Low magnetic field technology for space exploration

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Low magnetic field technology for space exploration

E.J. Iufer

To cite this version:

E.J. Iufer. Low magnetic field technology for space exploration. Revue de Physique Appliquée, Société française de physique / EDP, 1970, 5 (1), pp.169-174. �10.1051/rphysap:0197000501016900�.

�jpa-00243354�

LOW MAGNETIC FIELD TECHNOLOGY FOR SPACE EXPLORATION By ^E. J. ^IUFER (1),

National Aeronautics and Space Administration (Nasa), ^Ames Research Center, ^Moffet Field, California (U.S.A.).

Résumé. 2014 Une observation définitive de la morphologie ^du champ magnétique ^inter- planétaire ^ne dépend pas seulement de la reproductibilité ^{et de} l’intégrité spectrale ^des magnéto-

mètres envoyés ^dans l’espace, ^{mais aussi} de l’existence de véhicules ayant ^des champs perturbateurs négligeables.

Un étalonnage convenable des magnétomètres ^{et la} possibilité ^{de faire des mesures} magné- tiques ^dans l’espace ^sont essentiels pour obtenir ^un résultat. Étant ^{donné le} petit ^{nombre de} publications portant directement sur la technique ^de réalisation, bien des recherches concernant la mise au point ^de matériel pour des observations magnétiques ^{au cours de missions} spatiales

n’utilisent qu’une petite partie des connaissances disponibles. ^En conséquence, ^{il a fallu un} temps inutilement long pour faire accepter ^de nouvelles réalisations dans ce domaine ; ^{elles ont}

en outre, en ce qui ^{concerne les} caractéristiques magnétiques ^{et les essais, été} considérées avec

réticence, ^sans nécessité par certains.

Le but de cet article est de présenter ^{une revue} critique ^de l’état actuel des connaissances dans les domaines des mesures du vecteur champ magnétique ^{à basse} fréquence, ^{de la standar-}

disation des magnétomètres ^{et de la} réalisation de véhicules spatiaux ^{à faible} champ magnétique.

On décrit les caractéristiques ^des performances ^de l’appareillage ^{et les} techniques ^instru-

mentales utilisées pour la ^mesure du vecteur champ magnétique standardisé dans le domaine de 0,05 ^{à 60 000 nT} (1 ^nT

¹ gamma). ^{On établit une} comparaison ^{entre des modèles} pouvant

estimer les propriétés magnétiques des matériaux et montages pour véhicules spatiaux ^{à l’aide}

de nombreux résultats de laboratoire. On discute de façon détaillée les critères généraux ^{et les}

considérations permettant ^de comparer les techniques ^de blindage magnétique, ^de compensation

active du champ, ^et d’emploi de constituants non magnétiques. ^{Une revue des} principaux points

de l’étude qui ^{aboutit à la mesure du} champ magnétique ^de 0,25 ^{nT à} partir des véhicules du type Pioneer VI est indiquée ; ^on présente ^les caractéristiques ^{des véhicules et de} l’appareil- lage pour les missions interplanétaires ^futures.

Abstract.

Definitive observation of interplanetary magnetic ^fields depends ^not only

upon the development ^of high sensitivity spaceflight magnetometers ^{but also on the} availability

of spacecraft having negligible disturbance fields. Test results obtained in the Pioneer VI-IX program have demonstrated that through ^careful design, spacecraft magnetic ^{fields can be}

reduced by ^a factor of 25 so that spacecraft field levels of 10-6 gauss ^or less can be realized.

Since the literature contains little information on the magnetic properties ^of spacecraft parts

and materials, low-magnetism design principles and low-field magnetic testing techniques,

those recently concerned with low-magnetism design ^{may be} working ^with only ^a small fraction of the information currently ^{known. A critical} review of the current state-of-the-art for low-

magnetism spacecraft design and test is presented.

Introduction.

Analysis of data from magnetome-

ters used in space exploration ^{has changed the concept}

of the Earth’s magnetic field from that approximated by ^a dipole ^{in free} space ^to that of the magnetosphere.

