IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. NS-13, NO. 6, DECEMBER, 1966

A FLUXGATE MAGNETOMETER FOR THE

APPLICATIONS TECHNOLOGY SATELLITE

J. Dale Barry and

Institute of Geophysics

University of California,

ABSTRACT

A satellite-borne magnetometer used to de-

tectu magnetohydrodynamic wave propagation within

the magnetosphere is introduced. The instrunent

is a biaxial, closed-loop, fluxgate magnetometer.

The unit consists of the basic magnetometer plus

additional sections, including a data processor,

a field nulling section, and sensitivity selection

logic. The basic magnetometer is discussed

briefly, the additional sections in greater de-

tail. It is shown that the use of sum and diff-

erence amplifiers in the data processor enable

the derivation of magnetic field vectors trans-

verse and parallel to the spacecraft spin axis.

The field nulling section involves the use of an

offset-field-generator to apply discrete current

steps to the sensor offset winding in order to

null the ambient sensor field. The use of three

sensitivity levels is shown to maintain the

analog output within the dynamic range of the

instrument. An in-flight calibration section is

discussed and referenced to instrument stability

and sensitivity. A brief discussion of the ATS

satellite mentions the advantages of its orbit,

which is favorable for the study of long-term

magnetic field variations and f'or correlation

Robert C. Snare

and Planetary Physics

Los Angeles, California

with plasma and particle experiments also on-

board.

INTRODUCTION

A biaxial, closed-loop, fluxgate magneto-

meter was selected for use on the first Appli-

cations Technology Satellite. The satellite will

be placed in a synchrous equational orbit at 5.8

earth radii and 165 west longitude. This orbit

is advantageous for the goals of the magnetic-

field measurements as it eliminates the effects

of changing geographic and geomagnetic coordinate

system relations. The mnagnetometer will be used

to detect and measure magneto-hydrodynamic wave

propagation within the magnetosphere. In

addition to the study of low frequency field

variations, data will be provided for correlation

with plasmna and particle data from other instru-

ments onboard this same spacecraft.

Since the biaxial magnetometer is to be

placed on a spinning satellite in an average

ambient field of about 125y (lY=105 gauss),

the following design goals were essential: The

operational range of the instrument must be great

enough to allow a complete field reversal. The

sensitivity and resolution of the magnetometer

must be high enough to allow measurement of very

Manuscript received by NSG Sept. 11, 1966

326

BARRY AND SNARE: A FLUXGATE MGNETMETER3

small variations in the magnetic field. Even

though only two instantaneous magnetic field com-

ponents can be measured directly, the three

dimensional time dependent components of the

magnetic field must be derived within the in-

strument onboard the spacecraft.

To achieve these goals, an instrument was

designed with resolution and sensitivity of

0.05v/y and dynamic ranges of + 50y, + lOOy, and

+ 200y. With the use of an offset field gen-

erator, the total dynamic range is increased to

+925y and -675y. The three-dimensional time-

dependent components of the magnetic field are

derived from the two instantaneous measurements.

This is accomplished by using a sum and diff-

erence amplifier system which derives the field

components parallel and normal to the spacecraft

spin axis. A more complete discussion of this

system is included in a later section.

The instrument consists of two fluxgate

magnetometers coupled with four major subsystems.

These include: an offset field nulling system,

an in-flight calibration system, a sensitivity

switching system, and an analog data processing

system.

The basic magnetometer, its method of oper-

ation and the first two subsystems are very

similar to those of the magnetometer for the

OGO-E spacecraft. The OGO-E instrument is

described by Snare and Benjamin in another

article of this month's issue of the NSG Trans-

actions and should be referred to for a detailed

description of the basic magnetometer, offset

field nulling system and the in-flight calibrate

system. The instrument specifications are listed

in Table 1.

PRINCIPLES OF OPERATION

Considering Figure 1, the Z coordinate axis

is coincident with the spacecraft spin axis

rotating with frequency vs. The magnetometer

sensor axes are in a plane determined by the Y

and Z coordinate axes, and each sensor makes an

angle of 450 to the z axis. Thus as the satellite

spins, each magnetometer sensor measures time

varying components at an angle 450 to the di-

rection normal and parallel to the spin axis.

Let us assume that some constant ambient

field H is present at the sensors. Then with

respect to the three dimensional coordinate

system; 0 = constant, 0 < 0 < 2 Tr, and Hz=constant. Referring to Figure 1, the components

of H measured by the magnetometer sensors are

H = 4. (cos 0 + sin o)

Hb - (cos 0 - sin 0)vr1(1)

where the subscript a(b) denotes the magnetic

field component measured along the sensor

A(B) axis.

