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The Magsat Precision Vector Magnetometer

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The Magsat Precision Vector Magnetometer MARIO H. ACUNA THE MAGSAT PRECISION VECTOR MAGNETOMETER The Magsat preCISIon vector magnetometer was a state-of-the-art instrument that covered the range of ±64,OOO nanoteslas (nT) using a ±2000 nT basic magnetometer and digitally controlled current sources to increase its dynamic range. Ultraprecision components and extremely efficient designs minimized power consumption. INTRODUCTION stable and linear triaxial fluxgate magnetometer with a dynamic range of ±2000 nT (1 nT = 10-9 The instrumentation aboard the Magsat space­ weber per square meter). The principles of opera­ craft consisted of an alkali-vapor scalar magnetom­ tion of fluxgate magnetometers are well known and eter and a precision vector magnetometer. In addi­ will not be repeated here. The reader is directed to tion, information concerning the absolute orienta­ Refs. 1, 2, and 3 for further information concern­ tion of the spacecraft in inertial space was provided ing the detailed design of these instruments. by two star cameras, a precision sun sensor, and a To extend the range of the basic magnetometer system to determine the orientation of the sensor to the 64,000 nT required for Magsat, three digi­ platform, located at the tip of a 6 m boom, with tally controlled current sources with 7 bit resolution respect to a reference coordinate system on the and 17 bit accuracy were used to add or subtract spacecraft. automatically up to 128 bias steps of 1000 nT each. The two types of instruments flown aboard the The X, Y, and Z outputs of the magnetometer spacecraft provided complementary information were digitized by a 12 bit analog-to-digital con­ about the measured field. The scalar magnetometer verter that, in conjunction with the 7 bits asso­ measured the magnitude of the field independent of ciated with the digital current sources, yielded an its orientation with respect to the sensor, with an overall instrument resolution of ±0.5 nT. absolute accuracy that · is determined by atomic The ambient magnetic field was sampled 16 times constants and thus is not subject to change as a per second along each of the three orthogonal di­ function of time. On the other hand, the precision rections, and the digital current sources were up­ vector magnetometer measured the projections of dated at the same rate. The digital data corre­ the ambient field in three orthogonal directions sponding to the "fine" (12 bit) and "coarse" (7 with an absolute accuracy determined by calibra­ bit) information were fed directly to the spacecraft tions with respect to a standard; thus they were through a serial interface. The fine data were sam­ subject to error and drift. Accuracy goals for the pled at 16 samples per second; the coarse data were mission required a vector magnetometer capable of sampled at only 4 samples per second. This sam­ measuring the ambient field with a maximum error pling scheme took advantage of the fact that the of ± 1 part in 64,000 in magnitude and 5 arc-s in ambient magnetic field changed slowly over the orientation (1 arc-s = 0.00028°). The development Magsat orbit; hence, coarse updates were not of such an instrument within the constraints im­ needed as frequently as fine updates. The response posed by the spacecraft represents a major techno­ of the instrument to increasing and decreasing am­ logical achievement that would have been impossi­ bient fields is shown in Fig. 2. The current steps, ble without parallel developments in the areas of which are added or subtracted depending on the ultraprecise linear integrated circuits and miniature magnitude and direction of the external field, resistors. The design implemented for Magsat rep­ maintained the effective magnetic field seen by the resented an optimum compromise among many fluxgate sensor within its operating range of 2000 conflicting requirements and limitations imposed by nT. The stepping threshold was actually 1000 nT to available resources, reliability considerations, and allow the magnetometer to follow rapid changes in state-of-the-art electronics. This paper presents a the field between two adjacent updates of the bias brief description of this unique instrument. steps without loss of data. As can be seen from Fig. 1, the magnetometer INSTRUMENT DESCRIPTION electronics, analog-to-digital converters, and digi­ Figure 1 is a block diagram of the vector magne­ tally controlled current sources were implemented tometer. The heart of the instrument is a highly with redundant designs. This was also true for the 210 Johns Hopkins APL Technical Digest ~ 250 KHz clock A Sample A Coarse word gate A Attitude transfer Coarse data A system J----Shift clock A + t Fine data A r-- ---- ~ I I I Triaxial w..1 H}~~--'--------'---..L.....ll-____---, I fluxgate I sensor assembly ~...... ----t.-r--"""'T"----r---..,....,; I I L_ - -t- - _.J Fine word gate B Fine data B Thermal control ...