Satellites Links PDF

SATELLITE LINKS  SATELLITE LINKS SATELLITE LINKS USE MICROWAVES Characteristics of satellites. Satellites are regarded as a device that receives a signal from a ground station then: -amplifies it, -perhaps performs some signal processing, then retra...

SATELLITE LINKS  SATELLITE LINKS SATELLITE LINKS USE MICROWAVES Characteristics of satellites. Satellites are regarded as a device that receives a signal from a ground station then: -amplifies it, -perhaps performs some signal processing, then retransmits it to one or more ground stations. As mentioned, satellite links have a number of advantages over microwave links: -They can provide coverage to areas with difficult terrain (remote hills, etc), -The cost of implementation is largely independent of distance, -They are more cost-effective over long distances.  SATELLITE LINKS Orbits. An orbit is a movement of a body around a closed curved path. [ Orbits do not have to be circular. In fact, many are X elliptical in nature. Tz To describe an elliptical orbit, two properties of ellipses must be defined. Y 1. An ellipse has two radii. These are: The line 𝑎, (half of the x-axis) is known as the radius along 𝑥 The line, 𝑏, (half of the y-axis) is known as the radius along 𝑦. The longer radius is also called the ‘semi-major’ axis.  SATELLITE LINKS Orbits. 2. An ellipse has two foci. These are two points, 𝐹 and 𝐹 , They are equally spaced from the centre and always lie on the major axis. Consider the ellipse above-opposite. 𝑃 is a point on the ellipse’s circumference. The distance between 𝑃 and 𝐹 is 𝐿 , similarly the distance between 𝑃 and 𝐹 is 𝐿. For every point 𝑃 on an ellipse’s circumference, the sum of 𝐿 and 𝐿 is constant, i.e., for all points, 𝑃: 𝐿 +𝐿 =𝐾 E.g., Consider changing from 𝑃 to 𝑃 (below), Two new lengths: 𝐿 and 𝐿 may be computed but: 𝐿 +𝐿 =𝐿 +𝐿 =𝐾  SATELLITE LINKS Orbits – Space debris. The first man-made satellite was the Russian (Soviet Union) Sputnik 1 launched on 4 October 1957. -Sputnik returned crashing to Earth from a low Earth orbit. Ever since then, people have been placing satellites in orbit, which has led to an immense amount of space debris. As will be seen, in order to remain in orbit, the debris must move very fast. This poses a danger when launching rockets, etc, from Earth.  SATELLITE LINKS Kepler’s three laws of orbital motion. Satellite orbits have three properties as determined by Kepler (originally in the context of planets orbiting a sun or each other): 1.The orbit of a satellite is an ellipse with the Earth at one of the two foci. If no other forces are acting on the satellite, i.e., internal (locomotive) forces or gravity from other bodies apart from the Earth, the satellite will eventually develop a stable orbit. The size of the ellipse of orbit will depend on: -the satellite’s mass, -the satellite's angular velocity.  SATELLITE LINKS Kepler’s three laws of orbital motion. 2. A line segment joining a satellite and Earth sweeps out equal areas 𝐴 during equal intervals of time (see above & below). -Each value 𝐴 above is equivalent as is each time period 𝑡. -For this to occur, there must be changes in the satellite’s velocity as it makes the orbit. Consider now the terminology: PERILEE Perigee: The point in a satellite’s orbit when it is nearest Earth, APOGEE Apogee: The point in a satellite’s orbit when it is furthest from Earth.  SATELLITE LINKS Kepler’s three laws of orbital motion. Observe below that for Keppler’s second law to hold: -the velocity of the satellite at the perigee must be relatively fast, -the velocity of the satellite at the apogee must be relatively slow.  If A gets bigger, SATELLITE LINKS the Satellite must go Further out Kepler’s three laws of orbital motion. 3.The square of an orbital period of the satellite is proportional to the cube of the semi-major axis: 𝑃 ∝𝑎 𝑃 and 𝑎 are depicted opposite. Although 𝑎 is the semi-major axis, the larger it is, the greater the distance of the satellite from the Earth. A satellite designer may wish to have a certain orbital period for a certain application. E.g., multiple snapshots of the same place on Earth would require a short 𝑃, which in turn would require a low altitude as implied by a small value for 𝑎. Thus, a satellite’s altitude determines its period.  SATELLITE LINKS Kepler’s three laws of orbital motion. Keppler’s third law is fully expressed as: 4𝜋 𝑃 = 𝑎 𝜇 Where: 𝜇 = 3.986 × 10 km /s Furthermore, the degree to which an orbit is circular is given by its eccentricity 𝜖 as: 𝑟 −𝑟 𝜖= 𝑟 +𝑟 Where as indicated below: 𝑟 is the distance from the perigee to Earth’s centre, 𝑟 is the distance from the apogee to Earth’s centre.  SATELLITE LINKS Kepler’s three laws of orbital motion. In terms of satellite operation, some orbits are more eccentric than others. Numerically, eccentricity 𝜖 is bounded as follows: Elliptical: 0