Satellite Systems

5.3. Basics

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

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

32

2

)2( f

gRr

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.

32

2

)2( f

gRr

Satellite Period and Orbits

10 20 30 40 x106 m

24

20

16

12

8

4

radius

satellite period [h]velocity [ x1000 km/h]

synchronous distance35,786 km

Importance Parameters

• Inclination Angle d

– Angle between satellite’s orbit and the equatorof the earth.

– 0 inclination angle, if above the equator.

– In Elliptical path, the closest point to the earth is called Perigee.

Inclination angle

inclination d

d

satellite orbit

perigee

plane of satellite orbit

equatorial plane

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

Elevation angle

Elevation:angle e between center of satellite beam and surface

eminimal elevation:elevation needed at leastto communicate with the satellite

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..

Loss of Signal

• Loss L can be calculated as

– 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.

24

c

frL

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

Atmospheric Attenuation

Example: satellite systems at 4-6 GHz

elevation of the satellite

5° 10° 20° 30° 40° 50°

Attenuation of the signal in %

10

20

30

40

50

rain absorption

fog absorption

atmospheric absorption

e

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

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

Types of Satellite Orbits

earth

km

35768

10000

1000

LEO (Globalstar,

Irdium)

HEO

inner and outer VanAllen belts

MEO (ICO)

GEO (Inmarsat)

Van-Allen-Belts:ionized particles2000 - 6000 km and15000 - 30000 kmabove earth surface

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

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

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)

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

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

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