EELE 5451 Satellite Communications
Introduction • Applications include: Communications systems, Remote sensing (detection of water pollution, monitoring of weather conditions, search and rescue operations).
• Satellites first become a viable option due to the congestion on long distance cable routes and the difficulties encountered in attempting to relay television signals.
• The main objection to the use of satellites at that time had been the transmission delay that occurs on a satellite link. Specifically, speech signals sent by satellite incur around-trip transmission delay of approximately 270 ms. • A telephone subscriber using the satellite link for two-way telephony would therefore need to wait in excess of 540ms before receiving a response.
• The effect of time delay is compounded by the echo effect whereby a speaker hears an echo of his/her voice. We may overcome this problem by using an echo canceller, which is a device that subtracts an estimate of the echo from the return path. Time delay has also an effect on data communications.
History • Telestar 1 satellite first provided TV signals across the Atlantic in 1962. • Intelsat 1 in 1965 provided a telephony/television service between Europe and North America.
Advantages of Satellite Communications • Satellite links include point-to-point, point- to-multipoint and variations thereof. • The length of satellite link does not affect the cost of a service whereas cable link cost increases with distance. • Satellites provide a service in different terrain which would be inaccessible to cables.
• Satellites provide a service in remote areas would be prohibitive in terms of the cost of providing the cable link. • Satellites uniquely provide a service to mobile terminals such as ships, aircraft etc.
• Satellites offer a large channel capacity in each transponder channel. • Satellite television plays an important part in everyday life bringing major events and programs, directly into most peoples’ homes, either directly or via cable network.
Frequency Allocations for Satellite Services (ITU) • The world is divided into three regions: • Region 1: Europe, Africa, Former USSR, Monglia. • Region 2: North and South America and Greenland. • Region 3: Asia, Australia, and the south west Pacific.
Satellite services • Fixed satellite service (FSS) – Provides links for existing telephone network. – Provides links for transmission TV signals to cable companies for distribution over cable systems.
• Broadcasting satellite service (BSS) – Direct broadcast to the home, which is known as direct broadcast satellite (DBS) service or direct-to-home (DTH) service in Europe.
• Mobile satellite services – Land mobile, maritime mobile and aeronautical mobile. • Navigational satellite services – Global positioning systems. • Metrological satellite services – Search and rescue services.
Frequency Frequency Service band range GHz Ku band (12-18) (12-14)GHZ is • Direct broadcast satellite usually used for • Certain fixed satellite DBS. 14/12 where 14 GHZ is the uplink services frequency and 12GHZ is the downlink frequency. C band (4-8) (4-6)GHz is usually • Fixed satellite services used for FSS. 6/4 • No broadcast satellite where 6GHZ is the uplink frequency and allowed 4HZ is the downlink frequency. VHF band (0.1-0.3) • Certain mobile and navigational services • Data transfer from weather satellites L band (1-2) • Mobile satellite services • Navigational system
INTELSAT (International Telecommunications Satellites) • Includes more than 140 countries. • Satellites are in geostationary orbit. • Provide a transoceanic telecom. route.
U.S. Domsats • Domsats “Domestic satellites” are used to provide various telecoms services within a country. US Domsats are situated in geostationary orbit.
• Which provide a direct-to-home TV service can be classified broadly as high power, medium power, and low power according to the EIRP (Equivalent isotropic radiated power) as shown in table (1-4) page10. See fig. 1.5.
Satellite Orbits • Geostationary orbit (GEO) • Polar orbit (POES) • Elliptical inclined orbit
Geostationary orbit (GEO) • A satellite placed into a circular equatorial orbit may be made to appear stationary when viewed from the surface of the earth. Such an orbit may be called GEO, and satellites in that unique circular orbit become Geosynchronous Satellites.
• The GEO exists directly above the earth’s equator at an altitude of ≈ (35,855) km and Geosynchronous Satellites orbits the earth in the same time as one rotation of the earth (24 hours).
• Three geosynchronous satellites spaced equally around the earth, would provide continuous communications from one point on the earth to another. • Satellites in that orbit handle the bulk of military and commercial international telecommunications.
Advantages of GEO • Satellites remain almost stationary relative to the earth antennas. Therefore, a fixed ground antenna need not be fully steerable. • No necessity to switch from one satellite to another.
Advantages of GEO • Three satellites give global coverage except of Polar Regions. • There is effectively no relative movement between the satellite and a fixed earth station and, consequently no effective Doppler shift of the communication frequency is introduced. • Tracking of the satellite is simplified
Disadvantages of GEO • Weak signal after traveling over 35,000 km • Polar regions are poorly served • Signal sending delay is substantial (Propagation delay is 270 ms)
Polar orbit • A satellite in this orbit will travel its course over the graphical north and south poles and will effectively follow a line of longitude. • A polar orbiting satellite is rarely used for communication purposes because it is in view of a specific point on the earth’s surface for only a short period of time.
• Polar orbiting satellites are usually used for weather, rescue and search operations in conjunction with geostationary orbits.
Example: The Global Maritime Distress and Safety System (GMDSS) • GMDSS uses two types of satellite each with a very different type of orbit. – GMDSS distress altering may be achieved using the international emergency and locating system (COSPAS/SARSAT) polar orbiting satellites. – The GEO satellites which provide constant earth coverage for radio communications.
Elliptical inclined orbit • The inclination of a satellite orbit is the angle which exists between that orbit and the earth’s equator. Thus the GEO orbit is 0 ◦ inclined whilst the polar orbit is 90 ◦ inclined.
Angle of Elevation
• Angle of Elevation of the satellite is the angle subtended at the antenna between the satellite and the earth’s horizon.
• If the observer is standing on the equator directly beneath a geostationary satellite the angle of elevation will be a maximum of 90 degrees. If the observer now moves in any direction away from that point to the outer of the satellite footprint coverage area the angle of elevation will decreases until it will ultimately be zero.
• The minimum angle of elevation is 5 degrees. • The amplitude of the received signal will progressively decrease as the observer moves away from the 90 degrees elevation point.
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