Satellite communication

Topic: Transport and Communication in India · NCERT: Class 12, Ch 7 "Transport and Communication"; Class 12, Ch 7 "Transport and Communication"

Meaning

Satellite communication means using a satellite in space to pick up a signal from one place on Earth, make it stronger, and send it back down to many places at the same time. Satellites carry phone calls, TV, radio and data. Because a satellite can watch a very large area without a break, India also uses satellites for weather forecasting, disaster warning and border watch. This makes them important for both the economy and national security.

Explanation

How it works (signal up → strengthened → signal down)

• Step 1: Uplink. A ground station sends a signal (a phone call, TV programme or data) up to the satellite.
• Step 2: Transponder. The transponder is the part of the satellite that receives the signal, makes it stronger and sends it back.
• Step 3: Downlink. The signal comes back down to many receivers at once, such as dish antennas, TV sets and phone exchanges.
• Frequency bands. These are different ranges of radio frequency. INSAT uses the C, Extended C and Ku bands [5].
• The Ku-band works well with small dish antennas. This is why DTH (direct-to-home) TV uses it.

Two roles of a satellite

• A mode of communication in itself. It carries phone calls, TV and radio signals and data.
• It controls other modes of communication too. These all depend on satellite links:
• phone networks and TV broadcasting
• navigation of aircraft and ships

• banking networks

• Synoptic view. A satellite sees a very large area at one time and keeps watching it without a break.

• It watches the weather all the time, which helps weather forecasting.
• It tracks cyclones, floods and droughts, which helps disaster monitoring.
• It watches remote frontiers, which helps border surveillance.

Orbits: where the satellite sits

• Geostationary orbit. This orbit lies high above the Equator.
• A satellite here goes round Earth once a day. That is exactly how long Earth takes to spin once.
• Result: seen from the ground, the satellite seems to stay over one fixed spot. Dish antennas can point at one place in the sky and never need to move.

• Communication and weather satellites such as INSAT use this orbit.

• Polar orbit. This orbit passes over or near both poles.

• Earth turns underneath the satellite, so it slowly covers the whole planet.

• Remote sensing satellites such as IRS use this orbit. India launches them on its own PSLV (Polar Satellite Launch Vehicle).

• Inclined geosynchronous orbit. This orbit matches Earth's daily spin but is tilted to the Equator. NavIC uses it, at 29° [8].

India's two main satellite systems

• INSAT (Indian National Satellite System), set up in 1983. It is a multipurpose system, which means one system does many jobs.
• It is one of the largest domestic communication satellite systems in the Asia-Pacific region. It had nine operational communication satellites in geostationary orbit, with more than 200 transponders [5].
• Services [5]: telecommunications, TV broadcasting, satellite newsgathering, societal uses (education, health), weather forecasting, disaster warning, and Search and Rescue (picking up distress signals from ships, aircraft and people).
• TV reach (c. 2000): satellite TV covered over 65% of India's land area and over 80% of its population [6].
• Weather role [6]:
  • INSAT was the only source of repeated, wide-area weather observations over the Indian Ocean from geostationary orbit.
  • It measures cloud motion vectors (wind speed and direction worked out from how clouds move), cloud-top temperature, water vapour, and vertical profiles of temperature and humidity (how they change with height).
  • Result: better forecasts, early detection of the genesis (birth) of cyclones, and prediction of each cyclone's path.

• Start of the system: INSAT-1A (1982) failed early. INSAT-1B (1983) made the system work.

• IRS (Indian Remote Sensing satellites). These are for remote sensing, which means collecting information about Earth's surface from a distance using sensors on satellites.

• IRS-1A was launched in March 1988 from Baikonur, then in the Soviet Union and now in Kazakhstan.
• The data is received and processed at NRSC (National Remote Sensing Centre), Hyderabad.

Where it occurs

• Sriharikota (SHAR Centre), Andhra Pradesh. This barrier island on the Bay of Bengal coast is India's launch site. Rohini RS-1 was launched from here on 18 July 1980 on India's own SLV-3 [3].
• Volgograd Launch Station (now in Russia). Aryabhata, India's first satellite, was launched from here on 19 April 1975 on a Soviet "C-1 Intercosmos" rocket [2].
• Kourou, French Guiana. This site is on the north-east coast of South America, near the Equator. APPLE was launched from here on 19 June 1981 on Ariane-1 (V-3) [4].

