satellite communication ppt.ppt
- 1. Satellite communication UNIT-1 ANUJ KUMAR SHARMA Assistant Professor Faculty of Engineering & Technology Gurukul Kangri University Haridwar
- 2. Orbital Mechanics • We know that the path of satellite revolving around the earth is known as orbit. This path can be represented with mathematical notations. Orbital mechanics is the study of the motion of the satellites that are present in orbits. So, we can easily understand the space operations with the knowledge of orbital motion.
- 3. Orbital Elements • Orbital elements are the parameters, which are helpful for describing the orbital motion of satellites. Following are the orbital elements. • Semi major axis • Eccentricity • Mean anomaly • Argument of perigee • Inclination • Right ascension of ascending node The above six orbital elements define the orbit of earth satellites. Therefore, it is easy to discriminate one
- 4. Semi major axis The length of Semi-major axis (a) defines the size of satellite’s orbit. It is half of the major axis. This runs from the center through a focus to the edge of the ellipse. So, it is the radius of an orbit at the orbit's two most distant points.
- 5. Conti… • Both semi major axis and semi minor axis are represented in the figure. Length of semi major axis (a) not only determines the size of satellite’s orbit, but also the time period of revolution. • If circular orbit is considered as a special case, then the length of semi-major axis will be equal to radius of that circular orbit.
- 6. Eccentricity The value of Eccentricity (e) fixes the shape of satellite’s orbit. This parameter indicates the deviation of the orbit’s shape from a perfect circle. If the lengths of semi major axis and semi minor axis of an elliptical orbit are a & b, then the mathematical expression for eccentricity (e) will be
- 7. Mean Anomaly • For a satellite, the point which is closest from the Earth is known as Perigee. Mean anomaly (M) gives the average value of the angular position of the satellite with reference to perigee. • If the orbit is circular, then Mean anomaly gives the angular position of the satellite in the orbit. But, if the orbit is elliptical, then calculation of exact position is very difficult. At that time, Mean anomaly is used as an intermediate step.
- 8. Argument of Perigee • Satellite orbit cuts the equatorial plane at two points. First point is called as descending node, where the satellite passes from the northern hemisphere to the southern hemisphere. Second point is called as ascending node, where the satellite passes from the southern hemisphere to the northern hemisphere. • Argument of perigee (ω) is the angle between ascending node and perigee. If both perigee and ascending node are existing at same point, then the argument of perigee will be zero degrees • Argument of perigee is measured in the orbital plane at earth’s center in the direction of satellite motion.
- 9. Inclination The angle between orbital plane and earth’s equatorial plane is known as inclination (i). It is measured at the ascending node with direction being east to north. So, inclination defines the orientation of the orbit by considering the equator of earth as reference.
- 10. Right Ascension of Ascending node • We know that ascending node is the point, where the satellite crosses the equatorial plane while going from the southern hemisphere to the northern hemisphere. • Right Ascension of ascending node (Ω) is the angle between line of Aries and ascending node towards east direction in equatorial plane. Aries is also called as vernal and equinox. • Satellite’s ground track is the path on the surface of the Earth, which lies exactly below its orbit. The ground track of a satellite can take a number of different forms depending on the values of the orbital elements.
- 11. Orbital Equations Forces acting on Satellite A satellite, when it revolves around the earth, it undergoes a pulling force from the earth due to earth’s gravitational force. This force is known as Centripetal force (F1) because this force tends the satellite towards it. Mathematically, the Centripetal force (F1) acting on satellite due to earth can be written as
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- 13. Conti…..
- 14. Kepler’s Laws • We know that satellite revolves around the earth, which is similar to the earth revolves around the sun. So, the principles which are applied to earth and its movement around the sun are also applicable to satellite and its movement around the earth. • Many scientists have given different types of theories from early times. But, only Johannes Kepler (1571- 1630) was one of the most accepted scientist in describing the principle of a satellite that moves around the earth. • Kepler formulated three laws that changed the whole satellite communication theory and observations. These are popularly known as Kepler’s laws. These are helpful to visualize the motion through space.
- 15. Kepler’s First Law Kepler’s first law states that the path followed by a satellite around its primary (the earth) will be an ellipse. This ellipse has two focal points (foci) F1 and F2 as shown in the figure below. Center of mass of the earth will always present at one of the two foci of the ellipse.
- 16. Conti…. • For an elliptical path, the value of eccentricity (e) is always lie in between 0 and 1, i.e. 0 < e < 1, since a is greater than b. Suppose, if the value of eccentricity (e) is zero, then the path will be no more in elliptical shape, rather it will be converted into a circular shape.
- 17. Kepler’s Second Law Kepler’s second law states that for equal intervals of time, the area covered by the satellite will be same with respect to center of mass of the earth. This can be understood by taking a look at the following figure.
- 18. Kepler’s Third Law Kepler’s third law states that, the square of the periodic time of an elliptical orbit is proportional to the cube of its semi major axis length. Mathematically, it can be written as follows −
- 19. Conti…. • A satellite, when it revolves around the earth, undergoes a pulling force from the earth, which is gravitational force. Similarly, it experiences another pulling force from the sun and the moon. Therefore, a satellite has to balance these two forces to keep itself in its orbit.
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- 21. Orbital Slots • Here, a question may arise that with more than 200 satellites that are in geosynchronous orbit, how do we keep them from running into each other or from attempting to use the same location in space? • To answer this problem (question), international regulatory bodies like the International Telecommunications Union (ITU) and national government organizations like the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit, where the communications satellites can be located. • These locations are specified in degrees of longitude and are called as orbital slots. The FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C-band and Ku-band satellites due to the huge demand for orbital slots.