Rubidium Vapor Magnetometer for Near Earth Orbiting SpacecraftW. H. Farthing and W. C. Folz Citation: Review of Scientific Instruments 38, 1023 (1967); doi: 10.1063/1.1720960 View online: http://dx.doi.org/10.1063/1.1720960 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/38/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ultra-sensitive high-density Rb-87 radio-frequency magnetometer Appl. Phys. Lett. 104, 023504 (2014); 10.1063/1.4861657 Magnetic induction measurements using an all-optical 87Rb atomic magnetometer Appl. Phys. Lett. 103, 243503 (2013); 10.1063/1.4848196 Development of an optically pumped atomic magnetometer using a K-Rb hybrid cell and its application tomagnetocardiography AIP Advances 2, 032127 (2012); 10.1063/1.4742847 Evanescent wave magnetometer Appl. Phys. Lett. 89, 261113 (2006); 10.1063/1.2424657 Parametric modulation of an atomic magnetometer Appl. Phys. Lett. 89, 134105 (2006); 10.1063/1.2357553

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THE REVIEW OF SCIENTIFIC INSTRCMEKTS VOLUMlc 38, NCMBER 8 ,\lTGUST 1967

Rubidium Vapor Magnetometer for Near Earth Orbiting Spacecraft

W. H. FARTHING AND W. C. FOLZ

NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771

(Received 19 December 1966; and in final form, 30 March 1967)

This paper describes the instrumentation and in-flight performance of the rubidium vapor magnetometers being flown by the National Aeronautics and Space Administration on the POGO satellites. An optically pumped, self­oscillating rubidium magnetometer was selected as being most compatible with the objectives of the study and with the spacecraft capabilities. A four absorption cell configuration is used to reduce the effect of the null zones inherent in these instruments and to obtain accuracies compatible with the scientific objectives of the program. Scalar magnetic field data are obtained in both digital (PCM) and analog (frequency multiplex) form. Instrument performance parameters are monitored through both main frame and subcommutated PCM data. The first instru­ment orbited was aboard OGO-II which was launched on 14 October 1965. This instrument has returned a large quantity of data, and is still operating when sufficient spacecraft power is available. The accuracy of the data is determined, apart from orbit accuracy, by spurious phase shifts within the instrument. These arise from such sources as optical axis misalignment, electronic nonlinearities and frequency dependence, and propagation delay over the long cables connecting sensor and electronics. The magnitude of the resulting error is inversely proportional to the phase slope of the dual cell absorption line. The total effect in the POGO instrument of these sources of error is an accuracy of better than 1.5 'Y over the entire instrument range of 15 000 to 64 000 'Y.

I. INTRODUCTION

T HE Goddard Space Flight Center is placing a series of Orbiting Geophysical Observatories (OGO) into

orbit around the earth.! There are two classes of the observatory: the eccentric orbiters (EOGO) and the near­earth polar orbiters (POGO). This paper deals with the magnetometers being flown on the POGO missions, the first of which (OGO-II) was launched 14 October 1965.

The objectives of the experiment are four-fold. First, the data from OGO-II are being used to refine the pres­ently available analytical descriptions of the main geo­magnetic field. This objective is part of the World Mag­netic Survey program.2

The second objective of the experiment is to measure the secular change of the main field. This capability is provided by the long design lifetime of the spacecraft, and by the continuing nature of the OGO series. Knowledge of the secular change will permit investigations of the in­ternal source of the main geomagnetic field, presently thought to be a homogeneous dynamo operating within the liquid core of the earth.3

Thirdly, the data from the POGO magnetometers will be used as a complement to rocket observations in study­ing ionospheric currents which contribute secondary effects to the earth's magnetic field. The low perigee and polar orbit of POGO aid in determining the latitudinal distri­bution and intensity of these phenomena, while rocket observations provide vertical profiles.

Finally, the data from this experiment will be used in the continuing study of the interaction of the solar plasma

1 W. E. Scull and G. H. Ludwig, Proc. Inst. Radio Engrs. 50, 2287 (1962).

2 J. P. Heppner, The World Magnetic Survey, Space Sci. Rev. 2, No.3, 315 (1963).

aT. Rikitake, Electromagnetism and the Earth's Interior (Elsevier Publishing Company, Inc., New York, 1966), pp. 315-354.

