COMPUTER NETWORKING 2 LECTURE 6: satellites technology

Satellites Overview  History  How Satellites Work  Satellite Frequency Bands and Antennas  Orbit Distance, Pros & Cons, Applications  Types: Low-Earth-Orbit (LEOs) Medium-Earth-Orbit (MEOs) Global Positioning System (GPS) Geostationary (GEOs)

What is a satellite? A satellite is simply any body that moves around another (usually much larger) one in a mathematically predictable path called an orbit A communication satellite is a microwave repeater station in space that is used for telecommunication, radio and television signals The first man made satellite with radio transmitter was in 1957 There are about 750 satellite in the space, most of them are used for communication Picture from [4]

Communication satellites bring the world to you anywhere and any time…..

How Satellites Work 1. A Earth Station sends message in GHz range. (Uplink) 2. Satellite Receive and retransmit signals back. (Downlink) 3. Other Earth Stations receive message in useful strength area. (Footprint)

Working…  Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means  The two stations can use a satellite as a relay station for their communication  One Earth Station transmits the signals to the satellite. Up link frequency is the frequency at which Ground Station is communicating with Satellite  The satellite Transponder converts the signal and sends it down to the second earth station. This frequency is called a Downlink frequency

Elevation Elevation: angle  between center of satellite beam and surface  minimal elevation: elevation needed at least to communicate with the satellite footprint

base station or gateway Classical satellite systems Inter Satellite Link (ISL) Mobile User Link (MUL) Gateway Link (GWL) footprint small cells (spotbeams) User data PSTNISDN GSM GWL MUL PSTN: Public Switched Telephone Network Picture from [1]

Basics  elliptical or circular orbits  complete rotation time depends on distance satellite- earth  inclination: angle between orbit and equator  elevation: angle between satellite and horizon  LOS (Line of Sight) to the satellite necessary for connection  high elevation needed, less absorption due to e.g. buildings  Uplink: connection base station - satellite  Downlink: connection satellite - base station  typically separated frequencies for uplink and downlink

types of satellite orbits  Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit:  GEO: geostationary orbit, ca km above earth surface  LEO (Low Earth Orbit): ca km  MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ca km  HEO (Highly Elliptical Orbit) elliptical orbits

Types of orbits… ]

Geosynchronous-Earth-Orbit (GEO)  Orbit is sychroneous with the earths rotation.  From the ground the satellite appears fixed.  Altitude is about 23,000 miles.  Coverage to 40% of planet per satellite.

Basics of GEOs  Geostationary satellites are commonly used for communications and weather-observation.  The typical service life expectancy of a geostationary satellite is years.  Because geostationary satellites circle the earth at the equator, they are not able to provide coverage at the Northernmost and Southernmost latitudes.

Geostationary Earth Orbit (GEO)  Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates  This means GEO satellites remain in the same position relative to the surface of earth  Because of the long distance from earth it gives a large coverage area, almost a fourth of the earth’s surface  But, this distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication.  High transmit power needed and launching of satellites to orbit are complex and expensive.  Not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV transmission

Middle-Earth-Orbiting (MEO)  MEOs orbits between the altitudes of 5,600 and 9,500 miles.  These orbits are primarily reserved for communications satellites that cover the North and South Pole.  Unlike the circular orbit of the geostationary satellites, MEOs are placed in an elliptical (oval-shaped) orbit.

MEO systems  slower moving satellites  simpler system design  for many connections no hand-over needed  higher latency, ca ms  higher sending power needed  special antennas for small footprints needed

Medium Earth Orbit (MEO)  MEO satellites have a larger coverage area than LEO satellites  A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network  A MEO satellite’s distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite

LEO systems  Orbit ca km above earth surface  visibility of a satellite ca minutes  global radio coverage possible  latency comparable with terrestrial long distance connections, ca ms  many satellites necessary for global coverage  more complex systems due to moving satellites

Low Earth Orbit (LEO)  LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface  LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass  A network of LEO satellites is necessary for LEO satellites to be useful  Need for routing

LEOS  ISL Inter Satellite Link  GWL – Gateway Link  UML – User Mobile Link Picture from [1]

Applications… General applications  Weather forecasting  Radio and TV broadcast broadcasting  Military  Satellites for navigation and localization (e.g. GPS) In context of mobile communication  Global telephone backbones (get rid of large cables)  Connections for communication in remote places or developing areas (for researchers in Antarctica)  Global mobile communication (satellites with lower orbits are needed like LEO’s)

Advantages & Disadvantages The advantages of satellite communication over terrestrial communication are… The coverage area of a satellite is greater than that of a terrestrial system Transmission cost of a satellite is independent of the distance from the center of the coverage area Higher Bandwidths are available for use The disadvantages of satellite communication are… Cost involved in launching satellites into orbit is too high Satellite bandwidth is gradually becoming used up There is a larger propagation delay in satellite communication than in terrestrial communication