Satellite
Satellite — uses microwaves to carry digital signals from and to both ground based stations and between satellites.
General notes
- Satellites contain transponders that receive microwaves on one frequency, amplify and then transmit microwaves on a different frequency.
- A typical satellite contains hundreds and thousands of transponders.
- Communication satellites are usually geostationary (remain over the same spot on the Earth at all times). All geostationary satellites are directly above the equator at a height of approximately 35500km.
- Earth based satellite dishes in Australia (southern hemisphere) always face in a northerly direction. While in the northern hemisphere the dishes face in a southerly direction.
- Geostationary satellites are used for satellite TV and broadband Internet TV. Satellites are used for TV broadcasts, Satellite phones and global positioning.
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short video on how satellites work |
Examples of satellites
1. GLOBAL pOSITIONING sATELLITE system (gps)
- Global Positioning Satellite (GPS) system — uses a network of more than 24 satellites to orbit the globe in different directions to form a complete grid.
- Each satellite transmits a signal from 20 000km above the Earth that includes the satellites current position and the time the signal was transmitted.
- Receivers on the ground, such as cars and handheld navigators, receive the signal from multiple GPS satellites within range.
- To pinpoint any position on the globe requires signals from at least 3 satellites to be within range at any time. However, it is common for there to be up to 8 satellites.
- A triangular system is used to determine the current location of the receiver. The receiving GPS device calculates the time taken for each system to reach the device. As the signal travels at a constant speed the distance between each satellite and the receiving device can be calculated. The position of the satellite is known hence a series od spheres that can be constructed around each satellites known positions. The point where the spheres intersect on the Earth’s surface is the receiver’s current position.
- Most GPS devices are capable of plotting the position graphically on a map in real time and provide directions both graphically and using synthesised voice.
2. LOW EARTH ORBIT SATELLITES (LEOS)
- Low Earth Orbit Satellites (LEOS) — used for mapping and weather forecasting.
- These satellites travel at high speed at heights ranging from about 500 to 2000 km above the Earth’s surface.
- Typical LEOS satellite orbits the globe every 1 to 2 hours. Individual satellites are unable to provide uninterrupted coverage at any single position on the globe.
- There are two failed networks of low Earth satellites in operation — Iridium and Globalstar — originally created to provide global mobile phone and data communication services
aDVANTAGES AND DISADVANTAGES OF SATELLITES
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dISADVANTAGES |
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