1023

with the earth's magnetic field. While this interaction is not as striking at POGO altitudes, effects are present, and their observation will complement the observations of the eccentric orbiters.

II. SELECTION OF INSTRUMENTATION

Based upon the objectives and the capabilities of the OGO spacecraft, the instrumentation required for the pro­gram possesses the following characteristics:

(1) Scalar measurements to an accuracy of ±2'Y (h= 10-5 G). This is necessary to keep the total error including that in the position of the spacecraft to the goal of ±1O)' required to achieve an order-of-magnitude re­duction in the rms residuals from the spherical harmonic coefficients used in the analytical description of the main field.

(2) Instrumental range from 15000 to 64000)" the expected range for the nominal POGO orbit.

(3) An output compatible with digitization for storage on spacecraft tape recorders.

(4) Sufficient information bandwidth to monitor low frequency temporal and spatial fluctuations of the field.

(5) Power and weight requirements compatible with the spacecraft capabilities, considering that 20 or more ex­periments participate in each mission.

(6) Proven reliability consistent with a design lifetime of one year.

Of the types of magnetometers considered advanced enough in development at the time the selection of instru­mentation was made, only the optically pumped, self­oscillating magnetometer met the requirements. For in­stance, the saturable core, second harmonic (fluxgate) magnetometers could not possibly meet the accuracy re­quirements for scalar measurements at high fields. Proton

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1024 W. H. FARTHING AND W. C. FOLZ

LAMP EXCITATION

OSCILLATOR IOOMC

FIG. 1. Dual-cell, self-oscillating rubid­ium magnetometer.

__________ ~~t ______________________ _ MAIN BOOY r

L--------------+----l AMPLIFIER

'----LA:M::.:PL:IFI~ERJ------------- OUTPUT SIGNAL

precessional magnetometers, while they can be considered absolute instruments, have operational problems in at­tempting to cover such a broad range of fields in a data storage mode because of the inherently low signal-to-noise ratio. In addition, the information rate is necessarily low because of the requirement for periodic polarizing pulses. Finally, proton magnetometers are susceptible to audio noise from spacecraft and natural sources.

Other types of magnetometers existed in principle, but were not considered advanced enough in development for use on the POGO satellite.

Returning to the optically pumped atomic resonance magnetometers, one must consider the metastable helium type and the alkali vapor type. Considerable attention was given to metastable helium, even to the extent of developing a prototype instrument.4 However, the helium magnetometer at high fields cannot utilize the self-oscil­lating configuration, because its large gyromagnetic ratio results in frequencies too high for direct detection by available infrared detectors. Thus a servo loop with auto­matic search and lock circuitry is required. The difficulty in design of such an instrument for unattended operation is somewhat greater than that of the self-oscillator, and thus the alkali vapor was chosen over the meta­stable helium.

Of the different alkali vapors, rubidium 85, rubidium 87, and cesium are the most desirable from the standpoint of gyromagnetic ratio, resonant line width, and operating temperature. It was felt that the gyromagnetic ratio of rubidium 87 (approximately 7 Hz per gamma) would result in a frequency range over which it might be diffi­cult to maintain zero phase shift. Cesium, on the other hand, had not been used in magnetometers as extensively as either of the rubidium isotopes. Thus rubidium 85 was chosen, even though from the standpoint of resonant line width it is the poorest of the three.

III. PRINCIPLE OF SENSOR OPERATION

The rubidium vapor magnetometer is based on the phenomenon of optical pumping reported by Dehmelt in

4 J. P. Heppner, op. cit.

1957.5 The development of practical magnetometers fol­lowed rapidly, 6 evolving finally to the dual-cell, self­oscillating magnetometer shown in block diagram in Fig. 1. Two of these twin cell magnetometers are combined in each POGO magnetometer.

