Satellite

1. Satellite Communications

2. Overview
 Basics of Satellites
 Types of Satellites
 Capacity Allocation

3. Basics: How do Satellites Work
 Two Stations on Earth want to communicate through
radio broadcast but are too far away to use
conventional means.
 The two stations can use a satellite as a relay station
for their communication
 One Earth Station sends a transmission to the
satellite. This is called a Uplink.
 The satellite Transponder converts the signal and
sends it down to the second earth station. This is
called a Downlink.

4. Basics: Advantages of Satellites
 The advantages of satellite communication
over terrestrial communication are:
 The coverage area of a satellite greatly exceeds
that of a terrestrial system.
 Transmission cost of a satellite is independent of
the distance from the center of the coverage area.
 Satellite to Satellite communication is very
precise.
 Higher Bandwidths are available for use.

5. Basics: Disadvantages of
Satellites
 The disadvantages of satellite
communication:
 Launching satellites into orbit is costly.
 Satellite bandwidth is gradually becoming used
up.
 There is a larger propagation delay in satellite
communication than in terrestrial communication.

6. Basics: Factors in satellite
communication
 Elevation Angle: The angle of the horizontal of the
earth surface to the center line of the satellite
transmission beam.
 This effects the satellites coverage area. Ideally, you want
a elevation angle of 0 degrees, so the transmission beam
reaches the horizon visible to the satellite in all directions.
 However, because of environmental factors like objects
blocking the transmission, atmospheric attenuation, and the
earth electrical background noise, there is a minimum
elevation angle of earth stations.

7. Basics: Factors in satellite
communication (cont.)
 Coverage Angle: A measure of the portion of
the earth surface visible to a satellite taking
the minimum elevation angle into account.
 R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2)
= cos(β + θ)/cos(θ)
R = 6370 km (earth’s radius)
h = satellite orbit height
β = coverage angle
θ = minimum elevation angle

8. Basics: Factors in satellite
communication (cont.)
 Other impairments to satellite communication:
 The distance between an earth station and a satellite (free
space loss).
 Satellite Footprint: The satellite transmission’s strength is
strongest in the center of the transmission, and decreases
farther from the center as free space loss increases.
 Atmospheric Attenuation caused by air and water can
impair the transmission. It is particularly bad during rain
and fog.

9. Types of Satellites
 Satellite Orbits
 GEO
 LEO
 MEO
 Frequency Bands

10. Geostationary Earth Orbit (GEO)
 These satellites are in orbit 35,863 km above
the earth’s surface along the equator.
 Objects in Geostationary orbit revolve around
the earth at the same speed as the earth
rotates. This means GEO satellites remain in
the same position relative to the surface of
earth.

11. GEO (cont.)
 Advantages
 A GEO satellite’s distance from earth gives it a
large coverage area, almost a fourth of the earth’s
surface.
 GEO satellites have a 24 hour view of a particular
area.
 These factors make it ideal for satellite broadcast
and other multipoint applications.

12. GEO (cont.)
 Disadvantages
 A GEO satellite’s distance also cause it to have
both a comparatively weak signal and a time
delay in the signal, which is bad for point to point
communication.
 GEO satellites, centered above the equator, have
difficulty broadcasting signals to near polar
regions

13. Low Earth Orbit (LEO)
 LEO satellites are much closer to the earth
than GEO satellites, ranging from 500 to
1,500 km above the surface.
 LEO satellites don’t stay in fixed position
relative to the surface, and are only visible
for 15 to 20 minutes each pass.
 A network of LEO satellites is necessary for
LEO satellites to be useful

14. LEO (cont.)
 Advantages
 A LEO satellite’s proximity to earth compared to a
GEO satellite gives it a better signal strength and
less of a time delay, which makes it better for
point to point communication.
 A LEO satellite’s smaller area of coverage is less
of a waste of bandwidth.

15. LEO (cont.)
 Disadvantages
 A network of LEO satellites is needed, which can
be costly
 LEO satellites have to compensate for Doppler
shifts cause by their relative movement.
 Atmospheric drag effects LEO satellites, causing
gradual orbital deterioration.

16. Medium Earth Orbit (MEO)
 A MEO satellite is in orbit somewhere between 8,000
km and 18,000 km above the earth’s surface.
 MEO satellites are similar to LEO satellites in
functionality.
 MEO satellites are visible for much longer periods of
time than LEO satellites, usually between 2 to 8
hours.
 MEO satellites have a larger coverage area than
LEO satellites.

17. MEO (cont.)
 Advantage
 A MEO satellite’s longer duration of visibility and
wider footprint means fewer satellites are needed
in a MEO network than a LEO network.
 Disadvantage
 A MEO satellite’s distance gives it a longer time
delay and weaker signal than a LEO satellite,
though not as bad as a GEO satellite.

18. Capacity Allocation
 FDMA
 FAMA-FDMA
 DAMA-FDMA
 TDMA
 Advantages over FDMA

19. FDMA
 Satellite frequency is already broken into
bands, and is broken in to smaller channels
in Frequency Division Multiple Access
(FDMA).
 Overall bandwidth within a frequency band is
increased due to frequency reuse (a
frequency is used by two carriers with
orthogonal polarization).

20. FDMA (cont.)
 The number of sub-channels is limited by
three factors:
 Thermal noise (too weak a signal will be effected
by background noise).
 Intermodulation noise (too strong a signal will
cause noise).
 Crosstalk (cause by excessive frequency
reusing).

