Satellite OrbitsSatellite Orbits
Orbit ClassificationOrbit Classification
Size/PeriodSize/PeriodLocationLocationShapeShape
Size/PeriodSize/Period
Defined by semiDefined by semi--major axis (a)major axis (a)
Low Earth Orbit (LEO)Low Earth Orbit (LEO) Medium (High) Earth Orbit (MEO)Medium (High) Earth Orbit (MEO) SemiSemi--synchronous Orbitsynchronous Orbit GeoGeo--synchronous Orbitsynchronous Orbit
Orbit ClassificationOrbit Classification
LocationLocation
EquatorialEquatorial PolarPolar
Orbit ClassificationOrbit Classification
Shape (Conic Sections)Shape (Conic Sections)
Circular OrbitsCircular Orbits Elliptical OrbitsElliptical Orbits
Orbit ClassificationOrbit Classification
Parabolic TrajectoriesParabolic Trajectories Hyperbola TrajectoriesHyperbola Trajectories
Shape (Conic Sections)Shape (Conic Sections)
Orbit ClassificationOrbit Classification
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’
Geosynchronous Earth Orbits Geosynchronous Earth Orbits (GEOS)(GEOS)
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
Geosynchronous Earth Orbits Geosynchronous Earth Orbits (GEOS)(GEOS)
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
Antenna Lock Angles: Example Antenna Lock Angles: Example 11
Antenna Lock Angles: Example Antenna Lock Angles: Example 11
Antenna Lock Angles: Example Antenna Lock Angles: Example 22
Antenna Lock Angles: Example Antenna Lock Angles: Example 33
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