Orbital PertubationsOrbital Pertubations

AtmosphereOrbital Decay

Effect of the AtmosphereEffect of the Atmosphere

Height (km) Life

200 2 weeks

1000 1000yrs

36000 1Myrs

Effect of the Earth’s ShapeEffect of the Earth’s Shape

TOP VIEWTOP VIEW SIDE VIEWSIDE VIEW

15m15m

15m15m

7.5m7.5m 7.5m7.5m

EQUATOREQUATOR

12 714 km12 714 km

12 756 km12 756 km

N Pole

Orbital PertubationsOrbital PertubationsThese are subtle but accumulative effect on an orbit

• Nodal regression• The orbital plane effectively twists around the earth• This is because of the shape of the Earth. • This effect is known as Orbital Twist.

• Perigee Rotation• This effect appears as a twisting of the satellites position

relative to the Equator.• Therefore, satellites need to be managed, in effect ‘flown’.

Applications of OrbitsApplications of Orbits• LEO

• Earth Observation with resolution• Communications

• MEO Missions• Global Navigation Satellite Systems

• GEO • Communications• Earth Observation with persistence

• HEO • Communications• Earth Observation with persistence

Persistence………Persistence………

………….versus Resolution.versus Resolution

Orbital RequirementsOrbital Requirements

LEO

Coverage

Dwell Time

Revisit Time

Resolution & Power Requirements

GEO c.700

Achieving OrbitAchieving Orbit

Initial LaunchInitial Launch

Direct ascent into low altitude orbit

Data TableData TableEarth Gravitational

Force Mass of Earth Radius of Earth Orbital Height Orbital Velocity Orbital Period Energy Required to Achieve Orbit

6.67E-11 5.98E+24 6.378E+06 12 7900.7 84.7 3.133E+07

6.67E-11 5.98E+24 6.378E+06 19 7896.3 84.9 3.136E+07

6.67E-11 5.98E+24 6.378E+06 100 7846.8 86.5 3.175E+07

6.67E-11 5.98E+24 6.378E+06 200 7786.9 88.5 3.222E+07

6.67E-11 5.98E+24 6.378E+06 300 7728.4 90.5 3.267E+07

6.67E-11 5.98E+24 6.378E+06 400 7671.2 92.6 3.311E+07

6.67E-11 5.98E+24 6.378E+06 500 7615.2 94.6 3.354E+07

6.67E-11 5.98E+24 6.378E+06 600 7560.5 96.7 3.396E+07

6.67E-11 5.98E+24 6.378E+06 700 7506.9 98.8 3.436E+07

6.67E-11 5.98E+24 6.378E+06 800 7454.4 100.9 3.475E+07

6.67E-11 5.98E+24 6.378E+06 900 7403.0 103.0 3.514E+07

6.67E-11 5.98E+24 6.378E+06 1000 7352.7 105.1 3.551E+07

6.67E-11 5.98E+24 6.378E+06 1100 7303.3 107.3 3.587E+07

6.67E-11 5.98E+24 6.378E+06 1200 7255.0 109.4 3.622E+07

6.67E-11 5.98E+24 6.378E+06 20000 3888.6 710.6 5.498E+07

6.67E-11 5.98E+24 6.378E+06 35789 3075.6 1436.3 5.781E+07

Hohman Transfer OrbitHohman Transfer Orbit

Initial Orbit

Transfer Orbit

Final Orbit

On Orbit On Orbit ManoeuvreManoeuvre Basics Basics

• Attain initial on-orbit station.• Maintain assigned position (all Geo satellites).• Modify orbit to meet new mission requirements.

On-board motors (“thrusters”) are fitted to modify the satellite’s orbit to:

RepositioningRepositioning

• Primarily for GEO satellites .• Using fuel shortens life.• Takes weeks or months to complete the manoeuvre.• Takes capability away from users.

Moving satellite to a higher or lower orbit:

Skynet 5A Launch SimulationSkynet 5A Launch Simulation