2 - satellite orbits

7/28/2019 2 - Satellite Orbits

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2: Satellite Orbits

Satellite Communications IV(SCO401T)

Semester I 2013

Prof. M. MzyeceEmail: [email protected]

Office: 6-411E

T l 012 382 5339
mailto:[email protected]:[email protected]


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Satellite Orbits

An orbit is the trajectory (or path) followedby a satellite.

The trajectory is within a plane and shaped like

an ellipse.

Apogee: Point in trajectory at which satellite

attains its maximum distance from the earth.

Perigee: Point in trajectory at which satellite

attains its minimum distance from the earth.

Satellite moves more slowly in its trajectory

as the distance from the earth increases.


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Latitudes and Longitudes


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Orbital Inclination


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Types of Orbits

1. Highly elliptical orbits (HEO) Inclined elliptical orbits inclined at 64 to the equatorial

plane

Inclination allows satellite to cover regions of high

latitude for large fraction of orbital period as it passesto the apogee.

Example: Russian MOLNIYA satellite system (see next slide)

Period: 12 hours Satellite remains above regions under apogee for about 8

hours (24-hour coverage can be ensured with three phasedsatellites on different orbits)


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Orbit of a MOLNIYA satellite


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Types of Orbits (2) 2. Circular polar low earth orbits (LEO)

Altitude: Constant; Several hundred km

Inclination: Nearly 90

Period: 1.5 hours

Combined motion of satellite and earths rotationguarantee global long-term coverage (see next slide)

Examples: SPOT satellite

830 km altitude, 98.7 inclination, 101 minute period

IRIDIUM system 66 satellites at 780 km, global real-time communication

Non-polar orbits with inclination < 90 alsopossible: GLOBALSTAR, ECCO, etc.


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Circular polar low earth orbit

(LEO)


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Types of Orbits (3)

3. Circular medium earth orbits (MEO)Also called intermediate circular orbits (ICO)

Altitude: About 10,000 km

Inclination: 50

Period: 6 hours

Constellations of 10 to 15 satellites guaranteecontinuous global coverage

Example: ICO system (from Project 21 of INMARSAT)

Constellation of 10 satellites in two planes, 45 inclination


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Types of Orbits (4) 4. Circular orbits with zero inclination

(equatorial orbits) Most popular is the geostationary (or Clarke) satellite

orbit

Geostationary satellites most widely used of all satellites

Altitude: 35,786 km Satellite orbits earth in equatorial plane

(Note: Poor polar coverage)

Period: 24 hours (period of earths rotation)

Satellite appears as a fixed point in the sky

5. Hybrid orbits

Combines element of elliptical and circular orbits

E l ELLIPSO t


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Factors in Choice of Orbit Objectives and Constraints

Extent and latitude of area to be covered GEO provides global coverage, but coverage deteriorates towards the poles.LEO provides good coverage but limited in terms of space and time.

Angle of Elevation Higher in inclined elliptical or polar orbits than in geostationary orbits (enables

communication in shadowed areas)

Delay

Generally low(er) in LEO for nearby stations which are simultaneously visible tothe satellite, but can be exceedingly long (several hours) for distant stations ifonly store-and-forward transmission is possible.

Interference Orbit spectrum resource is limited i.e. both physical orbital space and RF

spectrum for space radio communications is limited. Geometry of orbiting satellite system varies with time and relative

geometries of different systems with respect to each other are also variableand difficult to synchronise/optimise.

Performance of Launchers The mass which can be launched decreases as the altitude increases.


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Keplers Laws

Define the laws of planetary motion. Keplers Laws apply generally to any two

bodies which interact through gravitation.

The more massive body is called the primary(body), and the other the secondary(body) orsatellite.

Artificial satellites orbiting the earth follow

the same laws governing the motion of themoon around the earth and the sun andother planets around the sun.


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Keplers First Law

The path followed by a satellite aroundthe primary will be an ellipse.

An ellipse has two focal points: F1 and F2.

The barycentre, the centre of mass of thetwo-body system, is always centred on oneof the foci.Always the centre of the earth in this case due

to enormous difference between the masses ofthe earth and the satellite.


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Keplers First Law (2)


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Keplers First Law (3)

Wheree

is the eccentricity,a

is the semimajor axis,b

is the semiminor axis.

The eccentricity and the semimajor axis are two of the orbital parameters specified

for satellites orbiting the earth.

For an elliptical orbit: 0

2 - satellite orbits

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