more satellite orbits introduction to space systems and spacecraft design space systems design
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More Satellite Orbits
Introduction to Space Systems and Spacecraft DesignSpace Systems Design
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Importance of Orbits to Mission
• When should you start analyzing orbits to satisfy mission requirements?
• Can the orbit effect any of the following in the mission design?
― Revisit time of satellite to a point on earth?― Amount of data that can be transferred between
the satellite and ground?― Space radiation environment?― Power generation for the satellite?― Thermal control on the satellite?― Launch costs?
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Orbit ReferencesMore Orbits
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Orbit Types
LEO – Low Earth Orbit
GEO – Geosynchronous Orbit
HEO – Highly Elliptical Orbit
N
S
N
S Around Equator
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Eclipse
(max eclipse)
Inclined(partial eclipse)
Terminator Orbit
(no eclipse)
- Twilight
Noon-Midnight
Sun
Orbit With Respect to SunMore Orbits
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More Orbits Orbit With Respect to Sun
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Sun
Winter Summer
Equinox
EquinoxEarth's Axis
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Orbit Period
Spacecraft Velocity
Spacecraft Orbital Velocity and Orbit Period More Orbits
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km/s mi/hourconvert
X (km/s) x 60s/m x 60m/hour x (1/1.6) mi/km = X (km/s) x 2250 mi/hour
V = (7.739 km/s) x 2250 mi/hour = 17,142 mi/hour
What is ISS altitude?
Spacecraft Orbital Velocity and Orbit Period -2
= 17,142 mi/hour
h = (150) (1.852) = 278 km
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Equations to Remember
Vcir = 631.3481 r-1/2 km/sec
Vesc = 892.8611 r-1/2 km/sec
r - is from center of the earth
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Changing Orbits
LEO – Low Earth Orbitfrom a shuttle launch – 280 km
N
S
280 kmV = 7.738 km/s
35,786 kmV = 3.0727 km/s
LEO
GEO
Want to Change OrbitLEO to GEO
How?
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1. Change to a GTO (GEO transfer Orbit)
For GTO
Want: Vp = 10.169 km/s Va = 1.606 km/s
2. Circularize orbit
Need
V = 3.0727 km/s for GEO Change V = 3.0727-1.606 =
1.4667 km/s
3. Burn at Va to increase V to 3.0727 km/s for circular orbit at GEO
How?
LEO
GEO
Va
Vp
35,786 kmV = 3.0727 km/s
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Other Ways to Change OrbitsMore Orbits
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Other Ways to Change OrbitsMore Orbits
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Now you have an orbit for your satellite.
1. Will it stay where you put it?
2. Is there anything that will change the orbit once you have it there?
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Special Effects on Orbits
Sun
Winter Summer
Equinox
Equinox
What happens to the orbit plane as the earth rotates around the sun?
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What effects the orbit?J2 effect
J22/J3 effectLunar gravitySolar gravity
Solar pressureAtmospheric drag
Orbits PerturbationsMore Orbits
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Solar Pressure/RadiationOrbits Perturbations
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Solar Pressure/Radiation
Using solar radiation for propulsion.
Solar Sails
Orbits PerturbationsMore Orbits
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Atmospheric Drag
Drag Coefficient
Orbits PerturbationsMore Orbits
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Ballistic Coefficient
How do they go through the atmosphere?
Which stays in orbit longer – a bowling ball or a soccer ball of the same size?
Bc = K (Mass/Cross Sectional Area)
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Atmospheric DragOrbits Perturbations
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Atmospheric DragOrbits Perturbations
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Atmospheric DragOrbits Perturbations
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Earth-moon tidal friction mechanismOrbits Perturbations
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Earth non-spherical effectOrbits Perturbations
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East-West drift occurs because the equator is not perfectly circular, so satellites drift slowly towards one of two longitudinal stable points.
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Orbit References – GEO Station KeepingMore Orbits
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Earth non-spherical effectOrbits Perturbations
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Due to luni-solar perturbations and the ellipticity of the Earth equator, an object placed in a GEO without any station-keeping would not stay there. It would start building up inclination at an initial rate of about 0.85 degrees per year. After 26.5 years the object would have an inclination of 15 degrees, decreasing back to zero after another 26.5 years
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N
S
Inclination
What is the effect of this?
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Earth non-spherical effectOrbits Perturbations
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N
S
< 90o
N
S
> 90o
< 900 Orbit
Inclination
> 900 Orbit
Orbits PerturbationsMore Orbits
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N
Prograde OrbitI < 90o
Oblatness causes rotation
clockwise
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Earth non-spherical effectOrbits Perturbations
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I > 90o
Oblatness causes rotation
counter clockwise
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Sun
Winter Summer
Equinox
Equinox
a
aaa
Orbit rotates to maintain same angle with sun
Sun Synchronous
Orbit
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Earth non-spherical effectOrbits Perturbations
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Sun Synchronous
Orbit
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Earth non-spherical effectOrbits Perturbations
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Earth non-spherical effectOrbits Perturbations
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Sun Synchronous Inclination
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Special Molniya orbit has a stable orbit that is used by Russians to have high latitude communications – 2 satellites.
Earth non-spherical effectOrbits Perturbations
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Earth non-spherical effectOrbits Perturbations
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Earth non-spherical effectOrbits Perturbations
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Effects are secular and accumulative
Which are Which are these?these?
J2 effectJ2 effectJ22/J3 effectJ22/J3 effectLunar gravityLunar gravitySolar pressureSolar pressureAtmospheric Atmospheric
dragdrag
Earth non-spherical effectOrbits Perturbations
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GEO satellites have drift due to non-spherical earth1.East-west drift2.North-south drift
Earth non-spherical effectOrbits Perturbations
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Orbit References – GEO Station KeepingMore Orbits
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Special Orbits
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N
S
N
S
Zero Inclination GEO Orbits
Satellite appears stationery to earth
observer
Inclination GEO Orbits
Satellite appears go N-S & EW in a figure 8 to
earth observer
Geostationary Orbit Geosynchronous Orbit
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GEO Orbits CharacteristicsMore Orbits
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N
S
What is the maximum latitude that a GEO satellite
can be viewed?
GEO Orbits CharacteristicsMore Orbits
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GEO Orbits CharacteristicsMore Orbits
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GEO Orbits CharacteristicsMore Orbits
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LEO Satellite Orbits
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FootprintFootprint
More Orbits LEO Orbits Characteristics
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LEO Orbits CharacteristicsMore Orbits
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LEO Orbits CharacteristicsMore Orbits
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How do you find Di? Use law of cosines.
What is elevation angle on ground antenna?
LEO Orbits CharacteristicsMore Orbits
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Objects in Orbit
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Objects in OrbitMore Orbits
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Importance of Orbits to Mission
• When should you start analyzing orbits to satisfy mission requirements?
• Can the orbit effect any of the following in the mission design?
― Revisit time of satellite to a point on earth?― Amount of data that can be transferred between
the satellite and ground?― Space radiation environment?― Power generation for the satellite?― Thermal control on the satellite?― Launch costs?
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Questions?
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