In each twin cell 5ensor an electrodeless discharge lamp driven by the rf oscillator produces resonance radiation which is then collimated and passed through an inter­ference filter which selects the rubidium D 1 spectral line at 7947 A and rejects the D2 line at 7800 A. The light is then circularly polarized before it is allowed to optically excite the vapor in the absorption cell.

Two identical sets of optics are employed with the di­rection of light propagation antiparallel, thus defining the "optical axis" of the system. The transmission of each absorption cell is monitored by a photodetector, amplified, and cross-fed back to solenoidal coils wound about the absorption cells coaxially with the optical axis.

A closed loop is thus formed which oscillates if the loop gain is greater than unity at that frequency where the loop phase shift is zero. The frequency of oscillation is a direct measure of the strength of the ambient magnetic field, i.e., the Larmor frequency of atomic magnetic moments pre­cessing about the ambient field direction.

The modulation of light transmission obtained through application of a Larmor frequency Hi field to an optically pumped absorption cell results from transitions between the discrete hyperfine energy states which exist in the rubidium atom when it is subjected to a magnetic field. Before the application of pumping light, the atoms can be considered to have a Boltzmann distribution among these energy states. Optical pumping polarizes the sample so that a nonequilibrium population distribution occurs.

The hyperfine splitting of rubidium 85 is shown in Fig. 2. The ground state (52St) of the rubidium atom splits into 7 Zeeman states from m=+3 to m= -3 for F=3, and into 5 states from m= +2 to m= -2 for F= 2. A similar splitting occurs in the first excited state (52Pt). The optical pumping introduces transitions from the ground

• H. G. Dehmelt, Phys. Rev. 105, 1487 (1957) . • Arnold L. Bloom, Appl. Opt. 1, 61 (1962).

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MAGNETOMETER 1025

state to the first excited state with the selection rule .6m = + 1 or -1, depending on both the sense of circular polarization and the direction of the optical axis com­ponent of the ambient magnetic field. Once in the excited state, the presence of the buffer gas in the absorption cell causes considerable disorientation within the excited state. Thus the decay back to any ground state sublevel is commonly assumed to occur with equal probability. Once in the m= +3 or m= -3 sublevel of the ground state, the selection rule forbids further pumping. Thus a non­equilibrium concentration of atoms in either the m = +3 or m = -" sublevel of the ground state is obtained.

To understand the modulation of transmitted light in the presence of an Hl field, consider the projections of the pumped atoms' precessing moments on the plane perpen­dicular to the direction of the ambient field. Prior to the application of the H1 field, these projections are randomly distributed in direction, i.e., there is no phase coherence of the moments.

In order to accomplish a modulation of the transmitted light, a phase coherent group of depumped atoms must be formed. Since the pumping process depends on the angle between the precessing atoms' magnetic moment and the light wave normal, the total absorption then varies peri­odically as the group of depumped atoms precesses. The Hi field, if its frequency is the Larmor frequency, forms the required group of depumped atoms by selectively depumping those atoms which have a certain directional relationship to the H1 field.

Specifically, the H1 field vector can also be projected onto the plane perpendicular to the ambient field, and this projection can be resolved into two rotating com­ponents, one of which rotates in the same sense and with the same angular velocity as the projections of the atoms' moments. :'\Jote that the magnitude of this rotating com­ponent is proportional to sinO, where 0 is the angle be­tween the optical axis and the ambient field direction.

The probability of depumping of an atom by the ro­tating field vector is a function of the angle measured in the perpendicular plane from the rotating component of the H1 field to that particular atoms' projected moment. Specifically, the probability of depumping is maximum when this angle is 90°.7

The phase of the precessing moment is not affected by the depumping, so the distribution of depumped atomic moments reflects the probability distribution of the de­pumping function. Thus a group of depumped atomic mo­ments also precesses about the ambient field direction,

7 Again ~he 9uestion ?f whether the angle is leading or lagging in the precesslOn IS determ~ned by the sense of polarization of the light, and by.whether the amblt;nt fie!d.has a component along the direction o! the hght or opposed to It. .It IS Important that the sense of polariza­tI?n be the same for bo!h Sides. of the twin c~ll magnetometer when vIew~d along the dIrectlOn of hght propagatlOn. This results in 90° lead .Ill on~ ce~l and 90° lag in the other cell, which is necessary to obtam oSCillatIOn at the center of the resonance line. •

