Short note: Satellite communication system

A satellite communication system uses an artificial satellite in orbit as a repeater in space: it receives signals from an earth station, amplifies them, changes their frequency and retransmits them to other earth stations over a wide area.

          [Satellite: transponder]
          ^ uplink        \ downlink
         /  (6 / 14 GHz)   v (4 / 12 GHz)
 [Earth station A]     [Earth station B]

Main parts

• Space segment: satellite with transponders (receive antenna, LNA, frequency down-converter, power amplifier such as TWTA/SSPA, transmit antenna), solar panels, batteries, and attitude/orbit control.
• Ground segment: earth stations with large dish antennas, high-power amplifiers (uplink), low-noise amplifiers (downlink), modems and multiplexers; plus telemetry, tracking and command (TT&C) stations.

Key features

• Geostationary orbit (GEO) at about 35 786 km above the equator; the satellite appears fixed, so antennas need not track. Three GEO satellites can cover almost the whole Earth.
• Different uplink and downlink frequencies avoid interference: C-band 6/4 GHz, Ku-band 14/12 GHz, Ka-band 30/20 GHz.
• Typical transponder bandwidth: 36 MHz.
• Multiple access: FDMA, TDMA, CDMA.
• One-way earth–satellite–earth delay about 240–270 ms for GEO.
• LEO and MEO constellations give lower delay (used for mobile broadband, GPS).

Advantages: wide coverage, broadcast capability, distance-independent cost, quick setup in remote and hilly areas (useful in Nepal). Disadvantages: high launch cost, propagation delay and echo, rain fade at high frequencies, limited lifetime (about 15 years).

Applications: TV broadcasting (DTH), telephony and VSAT networks, navigation (GPS), weather and remote sensing, internet.