英文作文介绍地球卫星的工作

A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency.In addition to communications satellites, there are other types of satellites:Weather satellites: These satellites provide meteorologists with scientific data to predict weather conditions and are equipped with advanced instruments
Earth observation satellites: These satellites allow scientists to gather valuable data about the earth's ecosystem
Navigation satellites: Using GPS technology these satellites are able to provide a person's exact location on Earth to within a few meters
What are the different kinds of orbits?
An orbit is the path that a satellite follows as it revolves around Earth. In terms of commercial satellites, there are three main categories of orbits:Geosynchronous Orbit (GEO): 35,786 km above the earth Orbiting at the height of 22,282 miles above the equator (35,786 km), the satellite travels in the same direction and at the same speed as the Earth's rotation on its axis, taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite is positioned over the equator in an assigned orbital location, it will appear to be "stationary" with respect to a specific location on the Earth.
A single geostationary satellite can view approximately one third of the Earth's surface. If three satellites are placed at the proper longitude, the height of this orbit allows almost all of the Earth's surface to be covered by the satellites.
Medium Earth Orbit (MEO): 8,000-20,000 km above the earth These orbits are primarily reserved for communications satellites that cover the North and South Pole
Unlike the circular orbit of the geostationary satellites, MEO's are placed in an elliptical (oval-shaped) orbit
Low Earth Orbit (LEO): 500-2,000 km above the earth These orbits are much closer to the Earth, requiring satellites to travel at a very high speed in order to avoid being pulled out of orbit by Earth's gravity
At LEO, a satellite can circle the Earth in approximately one and a half hours
GEO vs. MEO vs. LEOMost communications satellites in use today for commercial purposes are placed in the geostationary orbit, because of the following advantages:One satellite can cover almost 1/3 of Earth's surface, offering a reach far more extensive than what any terrestrial network can achieve
Communications require the use of fixed antennas. Since geosynchronous satellites remain stationary over the same orbital location, users can point their satellite dishes in the right direction, without costly tracking activities, making communications reliable and secure
GEO satellites are proven, reliable and secure - with a lifespan of 10-15 years
For a more comprehensive understanding of satellite advantages, see benefits of satellite.Satellite Architecture
Communications data passes through a satellite using a signal path known as a transponder. Typically satellites have between 24 and 72 transponders. A single transponder is capable of handling up to 155 million bits of information per second. With this immense capacity, today's communication satellites are an ideal medium for transmitting and receiving almost any kind of content - from simple voice or data to the most complex and bandwidth-intensive video, audio and Internet content.Diagrammatic Representation of a Satellite Orbital Location and Footprint
The location of a geostationary satellite is referred to as its orbital location. International satellites are normally measured in terms of longitudinal degrees East (° E) from the Prime Meridian of 0° (for example, Intelsat's IS-805 satellite is currently located at 304.5° E).The geographic area of the Earth's surface over which a satellite can transmit to, or receive from, is called the satellite's "footprint." The footprint can be tailored to include beams with different frequencies and power levels.Frequency Bands and Beams
Satellites transmit information within radio frequency bands. The frequency bands most used by satellite communications companies are called C-band and the higher Ku-band. Over the next several years, the use of a higher frequency band known as Ka-band is expected to increase. Modern satellites are designed to focus on different ranges of frequency bands and different power levels at particular geographic areas. These focus areas are called beams. Intelsat offers four beam types:Global: covering almost 1/3 of Earth's surface
Hemi: covering almost 1/6 of Earth's surface
Zone: covering a large landmass area
Spot: covering a specific geographic area
What is installed on the ground?
All communications with a geostationary satellite require using an earth station or antenna. Earth Stations may be either fixed (installed at a specific location) or mobile for uses such as Satellite News Gathering (SNG) or maritime applications. Antennas range in size, from large telecommunications carrier dishes of 4.5 to 15 meters in diameter, to VSAT antennas which can be as small as under one meter, designed to support services such as Direct to Home TV (DTH) and rural telephony.The antenna, itself, will generally be connected to equipment indoors called an indoor unit (IDU), which then connects either to the actual communications devices being used, to a Local Area Network (LAN), or to additional terrestrial network infrastructure.Network Topologies
Depending on the application, satellites can be used with different ground network designs or network topologies. At its simplest, satellite can support one-direction or two-direction links between two earth stations (called respectively simplex transmission and duplex transmission). More complex communications needs can also be addressed with more sophisticated network topologies, such as star and mesh.The following examples show some of the options available to customers for configuring their satellite networks:Simplex Transmission Applications for simplex services include broadcast transmissions such as:TV and video services
Radio services
Point-to-Point Duplex Transmission Applications for duplex services include:Voice Telephony transport
Data and IP transport (especially in asymmetric configurations)
Corporate networks
TV and Broadcast program contribution and distribution
Point-to-Multipoint Transmission (May be simplex or duplex, symmetric or asymmetric).Applications for point-to-multipoint services include:Corporate networks, including VSAT services and business television
Video and broadcast distribution, including Direct-to-Home Internet services
Mobile Antenna Service Applications for mobile antenna services include:Satellite News Gathering
Special Event Backhaul and Broadcasting
Maritime services
Star Network Applications for Star Networks include:Corporate Networks
Distance Learning
Mesh Network Applications for Mesh Networks include:National and International Telephony and Data networks
Rural Telephony
---------------------------第二篇Not so long ago, satellites were exotic, top-secret devices. They were used primarily in a military capacity, for activities such as navigation and espionage. Now they are an essential part of our daily lives. We see and recognize their use in weather reports, television transmission by DIRECTV and the DISH Network, and everyday telephone calls. In many other instances, satellites play a background role that escapes our notice:
Some newspapers and magazines are more timely because they transmit their text and images to multiple printing sites via satellite to speed local distribution.
Before sending signals down the wire into our houses, cable television depends on satellites to distribute its transmissions.
The most reliable taxi and limousine drivers are sometimes using the satellite-based Global Positioning System (GPS) to take us to the proper destination.
The goods we buy often reach distributors and retailers more efficiently and safely because trucking firms track the progress of their vehicles with the same GPS. Sometimes firms will even tell their drivers that they are driving too fast.
Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal (read this page for details).In this article, we will show you how satellites operate and what they do. You'll get to see what's inside a satellite, explore the different kinds of orbits and find out why the intended use of the satellite affects the choice of orbit. We'll even tell you how to see and track a satellite
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