F·3-----------52 Pin F·2'....,...-----------

F'2

'I • 466737 Ito -359 H.,z Hz Iz· 466137 Ho -215 Ho"z Hz ~ - 466131 Ho - 72 Hoi Hz ~ - 466137 110+ 72 1102 Hz Is' 466731 110+215 1101 Hz 1&- 466731 110+359 Hot Hz

<CI@:b + I +2

FIG. 2. Energy structure of rubidium 85 showing Zeeman splitting of the ground state, and corresponding absorption spectrum for F = 3 where Ho is expressed in gauss.

and as they precess the pumping rate varies at the Larmor frequency because of the modulation of the angle between the rotating moments and the direction of propagation of the light. The variation of pumping rate, or light ab­sorption, thus appears as a modulation at the Larmor frequency of the transmitted light. At exact resonance, this modulation either leads or lags the H1 signal by 90°. Note that the pumping probability depends on the pro­jection of the light wave normal onto the ambient field direction, and thus a cosO dependence is introduced into the gain of the process.

This picture of the process is considerably complicated when one considers that not only pumping into and de­pumping out of the m= +3 and m= -3 Zeeman states are of importance, but that the modulation process actu­ally involves 7 sets of precessing moments, each set pre­cessing at a slightly different frequency from the others. This leads to the 6-line Zeeman transition absorption spectrum of the rubidium 85 atom.

Since resolution of the closely spaced lines is not com­patible with signal-to-noise ratio requirements, operation over the composite structure yields a skewed composite resonance line. The dual gas cell configuration has a strong advantage over single cell magnetometers in that the sense of circular polarization is such that when one cell is pumped to the m= +3 state, the other is pumped to the m= -3 state, resulting in a reverse skew for the two ab­sorption cells. Since the dual cell loop transfer function includes the product of the individual absorption lines, a symmetrical two cell composite line is obtained. Thus the frequency of operation should not change under reversal of the ambient field with respect to the sensor.

The frequency of oscillation of the loop is determined

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1026 W. H. FARTHING AND W. C. FOLZ

by the open loop phase shift of all the components. If the phase shift from all sources other than the absorption cells is zero at any frequency, then the closed loop oscil­lates at that frequency where the phase shifts of the two absorption cells are equal and opposite. By the symmetry of the system, this frequency is the average of the six absorption frequencies of the Zeeman spectrum for F = 3. This yields for the theoretical oscillation frequency8

f= 4.66737H 0,

where f is given in cycles per second if the ambient field H 0 is expressed in gammas.

The discussion of the modulation mechanism shows that the loop gain depends upon the product sinO cosO. Thus null zones are to be expected for the ambient field direc­tion parallel with or at right angles to the optical axis. These null zones are termed "polar" and "equatorial," re­spectively, and are of obvious importance in the design of a magnetometer for use in space.

In order to minimize the width of the null zones, it is necessary to use high gain amplifiers in the system, the limitation then being the noise level of the system and the necessity of operating the amplifiers in a close to linear manner in optimum orientation where the signal level is high.

The width of the null zones is also quite dependent on the temperature of the lamp and the absorption cells. A combination of passive thermal design and active heaters is used to control the lamp temperature to 115±5°C and the cell temperature to 40±2°C. The heaters are made from bifilar wound resistance wire to avoid magnetic signature, and each heater dissipates approximately 1 W at 28 V when the thermistor-operated control circuitry closes the circuit. Passive temperature control is provided by aluminization of the sensors to provide a degree of iso­lation from the environment, and by enclosing the sensor in a thermally coated sphere whose absorption and emis­sion characteristics are known.

