4. 1. Definition:
Satellite is a physical object that orbits or revolves around
some celestial body.
In general Satellite is an artificial satellite stationed in
space for the purposes of telecommunications, military,
surveillance, etc
5. 2. History:
The first artificial satellite was the Soviet Sputnik-1, launched on
October 4, 1957, and equipped with an on-board transmitter that
worked on two frequencies, 20.005 and 40.002 MHz .
The first American satellite to relay communications was Project
SCORE in 1958, which used a tape recorder to store and
forward voice messages.
Telstar was the first active, direct relay communications satellite.
Belonging to AT &T.
7. Here we deal with the following concepts:
1.
2.
3.
4.
5.
6.
Orbit Fundamentals
Geosynchronous Satellites
Station Keeping
Attitude Control
Satellite Position
Satellite Launching
8. 1. Orbit Fundamentals:
Satellite keeps moving around the Earth in some orbital
pattern .
Orbit Fundamentals is based on
a. Orbit Shape
b. Direction of satellite’s revolution
c. Satellite Speed and Period
d. Satellite Angles
e. Satellite Repeaters
9. a. Orbit Shape
Satellite keeps moving around the Earth in some orbital
pattern called “Orbit Shape”.
Orbit Shape can be either
a. Circular Orbit
b. Elliptical Orbit
10. b. Direction of satellite’s revolution
1. Posigrade Orbit
i.e. satellites revolution=direction of Earth’s rotation
2. Elliptical Orbit
i.e. satellites revolution=against the direction of
Earth’s rotation
11. c. Satellite Speed and Period
The speed of the satellite is measured in miles per hour,
kilometer per hour , or knots.
Speed varies depending upon the distance of the satellite from
Earth.
Two types of Periods ----- 1. Sideral Period 2. Synodic Period
12. d. Satellite Angles
1. Angle of Inclination
Is the angle formed between the
equatorial plane and the satellite’s
orbital plane as the satellite enters the
northern hemisphere.
2. Angle of Elevation
Is the angle that appears between the
line from the Earth station’s antenna
to the satellite and the line between
the Earth station’s antenna and the
Earth’s horizon.
3. Polar Orbit
4. Equatorial Orbit
13. e. Satellite Repeaters
To use a satellite for communications relay or repeater
purposes ground station antenna must track or follow the
satellite as it passes overhead.
Height and speed only determines how long the satellite
can stay connected with the ground station.
Some time the satellite may disappear around the other
side of the Earth.
To solve this its be launched in a very long elliptical orbit.
14. 2. Geosynchronous Satellites:
A geostationary satellite revolves around the earth at a
constant speed once per day over the equator.
It appears to be in a fixed position to an earth-based
observer.
Usually geosynchronous satellites are placed at a distance
of 22,300 miles or 35,860 km above the Equator.
The satellite at that distance revolves around the Earth in
exact 24 hours.
Speed of the satellite=7000 miles/hour
15. Advantages of Geosynchronous Satellites:
Since the satellite remains apparently fixed, no special
earth station tracking antennas are needed
The antenna can simply be pointed at the satellite and
remain fixed.
Continuous communications are possible.
Most communication satellites used today are
geosynchronous satellites.
16. Disadvantages of Geosynchronous Satellites:
During an eclipse the Earth or moon gets between the
satellite and the Sun, is causes the sunlight to be blocked
from the solar panel.
So an eclipse shuts off all power to the satellite.
To avoid this backup batteries are used.
17. 3. Station Keeping:
Even with a very good launch the satellite can drift somewhat from its orbit. This is called “Orbital Drift”.
It is caused by a variety of forces like sun’s, moon’s
gravitational pull, etc.
The process of firing the rockets under ground control to
maintain or adjust the orbit is referred to as
“Station Keeping”
18. 4. Altitude Control:
Satellites have to be placed in some altitude for optimal
performances. This is called as Altitude Control.
Stabilizing the satellite is also called as Altitude Control.
Two types stabilization are there:
Spin Stabilization
Three axis Stabilization
Most common is the Spin Stabilization, where the satellite
spins around using the thrusters attached to it on its
primary axis.
19. 5. Satellite Positioning:
In order to use a satellite, it has to be positioned in space
properly, usually it a predetermined by design of the
satellite and is achieved during launch.
Once the position is known, the earth station antennas
have to pointed at the satellite for optimal transmission
and reception.
The location of a satellite is generally specified in terms of
latitudes and longitudes.
20. 6. Satellite Launching:
Satellites are placed into their orbits by mounting them on top of
rockets which literally shoot them into space.