The boundary of the Earth’s magnetic ^{field is} produced by ^{the solar} plasma flow which compresses the Earth’s field ^to about 15 Earth-radii (15 Re) ^{on the} day-time

side and extends it to greater than 80 Re ^on the night-

time side. Immediately beyond ^the magnetosphere,

one encounters the interplanetary medium where the ambient magnetic ^{field has a} quiescent value of about 5 X 10-5 _gauss due predominantly ^to the Sun. At greater ^solar distances, ^as illustrated ⁱⁿ figure 1, ^the interplanetary field becomes weaker and ^at Jupiter

its value is thought ^to be about 5 X 10-6 _gauss. The (1) ^Research Scientist, National Aeronautics and Space Administration, ^Ames Research Center, ^Moffett Field, Califomia 94035, Presented at Measurement of Low

Magnetic ^{Fields of} Spatial ^and Geophysical ^Interest Conference, Paris, France, May 19-25, ^1969.

FiG. 1.

Magnetic Fields in the Solar System.

ambient field _may not decrease significantly ^at greater ranges because of the galactic magnetic ^field.

Observation of the steady-state, temporal ^and spatial

fluctuations ofsuch weak magnetic ^{fields can be} comple- tely ^obscured by ^the spacecraft’s ^own magnetic ^field

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:0197000501016900

170

FLUXGATE MAGNETOMETERS

ELEKTRON 2, ⁴

EXPLORER VI, X, XI, XIV, XV, XVIII, ^XXI XXVI, XXVIII, XXXIII, XXXIV, ^XXXV

OGO 1- T7 OV 2-6 SP UT NIK 3 PIONEER VI-IX

PROTON PRECESSION MAGNETOMETERS VANGUARD I, ^III

ALKALI VAPOR MAGNETOMETERS

EXPLORER X, XVIII, XXI, XX TlIII, ^XXXIV

OGO I h 0 v 1-10

HELIUM MAGNETOMETERS MARINER IV, ^V

SEARCH COIL MAGNETOMETERS EXPLORER VI

OG0 I h

FIG. 2.

Spacecraft Having Magnetic

Field Experiments.

unless strict attention is given ^{to the} magnetic design

of the spacecraft. ^{The design and test} of low-magne-

tism spacecraft ^for magnetic exploration require ^the application ^of principles ^and techniques which, ⁱⁿ

many cases, may be ^{new to} structural and electronic

designers of space flight ^hardware.

As showns in figure 2, ^a large ^{number of scientific} spacecraft have flown magnetometers. ^{Of the} space- craft measurements reported ^{to date, the} spacecraft having the lowest magnetic field is the Pioneer with 2.5 X 10-l _gauss ^at the magnetometer ^{sensor located} 2 meters from the spacecraft ^center.

In the following sections, ^{the nature of} spacecraft fields, ^{their reduction and} measurement will be dis- cussed. Facility requirements ^and high sensitivity

instrument calibration methods will be described.

The Calculation ^of Spacecraft ^Fields.

^The mag- netic field of ^a spacecraft ^{is a} composite ^{of the indi-}

vidual static fields of all ofits parts which contain alloys of iron, ^{nickel or cobalt and the} active fields produced by electrical current loops.

Static fields arise from a heterogenous assembly ^of

small magnetized ^{bodies which can usually be charac-}

terized geometrically ^as ferromagnetic rods of small

length ^to diameter ratio. The high reluctance of space compared ^to that of the ferromagnetic ^{material causes}

the ^net magnetizing force inside the rods ^to be less than the applied ^field. Classically, this field diminution is attributed ^to free poles ^at the ends of the rods. As

a result, ^the hysteresis ^curve for the rod specimens

appears ^to be sheared clockwise ^when compared ^with

the hysteresis loop ^{of closed} rings. ^{The net} field, acting ^to magnetize ^{a rod is} given by :

where H is the net magnetizing ^force acting ^{on thé} rod,

Ho ^{is the} gross external magnetizing force, ^{N is the} demagnetization factor which depends ^{upon rod geo-}

metry, ^and I is the intensity ^of magnetization.

Because the ratio of H0/H ^for spacecraft parts ^is

usually large, ^one finds that spacecraft ^{induction and}

residual induction can be considered linear for external field exposures of 15 gauss ^{or more.} The ability ^of

a magnetized ^{rod to} produce ^{an external} magnetic

field is characterized by ^its magnetic moment, ^M.

