tromagnetic wave sent through the atmosphere (just as in terrestrial microwave). Figure 2.17 Satellite communication system Satellite Solar panels generating electricity to operate satellite Information 6 GHz 4 GHz Information signal signal Ocean High-frequency High-frequency electrical signal electrical signal Telecommunication Networks This signal is then sent up to a satellite in orbit around the earth. Specifically, the satellite is placed at 36,000 km (22,300 miles), and at that altitude it orbits the Earth at 6,870 mph. Moving at this speed, the satellite appears to be stationary when looked at from the equator, and SO it is said to be in a geo-stationary orbit. The satellite picks up the signal and does two things: (1) it acts as a repeater; and (2) it shifts the frequency (for example, from 6 GHz to 4 GHz) and sends it back down to earth in the direction of the receiver. Modern satellites are being built which can do more signal processing at the satellite itself, but we won't go into those details here. The signal leaves the satellite and heads back to the receiver on Earth. At that receiver, an antenna is applied that turns the incoming electromagnetic wave back to an electrical signal, and that electrical signal is returned to the original information signal by a device called a demodulator. Satellite communications operate in a number of frequency bands. Here are some of them: (1) C-band, which refers to 6-GHz uplink ("up" to the satellite) and 4-GHz downlink ("down" to the receiver); (2) Ku-band, which is 14-GHz uplink/12-GHz downlink; and (3) ACTS which refers to 30-GHz uplink/20-GHz downlink. Some of the many uses of satellite communications include satellite TV distribu- tion, live TV transoceanic links, telephone communications over the oceans, backup to fiber-optic links, and GPS (Global Positioning System). GPS refers to a system of satellites which enables anyone, using a hand-held device, to determine their exact position on our planet (very useful for ships, backpackers, and companies with l