Satgen 186  Satellite Stabilisation Part 1  by GM4IHJ        17th Oct 92
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You can only hear a satellite signal strongly , if you are in its antenna
beam . So satellites have two problems . Firstly there is no such thing as
an omni directional antenna - most so called antennas cover at best only
one plane Eg horizontal and have a very varied pattern in the vertical
plane. Secondly a satellite left to itself tumbles and wobbles about the
sky rarely pointing at the same place twice. So we need to stabilize or
fix the orientation of the satellite , either relative to the Earth for
sats in low orbit , or relative to the stars for sats in high orbits.
The simplest satellites use passive magnetic stabilisation, whereby one or
more permanent magnets inside the sat, interact with the Earth's magnetic
field turning the sat so that its internal magnetic poles align towards
the earths magnetic pole. By this means Pacsat with 4 stub antennas one on
each side of the -Z face, points that face at the earth when it is near
the north magnetic pole, and gives somewhat less good but useful alignment
when it is above the temperate zone of the Earths northern hemisphere.
But please note that this system means the -Z face and its antennas point
away from the earth when the sat is over the southern hemisphere.  So when
Amsat Argentina built Lusat they put the sat magnets the other way round.
Magnets do not however give total protection from satellite wobble. One
way to get a stable wobble free satellite is to spin it like a Top or
gyroscope. So in addition to their magnets Pacsat, Webersat , Lusat and
Dove are spun.The mechanism producing the spin is purely passive. The 4
separate antenna blades are painted black on one side and white on the
other. When the satellite is in sunlight Solar photons hit these antenna
blades, with at any one time the sun getting a clear view of the white
side of one blade and the black side of the one on the opposite side of
the -Z face. When a solar photon hits a black face it is absorbed , but
when it hits a white face it is reflected. The reflection produces more
thrust than the absorption, so the satellite slowly spins up . The spin is
not fast enough to fix satellite attitude in space but it does damp out
much of the tendency to tumble.
Uosats ( Uo14, Uo22, and Kitsat ) use a more sophisticated (and expensive
) stabilisation system called gravity gradient stabilisation. This is
achieved when several days after launch the Surrey sat is commanded to
unreel a weight on the end of a long cable. This has the effect of using
the earth's gravitationary field to keep the line formed by the weight and
the satellite  , pointing at the earth's centre. So Uosats are stabilized
passively to give good antenna pointing where ever they are around the
earth's north or south hemispheres.
With the bigger satellites to be described in the next issue of satgen,
the satellites have the capability to be spun round at quite high speed,
this brings some advantages and some ,perhaps ,  unexpected problems.

Duplicate bulletins.  My apologies to the many Amsat readers alarmed by
KD2BD's 5 week old duplication and recirculation of satgen 180. He claims
to send "his" bulletin everywhere, but this is no excuse for wasteful
duplication which greatly annoys other packet users. 73  GM4IHJ @ GB7SAN