Satgen294  Satellite Problem No7   Eclipse    by GM4IHJ       11th Nov 94
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In the distant past, eclipses of the Sun by the Moon have been blamed for
losing or winning battles,and , the overthrow of kings. More recently the
advent of artificial earth satellites, has gradually brought the practical
problems caused by eclipses , to the notice of most satellite users.
It is possible to put a satellite into low earth orbit so that it never
goes out of sunlight and into earth shadow. Careful choice of orbital
inclination and altitude can produce an orbit whose plane is perturbed
roughly 1 degree each day , so that the satellite orbit plane faces the
Sun for the whole year, as the earth does its annual circuit around the
Sun. But eventually atmospheric drag will reduce the satellites altitude
and after say 4 years of continuous sunshine, it suddenly starts to slip
deeper and deeper into earth shadow, each orbit. In the past this was
usually catastrophic, because the satellite battery was in poor shape
after 4 years of zero usage, and it did not like the sudden imposition of
heavy eclipse loading.
In many cases however it has never been practical to have the satellite in
the high inclination ( greater than 90 degree) orbit required for constant
solar illumination. So satellites were put into orbits where they met
eclipses in earth shadow right from the start directly after launch. These
eclipses were no problem provided they were relatively short duration.
Indeed the constant charge/discharge cycling kept the battery in good
shape for years. If however the eclipses got longer than say 15 or 20
minutes per orbit as they inevitably did at some times of year, it was
necessary for the Controllers to be ready to switch the satellite to
standby for a few hours, if heavy usage made it necessary to have a full
recharge before further usage. In consequence of this it has become common
practise to advise users of likely eclipse periods for low earth orbit
satellites, and take the satellites off the air temporarily if the battery
volts get too low.
The eclipse situation for the Phase 3 elliptical orbit high altitude
satellites can be much more serious. Eclipse sequences do not occur all
the time . Phase 3 sats can go for months with no eclipse problems . But,
when as is presently the case with Oscar 13, the satellite is experiencing
a month or two in which long eclipses are occuring daily, the Controllers
need to be able to close the satellite down at short notice if excessive
battery drain is occuring.
There is of course another side to the eclipse ( or non eclipse ) story.
In certain circumstances, a satellite can get too much sunshine. In the
case of low earth orbit satellites their spin and rapid change of aspect
with respect to the Sun, usually prevents serious overheating . But in the
case of the Phase 3 satellites which spin rapidly to maintain a fixed
aspect with respect to the Sun, in order to maximise solar power
production, excess solar heating of one face of the satellite could be a
severe problem. Phase 3 satellites avoid this situation because they are
built with internal heat exchange coils inside their structure. So that
excess local heating is rapidly spread all around the satellite by passive
tranfer through the coils. Thereby evening out the temperature highs and
lows and avoiding serious damage to delicate components. 73 de GM4IHJ @
GB7SAN