Echo 1 accomplished this by essentially serving as an enormous mirror, 10 stories tall, that could be used to reflect communications signals. There are two major classes of communications satellites, passive and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, and only a very small amount of the transmitted energy actually reaches the receiver. Since the satellite is so far above Earth, the radio signal is attenuated due to free-space path loss , so the signal received on Earth is very, very weak.
Active satellites, on the other hand, amplify the received signal before retransmitting it to the receiver on the ground. Telstar was the second active, direct relay communications satellite. Relay 1 was launched on December 13, , and it became the first satellite to transmit across the Pacific Ocean on November 22, An immediate antecedent of the geostationary satellites was the Hughes Aircraft Company 's Syncom 2 , launched on July 26, Syncom 2 was the first communications satellite in a geosynchronous orbit. It revolved around the earth once per day at constant speed, but because it still had north-south motion, special equipment was needed to track it.
Its successor, Syncom 3 was the first geostationary communications satellite. Syncom 3 obtained a geosynchronous orbit, without a north-south motion, making it appear from the ground as a stationary object in the sky. Beginning with the Viking program, [a] all Mars landers , aside from Mars Pathfinder, have used orbiting spacecraft as communications satellites for relaying their data to Earth.
The landers use UHF transmitters to send their data to the orbiters, which then relay the data to Earth using either X band or Ka band frequencies. These higher frequencies, along with more powerful transmitters and larger antennas, permit the orbiters to send the data much faster than the landers could manage transmitting directly to Earth, which conserves valuable time on the receiving antennas.
Communications satellites usually have one of three primary types of orbit , while other orbital classifications are used to further specify orbital details. As satellites in MEO and LEO orbit the Earth faster, they do not remain visible in the sky to a fixed point on Earth continually like a geostationary satellite, but appear to a ground observer to cross the sky and "set" when they go behind the Earth. Therefore, to provide continuous communications capability with these lower orbits requires a larger number of satellites, so one will always be in the sky for transmission of communication signals.
However, due to their relatively small distance to the Earth their signals are stronger. In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity. Low-Earth-orbiting satellites are less expensive to launch into orbit than geostationary satellites and, due to proximity to the ground, do not require as high signal strength Recall that signal strength falls off as the square of the distance from the source, so the effect is dramatic.
Thus there is a trade off between the number of satellites and their cost. In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions. A group of satellites working in concert is known as a satellite constellation.
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Two such constellations, intended to provide satellite phone services, primarily to remote areas, are the Iridium and Globalstar systems. The Iridium system has 66 satellites. It is also possible to offer discontinuous coverage using a low-Earth-orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part. Another system using this store and forward method is Orbcomm. MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 and 8 hours. One disadvantage is that a MEO satellite's distance gives it a longer time delay and weaker signal than a LEO satellite, although these limitations are not as severe as those of a GEO satellite.
Like LEOs, these satellites do not maintain a stationary distance from the earth. In various patterns, these satellites make the trip around earth in anywhere from 2 to 8 hours.
In , the first communications satellite, Telstar , was launched. It was a medium earth orbit satellite designed to help facilitate high-speed telephone signals. Although it was the first practical way to transmit signals over the horizon, its major drawback was soon realized. Because its orbital period of about 2. It was apparent that multiple MEOs needed to be used in order to provide continuous coverage.
To an observer on Earth, a satellite in a geostationary orbit appears motionless, in a fixed position in the sky. This is because it revolves around the Earth at Earth's own angular velocity one revolution per sidereal day , in an equatorial orbit. A geostationary orbit is useful for communications because ground antennas can be aimed at the satellite without their having to track the satellite's motion. This is relatively inexpensive. In applications that require a large number of ground antennas, such as DirecTV distribution, the savings in ground equipment can more than outweigh the cost and complexity of placing a satellite into orbit.
By , Hughes Space and Communications now Boeing Satellite Development Center had built nearly 40 percent of the more than one hundred satellites in service worldwide.
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Geostationary satellites must operate above the equator and therefore appear lower on the horizon as the receiver gets farther from the equator. This will cause problems for extreme northerly latitudes, affecting connectivity and causing multipath interference caused by signals reflecting off the ground and into the ground antenna. Thus, for areas close to the North and South Pole, a geostationary satellite may appear below the horizon.
Therefore, Molniya orbit satellites have been launched, mainly in Russia, to alleviate this problem. Molniya orbits can be an appealing alternative in such cases.
