bitwali.com 1 WIRELESS COMMUNICATION LEC 15-16

bitwali.com 2 TYPES OF SATELLITES Satellite Orbits  GEO  LEO  MEO  Molniya Orbit Frequency Bands

bitwali.com 3 GEOSTATIONARY EARTH ORBIT (GEO) These satellites are in orbit 35,863 km above the earth’s surface along the equator. 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. Satellite appears to be stationary over any point. Three Satellites can cover the earth (120° apart)

bitwali.com 4 GEO (CONT.) Advantages  A GEO satellite’s distance from earth gives it a large coverage area, almost a fourth of the earth’s surface.  GEO satellites have a 24 hour view of a particular area.  These factors make it ideal for satellite broadcast and other multipoint applications.

bitwali.com 5 GEO (CONT.) Disadvantages  A GEO satellite’s 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.  GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions

bitwali.com 6 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.66 satellites needed to cover. Earth stations must track satellite or have Omni directional antennas

bitwali.com 7 LEO (CONT.) Advantages  A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.  A LEO satellite’s smaller area of coverage is less of a waste of bandwidth. Disadvantages  A network of LEO satellites is needed, which can be costly.  Atmospheric drag effects LEO satellites, causing gradual orbital deterioration(خرابی ۔ زوال ۔ فساد).

bitwali.com 8 TELEDESIC Provides broadband Internet access for users all over the world Teledesic provides 288 satellites in 12 polar orbits with each orbit hosting 24 satellites. The orbits are at altitude of 1350 km. The system provides 3 types of communications Intersatellite communication allows 8 neighboring satellites to communicate with one another. Communication also possible between satellite and earth gateway station. Earth divided into tens of thousands of cells with each one assigned a time slot and the satellite focuses its beam to the cell at the corresponding time slot.

bitwali.com 9 MEDIUM EARTH ORBIT (MEO) A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earth’s surface. MEO satellites are similar to LEO satellites in functionality. MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours. MEO satellites have a larger coverage area than LEO satellites.

bitwali.com 10 MEO (CONT.) Advantage  A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network. Disadvantage  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.

bitwali.com 11 SATELLITE ORBITS

bitwali.com 12 SATELLITE ORBITS

bitwali.com 13 HEO - HIGHLY ELLIPTICAL ORBITS HEOs (i = 63.4°) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere) Depending on selected orbit (e.g. Molniya, Tundra, etc.) two or three satellites are sufficient for continuous time coverage of the service area. All traffic must be periodically transferred from the “setting” satellite to the “rising” satellite (Satellite Handover) Apogee The point in orbit of satellite most distant which is most distant from earth surface PerigeeThe point neaserst to earth centre in orbit of satellite

bitwali.com 14 VAN ALLEN BELTS The Van Allen belts are regions of electrons and protons held captive ( قیدی ) by the magnetic influence of earth. Radiation is concentrated and closest to the earth at the poles (aurora). Satellite orbits are designed to spend as little time as possible in the belts or avoid them completely. Satellites that travel in and around the belts may be damaged. Levels change because of magnetic storms and nuclear explosions.

bitwali.com 15 NGSO - NON GEOSTATIONARY ORBITS Orbit should avoid Van Allen radiation belts: Region of charged particles that can cause damage to satellite Occur at ~ km and ~ km

bitwali.com 16 FREQUENCY BANDS Different kinds of satellites use different frequency bands.  L–Band: 1 to 2 GHz, used by MSS (Mobile service satellite)  S-Band: 2 to 4 GHz, used by MSS, NASA, deep space research  C-Band: 4 to 8 GHz, used by FSS (Fixed Service Satellite)  X-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex: military and meteorological satellites  Ku-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS)  K-Band: 18 to 26.5 GHz: used by FSS and BSS (Broadcast service satellite)  Ka-Band: 26.5 to 40 GHz: used by FSS

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