For magnetizations along the axis of the rod, ^the magnetic ^{moment is the} product ^{of the} volume, V,

of the rod and its intensity ^of magnetization, ^{I. The} intensity of magnetization is related ^to the induction, B,

the demagnetization factor, N, ^{and the} applied field,

Hn, by :

This expression ^is approximate because both the inten-

sity ^of magnetization ^and permeability ^{are functions}

offlux density which is non-uniform in a rod.

Using ^{the above} expression, ^one may calculate the induced moment of a rod from :

Calculations using ^this approximate ^method provide

results accurate to an order of magnitude ^{or better.}

Although ^several ways ^are known to analyse ^the magnetic fields of bodies in two dimensions, ^{it is} only recently ^{that an} analysis ^{has been} performed ^which

treats magnetic ^{fields in} three dimension and accommo-

dates ^a field-dependent permeability. ^This analysis by ^A. Halacsy subdivides a magnetic body ^{into a}

finite number of elemental boxes, ^each having ^its magnetic ^moment concentrated at its center. Partial differential equations ^are written for the scalar _magne- tic potential ^{and the} permeability ^{of each box. The} partial differential equations ^are linearized and solved

by matrix inversion. This analysis ^{does not} require boundary conditions because by avoiding ^{the use of}

vector potential, ^no integration ^is necessary. The accuracy of this method increases ^as the number of elemental boxes increase. With this method, ^one requires only ^the hysteresis ^{curve and} geometrical shape

of a body ^to calculate its magnetic ^field.

A FORTRAN computer program which solves these equations ^has been written and successfully

run on an IBM 360/50 computer. Computed ^fields

for rods, ^{cubes and} spheres have been checked against laboratory measurement by the author and found to

have greater accuracy than fields calculated by pre- vious methods, figure ^3.

The calculation of total spacecraft ^{fields can be} accomplished by ^linear superposition ^{of the vector}

fields of subassemblies. Before these analytical ^tools

were available, ^the design oflow-magnetism spacecraft

was based on frequent and detailed measurements of

spacecraft ^{parts in a low field test} facility ^and by ^a general policy ^of avoiding ^all magnetic ^material

whenever possible. If sample parts ^are available, ^this policy is still considered the best choice. Analytical

methods should be used in predicting the fields of parts which have not been built or which are otherwise

not available for test.

FIG. 3.

Laboratory Measurements of Field of Cube Kovar Sample Compared ^to Calculated Values.

Magnetic Cleanliness.

For interplanetary ^mis- sions, the elimination of even milligrams ^of magnetic

material in a part is valuable if the number of times this part is used is large. ^The magnetic field of a transistor in a TO-5 nickel case with 1.4 mm Kovar leads can

be reduced from 60 X 10-5 _gauss to 3 X 10-5 _gauss

at 7.5 cm by placing ^{it in a} smaller TO-46 case made of nickel-silver. By strict avoidance of unnecessary

magnetic ^{material, the} properly designed low-magne-

tism spacecraft ^{can have} 1/25 ^{or less of the} magnetic

field produced by ^a conventionally designed ^counter-

part. This leads to the first principle ^of design-avoid magnetic materials where possible.

The second principle is that the acceptable magnetic

moment of a body ^{increases as a cube} of distance. For

example, ^a material used to encapsulate ^a magneto-

meter sensor can be unsuitable due to suspended par- ticles too small to be seen with the unaided _eye. If these particles produce ^{a field of} say 1 ^X 10-6 _gauss

at 10 _mm, then to obtain the same level of interference

at a distance of two meters (according ^to the inverse cube law) the effective moment of this material would have to be larger by ^a factor of 8 X 106. Rapid ^field decay ^with increasing ^{distance must} be considered in

dealing ^with spacecraft fields allowances because it is

possible ^to safely ^{relax the} magnetic ^moment specifi-

cation of parts and materials as magnetometer ^boom

lengths ^increase.

The Nature of Spacecraft ^Fields.

^{It has become}

convenient ^to arbitrarily subdivide the sources of space-

craft fields into four categories : ^soft remanence, hard remanence, current-loop and induction. Soft rema- nence refers ^to that portion ^of total residual induction which has sufficiently ^low coercive force as to be altered

by ^ambient momentary magnetic field exposures of 25 _gauss or less. Hard remanence refers to the balance of total residual induction which cannot, ^{or in} the case

of certain _necessary magnets, ^{must not} be removed by demagnetization treatment. Current-loop ^{stray fields}

are defined as those due to uncompensated leakage

fields resulting ^from electrical current flowing ^{in ca-} bling, solar-array assemblies and electronic assemblies.

