Satellite Orbits

LocationLocation

EquatorialEquatorial PolarPolar


Shape (Conic Sections)Shape (Conic Sections)

Circular OrbitsCircular Orbits Elliptical OrbitsElliptical Orbits


Parabolic TrajectoriesParabolic Trajectories Hyperbola TrajectoriesHyperbola Trajectories

Shape (Conic Sections)Shape (Conic Sections)


Orbit ClassificationsOrbit ClassificationsCircular OrbitsCircular Orbits

CharacteristicsCharacteristics Constant speedConstant speed Nearly constant altitudeNearly constant altitude

Typical MissionsTypical Missions Reconnaissance/Weather (DMSP)Reconnaissance/Weather (DMSP) MannedManned Navigational (GPS)Navigational (GPS) GeoGeo--synchronous (Comm sats)synchronous (Comm sats)

Orbit ClassificationsOrbit ClassificationsElliptical OrbitsElliptical Orbits

CharacteristicsCharacteristics Varying speedVarying speed Varying altitudeVarying altitude Asymmetric Ground TrackAsymmetric Ground Track

Typical MissionsTypical Missions Deep space surveillance (Pioneer)Deep space surveillance (Pioneer) Communications (Polar comm.)Communications (Polar comm.) Ballistic MissilesBallistic Missiles

Orbit ClassificationsOrbit ClassificationsParabolic/Hyperbolic TrajectoriesParabolic/Hyperbolic Trajectories

CharacteristicsCharacteristics Escaped Earth’s gravitational influenceEscaped Earth’s gravitational influence HeliocentricHeliocentric

Typical MissionsTypical Missions Interplanetary exploration (Galileo, Phobos, Interplanetary exploration (Galileo, Phobos,

Magellan)Magellan)

Low Earth Orbits (LEOs)Low Earth Orbits (LEOs)

Close to Earth (Close to Earth (100100~~15001500km)km) ~~2727,,000 000 km/hkm/h 9090min periodmin period

Space ShuttleSpace Shuttle Some Remote Sensing Some Remote Sensing

Satellites and weather satellitesSatellites and weather satellites ~~88,,000 000 Space Junks Space Junks –– satellites, satellites,

old rockets, metals, etc. old rockets, metals, etc.

Low Earth Orbits (LEOs)Low Earth Orbits (LEOs)

Operate at freq. Operate at freq. 11--22..5 5 GHzGHz Signal to noise should be better Signal to noise should be better

with LEOswith LEOs Shorter delays Shorter delays -- between between 1 1 -- 10 10

ms typicalms typical Because LEOs move relative to Because LEOs move relative to

the earth, they require trackingthe earth, they require tracking

Low Earth Orbits (LEOs)Low Earth Orbits (LEOs)Iridium Iridium mobile telephone Sat System having mobile telephone Sat System having

6666--sat with around sat with around 370 370 mile above earth (mile above earth (22..4 4 Kbps data)Kbps data)

SAT

ground stations

SAT

SAT

constellation

Medium Earth Orbits (MEOs)Medium Earth Orbits (MEOs)

Operate at freq. Operate at freq. 11..22--11..66 66 GHzGHz 5000 5000 -- 12000 12000 km above earth surfacekm above earth surface Latency (Delay) ~ Latency (Delay) ~ 70 70 -- 80 80 msms Special antennas for small footprints Special antennas for small footprints

neededneeded

Comparison with LEOsComparison with LEOs

GPSGPS--NavstarNavstar -- USAUSA DepDep.. ofof DefenseDefenseNAVSTARNAVSTAR 2121++66 SatsSats withwith aroundaround 95009500 milemileaboveabove earthearth

Slower moving satellites than LEOs Slower moving satellites than LEOs Less satellites needed than LEOs and Less satellites needed than LEOs and

simpler system designsimpler system design Higher sending power needed than Higher sending power needed than

LEOsLEOs

NearNear--synchronous Earth Orbits synchronous Earth Orbits

High Elev. (High Elev. (19000 19000 –– 25000 25000 miles)miles)NonNon--Synchronous with Circular Synchronous with Circular

OrbitsOrbits Slowly moving from west to eastSlowly moving from west to east

Geosynchronous Earth Orbits Geosynchronous Earth Orbits (GEOS)(GEOS)

Originally proposed by Originally proposed by Arthur C. ClarkeArthur C. Clarke

Satellite is always at the Satellite is always at the same position for a same position for a viewer on earthviewer on earth

‘Big‘Big--picture view’picture view’


Sat. must travel eastward at the same rotational Sat. must travel eastward at the same rotational speed as the earthspeed as the earth

The Orbit must be circularThe Orbit must be circular The Inclination of orbit must be zero.The Inclination of orbit must be zero.

nP 2

23

na

3/1

2

2

4

PaGSO

P=P=2323hrs hrs 5656min min 44..09 09 sec sec ==33..986005986005xx10101414

Km42164

KmaE 6378 Kmahh EGSOGEO 78635637816442


Angular separation about Angular separation about 22oo -- allows allows 180 180 satellites satellites

Three Sats are required for Three Sats are required for full earth coveragefull earth coverage

