This slide is regarding satellite systems, which come under Communications and network, explains the various satellites and their advantage and disadvantage

1. Satellite Systems
5.3. Basics

2. Basics
• Satellites orbit around the Earth either in
– Circular Path or
– Elliptical Path
• They maintain the circular orbital path using 2
forces
– The Attractive Force of the Earth Fg
– The Centrifugal Force Fc

3. Basics
• Satellites in circular orbits
– Attractive force Fg = m g (R/r)²
– Centrifugal force Fc = m r ²
m: mass of the satellite
R: radius of the earth (R = 6370 km)
r: distance of satellite to the center of the earth
g: acceleration of gravity (g = 9.81 m/s²)
: angular velocity ( = 2  f, f: rotation frequency)
• For a Stable orbit
Fg = Fc (mass of a satellite is irrelevant). We get
3
2
2
gR
(2 f )
r



4. Satellite Period and Orbits
• The distance of a Satellite to the earth’s
surface depends on its rotation frequency.
• If the distance is more then the rotation
frequency will be less.
• Geo-Stationary satellites have satellite period
of 24 hours and the distance 35,786 km.
3
2
2
gR
(2 f )
r



5. Satellite Period and Orbits
10 20 30 40 x106 m
24
20
16
12
8
4
radius
satellite
velocity [ x1000 km/h] period [h]
synchronous distance
35,786 km

6. Importance Parameters
• Inclination Angle d
– Angle between satellite’s orbit and the equator
of the earth.
– 0 inclination angle, if above the equator.
– In Elliptical path, the closest point to the earth
is called Perigee.

7. Inclination angle
d
inclination d
satellite orbit
perigee
plane of satellite orbit
equatorial plane

8. Important Parameters
• Elevation Angle e
– Angle between the center of the Satellite‘s
beam and the earth‘s surface.
– The area on earth where the satellite‘s signal
can be received is called footprint.
– LOS (Line of Sight) to the satellite necessary for
connection
 high elevation needed, less absorption due to
e.g. buildings
 Uplink: connection base station - satellite
 Downlink: connection satellite - base station

9. Elevation angle
Elevation:
angle e between center of
satellite beam and surface
e
minimal elevation:
elevation needed at least
to communicate with
the satellite

10. Loss of Signal
• Attenuation – Loss of Signal Power depending
on the following:
– Distance between the receiver on earth and the
satellite
– Satellite Elevation (If less than 10° no use)
– Atmospheric Conditions – Rain, Fog etc..

11. Loss of Signal
• Loss L can be calculated as
2
4







r f

c
L
– L : Loss of Signal
– r : distance between sender and receiver
– f : carrier frequency
– c : speed of light
• Power of the received signal decreases with
the square of the distance.
• If affects the maximum data rates achievable.

12. Satellite Link Budget
• It is needed to design optimum satellite
communication link.
• It considers the following:
– Antenna size
– Modulation technique availability
– Satellite power and Bandwidth
– Carrier noise
– Free space pass-loss
– Multipath propagation effects
– Atmospheric conditions
– Signal delays

13. Atmospheric Attenuation
Example: satellite systems at 4-6 GHz
5° 10° 20° 30° 40° 50°
elevation of the satellite
Attenuation of
the signal in %
50
40
30
20
10
rain absorption
fog absorption
atmospheric
absorption
e

14. Latency (Propagation Delay)
• Latency is the time delay between the actual
moment of a signal's broadcast and the time it
is received at its destination.
• The amount of latency depends on the
distance travelled and the speed of light.
• Eg. Geostationary orbit – 36000 kms away
One way propagation delay is = 36x106/ 3x108
= 0.12 seconds
Total round trip propagation delay is 0.24 seconds

15. Types of Satellite Orbits
• Four different types of satellite orbits can be identified
depending on the shape and diameter of the orbit:
• GEO: geostationary orbit, ca. 36000 km above earth
surface
• LEO (Low Earth Orbit): ca. 500 - 1500 km
• MEO (Medium Earth Orbit) or ICO (Intermediate
Circular Orbit): ca. 6000 - 20000 km
• HEO (Highly Elliptical Orbit) elliptical orbits

16. Types of Satellite Orbits
earth
35768
km
1000
10000
HEO
LEO
(Globalstar,
Irdium)
GEO (Inmarsat)
MEO (ICO)
inner and outer Van
Allen belts
Van-Allen-Belts:
ionized particles
2000 - 6000 km and
15000 - 30000 km
above earth surface

17. 5.3.1 GEO Satellites
• Orbit 35,786 km distance to earth surface, orbit in
equatorial plane (inclination 0°)
• complete rotation exactly one day, satellite is
synchronous to earth rotation.
• 3 satellites are enough to cover every part of earth
• Advantages
– Fixed antenna positions, no adjusting necessary
– Ideal for TV and Radio Broadcasting
– High life time – about 15 years.
– Large footprint. So no handover needed.
– No Doppler shift because of 0 movement

18. Disadvantages of GEO
• Northern, southern regions have problem of bad
elevation angle – need for larger antennas
• Shading of signals in cities due to large buildings
• High transmit power is needed – problem for
battery powered devices
• No global coverage, so cannot be used for small
mobile phones
• High latency of 0.25 seconds for one way makes it
unfit for voice and data communications
• Due to large footprints the frequencies cannot be
reused

19. 5.3.2 LEO Satellites
• Satellite period is about 95 to 120 minutes.
Orbits ca. 500 - 1500 km above earth surface
• Visible from earth for about 10-40 minutes
only.
• Has high elevation angle and high quality
communication link
• Further Classifications
– LEO with low bandwidth service (ca. 100bits/s)
– Big LEOs (ca. 1000 bit/s)
– Broadband LEOs (ca. 1 Mbit/s)

20. Advantages of LEO
• LEO provides bandwidth of 2400 bit/s (which
is sufficient of voice communication) with low
transmit power (1 w)
• Very low latency – ca.10 milli seconds
• Smaller footprints so better frequency reuse
• Provides higher elevation for polar regions and
provides better global coverage

21. Disadvantages of LEO
• Need for more number of satellites (50-200)
because of small footprint
• Mechanism for connection handover required
due to short time visibility with high elevation
• High number of satellites involves high
complexity
• Lifetime is shorter – 5-8 years only
• Routing from satellite to satellite or satellite to
base stations needed for global coverage

22. 5.3.3 MEO Satellites
• Orbits around 10000 km
• Advantages
– Requires 12 satellites to cover the earth
– Requires fewer handover
– Movement is slower
• Disadvantages
– Delay is about 70-80 ms
– Needs higher transmit power and special
antennas for smaller footprints