12 Types of Orbits

Dr. Puneeta Pandey

 

1. Aim of the Module

 

The purpose of this module is to understand the different types of orbits of satellites that are used for remote sensing of earth’s surface features.

 

2. Introduction

 

In the previous modules, we have studied about the different types of sensors that are mounted on a platform to view and image the objects and places on the surface of the earth. These platforms could be air-borne (e.g., balloons, helicopters, aeroplanes) or, space-borne (e.g. space shuttle, satellites). In space-borne remote sensing, sensors mounted on-board a spacecraft or satellite move around the earth in fixed path known as ‘orbit’. This path is generally elliptical, and depends on two important factors:

 

(a)  Gravitational pull of the earth

 

(b)   Velocity of the satellite

 

During the path of motion of satellite, the time taken to complete one revolution of the orbit is called the orbital period. The path on the earth’s surface corresponding to the satellite’s motion across the sky is called its ground track. As the earth below is rotating, the satellite traces out a different path on the ground in each subsequent cycle. As a result, remote sensing satellites often repeat their path after a fixed time interval. This time interval is called the repeat cycle of the satellite. Spatial and temporal coverage of earth is determined by the nature of the orbit of satellite.

 

3. Types of orbits

 

There are three types of orbit based on altitude, orientation and rotation of the satellite in relation to the earth. These are:

 

A. Polar Orbits

 

B.  Sun-synchronous Orbits

 

C.  Geostationary Orbits

 

In general, satellites are placed in any one of the three types of orbits mentioned above. The type of orbit determines the design of the sensor and its instantaneous field of view (IFOV). As we shall study in subsequent modules, IFOV is the area on the Earth which can be viewed at any particular moment in time. Thus, there are two earth orbits- low and high.

 

3.1 Low Earth Orbits

 

Low Earth Orbits are also called as Low level Earth observation satellites (LEO). These are the most common orbits for passive sensors in remote sensing that use sunlight as a source of energy for illumination. LEO satellites are sun synchronous, i.e., they remain fixed with respect to the sun. They are divided into three broad categories namely:

 

(i)   Polar orbits

 

(ii)   Near polar orbits

 

(iii)   Sun-synchronous orbits

 

 

3.1.1 Polar orbits

 

Polar orbits are low earth orbits with an altitude of 700 to 800 km, balancing the acceleration due to force of gravity and centrifugal force. A polar orbiting satellite is highly desirable for remote sensing applications since it views every part of the Earth’s surface. Such satellites are further sub-divided into equatorial orbiting satellites whose (orbits within the plane of the equator) and polar orbiting satellites (orbits in the plane of the earth’s polar axis).

 

An example is Polar Orbiting Environmental Satellites (POES) that are placed in circular sun-synchronous orbits with orbital periods of 98 to 102 minutes. They are launched into orbits at high inclinations to the Earth’s rotation, such that they pass across high latitudes near the poles.

Figure 1: Polar Orbiting satellites (http://noaasis.noaa.gov/NOAASIS/ml/genlsatl.html)

 

 

3.1.2 Near-polar orbits

 

In a near polar orbit, the orbital plane is inclined at a small angle with respect to the earth’s rotation axis. As a result, a satellite following a near polar orbit passes close to the poles. Such satellites almost cover the entire earth globally.

Figure 2: Near Polar Sunsynchronous Orbit (www.crisp.nus.edu.sg)

 

 

3.1.3 Sun-Synchronous orbits – These orbits are designed so that the satellite’s orientation is fixed relative to the Sun throughout the year, which implies that they cover each area of the world at a constant local time of day. These orbits are usually at an altitude between 600 to 800 km, and are used for Earth observation, solar study, weather predictions and reconnaissance studies. Most of the Earth observing missions such as NOAA polar orbiting meteorological satellites, Landsat, SPOT etc. use sun-synchronous satellites in low near polar orbits.

Figure 3: Sun-synchronous orbit (tornado.sfsu.edu)

 

Advantages of sun-synchronous orbits

 

•      Due to low altitude of sun-synchronous orbit, ground resolution is improved.