----Shift clock B Coarse data B Sun sensor Coarse word gate B assembly Calibration ~ Timing and ,Sample B }A power in ~ control ~~250 KHz clock B Data processor B ~ ~l~ Housekeeping }B power in ~ analog signals ---~ Fig. 1-Block diagram of the precIsion vector magnetometer. All subsystems are implemented with redundant designs to avoid loss of data in case of failures. digital processors and power converters. The block­ dition, its uniform expansion characteristics were redundancy approach provided a fundamental mea­ essential for achieving the angular stability re­ sure of reliability to this important instrument and quired. Figure 3 is a schematic diagram that shows eliminated from consideration single-point failures the construction of each sensor. Crucial elements in in the electronics. Selection of the particular system the Magsat fluxgate sensor design were the feed­ used (A or B in Fig. 1) for both the analog and back coil that nulls the external field, and the sen­ digital portions of the instrument was executed by sor core itself; they constitute the most important ground command. sources of error in terms of alignment stability as well as variation of scale factor with temperature. Any distortion or motion of the sensor core within TRIAXIAL FLUXGATE SENSOR the feedback coil represents an effective alignment The fluxgate sensor used in the Magsat vector shift. Structural deformations larger than 50 JLm magnetometer was a unique design that took ad­ were sufficient to exceed the alignment stability vantage of the ring core geometry. This particular tolerance. geometry exhibits superior performance characteris­ The fundamental strategy followed for the Mag­ tics in terms of noise and zero-level stability. In ad- sat vector magnetometer was to match the expan- EOut [V) 4095 8\ / E 3071 4 / c ::J o ~ :; 2047 -5~~B~"/I/ B- II oil ~I ~I.,;, .... I 'I '1,' .... 1" II I [nTx1000) ::J o 'I r l '1': t: r:;: f: f: r:': f:,. OJ / :/ :/ :/ :1 :/ :/ :,' 1/ :,' :/' /:/ ~ 1023 ~ v v V v -4...i. v ~ v { I ( Fig. 2-lnstrument response for increasing (solid curve) and decreasing (dashed curve) / 1 ----~~B~-- ambient field. Note the large / -8 o safety margin in the switching o 57 58 59 60 61 62 63 64 65 66 67 68 125126 127 f--+---nl I II II III I 1 til I I threshold (half scale). o 1 2 58 59 60 61 62 63 64 65 66 67 68 69 126 127 f- - -t - -+ -- -u- - t - -+ - -t - -t - -j -- t - -+ -- t - -I- - -1- - + -- -il- - + - -1 Field offset generator step (coarse output) Volume 1, Number 3, 1980 211 quired of the analog signal processing circuits dic­ tated the use of ultraprecise components, some of o Feedback coil which had to be specifically developed for Magsat. Center support ~ The maximum allowable error voltage for the digi­ A CO" tally con trolled current sources was only 125 fJ- V so considerable attention had to be paid to the prob­ lem of thermally generated electromotive forces that are produced across dissimilar metal junctions. UCJ~ 0 Circuit layouts with a minimal number of solder joints, the proximity of essential conductors in order to minimize thermal gradients, and the use of Fig. 3-lndividual fluxgat~ m~gnetomet~r sens~r construction. All parts have Identical expansion coeffi­ cadmium-tin alloys for soldering and assembly re­ cients. duced the instrument errors to an absolute mini­ mum. The resistors used in the digital current sources as well as the magnetometer feedback cir­ sion coefficients of all the materials used in the cuit were ultraprecise, hermetically sealed units construction of the sensor assembly (core, feedback with an absolute temperature coefficient of less coil: and support structure) so that differential than 0.5 ppm/oC and a long term stability of 20 stresses induced by temperature variations were ppm/year. The calibration of the analog circuits re­ minimized. Since the sensor core expansion coeffi­ quired special test instrumentation with accuracies cient is approximately 10 ppmrC, platinum wire established by the National Bureau of Standards was used to wind the feedback coil, and the sup­ and resolution of about 1 ppm (a type of design port structures were machined from solid blocks of usually restricted to laboratory-grade instruments). Macor, a machinable glass-ceramic. The triaxial Extensive reliability and performance trade-off sensor assembly was mounted on a temperature-sta­ analyses were carried out to ensure mission success. bilized baseplate whose temperature was actively The digital processors were implemented with ± maintained at 25 1°C. The attitude transfer CMOS digital integrated circuits to reduce power system mirrors were mounted under this baseplate. consumption to a minimum. Only one processor Their expansion coefficient was nearly zero; hence, was powered at any given time by its respective active thermal control of this crucial interface was power converter; the same was true for the analog needed.
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