Q14Principle of Electronic Communication

Basic Satellite Communication System

A basic satellite communication system consists of three main components: the uplink Earth station, which transmits a modulated carrier signal upward toward the satellite at a specified uplink frequency; the satellite transponder, an onboard repeater that receives the uplink signal, amplifies it, shifts it to a different downlink frequency (to avoid interference between the strong uplink and the much weaker downlink signal), and retransmits it back toward Earth; and the downlink Earth station, which receives, amplifies, and demodulates the downlink signal to recover the original transmitted information. Additional supporting components include the satellite's own power supply (solar panels and batteries), attitude and orbit control systems, and ground-based tracking, telemetry, and command (TT&C) stations that monitor and control the satellite's health and orbital position.

A complete satellite link budget calculation must additionally account for free-space path loss (which increases with the square of distance and with carrier frequency, making satellite links particularly sensitive to path loss given their very long uplink/downlink distances), atmospheric and rain attenuation (especially significant at higher frequency bands such as Ku and Ka band), and the required link margin to maintain acceptable signal quality despite these losses and any additional interference, all of which must be balanced against the satellite's limited available transmit power and the ground station's antenna gain when designing a practical satellite communication link.

Satellites are also broadly classified by orbit type, with GEO satellites (as discussed elsewhere in this examination) commonly used for broadcast television and weather monitoring given their fixed apparent position, while LEO constellations are increasingly used for broadband internet and low-latency communication services.