Thermopile

lal ARRANGEMENT OF THERMOPILES OF SYMPHONIE STATIC EARTH SENSOR

Fig. 6-33. Static Earth Sensors lal ARRANGEMENT OF THERMOPILES OF SYMPHONIE STATIC EARTH SENSOR

SENSOR b FOV

toi RADIOMETRIC BALANCE HORIZON SENSOR USED FOR ERTS/LANDSAT

Fig. 6-33. Static Earth Sensors

63 Magnetometers

Bruce T. Blaylock

Magnetometers are widely used as spacecraft attitude sensors for a variety of reasons: they are vector sensors, providing both the direction and magnitude of the magnetic field; they are reliable, lightweight, and have low power requirements;

they operate over a wide temperature range; and they have no moving parts. However, magnetometers are not accurate inertia] attitude sensors because the magnetic field is not completely known and the models used to predict the magnetic field direction and magnitude at the spacecraft's position are subject to relatively substantial errors, as discussed in Section S.l. Furthermore, because the Earth's magnetic field strength decreases with distance from the Earth as 1/r3, residual spacecraft magnetic biases eventually dominate the total magnetic field measurement, generally limiting the use of magnetometers to spacecraft below 1000 km; however, attitude magnetometers were flown successfully on RAE-1 at an altitude of 5875 km.

As illustrated in Fig. 6-34, magnetometers consist of two parts: a magnetic sensor and ah electronics unit that transforms the sensor measurement into a usable format. Magnetic field sensors are divided into two main categories: quantum magnetometers, which utilize fundamental atomic properties such as Zeeman splitting or nuclear magnetic resonance; and induction magnetometers, which are based

on Faraday's Law of Magnetic Inductance. Faraday's law is the observation that an electromotive force (EMF), E, is induced in a conducting coil placed in a time-varying magnetic flux, d$>a/d/, such that the line integral of E along the coil is

The two types of induction magnetometers are search-coil and fluxgate magnetometers. In a search-coil magnetometer, a solenoidal coil of N turns surrounds a ferromagnetic core with magnetic permeability /i, and cross-sectional area A. The EMF induced in the coil when placed in a magnetic field produces a voltage, V, given by

where B L is the field component along the solenoid axis. The output voltage is dearly time dependent and can be rewritten for a coil rotating at a fixed frequency, /=«/2w, about an inertially fixed axis normal to a constant field B0 as

Search-coil magnetometers based on the above principle are used mainly on spin-stabilized spacecraft to provide precise phase information. Because the search coil is sensitive only to variations in the component of the field along the solenoid axis, any spacecraft precession or nutation will greatly complicate the interpretation of data [Sonett, 1963].

The second type of magnetic induction device is the fluxgate magnetometer, illustrated in Fig. 6-35. The primary coil with leads and P2 is used to alternately drive the two saturable cores SC, and SC2 to states of opposite saturation. The presence of any ambient magnetic field may then be observed as the second harmonic of the current induced in the secondary coil with leads S, and S2. The purpose of the two saturable cores wound in opposite directions is to cause the secondary coil to be insensitive to the primary frequency. Other geometries used to achieve primary and secondary decoupling utilize helical and toroidal cores.

Search Coil Magnetometer Diy

Fig. 6-3S. Dual-Core Fluxgate Magnetometers With Primary and Secondary Induction Coils.

(Adapted From Geyger {19641)

Fig. 6-3S. Dual-Core Fluxgate Magnetometers With Primary and Secondary Induction Coils.

(Adapted From Geyger {19641)

The functional operation of a fluxgate magnetometer is illustrated in Fig. 6-36. If the voltage across the primary coil has a triangular waveform of frequency 2n/T and the amplitude of the resultant magnetic intensity is HD, then the core elements saturate at a flux density of ± Bs, when the magnetic intensity reaches ± Hc. The net magnetic intensity is displaced from zero by the ambient magnetic intensity, AH. The secondary coil will experience an induced EMF, Vs, while the core elements are being switched or the magnetic flux density is being gated from one saturated state to the other (hence the name fluxgate). Vs consists of a train of pulses of width KtT, separated by time intervals K2T or (1 - KJT where

The ambient magnetic intensity may then be derived from the pulse spacing in the fourth graph of Fig. 6-36 as

To model the response of the magnetometer electronics, Vs is expressed in a Fourier series as

ys = A Z, [' — exP( — i2wnK2) 1--««(2™^;) (6-8)

2BS KXT

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