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Date: 23 Oct 91 17:52:59 GMT
From: ssc-vax!bcsaic!hsvaic!  (Dani Eder)
Subject: Re: HST gyroscopes

roberts@CMR.NCSL.NIST.GOV (John Roberts) writes:

>>The reason is simple - the gyros are not used for position sensing, they
>>are used for pointing the spacecraft.  Spinning the gyro one way makes
>>the spacecraft rotate the other, since angular momentum is conserved.  An
>>optical gyro can only measure rotation, not cause it.
>>   -Lou Scheffer

To expand upon this, presumably they were intelligent enough to install a
'number of revolutions' sensor on the gyroscopes.  Then by knowing the ratio
of the moment of inertia of the gyro to the moment of inertia of the
entire telescope, they could get a fairly accurate estimate of how far
the telescope has turned by counting gyroscope revolutions.

For accurate pointing, there are three "fine guidance sensors" which use
the perimeter of the telescope field of view projected onto charge-coupled
devices to sense the positions of stars.  Coupled with the very large
star catalog that was compiled by the Space Telescope Science Institute,
they can determine which way they are pointing.  So the telescope itself
serves as it's own best position sensor.

>Magellan also has thrusters that it uses periodically to "desaturate"
>its reaction wheels. HST uses some electromagnetic device that I've never
>seen an explanation of, which interacts with Earth's magnetic field,
>for the same purpose.

They are called magnetic torque rods, and all they are is rods with
a winding that forms a long thin electromagnet when the winding has
current run through it.  The magnet then does what any magnet does,
it tries to align it's poles with the opposite poles of the Earth's
magnetic field, producing torque on the spacecraft.

There are 4 rods on the sides of the HST, and they are arranged
pointing in different directions so you can get whatever torque
direction you want by some suitable combination of them.

Dani Eder/Boeing/Advanced Civil Space/(205)464-2697(w)/232-7467(h)/
Rt.1, Box 188-2, Athens AL 35611/Member: Space Studies Institute
Physical Location: 34deg 37' N 86deg 43' W +100m alt.

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