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Wireless WANs: Satellite Networks

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Brief history of satellite communication

Name Date of launch

note

SPUTNIK I October 4, 1957

the world's first orbital spacecraft. Nov 1957, Sputnik 2 and a dog escape earth and enter outerspace

SCORE December 18, 1958

The first communication satellite which broadcasted a Christmas message for 12 days until the batteries failed

Echo 1 August 12, 1960

a passive reflector satellite, the technology was soon abandoned

April 12, 1961 First man in space

Telstar 1962 First telecommunication satellite, first real-time active

Intelsat 1964-1979 geosynchronous earth orbit ,open to use by all nations

Inmarsat 1979 used in international shipping

ACTS 1993 spot beams, on-board storage and processing, and all digital transmission

DirecTV 1994 begins Direct Broadcast to Home

Iridium Motorola was supposed to provide mobile telephone service

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16-2 SATELLITE NETWORKS

A satellite network is a combination of nodes, some of which are satellites, that provides

communication from one point on the Earth to another. A node in the network can be a satellite, an

Earth station, or an end-user terminal or telephone.

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Figure Satellite orbits

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Table 1 Satellite frequency bands

Sky UK, Eutelsat 28A; Ku band

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What is the period of the Moon, according to Kepler’s law?

Example 16.1

Here C is a constant approximately equal to 1/100. The period is in seconds and the distance in

kilometers. The Moon is located approximately 384,000 km above the Earth. The radius of the

Earth is 6378 km. Applying the formula, we get.

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According to Kepler’s law, what is the period of a satellite that is located at an orbit approximately

35,786 km above the Earth?

Example 16.2

Solution

This means that a satellite located at 35,786 km has a period of 24 h, which is the same as the

rotation period of the Earth. A satellite like this is said to be stationary to the Earth. The orbit, as we

will see, is called a geosynchronous orbit.

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Figure 16.14 Satellite categories

Low Earth Orbit (LEO)

Medium Earth Orbit (MEO)

Geosynchronous Orbit (GEO)

GEO: EXACTLY 22 238 miles

MEO: typically around 8000 miles

HE

O: var.

LEO: typically between 500 and 1000 miles

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Figure 16.15 Satellite orbit altitudes

Geosynchronous Orbit (GEO) Satellite Systems

Advantages:

large area coverage, stay where they are at 35,786km (22,000miles)

above the Earth

satellite rotation is synchronous to earth

three satellites can cover the whole globe

low system complexityDisadvantages:

long propagation delay (~125 msec)

high transmission power is required

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Figure 16.16 Satellites in geostationary orbit

Medium Earth Orbit (MEO) Satellite Systems

Advantages:

slightly longer propagation delays (~40 msec)

slightly higher transmission power required

more expensive than LEOs but cheaper than GEOsDisadvantages:

coverage spot greater than a LEO, but still less than a GEO

still the need to be in rotation to preserve their low altitude 6-8 hours to

circle the earth.

multiple MEO satellites are still needed to cover a region continuously

handovers and satellite tracking are still needed, hence, high complexity

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Global Position System (GPS) Operated by the US Department of Defense.

Orbiting at an altitude about 18,000km

Consists of 24 satellites in 6 orbits; 32 by Dec

2012

At any time, about 9 (>4) satellites are visible

from any point on Earth

A GPS receiver has an almanac that tell the

current position of each satellite

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Figure Trilateration

If we now our distance from three points, we know exactly where we are. (three circles

meet at one signal point)

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Application of GPS

Military forces

Navigation

Clock synchronization, CDMA cellular system

Low Earth Orbit (LEO) Satellite Systems

Advantages:

short propagation delays (10-15 msec)

low transmission power required

low price for satellite and equipmentDisadvantages:

small coverage spot

they have to be in rotation to preserve their low altitude (90 mins period)

a network of at least 6 LEO satellites is required to cover a region

continuously

high system complexity due the need for handovers and satellite tracking

Low Earth Orbit (LEO) Satellite Systems

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LEO satellites have polar orbits

Altitude is between 500-2000 km

Rotation period of 90-120 min.

An LEO system has a cellular type of access

Footprint has a diameter of 8000 km.

Delay < 20 ms, accept for telephony

Work together as a network, connected through intersatellite links

(ISLs)

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Figure LEO satellite system

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Little LEO, under 1GHz, for low date rate message

Big LEO: between 1-3 GHz, Globalstar and Iridium system

Broadband LEO provide communication similar to fibre

optic network. Teledesic

Three categories of LEO

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Figure Iridium constellation

The Iridium system has 66 (planning was 77)

satellites in six LEO orbits, each at an altitude of 750

km.

Iridium is designed to provide direct worldwide voice and data communication using

handheld terminals, a service similar to cellular telephony but on a global scale

( including poles, oceans and airways).

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http://www.polartrec.com/expeditions/glacial-movement-and-seismicity/

journals/2010-05-06

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Figure Teledesic

Teledesic has 288 satellites in 12 LEO orbits, each at an altitude of 1350

km.

Internet in the sky. Teledesic officially

suspended its satellite construction work on

October 1, 2002.

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Use Kepler’s formula to find the period and altitude for an Iridium satellite

and Globalstar satellite.

Iridium satellites are orbiting at 750 km above the earth surface.

Globalstar satellites are orbiting at 1400 km above the earth surface.

The radius of the earth 6378 km

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Iridium satellites are orbiting at 750 km above the earth surface. Considering

the

radius of the earth 6378 km, the radius of the orbit is then (750 km + 6378 km)

= 7128 km.

Using the Kepler formula, we have

Period = (1/100) (distance) 1.5

= (1/100) (7128)1.5

= 6017 s = 1.67 hours

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Globalstar satellites are orbiting at 1400 km above the earth surface. Considering

the radius of the earth, the radius of the orbit is then (1400 km + 6378 km) = 7778

km.

Using the Kepler formula, we have

Period = (1/100) (distance) 1.5

= (1/100) (7778)1.5

= 6860 s = 1.9 hours

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The space shutter is an example of a LEO satellite. Sometimes, it orbits at

an altitude of 250 km.

a. Using a mean earth radius of 6378km, calculate the period of the

shuttle orbit.

b. Determine the linear velocity of the shutter along this orbit.

a. a = 6378 + 250 = 6628 km

T = 1/100 a1.5

= 5396 sec = 1.5 hours

b. The linear velocity is the circumference divided by the period

(2πa)/T = (41645)/(5396) = 7.72 km/s