21. FDMA (cont.)
 FDMA can be performed in two ways:
 Fixed-assignment multiple access (FAMA):
The sub-channel assignments are of a fixed
allotment. Ideal for broadcast satellite
communication.
 Demand-assignment multiple access (DAMA):
The sub-channel allotment changes based on
demand. Ideal for point to point communication.

22. TDMA
 TDMA (Time Division Multiple Access)
breaks a transmission into multiple time slots,
each one dedicated to a different transmitter.
 TDMA is increasingly becoming more
widespread in satellite communication.
 TDMA uses the same techniques (FAMA and
DAMA) as FDMA does.

23. TDMA (cont.)
 Advantages of TDMA over FDMA.
 Digital equipment used in time division
multiplexing is increasingly becoming cheaper.
 There are advantages in digital transmission
techniques. Ex: error correction.
 Lack of intermodulation noise means increased
efficiency.

24. AZIMUTH
ANGLE


25. PERIGEE
 In an elliptical orbit the distance of satellite
from the primary body varies , hence a point
in orbit when satellite nearest to the earth
(or) primary body is called “PERIGEE”
 Perigee depends upon eccentricity
= a (1-e)

26. APOGEE
 It is a point in orbit when the satellite is
farthest from the earth
 Its height is a(1+e)

27. AZIMUTH ANGLE(contd)


28. LOCATION OF A SATELLITE IN
ORBITAL PLANE


29. 

30. ORBITAL PERTURBATIONS


31. ORBITAL ELEMENTS


32. 

33. Right ascension of ascending node:
It’s the angle measured eastward in
the equatorial plane from the first point of Aries
line to the ascending node. The first point of
Aries is a line from the centre of the earth
through the centre of the satellite at equinox
and the ascending node is the point where the
or bit crosses the equatorial plane going from
south to north.

34. 

35. KEPLER’s THREE LAWS OF
PLANETRY MOTION
LAW 1:
The orbit of any smaller body about a larger
body is always an ellipse ,with the center of
mass of the larger body as one of the foci.
LAW 2:
The orbit of the smaller body sweeps out equal
areas in equal time
dA=dT

36. 

37. SUN TRANSIT OUTAGE
 When the sun passes through the beam of
an earth station antenna. The receiver noise
level rises significantly and interferes with the
normal operation .The sun transit time may
be very small but it is very costly to users as
it occurs during day time .

38. SPACECRAFT SUBSYSTEMS:
1. Altitude and orbital control
subsystems(AOCS)
a. Altitude (stabilizing space craft)
b. Orbital (orbit or path taken by the
satellite)
2. Telemetry , Tracking & Telecommand
a. Tracking – velocity,acceleration
&range measurements

39. b. Telecommand-
i) launch phase
ii)Solar panels
iii)AOCS
iv)communication system
3.Power subsystem:
i)primary power(solar cells and
solar panels)
ii)secondary power(Batteries
ni,cd,Agcd)

40. 4.Propulsion:
Based on FUEL
a) Chemical Thrusters
b) Electrical Thrusters
5.Thermal Control Subsystem:
a) Active Method
b) Passive Method
6.Structure

41. Earth station requirements:
 High gain in the direction of wanted signal
 Low gain in the direction of unwanted signal
 Low effective noise temperature
 High antenna efficiency
 Continuous satellite pointing
 Minimum performance variation caused by local wind
& weather
 Minimum variation in the illumination of the satellite
by Earth station
 High disconnection between orthogonally polarised
signal

42. SATELLITE LINK DESIGN:


43. 

44. NOISE power:


45. SYSTEM NOISE TEMPERATURE:


46. PROPAGATION EFFECTS IN
SATELLITE COMMUNICATION
 Atmospheric absorption
 Cloud attenuation
 Attenuation due to troposphere
 Faraday rotation
 Rain & snow attenuation
 Ionospheric scintillation
 interference

47. INTERFERENCE EFFECTS:


48. EQUIPMENT RELIABILITY:


49. SPADE:
 Single channel per carrier/ pulse code modulated
/multiple access/ Demand assignment/equipment.
eg: FDMA
- Demand assigned
- SCPC
- PCM
FDMA MCPC(mux)
SCPC( no mux)
FDMA Pre assigned
Demand assigned

50. TDMA
 ADVANTAGES:
- Digital scheme
- Resistant noise
- No requirement for a linear transponder
- No intermodulation products
- Can handle mixed traffic( voice, video ,data)

51. TDMA
DISADVANTAGES
 Chances for intersymbol interference
 Synchronization of transmission & receiving
earth stations
 Higher transmission power required to
achieve high bit rates to fully utilize the
transponder’s capacity

52. TDMA FRAME STRUCTURE:
 Preamble traffic:N
bits
1s 2s 3s 4s nxt fr
CBTR UW TTY SC vow vow Digital speed
channels
CBTR-carrier&bit tuning ,
recovery
TTY-teletype,
SC -service channel ,vow-voice
over wire

53. TDMA SYNCHRONIZATION:
- Maintaining synchronization of existing
network of earth station
- New earth station entering a network &
bringing it into synchronization
METHODS:
1)Satellite loop back synchronization
2)co-operative synchronization

54. CDMA
ADVANTAGES
- The entire bandwidth is available for entire
time
- Anti jamming capability
- Digital transmission with TDM possible
- Shift encoding
DISADVANTAGES
- Network timing required
- Synchronization required between participating Earth
stations