What is a satellite? • A satellite is an object in space that orbits or circles around a bigger object • There are two kinds of satellites: ➢ natural (such as the moon orbiting the Earth) or ➢ artificial (such as the International Space Station orbiting the Earth). • Man-made satellites are highly specialized wireless receiver transmitters that are launched by a rocket and placed in orbit around the earth. • These satellites have wide applications in many fields including weather forecasting, TV broadcast, Navigation (GPS) etc. • there are four kinds of satellite according to the orbit in which they travels: a) Geosynchronous satellite: 1. A satellite in the geosynchronous orbit is known as Geosynchronous satellite. 2. a geosynchronous satellite revolves around the planet at the same speed at which the planet rotates on its axis. That’s the reason why this kind of satellite appears to be in the same region in the sky (at a given time of the day) when viewed from a particular position on Earth. 3. A geosynchronous orbit (sometimes abbreviated 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) b) Geostationary satellite: 1. A satellite in Geostationary orbit is known as Geostationary Satellite. 2. A geostationary equatorial orbit (GEO) is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately 42,164 km (26,199 mi) (measured from the center of the Earth). A satellite in such an orbit is at an altitude of approximately 35,786 km (22,236 mi) above mean sea level. 3. It maintains the same position relative to the Earth's surface.
4. A geostationary satellite is a ‘type’ of geosynchronous satellite. 5. The only difference between the two is that while a geosynchronous satellite may or may not be following an inclined orbit (with respect to the equatorial plane), a geostationary satellite has to follow a noninclined orbit. c) Medium Earth orbit satellite( MEO): MEO satellites operate at a distance of about 5,000-12,000 km from the earth’s surface. d) Low Earth Orbit satellite (LEO): 1. LEO satellites operate at an altitude of 500-1500 km. 2. The LEO satellites require traveling at high speed so that it does not get pulled out of its orbit by earth’s gravity. e) Highly Elliptical Orbit (HEO) satellite: 1. Satellites in HEO have orbits that are close to the Earth at one point of their orbit, but are much farther away from the Earth at other times.
Difference between communication satellite and communication relay: • For communication satellites the range is much higher than that of communication relay. Communication Satellite can cover up to several thousands of kilometers. • For communication relay, the uplink and the downlink frequency is the same. But for communication satellites the uplink and the downlink frequencies are different in order to avoid interference. • The satellite relays are inherently wide-area broadcast, i.e. the point-tomultipoint whereas all the terrestrial relays are point-to-point. • The satellite circuits can be installed rapidly but communication relay circuit cannot be installed rapidly. • The mobile communication can be easily achieved by satellite communications rather than communication as it has a unique degree of flexibility in interconnecting mobile vehicles. What are the basic elements of satellite communication? explain with neat diagram The basic element of satellite communication are earth station terrestrial system and users. The basic structure of a satellite communication is shown in the above figure This consists of many earth stations on the ground and there are linked with a satellite in space. The users is connected to the earth station through a terrestrial station and this network may be a telephonic switch or a well-established link to a earth station. Basic Elements shown in the figure are
a) b) c) d) Earth Station Satellite Terrestrial System User 1) The user generates a base band signal that is proceeds through a terrestrial network and transmitted to a satellite at the earth station. 2) The satellite consists of a large number of repeaters in the space that perform the reception of modulated RF carrier in its uplink frequency spectrum from all the earth station in the present networks, amplifier these carriers and retransmits them back to the Earth Station in the down link frequency spectrum. 3) In order to avoid the interference downlinks frequency spectrum should be different from uplink frequency spectrum. 4) The signal at the receiving earth stations in processed to get back base band signal, it sent to the user through a terrestrial network. The elements of satellite communication are also divided into two segments: the communication takes place between space segment subsystems and earth segment subsystems through communication links. Space Segment includes: • • Satellite (transponders etc) Means for launching satellite