The analog data processor contains the sum

and difference amplifiers which derive the com-

ponents normal and parallel to the spin axis.

These are

H = 1 (Ha+ Hb)P f2a

1H = (Ha(2)

327

IEEE TRANSACTIONS ON NIULEAR SCIE

where the subscript p(n) denotes the parallel

(normal) component. The p(n) component is

sampled at a rate of 3.12 samples/sec (6.24

samples/sec).

As the coordinate system rotates, Hn has

values ranging from a minimum when H is in the

plane of X and Z, to a maximum when H is in the

plane of Y and Z. Thus the magnitude of H is

given by2 2 '

IHI = (H +EH max) (3)p n

and the angle Q by

@ = tan [ (Ha - Hb)/(H + Hb) ax (4)

and the angle 0 by

0 =2rvt (5)

where vs is the nominal satellite spin frequency

of 1.67 cps.

The three dimensional, time dependent, com-

ponents of the magnetic field are just

Hx (t) = IHI sin 0 cos 0 X

Hy (t) = IHI sin G cps 0 y (6)

Hz (t) = IHI cos S z

That is, in terms of the two dimensional mea-

sured components,

Hx Ha -Rb cos 2rsvT X

H H- Ia b sin 29TT\o t Y

It is apparent from the actual position of

the sensors as shown in Figure 2 that the satel-

lite rotation will actually cause the sensors to

rotate along the perifery of a circle rather

than coincident with the spin axis as shown in

Figure 1. Thus it might appear that a dis-

crepancy between the theoretical description and

the actual experiment exists. This is removed

upon considering the magnetohydrodynamic diss-

turbances to be measured.

We expect to measure waves for which,

2x103 cps < v < 2x10 1 cps, where v is the

frequency of the magnetic disturbances. Let us

assume that the wave may be described by exp

(ik-i2TTv) where k indicates the direction of wave

propagation. Also, k is constant and at one

instant of time will lie along one of the sensor

axes., and the wave velocity is given by the

Alfven velocity, VA. In order to remove the

discrepancy, it is necessary that the wave length

of the disturbance be large compared to the dis-

tance between two consecutive samplings by the

magnetometer. This is to insure that the sep-

aration of the sensors from the spin axis is

immaterial to the measurement of the magnetic

disturbances. Assuming the worst possible case,

that is, a disturbance of frequency v = .2cps

whose wave front is normal to the spin axis, and

the usual sampling rate SR = 6.24 samples/sec,

then this relation may be written as

A dx V- > >

d

v SR

where VA is the Alfven velocity, and d is the

sensor to spin axis separation. Assuming that

the Hydrogen ion particle density is about

1.67x103 g/cm3 B is about 125y, and d is 200

cm, which are all valid assumptions at 5.8 earth

radii, then the left hand term is about 106larger than the right hand term. Thus the

D%esf328

BARRY AND SNARE: A FLUXGATE MAGNLEM 3

inequality holds and the sensor to spin axis

separation is immaterial with respect to the

wave disturbance detection. We could derive a

similar inequality to show that the sensor to

spin axis separation is immaterial with respect

to gradients within the disturbance and their

detection.

OFFSET FIELD GENERATOR

As discussed earlier, the total field is

directly measured as two vector components.

These two components are H , parallel to the

spin axis, and Hn normal to it. As shown by

equation (3) and the accompanying discussion

the total field is a vector sum of H andp

H . The necessity of using H is duenmax n max

to its modulation by the spin frequency.

If the value of H is greater than thep

dynamic range of the basic magnetometer, the

offset-field-generator applies discrete step

offset currents to windings on the magnetometer

sensor. This is, in effect, applying offset

magnetic fields on the sensor to null ambient

fields.

The offset-field-generator produces a total

of 64 steps, each with a value of 25y. Each

magnetometer can be offset by a total value of

+925y and -675y. The displacement is to account

for the nominal field value of +125y expected

at the spacecraft position. For a more complete

description of the offset-field-generator see

Snare and Benjamin (1966).

SENSITIVITY SWITCHING

Consider the case when the spin modulated

functions, Hn is large enough to drive the basic

magnetometer off scale on the peaks. This sit-

uation would normally be detected by the offset-

field-generator system and nulling fields applied

to keep the output on scale. This would result

in the offset-field-generator stepping up and

down during each period of spacecraft rotation.

To prevent this offset generator stepping,

the basic magnetometer dynamic range is in-

creased from + 50y to + lOOy and + 200y for

full scale values. A block diagram of the basic

magnetometer including the sensitivity switching

elements is shown in Figure 3. The increased

dynamic range is switched in whenever the upper

level and the lower level detectors of the

offset-field-generator are activated within 0.1

sec of another.

