This document discusses different types of orbits used for satellites, including geostationary orbit, low Earth orbit, medium Earth orbit, and polar orbit. It provides details on the characteristics of each orbit type, such as altitude, period of revolution, coverage capabilities, and perturbations affecting the orbit. Geostationary orbit allows a satellite to appear stationary over one position on Earth, with an altitude of about 36,000 km. Low Earth orbit is below geostationary altitude, ranging from 160 to 2,500 km, requiring multiple satellites for global coverage. Medium Earth orbit is between low Earth and geostationary orbits. Polar orbit has an inclination near 90 degrees and is useful for sensing applications.

1. SATELLITE
COMMUNICATION
TYPES OF ORBITS
LECTURE-2
ASST. PROF. SANDIP DAS

2. SIDEREAL DAY

3. GEOSTATIONARY ORBIT
• From Kepler’s third law we know that there is a fixed relationship between orbit radius and the
orbit period of revolution.
• If the orbit radius is chosen-
• If the orbit radius is chosen so that the period of revolution of the satellite is exactly set
to the period of the earth’s rotation, one mean sidereal day, a unique satellite orbit is
defined.
• If the orbit is circular (eccentricity = 0).
• If the orbit is in the equatorial plane (inclination angle = 0◦).
• The satellite will appear to hover motionless above the earth at the sub-satellite point above the
equator. This important special orbit is the geostationary earth orbit (GEO).

4. GEOSTATIONARY ORBIT
• Considering the orbit as circular, according to Kepler’s third law, the orbit radius for the GEO, 𝑟𝑠, is found
as-
𝑟𝑠=
𝜇
4𝜋2
1
3
𝑇
2
3=
3.986004∗105
4𝜋2
1
3
(86164.09)
2
3
=42164.17 Km
Where, T= 1 mean sidereal day= 86164.09 sec
• The geostationary height (altitude above the earth’s surface), ℎ 𝑠, is given by
ℎ 𝑠= 𝑟𝑠 − 𝑟𝐸= 42164-6378= 35786 Km≈36000 Km
Where, 𝑟𝐸= 6378 Km= equatorial earth radius

5. GEOSTATIONARY ORBIT
• The geostationary orbit is an ideal orbit that cannot be achieved for real artificial satellites
because there are many other forces besides the earth’s gravity acting on the satellite.
• A ‘perfect orbit’, i.e., one with e exactly equal to zero and with inclination exactly equal to 0◦, cannot be
practically achieved without extensive station keeping and a vast amount of fuel to maintain the precise
position required.
• typical GEO orbit in use today would have an inclination angle slightly greater than 0 and possibly an
eccentricity that also exceeds 0.
• The ‘real world’ GEO orbit that results is often referred to as a geosynchronous earth orbit (GSO) to
differentiate it from the ideal geostationary orbit.

6. GEOSTATIONARY ORBIT

7. LOW EARTH ORBIT
• Earth satellites that operate well below the geostationary altitude, typically at altitudes from 160 to
2500 km, and in near circular orbits, are referred to as low earth orbit or LEO satellites.
• The low earth orbit satellite has several characteristics that can be advantageous for communications
applications-
• requires earth terminal tracking
• approx. 8 to 10 minutes per pass for an earth terminal
• requires multiple satellites (12, 24, 66, . . . ) for global coverage
• The non-spherical shape of the earth will cause two major perturbations to the LEO orbit-
• The point on the equator where the LEO satellite crosses from south to north (the ascending node) will
drift westward several degrees per day.
• Rotates the orientation of the major axis in the plane of the orbit, either clockwise or counterclockwise.

8. LOW EARTH ORBIT

9. MEDIUM EARTH ORBIT
• Satellites operate in the range between LEO and GSO, typically at altitudes of 10 000 to 20 000 km.
• similar to LEO, but at higher circular orbits.
• 1 to 2 hours per pass for an earth terminal.
• used for meteorological, remote sensing and position, location applications.

10. POLAR ORBIT
• A circular orbit with an inclination near 90◦ is referred to as a polar orbit.
• Polar orbits are very useful for sensing and data gathering services, because their orbital characteristics
can be selected to scan the entire globe on a periodic cycle.
• Landsat, for example, operated with an average altitude of 912 km, and an orbital period of 103
minutes, tracing out 14 revolution each day.
• Each day the orbit shifted about 160 km west on the equator, returning to its original
position after 18 days and 252 revolutions.