The POGO twin cell magnetometer has equatorial null zones of less than 7° half angle, and polar null zones of less than 15° half angle. When placed in the four cell con­figuration, null zones are all but eliminated, since the re­sultant composite null zone is the intersection of the two disk shaped equatorial null zones of the individual sensors. The two optical axes are crossed at 55° to insure that the polar nulls do not overlap and to achieve a small, though not optimum, composite null zone cross section.

IV. SOURCE OF INSTRUMENT ERROR

The accuracy of the instrument is determined by the extent to which unwanted phase shifts in the system can

8 L. C. Balling, Bull. Am. Phys. Soc. 2, Vol. 10, 28 (1965). The constant and the data of Fig. 2 are derived from substitution of data reported here into the Breit-Rabi formula.

be avoided, since such phase shifts inevitably result in frequency shifts from the ideal. Such phase shifts may be either orientation dependent or frequency dependent, or both orientation and frequency dependent.

The first type is a spurious phase shift caused by mis­alignment of the Hl coil axis with the optical axis. The magnitude of the phase shift becomes quite large when the field direction is in the vicinity of a polar null zone, even for small angular misalignments. The winding of the Hi coil and positioning of the optical components must be tightly controlled. Even so, this source of error remains the most serious in the twin cell sensor. As will be seen, the use of the four cell configuration reduces the seriousness of this source of error.

Frequency-dependent phase shifts arise from at least three sources, the wide band amplifiers, the photo de­tectors, and the propagation delay over the long cables which interconnect the sensor and the amplifiers.

The expected field range in the nominal POGO orbit results in a Larmor frequency ranging from 75 to 300 kHz. The basic amplifiers must be free of phase shift over this range. While it is possible to achieve this with extremely wide band amplifiers, the resultant noise bandwidth would be objectionable. It is more desirable to use amplifiers with appreciably less bandwidth, and to shape the gain characteristic to keep the phase reasonably close to zero within the expected frequency range. In the electronics for the POGO instrument, this gain shaping is provided by tuned circuits. The component values are selected ex­perimentally, and provide phase correction over the final 50 kHz of the frequency range.

The photodetector is the second source of frequency-de­pendent phase shift. The detector used must be comparable in size with the light source, and consequently has a large junction capacitance. Since it is operated under a slight reverse bias, it also has a large source resistance. In order to prevent a full 90° of phase shift from the detector alone, it is operated into a low impedance preamplifier. However, operation into such a low impedance that the phase shift from this source is negligible over the whole range re­quires an impedance of the order of 100 fl, and results in a very unfavorable signal-to-noise ratio for the system be­cause of the resulting wide noise bandwidth and relative attenuation of the Larmor signal. Thus the amplifier input impedance is chosen to allow a controllable phase shift which is then compensated later in the amplifier. The roll-off of .the photocell-preamplifier combination in the POGO magnetometer is set to slightly greater than the upper frequency limit of the instrument, and it reasonably well approximates a single pole per preamplifier, that is 45° phase shift at the roll-off frequency.

Finally, frequency-dependent phase shifts are intro­duced because of the long cables normally used to inter­connect the sensor with its electronics so that magnetic

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.\IAGNETOMETER 1027

TEMPERATURES (2~ END SIGNAL r--------------------------------i .. ~AMPUTUDE (I)

l' ~~

FIG. 3. Block diagram of complete instrument.

1

LOCK

transistor cases and other parts do not influence the instru­ment. For the POGO instrument, the total length of cabling associated with one amplifier is approximately 16.5 m, resulting in a linear phase shift which approaches go at the high frequency end. This effect is also partially compensated for in the amplifiers.

The final class of spurious phase shifts, i.e., those which are both frequency and orientation dependent, arise from nonlinearities in the electronic components. The latter stages of amplification are necessarily operating in a large signal mode a large part of the time. Since the bandwidth is not extremely wide, one can expect some variable phase shift from this source, though it can be kept quite small if the amplifiers are prevented from saturating.