The term induction refers here to magnetic ^induction

of materials while they ^are exposed ^{to a} magnetizing force, ^H.

SOFT REMANENCE. - It is customary ^to demagnetize

the spacecraft just before launch in a coil facility. ^This

process is performed ^{to remove all soft remanence of}

the spacecraft. Demagnetizing ^{fields in} the order of 50 _gauss are commonly used. After this process, the soft remanence is in its lowest value. Exposures ^due

to handling, shipping ^and launching may expose the

spacecraft ^to fields of 5 _gauss or greater. ^These expo-

sures will increase the spacecraft ^{soft remanence fields.}

If the _exposure field was fairly ^{uniform over the volume}

of the spacecraft, ^the resulting magnetizations ^{of the}

assemblies will be approximately parallel ^{and the}

FIG. 4.

Typical Results of Spacecraft Field Measurements.

resulting field will be approximately dipolar ^{in nature}

and will obey ^the inverse cube law of attenuation.

Because the soft remanent fields are produced by ^ran-

dom _exposures and are somewhat unstable with time,

very low limits are set on the amount of magnetic

material permitted ⁱⁿ spacecraft construction. Space-

craft specifications ^are frequently ^{set such} that random exposures of 5 gauss ^or less produce magnetic ^fields

which are at or below the threshold of the spacecraft

magnetometer. ^{The soft remanence of the} magneto-

meter instrument flown on Pioneer IX represents ^the state-of-the-art for low-magnetism design. ^{This ins-}

trument weights ^2.5 kg, ^{contains a} power supply ^and

over 1 000 integrated ^{circuits. The} change ^{in moment}

for this instrument due to a 5 _gauss exposure ^was 8 X 10-6 _gauss at one meter. The _progress in low-

magnetism spacecraft design ^{is shown in} figure ^4.

HARD REMANENCE. - Hard remanence fields are

produced ^from relays, solenoids, traveling ^{wave tube}

and certain magnets ^{used in} experiments. Traveling

wave tubes use magnets ^of high energy product ^and

great stability. If the orientation of these highly ^stable

field sources is fixed relative to the flight magnetometer, the external field ^can be reduced by 1) installing ^mat-

ched pairs ^to effect mutual cancellation; 2) adding

equally ^stable compensating magnets; ^or 3) by precisely

measuring ^{the field and} removing its contribution from

172

the scientific data analytically. ^In practice, ^{all three} techniques ^{are used. Permanent magnet fields can be}

reduced by 20-to-1 and made to be stable during

environmental _exposures.

CURRENT-Loops.

Magnetic fields due ^to the flow of currents can be minimized _very effectively by ^redu- cing the effective area encircled by ^{current. This is} accomplished by carrying ^{current in coaxial or twisted} pair cables. Because of the high conductivity ^mate-

rials used, ^{care must be taken to avoid current flow in} spacecraft structures. For individual electronic assem-

blies consuming ^{2-4 watts of} power, stray ^{fields on the} order of 5 X 10-7 _gauss have been routinely ^achieved

at 1 meter.

The solar _array used on the Pioneer VI series consis- ted of 10,368 solar cells mounted ^{on a} cylindrical

surface having diameter of 95 cm and length ^{of 89 cm.}

A cell temperature ^{of 277 OK} is maintained by ^{a rota-}

tion of approximately ^one per second about ^an axis normal to the sun line. This _array provides ^{a nominal}

current 1.7 ampere ^at 31.5 volts when illuminated with

one sun. Compensation ^{was achieved} by ^back wiring

each series string ^{of cells as} illustrated in figure ^{5. The}

FIG. 5.

Pioneer Solar Array ^Panel Wiring Showing Compensating ^{Positive Lead.}

stray ^fields produced by ^this array ^was 2 X 10-7 _gauss when measured ^{at a} distance of two meters from the

geometric ^{center of the} cylinder.

Figure ⁶ illustrated the state-of-the-art for existing interplanetary spacecraft magnetic ^{fields and contrasts}

these values with those required ^{for a} Jupiter ^mission.

