CONCEPT – SATELLITE ORBIT TYPES

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CONCEPT – SATELLITE ORBIT TYPES

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• Various types of orbits: There are many different satellite orbits possible, depending upon its application.

1. Those used for direct broadcast television use a Geostationary orbit. Many communications satellites also use it.
2. Those used for satellite phones may use Low Earth orbiting systems. Global Positioning (GPS) system satellites occupy a relatively low Earth orbit.
3. There are other types of satellite from weather satellites to research satellites and many others. Each will have its own type.
4. The actual satellite orbit that is chosen will depend on factors including its function, and the area it is to serve. In some instances the satellite orbit may be as low as 100 miles (160 km) for a Low Earth Orbit LEO, whereas others may be over 22 000 miles (36000 km) high as in the case of a GEostationary Orbit GEO. The satellite may even have an elliptical rather than a circular orbit.

• Gravity’s pull and satellite orbits: As satellites orbit the Earth, they are pulled back in by the force of the gravitational field. They can fall back to Earth, burning up in the upper reaches of the atmosphere. But the motion of the satellite rotating around the Earth has a force associated with it pushing it away from the Earth. For any given orbit there is a speed for which gravity and the centrifugal force balance each other and the satellite remains in a stable orbit.
• Higher and lower: The lower the satellites orbit the Earth, the stronger the gravitational pull, and the faster it must travel to balance it out. The higher it is, the lower is the gravity’s pull, and the satellite velocities are correspondingly less.
• Speeds needed (for the counteracting force): For a very low orbit of around 160 km a velocity of about 28,000 km per hour is needed. That makes one orbit possible in around 90 minutes. At an altitude of 35,200 km, a velocity of 11,200 kmph is needed giving an orbit time of about 24 hours.

• Circular and elliptical orbit definitions: A satellites orbit the Earth in one of two basic types of orbit –

1. Circular satellite orbit: The distance from the Earth remains the same at all times.
2. Elliptical satellite orbit:  The elliptical orbit changes the distance to the Earth
3. Now we take a look at the most popular of orbit types.

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• SWEET SPOTS AND LAGRANGE POINTS

1. When a satellite reaches exactly 42,164 kilometers from the center of the Earth (about 36,000 kilometers from Earth’s surface), it enters a “sweet spot” in which its orbit matches Earth’s rotation. Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south. This special, high Earth orbit is called geosynchronous.
2. A satellite in a circular geosynchronous orbit directly over the equator (eccentricity and inclination at zero) will have a geostationary orbit that does not move at all relative to the ground. It is always directly over the same place on the Earth’s surface. A geostationary orbit is extremely valuable for weather monitoring because satellites in this orbit provide a constant view of the same surface area.
3. Other orbital “sweet spots,” just beyond high Earth orbit, are the Lagrange points. At the Lagrange points, the pull of gravity from the Earth cancels out the pull of gravity from the Sun. Anything placed at these points will feel equally pulled toward the Earth and the Sun and will revolve with the Earth around the Sun.
4. Of the five Lagrange points in the Sun-Earth system, only the last two, called L4 and L5, are stable. A satellite at the other three points is like a ball balanced at the peak of a steep hill: any slight perturbation will push the satellite out of the Lagrange point like the ball rolling down the hill. Satellites at these three points need constant adjustments to stay balanced and in place. Satellites at the last two Lagrange points are more like a ball in a bowl: even if perturbed, they return to the Lagrange point.

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• Geostationary orbit: One very popular orbit format is the geostationary. It is used by many applications including direct broadcast as well as communications or relay systems.

