Satellites orbits

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04/19/23 1Dr. Hassan Yousif

Satellites orbits

• Satellite Orbits GEO LEO MEO HEO HAPs(High Altitude Platform )

LEO 500 -1000 km

GEO 36,000 km

MEO 5,000 – 15,000 km

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Geostationary Earth Orbit (GEO)

• These satellites are in orbit 35,863 km• 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.

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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

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Low Earth Orbit (LEO)

Dr. Hassan Yousif

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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.

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LEO (cont.)

Dr. Hassan Yousif

Doppler Shift

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Medium Earth Orbit (MEO)

• A MEO satellite is in orbit 8,000 km -18,000 km

• 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.

• A.k.a. Intermediate Circular Orbits (ICO),04/19/23 10Dr. Hassan Yousif

Highly Elliptical Orbit (HEO)

• Known as Molniya Orbit Satellites Used by Russia for decades. Molniya Orbit is an elliptical orbit. The

satellite remains in a nearly fixed position relative to earth for eight hours.

A series of three Molniya satellites can act like a GEO satellite.

Useful in near polar regions.

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Highly Elliptical Orbit (HEO)

Dr. Hassan Yousif

Other Orbits (cont.)

• High Altitude Platform (HAP) One of the newest ideas in satellite

communication. A blimp or plane around 20 km above

the earth’s surface is used as a satellite. HAPs would have very small coverage

area, but would have a comparatively strong signal.

Cheaper to put in position, but would require a lot of them in a network.

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Kepler’s Work• Tycho Brahe led a team which

collected data on the position of the planets (1580-1600 with no telescopes).

• Mathematician Johannes Kepler was hired by Brahe to analyze the data.

• He took 20 years of data on position and relative distance.

• No calculus, no graph paper, no log tables.

• Both Ptolemy and Copernicus were wrong.

• He determined 3 laws of planetary motion (1600-1630).

Kepler’s First Law

• The orbit of a planet is an ellipse with the sun at one focus.

A path connecting the two foci to the ellipse always has the same length.

Orbital Description

• An ellipse is described by two axes.– Long – semimajor (a)– Short – semiminor (b)

• The area is ab (becomes r2 for a circle).

b

a

Orbital Speed

• The centripetal force is due to gravity.– GMm/r2 = mv2/r– v2 = GM/r

• Larger radius orbit means slower speed.

• Within an ellipse larger distance also gives slower speed.

Kepler’s Second Law

• The line joining a planet and the sun sweeps equal areas in equal time.

The planet moves slowly here.

The planet moves quickly here.

t

t

Orbital Period• The speed is related to the period in a circular

orbit.– v2 = GM/r– (2r/T)2 = GM/r– T2 = 42r3/GM

• Larger radius orbit means longer period.

• Within an ellipse, a larger semimajor axis also gives a longer period.

Kepler’s Third Law

• The square of a planet’s period is proportional to the cube of the length of the orbit’s semimajor axis.– T2/a3 = constant– The constant is the same for all objects orbiting

the Sun

semimajor axis: a

direction of orbit

The time for one orbit is one period: T

VSAT• VSAT stands for Very Small Aperture Terminals. It

allows for the provision of Broadband services• using Satellite technology. This technology has

been deployed for many years and was• traditionally reserved for large

telecommunication operators. Modern VSAT technology now

• allows small businesses and residential users to avail of the opportunity to receive Broadband

• services via satellite.

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• VSAT is a platform, which uses satellite terminals upon which broadband application services

• and electronic commerce activity have the potential to advance and develop. VSAT offers

• satellite based, ‘always-on’ Internet access.• A VSAT network consists of three components:• ■ A central hub• ■ A satellite• ■ A virtually unlimited number of VSAT user

terminals.

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Wireless local loop (WLL)

• What is WLL?- WLL is a system that connects subscribers to the local telephone station wirelessly.

• Systems WLL is based on:– Cellular– Satellite (specific and adjunct)– Microcellular

• Other names– Radio In The Loop (RITL) – Fixed-Radio Access (FRA).

A general WLL setup

WLL services• Desirable:

– Wireless feature should be transparent– Wireline Custom features

• Other:– Business related

• Hunt groups,• Call transfers• Conference calling

– Calling cards, coin phones– V.29 (9600bps)– ISDN (64kbps)

Advantages of WLL over Wired Approach

• Cost – wireless systems are less expensive due to cost of cable installation that’s avoided

• Installation time – WLL systems can be installed in a small fraction of the time required for a new wired system

• Selective installation – radio units installed for subscribers who want service at a given time– With a wired system, cable is laid out in anticipation of

serving every subscriber in a given area

Propagation Considerations for WLL

• Most high-speed WLL schemes use millimeter wave frequencies (10 GHz to about 300 GHz)– There are wide unused frequency bands available above

25 GHz

– At these high frequencies, wide channel bandwidths can be used, providing high data rates

– Small size transceivers and adaptive antenna arrays can be used

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Thank you