Occasionally, the rocket will contain more than one satellite. Here the
main satellite is called as “Initial Payload” and others as “Secondary
payload”.
The satellite is first put into what is called a “transfer orbit”, a highly
elliptical orbit that permits adjustments to the satellite to be made
prior to its being placed into final position.
23. Satellites
Natural
Natural
Satellites
Satellites
E.g.: Moon
E.g.: Moon
Natural
Natural
Satellites
Satellites
Based on
Orbiting
the Earth
Geostationary
Geostationary
Satellites
Satellites
Medium Earth
Medium Earth
Orbiting Satellites
Orbiting Satellites
Low Earth
Low Earth
Orbiting Satellites
Orbiting Satellites
Highly Elliptical
Highly Elliptical
Orbiting Satellites
Orbiting Satellites
Polar
Polar
Satellites
Satellites
Based on
Application
Remote Sensing
Remote Sensing
Satellites
Satellites
Meteorological
Meteorological
Satellites
Satellites
Communication
Communication
Satellites
Satellites
Navigation
Navigation
Satellites
Satellites
Scientific and
Scientific and
Military Satellites
Military Satellites
24. A. Natural Satellites:
A natural
satellite or moon is
a celestial
body that orbits a planet or smaller body, which is called
the primary. Technically, the term natural satellite could refer
to a planet orbiting a star, or a dwarf galaxy orbiting a major
galaxy.
E.g.: Moon
Fig : Jupiter's Moon
25. B. Based on Orbiting the Earth:
There are five types.
1. Geostationary Satellites
- Satellites are placed above the equator at a distance of about
36000 km.
- Almost today all satellite orbiting the Earth are of this type
2. Medium Earth Orbiting Satellites
- Operate at a distance of about 5,000-12,000 km.
- Up to now there has not been many satellites in this class.
26. 3. Low Earth Orbiting Satellites
-
Are placed at an altitude of 5,00-1,500 km.
Typical duration of them are 95-120 minutes.
They try to ensure a high elevation for every spot on Earth to provide
high quality communication link.
Uses advanced compression schemes, transfer rate of 2,400 bits/sec
can be enough for voice communication.
4. Highly Elliptical Orbiting Satellites
-
Comprises of all satellites with a relatively low-altitude perigee and
an extremely high-altitude apogee.
It has the advantage of long dwell times at a point in the sky during
the approach to and descent from apogee.
E.g.: US’s Sirius Satellite
27. 5. Polar Satellites
-
These satellites orbit from Northern Hemisphere to Southern
hemisphere. E.g.: PSLV, Polar Wind(USA)
They follow highly elliptical orbit, inclined about 86 o with an orbital
period of 18 hours
It gathers multi-wavelength imaging of the aurora, and measures the
entry of plasma into the polar magnetosphere, etc..
28. C. Based on Applications :
There are five types.
1. Remote Sensing Satellites
- Are a series of Earth Observation satellites, which observes
weather, landscapes, atmosphere, oceanic surface, climate changes,
urban planning, etc..
- Two types of remote sensing --- 1. Active 2. Passive
29. 2. Meteorological Satellites
- a type of satellite that is primarily used to monitor the weather and
climate of the Earth.
- Satellites can be either polar orbiting, or geostationary, etc..
- It sees clouds and cloud systems, City lights, fires, effects of
pollution, auroras, sand and dust storms, snow cover, ice mapping,
boundaries of ocean currents, energy flows, etc., are other types of
environmental information collected using weather satellites.
30.
31. 3. Communication Satellites
- They aid telecommunications, as by reflecting or
relaying a radio.
-
have been a significant part of domestic and global
communications since the 1970s.
-
Uses --- Telephony, Satellite TVs, Satellite Internet,
Satellite Radio, Aircraft communications, etc..
32. 4. Navigation Satellites
-
Global Navigation Satellite Systems (GNSS) is the standard
generic term for satellite navigation systems that provide
autonomous geo-spatial positioning with global coverage.
-
allows small electronic receivers to determine their location
(longitude, latitude,
and altitude)
to
within
a
few meters using time signals transmitted along a line-of-sight by
radio from satellites.
-
Receivers on the ground with a fixed position can also be used to
calculate the precise time as a reference for scientific experiments.
-
As of 2009, the United States NAVSTAR Global Positioning
System (GPS).) is the only fully operational GNSS.
34. 5. Military and Scientific Satellites
-
A military satellite is an artificial satellite used for a military
purpose, often for gathering intelligence, as a communications
satellite used for military purposes, or as a military weapon.
