The azimuth and elevation angles of the ground station antenna are termed as look angles. 18. Write short notes on station keeping. It is the process of maintenance of satellite’s attitude against different factors that can cause drift with time. Satellites need to have their orbits adjusted from time to time, because the satellite is initially placed in the correct orbit, natural forces induce a progressive drift. 19. What are the geostationary satellites? The satellites present in the geostationary orbit are called geostationary satellite. The geostationary orbit is one in which the satellite appears stationary relative to the earth. It lies in equatorial plane and inclination is ‘0’. The satellite must orbit the earth in the same direction as the earth spin. The orbit is circular. 20. What is sun transit outage. The sun transit is nothing but the sun comes within the beam width of the earth station antenna. During this period the sun behaves like an extremely noisy source and it blanks out all the signal from the satellite. This effect is termed as sun transit outage.
PART B 1.Explain about kepler laws in detail Kepler's laws are: 1. The orbit of every planet is an ellipse with the Sun at one of the two foci. 2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. 3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. Kepler's laws are strictly only valid for a lone (not affected by the gravity of other planets) zeromass object orbiting the Sun; a physical impossibility. Nevertheless, Kepler's laws form a useful starting point to calculating the orbits of planets that do not deviate too much from these restrictions. First Law "The orbit of every planet is an ellipse with the Sun at one of the two foci." Figure 1: Kepler's first law placing the Sun at the focus of an elliptical orbit An ellipse is a particular class of mathematical shapes that resemble a stretched out circle. (See the figure to the right.) Note as well that the Sun is not at the center of the ellipse but is at one of the focal points. The other focal point is marked with a lighter dot but is a point that has no physical significance for the orbit. Ellipses have two focal points neither of which are in the center of the ellipse (except for the one special case of the ellipse being a circle). Circles are a special case of an ellipse that are not stretched out and in which both focal points coincide at the center.
Symbolically an ellipse can be represented in polar coordinates as: where (r, θ) are the polar coordinates (from the focus) for the ellipse, p is the semi-latus rectum, and ε is the eccentricity of the ellipse. For a planet orbiting the Sun then r is the distance from the Sun to the planet and θ is the angle with its vertex at the Sun from the location where the planet is closest to the Sun. At θ = 0°, perihelion, the distance is minimum At θ = 90° and at θ = 270°, the distance is At θ = 180°, aphelion, the distance is maximum The semi-major axis a is the arithmetic mean between rmin and rmax: so The semi-minor axis b is the geometric mean between rmin and rmax:
so The semi-latus rectum p is the harmonic mean between rmin and rmax: The eccentricity ε is the coefficient of variation between rmin and rmax: The area of the ellipse is The special case of a circle is ε = 0, resulting in r = p = rmin = rmax = a = b and A = π r2. Second law Figure 3: Illustration of Kepler's second law. "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time." The planet moves faster near the Sun, so the same area is swept out in a given time as at larger distances, where the planet moves more slowly. The green arrow represents the planet's velocity, and the purple arrows represents the force on the planet.