The Molniya orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. Elevation is the extent of the satellite's position above the horizon. Thus, a satellite at the horizon has zero elevation and a satellite directly overhead has elevation of 90 degrees. The Molniya orbit is designed so that the satellite spends the great majority of its time over the far northern latitudes, during which its ground footprint moves only slightly. Its period is one half day, so that the satellite is available for operation over the targeted region for six to nine hours every second revolution.
In this way a constellation of three Molniya satellites plus in-orbit spares can provide uninterrupted coverage.doubciepunkvo.tk
The first satellite of the Molniya series was launched on April 23, and was used for experimental transmission of TV signals from a Moscow uplink station to downlink stations located in Siberia and the Russian Far East, in Norilsk , Khabarovsk , Magadan and Vladivostok. In November Soviet engineers created a unique system of national TV network of satellite television , called Orbita , that was based on Molniya satellites. These orbits are sun synchronous, meaning that they cross the equator at the same local time each day. The bandwidth available from a satellite depends upon the number of transponders provided by the satellite.
Each service TV, Voice, Internet, radio requires a different amount of bandwidth for transmission. This is typically known as link budgeting and a network simulator can be used to arrive at the exact value. Allocating frequencies to satellite services is a complicated process which requires international coordination and planning. To facilitate frequency planning, the world is divided into three regions: Region 1: Europe, Africa, what was formerly the Soviet Union, and Mongolia Region 2: North and South America and Greenland Region 3: Asia excluding region 1 areas , Australia, and the southwest Pacific.
Within these regions, frequency bands are allocated to various satellite services, although a given service may be allocated different frequency bands in different regions. Some of the services provided by satellites are:. The first and historically most important application for communication satellites was in intercontinental long distance telephony.
The fixed Public Switched Telephone Network relays telephone calls from land line telephones to an earth station , where they are then transmitted to a geostationary satellite. The downlink follows an analogous path. Improvements in submarine communications cables through the use of fiber-optics caused some decline in the use of satellites for fixed telephony in the late 20th century.
Satellite communications are still used in many applications today. Remote islands such as Ascension Island , Saint Helena , Diego Garcia , and Easter Island , where no submarine cables are in service, need satellite telephones. There are also regions of some continents and countries where landline telecommunications are rare to nonexistent, for example large regions of South America , Africa , Canada, China , Russia, and Australia.
Satellite communications also provide connection to the edges of Antarctica and Greenland.
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Other land use for satellite phones are rigs at sea, a back up for hospitals, military, and recreation. Ships at sea, as well as planes, often use satellite phones. Satellite phone systems can be accomplished by a number of means. On a large scale, often there will be a local telephone system in an isolated area with a link to the telephone system in a main land area. There are also services that will patch a radio signal to a telephone system. In this example, almost any type of satellite can be used.
Satellite phones connect directly to a constellation of either geostationary or low-Earth-orbit satellites. Calls are then forwarded to a satellite teleport connected to the Public Switched Telephone Network. As television became the main market, its demand for simultaneous delivery of relatively few signals of large bandwidth to many receivers being a more precise match for the capabilities of geosynchronous comsats.
Most satellites used for direct-to-home television in Europe have the same high power output as DBS-class satellites in North America, but use the same linear polarization as FSS-class satellites. Examples of these are the Astra , Eutelsat , and Hotbird spacecraft in orbit over the European continent. Fixed Service Satellites use the C band , and the lower portions of the K u band. They are normally used for broadcast feeds to and from television networks and local affiliate stations such as program feeds for network and syndicated programming, live shots , and backhauls , as well as being used for distance learning by schools and universities, business television BTV , Videoconferencing , and general commercial telecommunications.
FSS satellites are also used to distribute national cable channels to cable television headends.
Satellite Communications Systems: Systems, Techniques and Technology, 5ed
Analog and digital modulation and multiple access techniques and their implementation in satellite communication systems. Emphasis on the matched filter and calculation of the probability of error in digital communication systems. Detailed examination of the satellite transponder. Emphasis on transponder signal processing and the effects of nonlinearities in satellite amplifiers.
Development of satellite networks based using multiple access techniques. Digital Video Broadcasting and applications. This lesson aims in understanding the methods of analysis and design of satellite communication systems. This course provides the necessary knowledge of the basic principles and characteristics of satellite communications networks, and the field of efficient implementation.