By increasing ^{the boom} length, the level of magnetic

cleanliness is only slightly stricter for individual assem-

blies on the Jupiter ^mission.

SHIELDING.

Reducing ^the magnetic ^fields of space-

craft parts by ferromagnetic shielding ^{is not} permitted

as a general ^rule. Shielding ^to reduce external effects of permanent magnets ^or electromagnets ^{should be}

considered only ^{in cases where} the field of the magnet is relatively ^unstable. Often, the shield itself would be more unstable than the magnet it encloses and hence

causes a larger rather than a smaller field uncertainty

at the spacecraft magnetometer. Effective and light- weight ^shields require ^careful design ^{and the} high

nickel content alloys employed ^are significantly degra-

ded by ^cold working ^or by stressing after final annea-

ling. Shielding, however, ^{continues to be} suggested

as a panacea for reducing the fields of motors, relays

and similar parts by ^{those not} yet familiar with the

principles of low-magnetism spacecraft design. ^Small

shields have been used in isolated cases to prevent unwanted coupling ^of signals between circuits. The

shielding ^{used in} these instances should be nearly spherical ⁱⁿ shape ^{and be as small as} possible.

Magnetic Test Facilities.

A single ^{axis set of air}

core coils _may be used to produce ^a magnetic ^field matching ^the magnitude ^and uniformity ^{of Earth’s}

field but directed antiparallel ^{to the} ambient field.

However, ^{it is more common to use a} triaxial coil _sys-

tem that cancels the orthogonal components ^{of the} ambient field. Coil design ^{has its} origins in the work of Ampere dating ^back nearly ¹⁵⁰ year. Ampere’ss single ^{circular current} loop ^{had a} very small volume of field uniformity. ^{Some 30} years later, ^Helmholtz

and possibly others discovered that placing ^{two identical}

circular coils in parallel planes, ^{on a common} axis, spaced

at a distance equal ^to one-half their diameters, ^would produce ^a highly uniform field. The region ^of high uniformity is characterized by ^{a minimum} gradient ⁱⁿ

the axial component ^{of the field} produced by ^{the coil.}

AMES RESEARCH CENTER FACILITY.

The small coil

facility ^{at NASA} Ames Research Center is an adapta-

tion of a design originated by Rubens which consisted

FIG. 6.

Properties ^{of Low} Magnetism Spacecraft.

of five equally spaced ^{square coils} forming ^{a cube} (the

coil turns were in the ratio 19 :4 :10 :4 :19) ^and produced

a volume of ::1::: 0.1 % uniformity having ^the approxi-

mate shape ^{of a} cylinder oflength ^{0.45 d and diameter}

of 0.20 d (where d ^{is the} length of one side of the coil).

The method of Rubens and a digital computer ^were used to develop ^{a new} design ^of improved ^radial uniformity having ^a different ratio of coil ^turns and

unequal ^coil spacing. The Ames cubic coil ^uses three coil sets nested together ^which independently ^annul

the vertical, North-South and East-West components

of the Earth’s field. Alining the North-South axis

parallel ^to the local magnetic meridian reduces the

required output of the East-West axis sufficiently ^to provide proper uniformity ^with only ^{a Helmholtz} pair.

It is on this basis that the Ames cubic coil system ^uses 12 coil loops instead of 15. It has been found that the measured transfer function of the coil system ^{at it}

center agrees with the theoretical transfer function to

within 0.016 %. Although the theoretical improve-

ments predicted for this coil design ^were partially ^offset by ^the practical limitations imposed by fabrication,

these degradations ^{did not} reduce the volume of uni-

formity ^to less than theoretically predicted ^{for the} original ^Rubens’ design.

The values of theoretical performance ^were compu- ted on an IBM 7094 digital computer using ^{a For-}

tran IV adaptation of the MAFCO computer pro- gram developed ^{at the} University of California Law-

rence Radiation Laboratories.

The facility is housed in a 15 by ^{60 foot} nonmagnetic building ^located approximately ^0.6 mile North of Ames Research Center at Moffett Field, California, ^as

shown in figure ^{7. This site was} selected for its low level of magnetic ^{noise. In most cases, the noise at}

power frequencies ^{can be} eliminated by output filters, provided saturation of the magnetometer preamplifier

does not occur. A significant ^{source of noise is move-}

FIG. 7.

