Introduction to Satellite Orbits
Dinesh ManandharCenter for Spatial Information Science
The University of Tokyo
Contact Information: dinesh@iis.u-Tokyo.ac.jp

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 1

Basics of Satellite Orbits• The speed of a satellite is• Determined by the satellite’s orbit• Closely tied to the satellite’s altitude.

• A satellite’s orbit does not depend on its mass

• All objects with the same velocity (speed and direction) at a given pointin space follow the same orbit.

• Satellites close to the Earth move faster than those at higher
altitudes• ISS moves faster than a GPS satellite when viewed from earth• Satellites in low earth orbits (hundreds of kilometers above the Earth)
move rapidly relative to the Earth, completing an orbit in 1.5 to 2 hours.
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 2

Basics of Satellite Orbits• Satellites in higher orbits move at slower speeds than those in lowerorbits• Distance they travel in one orbit is longer.
• The time required for a satellite to orbit (the orbital period) increases withaltitude.
• Only one altitude (36,000 km)permits satellites to orbit at the same rate at
which the Earth rotates, such satellites are called geosynchronous.

• Once in orbit, a satellite does not need constant powering to remain inflight, as airplanes do. Satellites use small onboard rocket engines tomaneuver in space.
• A satellite’s orbit always lies in a plane that passes through the center ofthe Earth.
• The angle between that plane and the plane of the equator is called theorbit’s inclination.
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 3

Basics of Satellite Orbits• Ground Track: The part of the earth that lies beneaththe satellite orbit• A satellite in an orbit with inclination angle “theta”
cannot pass directly over any location on Earth withlatitude greater than “theta”.
• A satellite launched from a site at latitude “theta”
follows an orbit with inclination greater than orequal to “theta”.
• From a launch site at latitude “theta” it is notpossible to launch a satellite into an orbit withinclination less than “theta”.
• A launch site that is not on the equator cannot placea satellite directly into an equatorial orbit.
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 4

Altitude vs. Orbital Speed vs Orbital Period
Altitude (km)Orbital Speed (km/s)

Orbital Period (min)

200

7.8

88.3

500

7.6

94.4

1,000

7.4

104.9

5,000

5.9

201.1

10,0004.9

347.4

20,200 (semisynchronous)

3.9

718.3

35,800 (Geosynchronous)

3.1

1436.2 (24 hours)

Speed needed to keep an object on orbit does not depend on its mass. Any object smallor large with the same velocity will travel on the same orbit.
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 5

Basics of Satellite Orbits : Elliptical Orbits

a2 = b2 + c2
focus

where:
Major Axis = 2a, Line that contains two fociMinor Axis = 2b, Line perpendicular to Major Axis
Distance between the two foci = 2c

Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

caFocusbSlide : 6

Basics of Satellite Orbits : Elliptical Orbits
Elliptical Orbit

a2 = b2 + c2
where:
Major Axis = 2a, Line that contains two fociMinor Axis = 2b, Line perpendicular to Major Axis
Distance between the two foci = 2cPerigee

Earthca

FocusApogeeb
Circular Orbit

Perigee: the point on an elliptical orbit when the satellite is the closest to the earth
Apogee: the point on an elliptical orbit when the satellite is the farthest to the earth
A satellite when near to the perigee moves fasterA satellite when near to apogee moves slower
The speed of a satellite depends not only on Altitude but also the shape of the orbit (length of major-axis)
Dinesh Manandhar, CSIS, The University of Tokyo, dinesh@iis.u-tokyo.ac.jp

Slide : 7