CHAPTER 13

---

1. Direct broadcast satellites transmit TV programs directly to homes. These systems operate in the frequency band 17/12GHz. However, due to the wide penetration of cable TV, direct broadcast satellite technology has not taken off well in India, though DBS is extensively used in North America and Europe.

2. Remote sensing satellites are used for a variety of applications: to find out the areas in which natural resources are available, to determine the water table under the earth's surface, to analyze the fertility of lands, and so on. Remote sensing satellites have sensors that operate in the infrared and near- infrared bands. The satellite imagery is sent to the ground stations. The imagery is analyzed based on the application. Indian Space Research Organization (ISRO) launched the IRS (Indian Remote Sensing Satellite) series satellites exclusively for remote sensing.

3. In satellite communication, efficient use of the bandwidth is very important, as bandwidth is much costlier than the terrestrial media bandwidth. Low bit rate coding of voice is done using ADPCM, LPC, and CELP techniques.

4. In satellite-based video surveillance, there should be a system that captures the video and transmits it to the ground continuously. The video camera captures the scene and digitizes it using a standard coding technique such as MPEG2 or MPEG4 and transmits to the Earth station. The minimum data rate to be supported by the link is 1Mbps to obtain reasonably good quality video.

5. In a satellite network, the round-trip delay is about 0.5 second. If stop-and- wait protocol is used on such a network for data communication, the satellite channel is not used effectively. After a packet is transmitted from one side, it takes 0.5 seconds to reach the destination (assuming that there is no other delay). Then the acknowledgement will be sent by the other station, and it will be received by the station after another 0.5 seconds. So, effectively one packet is transmitted every one second!

---