{"id":73,"date":"2019-03-08T06:05:42","date_gmt":"2019-03-08T06:05:42","guid":{"rendered":"http:\/\/esp06.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=73"},"modified":"2019-03-08T06:18:07","modified_gmt":"2019-03-08T06:18:07","slug":"types-of-orbits","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/esp06\/chapter\/types-of-orbits\/","title":{"rendered":"Types of Orbits"},"content":{"raw":"
\r\n\r\n \r\n\r\n1. Aim of the Module<\/strong>\r\n\r\n \r\n

The purpose of this module is to understand the different types of orbits of satellites that are used for remote sensing of earth\u2019s surface features.<\/p>\r\n \r\n\r\n2. Introduction<\/strong>\r\n\r\n \r\n

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 \u2018orbit\u2019. This path is generally elliptical, and depends on two important factors:<\/p>\r\n \r\n\r\n(a)\u00a0 Gravitational pull of the earth\r\n\r\n \r\n\r\n(b)\u00a0\u00a0 Velocity of the satellite\r\n\r\n \r\n

During the path of motion of satellite, the time taken to complete one revolution of the orbit is called the orbital period<\/em><\/strong>. The path on the earth\u2019s surface corresponding to the satellite\u2019s motion across the sky is called its ground track<\/em><\/strong>. 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<\/em><\/strong> of the satellite. Spatial and temporal coverage of earth is determined by the nature of the orbit of satellite.<\/p>\r\n \r\n\r\n3. Types of orbits<\/strong>\r\n\r\n \r\n

There are three types of orbit based on altitude, orientation and rotation of the satellite in relation to the earth. These are:<\/p>\r\n \r\n\r\nA. Polar Orbits\r\n\r\n \r\n\r\nB.\u00a0 Sun-synchronous Orbits\r\n\r\n \r\n\r\nC.\u00a0 Geostationary Orbits\r\n\r\n \r\n

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\u00a0in 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- <\/span>low<\/em><\/strong> and <\/span>high<\/em><\/strong>.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n\r\n3.1 Low Earth Orbits<\/strong>\r\n\r\n \r\n

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:<\/p>\r\n \r\n\r\n(i)\u00a0\u00a0 Polar orbits\r\n\r\n \r\n\r\n(ii)\u00a0\u00a0 Near polar orbits\r\n\r\n \r\n\r\n(iii)\u00a0\u00a0 Sun-synchronous orbits\r\n\r\n \r\n\r\n \r\n\r\n3.1.1 Polar orbits<\/strong>\r\n\r\n \r\n

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 <\/em><\/strong>whose (orbits within the plane of the equator) and polar orbiting satellites <\/em><\/strong>(orbits in the plane of the earth\u2019s polar axis).<\/p>\r\n \r\n

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.<\/p>\r\n\r\n<\/div>\r\n

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Figure 1: Polar Orbiting satellites (http:\/\/noaasis.noaa.gov\/NOAASIS\/ml\/genlsatl.html)<\/p>\r\n \r\n\r\n \r\n\r\n3.1.2 Near-polar orbits<\/strong>\r\n\r\n \r\n

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.<\/p>\r\n\"\"\r\n

Figure 2: Near Polar Sunsynchronous Orbit (www.crisp.nus.edu.sg)<\/a><\/p>\r\n \r\n\r\n \r\n

3.1.3 Sun-Synchronous orbits - <\/strong>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.<\/p>\r\n\r\n<\/div>\r\n

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Figure 3: Sun-synchronous orbit (tornado.sfsu.edu<\/em>)<\/p>\r\n \r\n\r\nAdvantages of sun-synchronous orbits<\/strong>\r\n\r\n \r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Due to low altitude of sun-synchronous orbit, ground resolution is improved.\r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Regular scanning resolution along the ground track is obtained.\r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Global coverage of entire Earth can be achieved.\r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Low altitude permits both a large ground swath and a good ground resolution.\r\n\r\n \r\n\r\n \r\n\r\nLimitations of sun-synchronous orbits<\/strong>\r\n\r\n \r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Poor temporal observation with only one sun-synchronous satellite.\r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Although satellites in sun-synchronous orbits pass over polar region on every period, but not same equatorial regions.\r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Generally used for telecommunication, not for Earth observations.\r\n\r\n\u2022\u00a0\u00a0\u00a0\u00a0\u00a0 Limited applications of POES for weather forecasting.\r\n\r\n \r\n\r\n \r\n\r\n3.2 High level satellites<\/strong>\r\n\r\n \r\n

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\u00a0the 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.<\/span><\/p>\r\n\r\n<\/div>\r\n

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Figure 4: Geostationary satellites (www.crisp.nus.edu.sg)<\/p>\r\n \r\n\r\n \r\n\r\n3.2.1 Geosynchronous satellites<\/strong>\r\n\r\n \r\n

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.<\/p>\r\n \r\n\r\n3.2.2 Geostationary satellites<\/strong>\r\n\r\n \r\n

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.<\/p>\r\n \r\n\r\nSome examples of geostationary satellites are:\r\n\r\n<\/div>\r\n