Satellites and their Orbits

TOPIC: Awareness in the Orbits fields of IT, Space, Computers, robotics, nano-technology, bio-technology and issues relating to intellectual property rights.

Types of Orbits according to shape

Circular Orbit
Elliptical Orbit

Types of orbits according to height ( Orbits )

Low Earth Orbit- The altitude is between 160- 2,000 Km. For example, Hubble Space Telescope, International Space Station (ISS)
Medium Earth Orbit- It is also known as Intermediate Circular Orbit (ICO). It lies above the LEO and below the geostationary orbit. 2,000 Km- 36,000 Km.
Geostationary Orbit- The orbit is circular.
– It is in the equatorial plane (directly above the equator and thus its inclination is zero).
– The angular velocity of the satellite is equal to the angular velocity of the earth.
– The period of revolution of the satellite is equal to the period of rotation of the Earth.
– The satellite finishes one revolution around the Earth in exactly one day.
– There is only one geostationary orbit.
– It is also known as the Geostationary Earth Orbit or the Geosynchronous Equatorial Orbit (GEO).
– It is at a height of 36,000 Km from the Earth ( 35,786 Km to be precise).
– The satellite follows the direction of Earth’s rotation.
– The satellite appears stationary from the Earth.
High Earth Orbit- It lies beyond 36,000 Km.
Geosynchronous Orbit- It is not circular.
– It is not in the equatorial plane but is in the inclined plane.
– Angular velocity of the satellite is equal to the angular velocity of the Earth.
– Period of revolution of the satellite is equal to the period of rotation of the Earth.
– The satellite finishes one revolution around the Earth in exactly one day.
– There are many geosynchronous orbits.
Polar Orbit- It passes above or nearly above both the poles of the Earth.
– It has an inclination of 90º to the equator.
– The orbit is used for Earth mapping, Earth observation etc.
– For Example, Indian Remote Sensing Satellites (IRS).
Sun Synchronized Orbit- It is a special case of the polar orbit.
– The satellite travels from the North to South poles as the Earth turns below it.
– The satellite passes over the same part of the Earth at roughly the same local time each day.
– This orbit can make communication and various forms of data collection very convenient.
– They are generally medium or low orbits.
– Uses include for surface analysis and espionage.
Example– Resource Satellites, Carto Satellites, Ocean Satellites.
RISAT-1, RISAT-2 (Radar Imaging Satellite)
RISAT-2 (a coastal surveillance satellite) was launched before RISAT-1(2012) in 2009 the aftermath of the 2008 Mumbai Terror Attacks.
Indian Mini Satellite (IMS)- part of spying system
Technology Experiment Satellite (TES)- It is also an IRS. It is an experimental satellite to demonstrate and validate, in orbit, technologies that could be used in the future satellites of ISRO.

Shifting Orbits

When satellite reaches the apogee* position, an apogee motor is fired and the satellite moves to another orbit.
* apogee- the point in the orbit of the moon or a satellite at which it is furthest from the earth.

Naming of satellites

All geostationary satellites are called INSAT/GSAT i.e. Indian National Satellite used for communication, weather etc. They are launched using Geosynchronous Satellite Launch Vehicle (GSLV).
All remote sensing satellites are polar orbiting satellites. They are launched using a Polar Satellite Launch Vehicle (PSLV). For example, CARTOSAT, RISAT, OCEANSAT, RESOURCESAT.

How are satellites disposed off?

There are two methods by which a satellite can be disposed off. Either of the two can be employed.

Bring the satellite down to a lower orbit and burn it in the Earth’s atmosphere.
Take the satellite above its orbit and park it there.

A height of ∼ 38,000 Km is known as the graveyard orbit/ junk or the disposal orbit.
All geostationary satellites are shifted to this orbit after their useful life because they are too bulky to be brought down.
Polar orbiting satellites are smaller in size and are at a lower height.
So they are simply brought down as their volume is less.
This makes them burn in the earth’s atmosphere.

How life of a satellite is calculated?

The life of a satellite is calculated by the amount of fuel required by it to fulfill its functions.
Once the fuel is finished, the satellite can be shifted above or below the orbit for disposal.

What is the fuel used?