Today’s discussion topic is satellite orbits. Before going to explain what the satellite orbits are and how many type of these orbits. You have to know about the satellite remote sensing. What actually the satellite remote sensing is?
Satellite remote sensing is the measurement of properties of object on the earth by using the data from instruments like on board aircraft and satellites. Satellite remote sensing provides the consistent and repetitive monitoring of earth so that we can monitor the global changes in both the earth and the atmosphere. After the brief introduction of satellite remote sensing, we move towards the satellite orbits. Satellite orbits are the orbit where we place a satellite that orbits the earth. There are three types of satellite orbit including Low earth orbit (LEO), Medium earth orbit (MEO) and High earth orbit (HEO).
Now the question arise what are the LEO, MEO, HEO and what type of satellites are launched in LEO, MEO, HEO.
High Earth orbit (HEO)
High earth orbit is about 36000 km above the earth surface. Satellite in this orbit has a fixed position relative to the earth. It takes parallel measuremnts in a limited area from low to middle latitudes. High earth orbit is also known as geosynchronous orbit. Satellites in this orbit rotate in the same direction as the rotation of the earth. Its direction of rotation is always posigrade. Satellites in this orbit have zero inclination and eccentricity. Satellite in circular geosynchronous orbit is directly over the equator will have a geostationary orbit that does not move at all relative to the ground. Geo-synchronous orbit enters a sweet spot that allows the satellite to remains in the fixed position relative to the earth.
Satellite in high earth orbit completes its one revolution in 24 hrs. Satellite in this orbit cannot provide complete coverage of the earth. It covers approximately 42% of the earth surface. A constellation of three satellites equally spaced around the globe cover the equator and latitude up to 81o both north and south.
Satellite in this orbit is valuable for weather monitoring and television broadcast as it gives the constant view of same area. Every few minutes, geostationary satellite like Geostationary Operational Environmental Satellite (GOES) gives the information about wind, cloud, water vapor present in the atmosphere.
Medium Earth Orbit (MEO)
Medium earth orbit is about 20,200 km above the earth surface. It is closer to the earth. There are two types of Medium earth orbit i.e. Semi-synchronous orbit and Molniya orbit.
Semi-synchronous orbit is nearly circular. Satellites in this orbit have low inclination and low eccentricity. Satellites in this orbit complete its one revolution in 12 hrs. As the satellite move, the earth rotates underneath it. Satellite in this orbit crosses over the same two spots on the equator every day. It is highly predictable and consistent. Global Positioning System (GPS) use the semi-synchronous orbit.
Molniya orbit is the commonly used medium earth orbit. It is invented by Russians for observing high latitude especially polar regions. Satellites in this orbit have high inclination i.e. 63.4o and high eccentricity i.e. 0.722 to maximize the viewing time over high latitude. The satellite in this orbit completes its one revolution in 12 hrs. and it covers the same location every day and night. Russian communication satellite and Sirius radio satellite use Molniya orbit.
Low Earth Orbit (LEO)
Low earth orbit is about 1000 km above the earth surface. It is very closer to the earth. The satellite moves very quickly when it is closer to the earth. As it moves away, its speed slows. Low earth orbit cross close to the pole. It has ascending and descending node. Ascending node is the point where satellite crosses the equatorial plane going north and descending node is the point where the satellite crosses the equatorial plane going south.
Satellites in low earth orbit at a steep inclination relative to the equator. Satellite in this orbit rotates in the opposite direction as the rotation of the earth so its direction of rotation is retrograde. In low earth orbit, satellite’s orbit and earth’s rotation are combined; they form an s-shaped path relative to the map of earth’s surface. Like geosynchronous orbit, low earth orbit enters a sweet spot that allows the satellite to stay in one time.
In highly inclined orbit, satellite moves around the earth from pole to pole. Satellite in this orbit completes its one revolution in 99 minutes. During its one half of the orbit, it covers the daytime side of the earth and at the pole satellite crosses over the nighttime side of the earth. Low earth orbit is known as Sun-synchronous orbit, it crosses over the equator at the same local time each day and night. NASA’s earth observing satellite use low earth orbit and satellite in this orbit allow consistent scientific observation.