GEO ProsGEO Pros

GEO satellites are good for com. due wide GEO satellites are good for com. due wide area coveragearea coverage

No tracking is requiredNo tracking is requiredNo DopplerNo Doppler No need to switch from one Sat. to anotherNo need to switch from one Sat. to another

GEO ConesGEO Cones

Sophisticated and heavy propulsion Sophisticated and heavy propulsion devices devices high precision spacemanshiphigh precision spacemanship

High Latency High Latency Delay ~ Delay ~ 00..55--00..6 6 ss High TX/RX power requirementsHigh TX/RX power requirements

SYNCOM SYNCOM 22

Picture from NASAPicture from NASA

Antenna Lock AnglesAntenna Lock Angles The look angles for the ground station antenna The look angles for the ground station antenna

are the azimuth and elevation angles required at are the azimuth and elevation angles required at the antenna so that it points directly at the satellitethe antenna so that it points directly at the satellite

Antenna Lock AnglesAntenna Lock AnglesRequired InfoRequired Info

Antenna Lock AnglesAntenna Lock Angles

Antenna Lock Angles: Example Antenna Lock Angles: Example 11

Limits of VisibilityLimits of Visibility Line of sight for an Line of sight for an

earth station that earth station that defines the farthest defines the farthest satellite away that can satellite away that can be seen looking east be seen looking east or west of the earth or west of the earth station.station.

Limits of VisibilityLimits of Visibility In practice, the lower limit of visibility is set to In practice, the lower limit of visibility is set to 55oo in in

order to reduce the effect of noise picked up from order to reduce the effect of noise picked up from the earth (atmospheric conditions + ground the earth (atmospheric conditions + ground reflectionsreflections

Explain why: Explain why: geosynchronous satellites are geosynchronous satellites are

not used for not used for communications at the communications at the

earth pole locationsearth pole locations

Atmospheric attenuationAtmospheric 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

Polar OrbitPolar Orbit One type of LEOOne type of LEO High inclination (High inclination (8080oo--100100oo)) Can scan the entire surface due Can scan the entire surface due

to earth rotation (eastto earth rotation (east--west) and west) and satellite orbit (northsatellite orbit (north--south)south)

Polar OrbitPolar Orbit

Satellite passes through the earth's shadow and permits viewing of the entire earth’s surface each day with a single satellite

Polar OrbitPolar Orbit

National Oceanic and Atmospheric Administration (NOAA) http://www.noaa.gov/

SunSun--synchronous Orbitsynchronous Orbit These orbits allows a satellite to pass over a section of the These orbits allows a satellite to pass over a section of the

Earth at the same time of day. Earth at the same time of day. Since there are Since there are 365 365 days in a year and days in a year and 360 360 degrees in a degrees in a

circle, it means that the satellite has to shift its orbit by circle, it means that the satellite has to shift its orbit by approximately one degree per day. approximately one degree per day.

These satellites orbit at an altitude between These satellites orbit at an altitude between 700 700 to to 800 800 km. km.

SunSun--synchronous Orbitsynchronous Orbit

These orbits are used for satellites that need a These orbits are used for satellites that need a constant amount of sunlight.constant amount of sunlight.

Sun synchronous have 95 – 105o inclination) Procession of orbital plane synchronized with

the earth’s rotation so satellite is always in view of the sun

Elliptical (Molnyia) OrbitElliptical (Molnyia) Orbit

Perigee (closest), Apogee (farthest)Perigee (closest), Apogee (farthest) Period: ~Period: ~1212hrshrs Polar coverage Polar coverage Communication satellites for north Communication satellites for north

and south regionand south regionPerigee

Apogee

ORBITAL MECHANICS: GROUND TRACES

GROUND TRACES

THE POINTS ON THE EARTH’S SURFACE OVER WHICH A SATELLITE PASSES AS IT TRAVELS ALONG ITS ORBIT

PRINCIPLE : GROUND TRACE IS THE RESULT OF THE ORBITAL PLANE BEING FIXED AND THE EARTH ROTATING UNDERNEATH IT

AMPLITUDE OF GROUND TRACE (LATITUDE RANGE) IS EQUAL TO THE ORBITAL INCLINATION

MOVEMENT OF GROUND TRACE IS DICTATED BY THE SATELLITE ALTITUDE AND THE CORRESPONDING TIME FOR IT TO COMPLETE ONE ORBIT

ORBIT 4ORBIT 3

ORBIT 2ORBIT 1

EQUATORIAL ORBIT

INCLINED ORBIT

ANGLE OF INCLINATION(0 DEG. FOR EQUATORIAL)

MULTIPLE ORBITS

SAT

EARTH MOTION BENEATH SATELLITE

Ground TracksGround TracksWestward RegressionWestward Regression

0 30-30-60-90-120

ABC

A - time zeroB - after one orbitC - after two orbits

60

Ground TracksGround TracksEccentricityEccentricity

Ground Track for Molnyia orbiteccentricity = .7252

Ground tracks Ground tracks InclinationInclination

60

30

0

30

60

45N

45S

Inclination = 45 degrees Eccentricity ~ 0

Satellite Orbits

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