Satellite Technology by Martin Halliwell

Satellite technology
Overview
• What is a satellite?
• The key elements of orbital position
• Satellite manufacturers and design
• The components of a satellite: payload and bus
• Digital versus analogue
• How do we control a satellite?
• The life of a satellite
• The way to geostationary orbit
• Your questions
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What is a satellite?
“Natural or artificial object that revolves around
a larger astronomical object, usually a planet.
–The Moon is the most obvious example.”
Space Station
Moon
Hubble
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What are artificial satellites used for?
• Telecommunications
• Weather forecasting
• Scientific missions
• Earth Observation
• Military purposes
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How do satellites “fly”?
The higher the release
speed the further the
ball will fly …
… with sufficient speed,
the ball will continue to
orbit around the Earth,
with no need for additional
propellant!
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What are the possible orbits?
• A satellite always moves in a fixed
plane, the “orbital plane”.
• The orbital plane always passes
through the center of the Earth
The speed and the orbital period of a satellite
vary according to their distance from the Earth’s
surface:
Space Station: altitude ~350 km
27700 km/hour, 1 period is 92 minutes
ASTRA spacecraft: altitude ~36000 km
11070 km/hour, 1 period is 24 hours
The Moon: altitude ~380000 km
3600 km/hour, 1 period is ~28 days
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Why is geostationary orbit so attractive?
The geostationary orbital period of the satellite is equal to
the Earth’s rotation period, so your TV dish at home can be fixed!
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How are the orbital positions defined?
We
st l
on
g
Greenwich
ASTRA at orbital position 19.2° East
itud
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1 9. 2 °E
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as
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Access to orbital positions and frequencies
• Frequency allocations are agreed at the ITU
– a United Nations agency
– periodic meetings of the World Radio Council agree
amendments to the allocations
• Frequencies are classified in a number of ways
– “Planned”
• FSS
• BSS
– “Unplanned”
• FSS
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Frequency allocation
• Planned
– Frequencies allocated to given countries
– At specific orbital positions
– co-ordinated
• Unplanned
– Available on “first-come, first-served” basis
– must be co-ordinated
• Application for use must be made through state agency
(e.g. FCC, UKRA, Luxembourg Govt.)
– Position must be brought into use within a specified time
period
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Security of access
• After being brought into use, an operator has a
high degree of security
– automatic renewal after first 15 year period, under
ITU rules
• Interruption of existing business is considered
unlikely
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Cost of access
• Cost of access varies according to jurisdiction
– annual management fee (US, Gibraltar)
– franchise fee (Luxembourg)
• related to profitability
• Typically relatively modest
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How to maintain the orbital position?
Periodic stationkeeping manoeuvres are needed …
… because natural forces are continuously perturbing
the satellite position
The Sun and Moon affect the orbit
The earth’s equator is not a perfect circle!
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What is Satellite Co-location?
150 Km
Co-location allows ASTRA to maintain
several satellites in the same “box”
0.1°
19.2°
0.1°
150 Km
It is interesting because …
Î
only one dish is needed to receive TV channels
from all satellites
Î
backup capacity is directly available
Î
it allows an efficient utilization of the
geostationary ring
The ASTRA satellites at 19.2°East
seen from Betzdorf
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How to perform station-keeping manœuvres?
The Propulsion
Subsystem
Î
Thrusters “push” the satellite according to
commands sent from the control center
Î
Enough propellant for ~15 years
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Who manufactures the SES satellites?
It’s not SES!
SES closely follows the design and manufacturing!
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What does a communication satellite look
like? (1/2)
2 basic designs
3-axis satellite
Spinner satellite
Î
Simple
Î
More channels
Î
Light
Î
More power
Î
Inexpensive
Î
Longer lifetime
Î
Quick to build
Î
More coverage
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26 m
7m
What does a communication satellite look
like? (2/2)
2.8 m
2.2 m
750 kg (in orbit)
2300 kg (in orbit)
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Payload and Bus
The Payload …
Î
is the reason why the satellite is there, and how
we make money!
Î
includes the components to achieve the core
satellite objective
The Bus …
Î
includes all the functions needed to support
the payload:
1.
Attitude control
2.
Orbit control
3.
Power management
4.
Thermal control
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Payload (1/2)
The Payload …
Î
performs reception, amplification and
retransmission of a signal from the
ground
TWTA
TWTA
RECEIVER
TWTA
TWTA
D/C
TWTA
TWTA
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Payload (2/2)
Î
The Amplifiers have a high degree of
flexibility and redundancy
Î
The shape and pointing of the Antenna
define the “footprint”
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Digital vs Analogue
The Payload can carry analogue or digital transmission …
Analogue to Digital
Conversion
Digital transmission offers more capacity per transponder by means of compression
01110011011
10010101010
11100110101
10001010...
10010101010
01110011011
11100110101
10001010...
110
10...
100
00...
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Keeping the right attitude …
The Attitude Control
Î
Ensures continuous spacecraft pointing accuracy
Î
Is very sensitive:
1 degree depointing = 600 km of footprint change!
Î
Is autonomous
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Providing the electricity …
Electrical Power is …
Î
needed to amplify and transmit the signals down
to Earth
Î
provided by “wings” continuously tracking the sun
and covered with solar cells …
Î
… or by batteries during the eclipses
(March/September)
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Controlling the temperatures …
The Thermal Subsystem
Î
keeps all the equipment within qualification
limits
Î
fights the huge temperature gradient
Î
is made of active (heaters) and passive
(blankets/radiators) devices
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The life of a satellite
Definition
Definitionof
of
requirements
requirements
Evaluation
Evaluation
Negotiation
Negotiation
End of life
~3 months
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Commercial
Commerciallife
lifeof
ofsatellite
satellite
10
Years
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Manufacturing (~2.5 years)
LEOP & IOT (~2months)
S/C
S/CHand-over
Hand-overto
toSES
SES
OSD
OSD
Transfer Orbits
6-8 days
D
1 day
Bus IOT
5-7 days
S/C controlled from manufacturer
S/C controlled from manufacturer
Payload IOT
Drift to o.p.
15 days
20-30 days
S/C controlled from SES
S/C controlled from SES
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The way to geostationary orbit (1/2)
The launcher …
Î must
be powerful …
Î because
the heavier
the satellite can be…
Î more
payload and fuel
can be loaded!
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The way to geostationary orbit (2/2)
The launcher will just do a part of the job …
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What happens after a nominal “end of life”?
Î
After ~15 years the satellite approaches end of propellant
Î
Station-keeping control is not possible anymore
Î
The remaining fuel is used to push the satellite into a
“graveyard” orbit, before complete switch off
Î
Graveyard orbit is ~ 300 km ABOVE geostationary orbit
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Thank you for your attention!
Do you have any questions?
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