•      Regular scanning resolution along the ground track is obtained.

•      Global coverage of entire Earth can be achieved.

•      Low altitude permits both a large ground swath and a good ground resolution.

 

 

Limitations of sun-synchronous orbits

 

•      Poor temporal observation with only one sun-synchronous satellite.

•      Although satellites in sun-synchronous orbits pass over polar region on every period, but not same equatorial regions.

•      Generally used for telecommunication, not for Earth observations.

•      Limited applications of POES for weather forecasting.

 

 

3.2 High level satellites

 

These orbits are followed by Geostationary or geosynchronous satellites that appear to be stationary with respect to the Earth since they are placed at a very high altitude (~ 36,000 km) so as to equalize the orbital period of the satellite to that of Earth’s rotation. Any sensor onboard a geosynchronous satellite views the same area of the Earth at all times. Examples include communications and weather satellites.

Figure 4: Geostationary satellites (www.crisp.nus.edu.sg)

 

 

3.2.1 Geosynchronous satellites

 

Earth-synchronous or geosynchronous satellites are placed into orbit so that their period of rotation exactly matches the Earth’s rotation period of 24 hours, and rotate from west to east direction. These satellites are placed in highly elliptical orbits with an inclination of 180 degrees to the equator.

 

3.2.2 Geostationary satellites

 

Geostationary orbit follows the equatorial plane of the Earth and such satellites are positioned directly over the equator so that it travels in the same direction as the earth’s rotation with the same period of 24 hours. The geostationary orbits are commonly used by meteorological satellites.

 

Some examples of geostationary satellites are:

  • GMS (Geostationary Meteorological Satellites) (Japan), over the Asia-Pacific region (140oE)
  • INSAT (Indian National Satellite System) (India)
  • GOES (Geostationary Operational Environmental Satellites) (USA), over the American continents.
  • METEOSAT (Europeean Space Agency), over Europe and Africa.
  • FY-2 (Fengyun-2) (China), over the Asia-Pacific region

 

Advantages of Geostationary Satellites for remote imaging – Since the geostationary satellites are placed at a very high altitude, they can view a very large area of the earth (about 45-50%); thus ideally suited for meteorological applications.

 

Disadvantages of Geostationary Satellites for remote imaging – Due to poor spatial resolution pertaining to high altitude, these satellites cannot be used for mapping purposes and have limited applications at higher latitude areas.

 

4. Conclusions

 

Thus, at the end of this module, you would have gained a general insight about the types of orbits as well as the factors determining the types of orbits. Broadly, there are two types of orbits based on altitude-low earth and high earth orbits. Low earth orbits are further categorized into polar, near-polar and sun-synchronous orbits; while, high-earth orbits are categorized into geosynchronous and geostationary orbits. The satellites pertaining to these orbits have various advantages over air-borne platforms and sensors such as global and repetitive coverage with reasonable costing; thus, these are widely used for research, mapping, monitoring of earth’s features and meteorological applications.

 

Learn More:

  • https://www.esa.int/Our_Activities/Space_Transportation/Types_of_orbits http://cirrus.larc.nasa.gov/SCOOL/orbits.html
  • http://www.jpl.nasa.gov/basics/bsf5-1.htm
  • http://www.csr.utexas.edu/tsgc/projects/stars/rsorbits.html
  • https://marine.rutgers.edu/cool/education/class/paul/orbits2.html http://nptel.ac.in/courses/105108077/module2/lecture6.pdf
  • http://meteorology.uonbi.ac.ke/sites/default/files/cbps/sps/meteorology/ http://noaasis.noaa.gov/NOAASIS/ml/genlsatl.html
  • www.crisp.nus.edu.sg
  • Lillesand, T. M., Kiefer, R. W., Chipman, J. W. (2004). “Remote sensing and image interpretation”, Wiley India (P). Ltd., New Delhi.
  • Sabins, F.F. 1997. Remote Sensing and Principles and Image Interpretation. WH Freeman, New York.