Nasa Ames Low-Field Magnetic ^Test Facility

12 Foot Cubic Coil Assembly.

ment of magnetic ^{bodies near the test area. For} example, moving automobiles ^can produce 1-gamma signals ^{at a} range of 100 yards, moving steel doors and

push ^{carts can} produce gamma fields at 50 feet, ^and

the hardware ^on one’s _person ^can produce gamma fields at ranges ^to 10 feet (1 gamma =10-5 gauss).

OTHER FACILITIES.

The NASA Ames Facility ^is designed primarily ^for magnetometer calibration ^and

subsystem magnetic properties testing. ^Much larger

coil facilities for spacecraft testing ^{exists at Nasa God-} dard, ^near Washington, ^{D.C. and near Los} Angeles,

California. The Goddard facilities include triaxial 12 meter and 6 meter high-uniformity Braunbek coils.

The California facility ^{has a 6 meter} Fanselau coil system.

FACILITY CALIBRATION.

To perform ^a proper d-c calibration of coil output, magnetic ^field intensity

and electrical current standards are required. ^The

most accurate reference standards for magnetic ^field

measurements use the gyromagnetic ratio of the proton.

These instruments can measure magnetic ^{fields to}

absolute accuracies of 0.001 %. ^A practical ^limitation

of this type of instrument is that the _accuracy is degra-

ded if the field level is less than 0.2 _gauss. By ^means

of the proton ^reference standards, reference coils and conventional d-c bridge measurement systems, ^calibra- tions with accuracies of + ^0.2 gamma + ^0.002 % ^of reading ^are performed ^{at Ames and are traceable to}

the National Bureau of Standards. This equipment

has been used to calibrate flight magnetometers, ^sole- noids and other portable ^coil systems ^{used as transfer} standards to other laboratories.

Measurement Philosophy.

ASSEMBLY MEASURE-

MENT.

Usually, the purpose ofmapping ^the magnetic

field of a piece ^of hardware is to provide ^{a basis for} predicting ^{the field} contributed by that hardware when mounted aboard the spacecraft ^with respect ^{to the} magnetometer. Normally, ^this represents ^a separation

distance that is much larger ^than the dimensions of the

assembly. ^{From a} data reduction standpoint, ^{it is}

desirable to measure under conditions simulating ^the position ^{of the} spacecraft magnetometer. ^{This nor-}

mally ^{is not} practical, since the field contributed by ^a single assembly ^{must be} only ^a fraction of the total field permitted ^{for an} integrated spacecraft ^{and would,} therefore, ^{have a} magnitude below the threshold of available magnetometers. ^{For this reason, measu-}

rements are commonly ^{taken at closer ranges, and}

estimates of the contributed fields are made using

inverse cube rules for a dipolar ^{source and} spherical

harmonic analysis ^for higher ^order multipolar ^sources.

The general ^{rule is to measure at a} range where the field of the assembly falls off as the inverse cube of distance. This _{range may} be from three to six times the largest linear dimension of the assembly.

Acceptance testing of spacecraft assemblies is accom-

plished ^{as follows :}

a) Measuring (mapping ^{the remanent} fields of the

assembly prior ^to magnetic treatment).

b) Mapping after the assembly has been dema-

gnetized.

c) Mapping after the assembly ^{has been} exposed

to a standard d-c exposure of 25 gauss.

d) Mapping the field due ^to energized circuits in

the assembly.

174

In acceptance testing ^the experimental or measure- ment uncertainty ^is usually kept ^{to 0.2} gamma RMS

or less in the 0.01 to 20 hertz band. Radial field

intensity ^is plotted ^{as a} function of azimuthal angle plots ^{in the} X-Y, ^{Y-Z and Z-X} planes ^{of the} specimen.

SPACECRAFT MEASUREMENT. - Both the _average value and the fluctuations in the geomagnetic ^field

must be considered during spacecraft magnetic ^field

measurements. For interplanetary spacecraft, only

the remanent and current-loop ^{fields are} significant

since the interplanetary ^{field is too weak to induce a}

measurable field. To accurately ^{measure the rema-}

nent fields it is customary ^to place ^the spacecraft ^{in a}

low-field magnetic ^coil facility ^{where the} geomagnetic

field has been reduced. Measurements indicate that the interplanetary ^{medium can be} adequately ^simula-

ted by attenuating ^the geomagnetic field 40 dB or more. The static value of the net uncompensated geomagnetic ^{field can} be removed from the output ^of the facility magnetometers by biasing ^solenoids.