-
Many cryptographic algorithms are used to encode the signals, use
special frequency ranges, advanced transmitting and receiving
equipments .
-
Scientific satellites gather data for scientific analysis. This
includes observations of the atmosphere of our planet, the stars,
the sun and other parts of space.
37. Communication Satellites are originators of
information.
They instead relay stations for other sources.
Here we deal with the following concepts:
1.
2.
3.
4.
Transponders
Satellite Frequency Allocations
Satellite Bandwidth
Increasing Channel Capacity
38. 1. Transponders
-
Satellite contains a receiver which picks up the transmitted
signal, amplifies it, and translates it into another frequency.
The transmitter-receiver combination in the satellite is known as
“Transponder”.
Uplink – Upto 6GHz
Downlink – Upto 4GHz
Typical transponder has a wide bandwidth. But use only a single
signal to minimize interference and to improve communication
reliability.
39. 2. Satellite Frequency Allocations
-
Most satellites operate in microwave frequency spectrum.
-
It is divided up into frequency bands which have been allocated
into satellite as well as other communications services such as
radar.
-
The most widely used satellite communications band is the C
band.
40. Fig: Frequency bands used in satellite communications
FREQUENCY
BAND
225-390 MHz
P
350-530 MHz
J
1530-2700 MHz
L
2500-2700 MHz
S
3400-6425 MHz
C
7250-8400 MHz
X
10.95-14.5 GHz
Ku
17.7-21.2 GHz
Kc
27.5-31 GHz
K
36-46 GHz
Q
46-56 GHz
V
56-100 GHz
W
41. 3. Increasing Channel Capacity
-
Although the transponders are quite capable, they nevertheless
rapidly become overloaded with traffic.
For these reasons, numerous techniques have been developed to
effectively increase the band-width and signal carrying
capacity of the satellite.
Two of these techniques are:
1. Frequency Reuse
2. Spatial Isolation
43. Regulators,
Regulators,
protection and
protection and
conditioning
conditioning
Charger and
Charger and
Batteries
Batteries
Solar Panel
Solar Panel
DC to all subsystem
Communication
Subsystem
Receiver
Frequency
Translator
Transmitter
DC/DC
DC/DC
Converters,
Converters,
DC/AC Inverters
DC/AC Inverters
Power
Sub
System
DC and AC to
special subsystem
Altitude
Control
Subsystem
Transponder
Other Transponders
I/Ps
Antenna
Antenna
from onSubsystem
Subsystem
board
sensors
Communications
Antennas
Telemetry,
Tracking, and
Control
Subsystem
Ctrl Sgls
to all
Telemetry Antenna subsystems
Propulsion
Subsystem
AKM
Jet Thrusters
44. Generally satellites have many subsystems which join
together for the fully operation of the satellite.
The various subsystems in a general communication satellites
are:
1. Power Subsystem
2. Communication Subsystem
3. Antenna Subsystem
4. Telemetry, Tracking, and Control Subsystem
5. Propulsion Subsystem
6. Altitude Control Subsystem
46. LNA
Demodulator
Demodulator
BPF
BPF
BPF
BPF
LO
LO
Base Band O/P
Down Converter
Receive Subsystem
Diplexer
Carrier
Oscillator
Antenna Subsystem
UP Converter
Driver
Demodulator
Demodulator
BPF
HPA
LO
GCE-Transmit
Transmit Subsystem
Fig: General Block Diagram of an Earth Station
Power
Subsystem
Base Band I/P
GCE-Receive
47. The earth station on the ground is the terrestrial base of the
system.
The earth station communicates with the satellite to carry
out designated mission.
It may be located at the end user’s facilities or may be
located with ground-based intercommunication links
between the earth station and the end user.
Many earth stations are now located on top of tall buildings
or in other urban areas directly where the end user resides.
48. The various subsystems in an earth station are:
1. Antenna Subsystem
2. Receive Subsystem
3. Transmit Subsystem
4. Ground Communication Equipment (GCE) Subsystem
1.
2.
GCE Transmit Subsystem
GCE Receive Subsystem
5. Power Subsystem
50. Satellite Routing
bi
le
U
(M se
UL r L
in
)
k
Mo
GWL
nk
Li
er
Us L)
i le U
ob (M
Gateway Link
(GWL)
M
Inter Satellite Link
(ISL)
Spot Beam
Foo
tP
Base Station
Or
Gateway
ri n
t
ISDN
PSTN
User Data
GSM
52. There are four types of satellite handovers. They are:
1. Intra-satellite Handover
2. Inter-satellite handover
3. Gateway Handover
4. Inter-system Handover