The magnitude of the frequency shift which results from a given phase shift is determined by the phase slope of the absorption line at the operating frequency. This slope varies markedly with orientation in a manner that has not been treated quantitatively. Qualitatively, it re­sults from the competition between the pumping and de­pumping mechanisms in the absorption cell. Close to a polar null zone, the pumping process dominates, resulting in a sharper single cell absorption line close to the theo­retical major transition frequency, for instance, from F = 3, m= -3 to F=3, m= -2. As the sensor is then rotated toward an equatorial null zone, the depumping process is dominant, producing a wider line closer to the desired average absorption frequency. The net result is a com­posite two cell phase slope which is considerably sharper at the equatorial null zone than at the polar. Since the frequency error is inversely proportional to the phase slope, orientation dependence for any phase shift results.

~r----.. t- nIY'8!TS lEI

SHIFT (2) TIMING (21

F~~IJ..l1-__ TO TAPE

I----..TO REALTIME '------'

I----..TQ TAPE I----..TO REALTIME

'rnrTTTT'1~

INHIBITS (6) SHIfT (2)

L-___ ....... : TIMING (2)

TO SPECIAL PURPOSE TELEMETRY

TEMPERATURES (2) SIGNAL AMPLITUDE MONITOR (I)

Finally, the splitting of the Zeeman levels, and thus the inverse phase slope of the resonance, are proportional to the square of the ambient field.

For the POGO twin cell magnetometer, the inaccuracy resulting from all these error sources varies from ±0.5 l' in 15 000 l' to ±2.5 l' at 64 000 1'. When placed in the four cell configuration, these figures become ±O.5 ')I and ± 1.5 ')I,

respectively.

v. THE FOUR CELL CONFIGURATION

In the four cell magnetometer, the two individual sensors are oriented with respect to each other so that the resultant null zone is formed by the intersection of the individual equatorial null zones, and the polar null zones are not allowed to overlap. In the POGO instrument, a composite null cone of elliptical cross section is obtained with a major axis of approximately 20° and a minor axis of 10°. The two sensors are locked in frequency by ac coupling at their clippers, and the filtered output signals from the two magnetometers are mixed before subsequent processing. This procedure results in an output frequency which is the average of the individual unlocked twin cell sensors weighted by their unlocked amplitudes. Since in­dividual errors are maximum close to the polar null zone, where the amplitude is low, an appreciable improvement in accuracy is obtained with the four cell configuration.

While the signal-to-noise ratio obtained from the instru­ment is quite high for optimum orientation, limitation of the noise bandwidth is necessary for digitization of the Larmor signal in poor orientation. Thus bandpass filters are employed outside the loop before the two sensor signals are mixed. After mixing, a four tooth comb filter

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1028 W. H. FARTHING AND W. C. FOLZ

Ir=======rfr§~~~~~~~~~~}TIMING PULSES

} INHIBITS (3) AND SHIFT tIl

f----+- TO TAPE RECORDERS

LARMOR FREQUENCY

f----+- TO REAL TIME

FIG. 4.fBlock diagram of digitizer.

l~~~~~~~~~~~~}INHIBITS (3) AND SHIFT (I)

is used, resulting in a final noise bandwidth of approxi­mately 60 kHz. The Larmor signal is then counted ac­curately whenever either signal-to-noise ratio inside the loop is greater than unity. This prevents any appreciable increase in the effective null zone because of inaccurate scaler operation. A block diagram of the complete system is shown in Fig. 3.

VI. DIGITAL DATA GENERATION

The OGO spacecraft includes in its telemetry system two wideband systems with nonretum to zero pulse code modulation. One system is normally assigned to real time operation and the other to data storage.

Data from spacecraft and experiments is time multi­plexed into a main frame of 128 nine bit words. Data from analog sources is converted by the spacecraft analog data handling assembly to eight bit digital words before appli­cation to the multiplexer, while digital data passes through digital transmission gates directly into the multiplexer.

Two of the 128 main frame channels are subcommutated into spacecraft instrumentation, and an experiment sub­commutator feeds a third channel.

The digital experiments must respond to spacecraft in­hibit (word select) and shift (bit select) signals by es­tablishing the proper output levels into the spacecraft's digital transmission gates. Each experiment must be so designed that it is compatible with the fonnat and bit rate of the spacecraft. The bit rate for the data storage equip­ment group is fixed, while the real time bit rate may be selected by ground command. The data storage bit rate on OGO-D to be launched in 1967, is 4000 bits per second, resulting in a frame period of 0.288 sec.