The stability ^{of the net field} represents ^{a more strin-} gent requirement that the level of ambient field reduc- tion. In order to measure spacecraft ^fields accurately

to 10-6 _gauss, with a signal-to-noise ^{ratio of} 1:1, ^{it is}

necessary ^to have a noise level of about 114 dB below the magnitude ^{of the} geomagnetic ^{field. Conse-} quently, ^the electrical and mechanical stability ^of

the coil system is critical. For example, tempera-

ture changes ^{of 0.1 OK can} change ^coil output by

1 X 10-s _gauss. Coil dimensions are controlled by stabilizing temperature ^{and coil current is controlled}

by constant current power supplies having regulation

of 0.002 % ^or better. The facility magnetometers having sensitivities _up to 100 microgauss full-scale are

equiped ^with low-pass ^filters.

SPECIAL TECHNIQUES.

^{In addition to} these tech-

niques, systems ^which sample ^and compensate ^the diurnal variation (250-500 microgauss) ^are employed

to stabilize the field during spacecraft measurements.

Even when all these methods are used, ^one frequently

finds that facility magnetometers will drift at the micro- gauss level in a matter of seconds. The effect of drift in magnetometer output has been minimized by ^remo- ving ^the spacecraft from the coil system, zeroing ^the

magnetometer, quickly placing ^the spacecraft ^{in the}

coils and then rotating ^the spacecraft during ^{the measu-}

FIG. 8.

Spacecraft Magnetic Measurement Scanning ^Modes.

rement. The author has managed ^to improve ^{on this}

method by eliminating the need for continually ^remo- ving ^the spacecraft ^from the coils. Figure 8 illustrates this new technique. ^Three separate ^and consecutive orientations of the spacecraft ^{are used. If one assumes}

the designated location of the spacecraft magnetometer

to be out the spacecraft X ^{axis with axes} X l, ^{Yl and} Z, parallel ^{to the} spacecraft X, Y, ^Z axes, then in the first

orientation, ^the field contribution due to the spacecraft

moments along ^{Y and Z will} produce outputs ^{in the} Y-Z moment magnetometers periodic ^with each space-

craft rotation and symmetrical ^{about zero.}

By performing ^{this test} using ^{the three} spacecraft

orientations shown, ^and by labelling ^the facility magne- tometer outputs with the instantaneous azimuthal

spacecraft position, ^{it is} possible ^to separate ^{the three}

geometrical components ^{of the} spacecraft ^{field at the}

magnetometers ^{from the} background. ^The facility

magnetometers ^are positioned ^at 1) ^the range of the

spacecraft ^sensor; 2) ^{at a range where the} spacecraft

field should be doubled; ^and 3) ^{at a} range where the field should be quadrupled. ^The customary ^radial- map magnetometers produce data which ^can be degra-

ded by large concentrations of magnetic materials that

change ^{their range to the sensor} with each rotation.

This new method, using ^moment magnetometers ^avoids this problem since all magnetic ^{sources remain at}

constant range with respect ^{to the moment} mag- netometers.

REFERENCES ANON, Magnetic Fields. Earth and Extraterrestrial,

Nasa SP-8017, ^march 1969.

ANON, Spacecraft Magnetic Torques, ^{NASA SP-8018,}

march 1969.

HALACZY (A.), Study ^to Develop ^Methods Predicting Spacecraft Magnetic Fields, ^Nasa CR-73256, ^1969.

IUFER (E. J.) ^ed. Proceedings ^of Symposium ^on Space Magnetic Exploration ^and Technology, Reno, Nevada, 1967, ^28-30.

IUFER (E. J.) ^{and DROLL} (P. W.), Space Magnetic ^Envi-

ronment Simulation for Spacecraft Testing. Paper presented ^at ASTM/IES/AIAA ^Second Space ^Simu-

lation Conference, ^Am. Soc. Testing Mats., ^1967.

RUBENS (S. M.), Cube-Surface Coil for Producing ^Uni-

form Magnetic Field, ^Review of Scientific Instruments, Sept. ^1945.

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