• APPLE was placed in geosynchronous orbit at 102° E, over Indonesia. It had 2 C-band transponders [4].

• Baikonur (now Kazakhstan). IRS-1A was launched from here in March 1988.

• NRSC, Hyderabad, Telangana. This is where IRS data is received and processed.
• Indian villages under SITE (1975-76). NASA's ATS-6 satellite was lent to India from 1 August 1975 to 31 July 1976 [7].
• It showed educational TV in 6 clusters of about 400 villages each, about 2,400 villages in all, for about 4 hours a day [7].

• Morning (1.5 hours): for school classrooms. Evening (2.5 hours): for adult education [7].

• Indian Ocean. INSAT gave the only geostationary weather coverage of this region [6].

• NavIC coverage area. This covers India plus the area up to 1,500 km beyond its borders [8].

• Its 3 geostationary satellites sit at 32.5° E, 83° E and 129.5° E [8].

• Who runs it: ISRO (Indian Space Research Organisation), founded in 1969.

Don't confuse with

• INSAT vs IRS. INSAT is in geostationary orbit and is used for communication, TV and weather. IRS is used for remote sensing of Earth's surface, launched on the PSLV into polar orbit, and its data is processed at NRSC Hyderabad.
• Geostationary vs polar orbit. A geostationary satellite sits over one fixed spot above the Equator, which suits communication. A polar satellite passes near both poles and slowly covers the whole Earth, which suits mapping.
• Aryabhata vs Rohini RS-1. Aryabhata (1975) was India's first satellite, but it went up on a Soviet rocket [2]. Rohini RS-1 (1980) was the first satellite India put into orbit with its own rocket, SLV-3, from Sriharikota [3].
• SITE vs APPLE. SITE used an American satellite (NASA's ATS-6) [7]. APPLE was ISRO's first home-built experimental communication satellite [4].

Prelims Hooks

• APPLE (Ariane Passenger Payload Experiment, 19 June 1981) was launched on Ariane-1 from Kourou, French Guiana, not from Sriharikota. It was placed at 102° E [4].
• SITE (1975-76) used NASA's ATS-6, not an Indian satellite. It reached 6 clusters of about 400 villages each [7].
• INSAT (set up in 1983) is in geostationary orbit and uses the C, Extended C and Ku bands [5]. INSAT-1A (1982) failed, and INSAT-1B (1983) made the system work.
• IRS-1A: launched March 1988 from Baikonur (now Kazakhstan). IRS data goes to NRSC Hyderabad.
• Order of milestones: ISRO (1969) → Aryabhata (1975) → SITE (1975-76) → Rohini RS-1 (1980) → APPLE (1981) → INSAT-1B (1983) → IRS-1A (1988).
• NavIC (earlier IRNSS) has 7 satellites (3 geostationary + 4 inclined geosynchronous) and covers India plus 1,500 km beyond its borders. It offers an SPS for ordinary users and an RS for strategic users [8].

Mains Points

• Satellites are strategic infrastructure (GS-III).
• Their synoptic view supports weather forecasts, cyclone tracking, disaster warning and border surveillance.

• INSAT was once the only geostationary source of weather data over the Indian Ocean [6]. This shows why India needs its own satellites for disaster management and security.

• Satellites close the development gap in remote areas.

• SITE and INSAT took TV and education to villages where land-based networks were weak.
• By about 2000, satellite TV reached over 80% of India's population [6].

• Today, satellite internet licences for private companies can do the same for hilly, island and border areas, which supports Digital India.

• A path to self-reliance. India moved step by step from depending on others to doing it alone:

• Foreign rockets: Aryabhata on a Soviet rocket, APPLE on Ariane.
• Foreign satellite: SITE on NASA's ATS-6.
• Own rockets: SLV-3, then PSLV.
• Own systems: the GSAT series and NavIC. NavIC reduces India's dependence on the American GPS and gives secure positioning to the armed forces.

Related concepts

• INSAT
• Aryabhata
• Rohini satellites
• APPLE satellite

Read more

• Satellite communication and India's early satellites