a current amplifier is shown to have significant advantages over the more
conventional unloaded or second-harmonic tuned voltage coupling. The short-circuit
output current impulses are independent of changes in the excitation level, and sensor
performance is not degraded by cable and coil stray capacitances or by resistive
loads from the feedback circuit. The current impulse amplitude is proportional
to the average-to-minimum secondary self-induction ratio minus 1, and a virtual
current in the secondary coil corresponding to the external magnetic field.