1. Its advantage is that the satellite remains in the same position throughout the day, and antennas can be directed towards the satellite and remain on track. Its useful for applications such as direct broadcast TV where changing directions for the antenna would not be practicable.
2.  As satellites in geostationary orbit continuously cover a large portion of the Earth, this makes it an ideal orbit for telecommunications or for monitoring continent-wide weather patterns and environmental conditions. It also decreases costs as ground stations do not need to track the satellite.
3. A constellation of three equally spaced satellites can provide full coverage of the Earth, except for the polar regions.
4. As the height of a satellite increases, so the time for the satellite to orbit increases. At a height of 35790 km, it takes 24 hrs for the satellite to orbit. This type of orbit is known as a geosynchronous orbit, i.e. it is synchronized with the Earth.
5. One particular form of geosynchronous orbit is known as a geostationary orbit. In this type of orbit the satellite rotates in the same direction as the rotation of the Earth and has an approximate 24 hour period. This means that it revolves at the same angular velocity as the Earth and in the same direction and therefore remains in the same relative position.
6. The only way in which an orbit that rotates once per day can remain over exactly the same spot on the Earth's surface is that it moves in the same direction as the earth's rotation. Also it must not move north or south for any of its orbit. This can only occur if it remains over the equator.
7. Even when satellites are placed into a geostationary orbit, there are several forces that can act on it to change its position slowly over time. Factors including the earth's elliptical shape, the pull of the Sun and Moon and others act to increase the satellite orbital inclination. In particular the non-circular shape of the of the Earth around the Equator tends to draw the satellites towards two stable equilibrium points, one above the Indian Ocean and the other very roughly around the other side of the World.. This results in what is termed as an east-west libration or movement back and forth.
8. To overcome these movements, fuel is carried by the satellites to enable them to carry out "station-keeping" where the satellite is returned to its desired position.
9. A single geostationary satellite can see approximately 42% of the Earth's surface, and a constellation of three satellites equally spaced around the globe can provide complete coverage around the equator and up to latitudes of 81° both N & S.

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• Geosynchronous orbit:

1. A geosynchronous orbit (GSO) is an orbit around Earth of a satellite with an orbital period that matches Earth's rotation on its axis, which takes one sidereal day (23 hours, 56 minutes, and 4 seconds).
2. For an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day.
3. Over the course of a day, the object's position in the sky traces out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity.
4. A geosynchronous orbit is 35,786 km above the Earth's surface. Those closer to Earth orbit faster than Earth rotates, so from Earth, they appear to move eastward while those that orbit beyond geosynchronous distances appear to move westward.
5. A special case of geosynchronous orbit is the geostationary orbit, which is a circular geosynchronous orbit inclined 0° to Earth's equatorial plane (directly above the Equator).
6. At times, the term geosynchronous Earth orbit (GEO) may be a synonym for geosynchronous equatorial orbit.
7. Elliptical geosynchronous orbits are used in communications satellites to keep the satellite in view of its assigned ground stations and receivers. A satellite then appears to oscillate in the sky from the viewpoint of a ground station, tracing an analemma in the sky.
8. An analemma is a diagram showing the variation of the position of the Sun in the sky over the course of a year, as viewed at a fixed time of day and from a fixed location on the Earth.

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• Geostationary Transfer Orbit: This is an elliptical Earth orbit used to transfer a spacecraft from a low altitude orbit or flight trajectory to geostationary orbit. The apogee is at 36,000 km. When a spacecraft reaches this point, its apogee kick motor is fired to inject it into geostationary orbit.
• Low Earth Orbits: A Low Earth orbit (LEO) is normally at an altitude of less than 1000 km and could be as low as 160 km above the Earth. Satellites in this circular orbit travel at a speed of around 7.8 km per second. At this speed, a satellite takes approximately 90 minutes to circle the Earth. In general, these orbits are used for remote sensing, military purposes and for human spaceflight as they offer close proximity to the Earth’s surface for imaging and the short orbital periods allow for rapid revisits. The International Space Station is in low Earth orbit.
• Medium Low Earth Orbits: This orbit takes place at an altitude of around 1000 km and is particularly suited for constellations of satellites mainly used for telecommunications. A satellite in this orbit travels at approximately 7.3 km per second.
• Polar Orbits: These pass over the Earth’s polar regions from north to south. The orbital track of the satellite does not have to cross the poles exactly for an orbit to be called polar, an orbit which passes within 20 to 30 degrees of the poles is still classed as a polar orbit. These orbits mainly take place at low altitudes of between 200 to 1000 km. Satellites in polar orbit look down on the Earth’s entire surface and can pass over the North and South Poles several times a day. Polar orbits are used for reconnaissance and Earth observation. If a satellite is in polar orbit at an altitude of 800 km, it will be travelling at a speed of approximately 7.5 km per second.
• Sun Synchronous orbits: These are low-earth, near polar orbits synchronous with the Sun. A satellite here will usually be at an altitude of between 600 to 800 km. Generally these orbits are used for Earth observation, solar study, weather forecasting and reconnaissance, as ground observation is improved if the surface is always illuminated by the Sun at the same angle when viewed from the satellite. A Sun-synchronous orbit can place a satellite in constant sunlight, which allows the solar panels to work continually.

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