The magnetometer on POGO, in order to utilize the PCM tape storage, has two digitizers which count the Larmor frequency alternately and store the count for readout on command. A digitizer block diagram is shown in Fig. 4. The gating period is derived from the space­craft low frequency timing assembly, and is set to ! sec to ensure that each sample is read into the telemetry sys-

tem at least once, and to yield a reasonable sample reso­lution of ±0.43 'Y. The stability of the timing system is specified one part in lOS, and it actually performs somewhat better, so that it is a source of relatively small error in the data.

The sample resolution of ±0.43'Y is not adequate for some purposes. To enable the addition on the ground of successive samples retaining ± 1 count accuracy, the gating period is synchronized with zero crossovers of the Larmor frequency. The gated Larmor frequency is then delayed by 2 J,tsec to ensure that every Larmor cycle is counted by one and only one scaler.

The maximum expected count per sample is 150000 (300 kHz for i sec) which requires 18 bits for storage. Thus two main commutator channels per scaler are re­quired. Upon command, the eighteen bits are scanned by an experiment commutator. If a digitizer is interrogated by the spacecraft during the count period, the output of the commutator is inhibited, and all zeros are read into the spacecraft commutator to prevent ambiguous data.

The experiment output commutator has 27 positions. Positions 1 to 18 scan the 18 bits of Larmor data, and positions 19 to 27 scan experiment status information which is assigned to an experiment subcommutator chan­nel on the spacecraft commutator. Since these data are called for only 1/128 as often as the Larmor data, the experiment commutator resets to position 1 on the leading edge of the inhibit calling for the first 9 bits of Larmor data. The commutator then steps sequentially in response to the gated shift pulses.

Positions 19 to 27 monitor the status of the experiment heaters, the lamp ignition, the status of the switch which applies Larmor signal directly to the special purpose fre­quency multiplex telemeter, and the status of the comb filters.

VII. ANALOG DATA

The spacecraft analog data handling assembly men­tioned earlier is used to monitor the signal amplitudes in

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MAGNETOMETER 1029

each of the two sensors. Two main frame channels are assigned to this function. These data serve to eliminate possibly noisy data from the tapes finally processed on the ground. Six subcommutated analog channels monitor critical sensor temperature (4) and the status of space­craft power commands to the experiment (2). The analog data handling assembly accepts from zero to 5.12 V, and thus all data must be calibrated over this range. Two power commands are unambiguously voltage coded onto each power status monitor.

The special purpose telemeter mentioned earlier is also used to transmit magnetic field data. This is a separate frequency multiplex telemeter of 5 channels and 100 kHz bandwidth provided by the spacecraft whenever a receiv­ing station is in range of the spacecraft. The Larmor fre­quency is divided by four in order to stay within the band­width limitations of the system. It is then combined with the other channels before being applied to the system's phase modulator. While these data are only available in real time, the information bandwidth is quite wide as compared with that of the recorded data. The system can thus be used to take advantage of the sensor's inherently fast response to field fluctuations.

vm. MECHANICAL CONFIGURATION

The stringent requirements for accuracy demand that the experiment not be subjected to interfering fields from the spacecraft. Thus the sphere containing the sensor is mounted at the end of a 6-m boom, where the spacecraft permanent field contribution is less than 0.5 /'. This con­figuration is shown in the artist's conception of the de­ployed OGO shown in Fig. 5.

FIG. 5. Deployed OGO. The magnetometer sensor is housed in the sphere at the end of one long boom. Electronics housed inside main body.

FIG. 6. View of sensor with cover removed.

The two rf oscillators for lamp excitation are mounted on the boom 56 cm inboard from the sphere. The remaining electronics are housed in a 20-cm cubic container inside the main spacecraft body, with interconnecting cabling routed through the hollow boom.