The instrument first goes from + 50y full

scale to + lOOy full scale. If the level de-

tectors are again activated within 0.1 sec of

one another, the dynamic range is then increased

to + 200y. The increased dynamic range is applied

for one minute and the magnetometer then returns

to the lowest dynamic range. The effect of the

dynamic range switching is to maintain high

resolution for most magnetic field variations

and yet be able to accomodate any large vari-

ations and field values which may occur.

ANALOG DATA PROCESSOR

In order to transform the magnetic field

components measured along each sensor axis into

the components normal and parallel to the spin

axis, onboard circuitry is utilized. The analog

data processor contains sum and difference

amplifiers followed by filters which prevent

1966 329

330 IEE TRANSACTIONS ON NUCLEAR SCIENCE

frequency aliasing of the telemetered signals.

The sum and difference amplifier input is

through closely matched, low offset, differential

pair transistors. Total offset through the

amplifier is held below a few millivolts with

frequency response better than 1% from DC to

approximately 100 cps.

The active filters are conventional types

and provide sharp cutoff and maximum energy

transfer within the pass band.

IN-FLIGHT CALIBRATE

The in-flight-calibrate circuitry is acti-

vated every 44 minutes by telemetry system sync

signals and applies calibration currents to IFC

windings on the sensor.

The in-flight-calibrate sequence has eight

steps which are arranged to check the value of

one offset-field-generator step, the operation

of the sum and difference amplifiers, and to

demonstrate the end to end offset through the

telemetry and data acqusition systems.

ACGKNOWLEDGMENTS

The principal experimentor for this instru-

ment is Professor Paul J. Coleman, Jr. of the

Department of Planetary and Space Science

of the University of California at Los Angeles.

Early development of the magnetometer was funded

under NASA Grant NSG 249-62. Project funding

was from NASA Contract NAS5-9570. Instrument

detailed design, packaging and fabrication was

by Marshall Laboratories, Torrance, California

under University of California Contract 79633-0.

BIBLIOGRAPHY

(1) Snare, R. C., and Benjamin, C. R., A

Magnetic Field Instrument For The OGO-E

Sp2acecraft, IESE Transactions on Nuclear

Science, (1966).

TABLE 1

INSTRUMENT SPECIFICATIONS

Dynamic Range: Basic Magnetometer +50y, tlOOy and +200y

Offset -field-generator +925y to -675y in 25y steps

Bandwidth: Basic Magnetometer DC to 100 cps

Accuracy: + 0.125y

Output Voltage: 0.0 to +5.0 VDC

Noise Level: O. ly

Operating Power: +23.2 VDC + .5v, 130 ma

Physical Dimensions: Electronic Package 9.15 x3.80x3.7 in

Sensor Assembly 3.50x3.125x3.25 in

Weight: Electronics 3.05 lb

Sensor 0.70 lb

T-uDO.ecmber330

BARRY AND SNARE: A FLUXGATE MAGNETOMER

SPINAXIS

APOGEEENGINE- I

PANEL

Fig. 1. An illustration of the relationshipbetween the three dmensional spacecraft co-ordinate system and the two magnetareter axes.

Fig. 2. An illustraticn of the ApplicationsTechnology Satellite and the position of themagnetaneter sensor. Ihe sensor is positionedabout 100 an away frmn the spin axis.

2.5 V

Fig. 3. Schematic of the basic magnetaieter.The sensitivity switching system is indicated-as the series of three switches connectedwithin feedback loop.

1966 331

,y

IEEE TRANSACMIONS ON NUCLEAR SCIENCE

AUTHORS

J. Dale Barry (S'66) was bornin Washington, D.C., on Febru-ary 8, 1942. He received theB.S. degree in nuclear physicsfrom the University of Califor-nia at Los Aogeles (UCLA) in1964, and the M.S. degree inpbysics from California StateCollege, Los Angeles, in 1966.He is currently working towardsthe Ph.D. degree in apace phy-

sics at UCLA.Since 1963, he has worked at the UCLA Space

Center as a Research Assistant at the low magneticfield laboratory, where he is currently engagedin the ATS and OGO-E magnetometer program. Mr.Barry is a mem*ber of the American Geophysical Union.

Robert C. Snare was born inWaco, Tex., on July 23, 1931.He received the B.S.E.E. degreefrom the University of Tecasat El Paso in 1954 and laterdid graduate work at the Uni-versity of Washington, Seattle.

He has had varied aerospaceexperience which includes cir-ouit design, missile and satel-lite testing, and launch sys-

tes integration. In 1964 he joined the staffof the Space Science Center of the University ofCalifornia at Los Angeles as a Project Engineer.

332