Andy Stamp Department of Communication & Electronic Engineering UoP 2002 SATELLITE ECLIPSE Satellites need power, and solar panels are the most frequently found method of power generation. On board power is generated as long as the satellite has a clear view of the sun, but for about a quarter of each year the solar cells are in darkness for some portion of the day. During Eclipse the Earth obfuscates the satellites primary power source and the system has to rely on batteries. Eclipse due to Earth The orbital plane of Earth (the plane of the ecliptic) and that of our Sun are different. The Earth moves around the sun once a year, and as it does so its declination angle (i.e. the angle between the equatorial plane of Earth and the Sun itself) varies sinusoidally. At the peaks of the sinusoid, the declination angle (δ is 23.5o , and the rate of climb of the sun in the sky is almost imperceptible. These times are known as Solstice (meaning the sun stops). When the sun is at its highest in the sky we enjoy the summer solstice (21st June), whilst when the sun is at its lowest we have the winter solstice (21st December). At these times the satellite has a clear view of the sun whatever the satellites orbital position. However, at other times in the cycle, the sinusoid crosses the equator (and the plane of the ecliptic). At these times we enjoy days with equal amounts of day and night (equinox). When the sun crosses the equator on a downward journey we enjoy the autumnal equinox (21st September), When the sun crosses the equator on a upward journey we enjoy the vernal equinox (21st March). At equinox, the Earth is directly in line between the satellite and the sun for a time equal to the angle subtends by the satellite from the Earth (17.3o). So the satellite is in the dark at equinox for; times one sidereal day (i.e. 86164.2 seconds) Hence the eclipse at equinox lasts about 72 minutes Eclipse's start when the angle of the sun just clips the angle subtended by the satellite from Earth (i.e. 8.7o) and this corresponds to 21 days before or after equinox. In practice this becomes 23 days either side of equinox (twice a year too), and so the satellite is without power at some time for 94 days of the year. The duration of eclipse varies for a few seconds to 72 minutes. Around the time of eclipse (6 days) the sun also fills the earth stations antenna beam and causes an outage due to prohibitively high antenna temperatures for about 10 minutes. If the satellite longitude is East of the Earth station, then the satellite eclipse occurs in the Earth station's daylight hours (when you want to use the system most).

SATELLITE COMMUNICATIONS DEPARTMENT OF COMMUNICATION AND ELECTRONIC ENGINEERING

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a = 149,598,600 km Semi-major axis

b= 0.99986 IAU

VERNAL EQUINOX ST MARCH 21 d = 0.996 IAU WINTER 89.00 Days SPRING 92.81 Days

b

Perihelion January 2nd d = 0.983 IAU

1 IAU WINTER SOLSTICE ST DECEMBER 21

d = 0.984 IAU SUMMER SOLSTICE ST June 21 d = 1.016 IAU

a T = 365.256 Days Autumn 89.82 Days

e= 0.01673

Aphelion July 2nd d = 1.017 IAU

SUMMER 93.62 Days AUTUMNAL EQUINOX SEPTEMBER 21ST

The Orbit of Earth around the Sun, indicating the seasons and ecliptic tilt W.R.T. the Sun SUMMER SOLSTICE ST June 21 d = 1.016 IAU

AUTUMNAL EQUINOX ST SEPTEMBER 21 d = 0.996 IAU

VERNAL EQUINOX ST MARCH 21 d = 0.996 IAU

23½O 23½O

WINTER SOLSTICE ST DECEMBER 21 d = 0.984 IAU

SATELLITE COMMUNICATIONS DEPARTMENT OF COMMUNICATION AND ELECTRONIC ENGINEERING

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SUMMER SOLSTICE 23½O 21/6

+8.7O

VERNAL EQUINOX -8.7O

VERNAL EQUINOX

AUTUMNAL EQUINOX 21/9

21/3

21/12 -23½O 21 Days

WINTER SOLSTICE

Angle of the Sun relative to the ecliptic and angles/dates when eclipse occurs

70 MINUTES 60 MINUTES

30 MINUTES

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEPT

OCT

NOV

DEC

42 DAYS TWICE A YEAR Duration of Eclipse and time of Occurrence

SATELLITE COMMUNICATIONS DEPARTMENT OF COMMUNICATION AND ELECTRONIC ENGINEERING

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