Figure 6 shows the sensor package with the front half of the sphere removed. The center of the null cone is perpendicular to the plane defined by the two optical axes, and in orbit is directed downward by 25°, and away from the spacecraft. Under the assumption of perfect behavior of the spacecraft attitude control system, this would result in minimal loss of data due to orientation. Even on OGO-II, in which case the attitude control system failed, the data lost from poor orientation have been negligible.

The sensor package, including the last section of the boom and its cabling, weighs 4.4 kg. The main body package weighs 5.1 kg. Total power dissipation, excluding thermal control, is about 8 W, of which nearly 6 W is required to drive the rubidium lamps.

IX. PERFORMANCE IN ORBIT

In spite of the spacecraft attitude control system prob­lems encountered in the launch of OGO-II, the rubidium magnetometer has functioned quite well. The instrument operation was normal in every respect for approximately the first six months of the satellite's lifetime, when a failure of one scaler power supply caused loss of the special purpose telemetry signal and half of the PCM data. The reduction in scientific usefulness of the data received from the remaining scaler is minor however, since the inclusion of two scalers in the system was primarily for redundancy. The instrument as of March 1967 is still returning data in this reduced capacity whenever reliable spacecraft power

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1030 W. H. FARTHING AND W. C. FOLZ

)::

LAT. 62 64 LON. -347 -181 ALT. 1099 626

50000

-.

5 -53 -180 -180 421 712

-74 -28 16 -17 -12 -14 1199 1495 1445

~ 40000 i t\

1\ / \ Li: ...J 30000

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10000 G.M;r. 16 45 17 0 17 15 17 30 17 45 180 18 15

DATE 1965/10115

POGO TOTAL FIELD

FIG. 7. One full orbit of digital data from OGO-II. Note data loss at low field due to high apogee of orbit.

is available, and there is no further indication of deteriora­tion in the performance of the instrument. Typical dis­plays of digital data obtained from OGO-II are shown in Figs_ 7, 8, and 9. Figure 7 contains data for a complete orbit during which at one point the magnetic field was not within the instrument range. This came about because the orbit achieved by OGO-II was considerably higher than had been planned. Figure 8 shows an expanded view of the region of data loss. The instrument first becomes incap-

LAT. -28 ·20 -13 -5 I 9 16 LON. -12 -12 ·13 -13 -13 -13 -14

r~:~~LtlE' 14000~· -

G.MI. 180 182 \e5 187 1810 1812 1815 29.99 29.99 29.99

DATE 1965110115

POGO TOTAL FIELD

FIG. 8. Expanded view of regio~ of data loss of Fig. 7.

E ·lL

~ w a

LA! 51 ·60 -69 ·78 ·86 ·82 LON. 35 .35 37 42 79 .173 ALT. 1362 1445 1493 1525 1542 1542

20.00

10.00 "",,-"".

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0

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OATE 1965111/10

FIG. 9. Difference field between measured and calculated field. Calculated field is obtained from spherical harmonic expansion ob­tained from three days of OGO-II data.

able of indicating the magnitUde of the magnetic field when the signal level from the bandpass filter drops too low for the decision circuitry which determines which tooth of the comb filter to select. Thus the transition from good data to noise is quite abrupt. Figure 9 shows the difference field between tbe experiment data and a calcu­lated field derived from a set of spherical harmonic co­efficients. This set of coefficients is generated from OGO-II magnetometer data.9 The effect of quantization noise is very apparent on this plot.

Future launches of these magnetometers will be instru­mental in further defining the secular change and shorter period fluctutations of the geomagnetic field. The data ob­tained will provide a clearer picture of the physical processes governing the magnetosphere.

ACKNOWLEDGMENTS

The authors wish to acknowledge Varian Associates of Palo Alto, California, who provided the sensors for this experiment and Charles H. Ehrmann of Goddard Space Flight Center, who provided the digitizers.

9 J. C. Cain, "First Magnetic Field Results From the OGO-If Satellite" COSPAR Ninth Plenary Meeting and Seventh Inter­national' Space Scie~ce Symposium (10--19 May 1966), Vienna, Austria.

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132.174.255.116 On: Sat, 20 Dec 2014 15:49:46