Communication Satellites

Communication Satellites • Communication Satellites can be thought of as a big microwave repeater in the sky. • Satellites are transponders (units that receive on one frequency and retransmit on another) that are set in geostationary orbits directly over the equator. • These geostationary orbits are 36,000km from the earth’s surface. • Satellite networks are like cellular networks as they divide the planet into cells.

Communication satellites contd… • Satellite can provide transmission capability to and from any location on earth, no matter how remote. • This makes high-quality communication available to the undeveloped parts of the world without requiring a huge investment in ground based infrastructure.

• Orbits: The path in which a satellite travels around the earth is called the orbit of the satellite. • Period: The time required for a satellite to make a complete trip around the earth is called period of the satellite. • Footprint: The footprint is the “shadow” that the satellite can transmit to, the shadow being the area that can receive the satellite’s transmitted signal.

• Uplink: The uplink is the transmitter of data to the satellite. • Downlink: The downlink is the receiver of data. • Earth Stations: Uplinks and downlinks are also called earth stations because they are located on the earth. • Van Allen Belt: A Van Allen Belt is a layer that contains charged particles. A satellite orbiting in one of these two belts would be totally destroyed by the energetic charged particles. The MEO orbits are located between these two belts.

Frequency Bands for Satellite Communications • The frequencies reserved for satellite microwave communication are in the gigahertz range. Band L S C Ku Ka

Downlink GHz 1.5 1.9 4.0 11.0 20.0

Uplink GHz Bandwidth MHz 1.6 15 2.2 70 6.0 500 14.0 500 30.0 3500

Categories of Satellites Satellites

Geostationary Earth Orbit Satellites (GEO)

Middle Earth Orbit Satellites (MEO)

Low Earth Orbit Satellites (LEO)

GEO Satellites

• The satellites which move at the same speed as the earth so that it seems to remain fixed above a certain spot are called geostationary satellites. • Because orbital speed is based on the distance from the planet, only one orbit can be geostationary. • But one geostationary satellite can not cover the whole earth. • One satellite in orbit has line-of-sight contact with a vast no. of stations, but the curvature of the earth still keeps much of the planet out of sight. • It takes a min. of 3 satellites equidistant from each other in geostationary earth orbit to provide full global transmission.

MEO Satellites • Medium-Earth-Orbit satellites are positioned between the two Van Allen Belts. • A satellite at this orbit takes approximately 6-8 hours to circle the earth. • Because they have a smaller footprint on the ground and require less powerful transmitters to reach them. • Examples are: GPS

GPS • Global positioning System orbits at an altitude of about 18,000 km above the earth. • The system consists of 24 satellites and is used for land, sea and air navigation to provide time and locations for vehicles and ships. • GPS uses 24 satellites in 6 orbits. The orbits and the location of the satellite in each orbit are designed in such a way that at any time 4 satellites are visible from any point on earth.

LEO • The LEO satellites are located at an altitude between 500 & 2000 km, with a rotation period of 90 to 120 min. • The satellite has a speed of 20,000 to 25,000 km/h. • An LEO system usually has a cellular type of access, similar to the cellular telephone system • The footprint normally has a diameter of 8000km. • Because LEO are close to earth, the round trip time propagation delay is normally less than 20ms, which is acceptable for audio communication.

LEO contd.. • An LEO system is made of a constellation of satellites that work together as a network; each satellite acts as a switch. • Satellites that are close to each other are connected through intersatellite links (ISLs). • A mobile system communicates with the satellite through a user mobile link (UML). • A satellite can also communicate with an earth station through a gateway link (GWL).

LEO Little Broad Band Big Operate under 1GHz Used for lowdata-rate

Operate between 1GHz & 3 GHz e.g. Globestar & Iridium

Provide communication similar to fiberoptic networks. e.g. Teledesic

Iridium Telecom System • The Iridium Telecom System is a new satellite system that will be the largest private aerospace project. • It is designed to provide direct worldwide voice and data communication using handheld terminals, a service similar to cellular telephony but on a global scale • It relies on satellites in lower earth orbit (LEO). • The satellite will orbit at an altitude of 900-10000 km in a polar, non-stationary orbit. • User’s handset will require less power and will be cheaper than cellular phones. • There will be 100% coverage of the earth.

Globalstar • Globalstar is another LEO satellite system. • The system uses 48 satellites in 6 polar orbits with each orbit hosting 8 satellites. • The orbits are located at an altitude of almost 1400km. • The globalstar system is similar to the Iridium system; the main difference is the relaying mechanism • Communication b/w two distant users in the Iridium system requires relaying b/w several satellites. • Globalstar communication requires both satellites & earth stations, which means that ground stations can create more powerful

Teledesic • Teledesic is a system of satellites that provide fiber-optic like (broadband channels, low error rate & low delay) communication. • Its main purpose is to provide broadband Internet access for users all over the world. • It is sometimes called “Internet in the Sky”.