Ein Transponder auf einem Satelliten empfängt Daten und strahlt sie wieder ab. Zum Beispiel kann eine Erdfunkstelle Datensignale für Fernsehen an einen geostationären Satelliten senden, die dieser zurück auf die Erde schickt. Jede geeignete Satellitenantenne mit Sichtverbindung zum Satelliten kann das Signal empfangen. (de)
A communications satellite's transponder is the series of interconnected units that form a communications channel between the receiving and the transmitting antennas. It is mainly used in satellite communication to transfer the received signals. A transponder is typically composed of: * an input band-limiting device (an input band-pass filter), * an input low-noise amplifier (LNA), designed to amplify the signals received from the Earth station (normally very weak, because of the large distances involved), * a frequency translator (normally composed of an oscillator and a frequency mixer) used to convert the frequency of the received signal to the frequency required for the transmitted signal, * an output band-pass filter, * a power amplifier (this can be a traveling-wave tube or a solid-state amplifier). Most communication satellites are radio relay stations in orbit and carry dozens of transponders, each with a bandwidth of tens of megahertz. Most transponders operate on a bent pipe (i.e., u-bend) principle, sending back to Earth what goes into the conduit with only amplification and a shift from uplink to downlink frequency. However, some modern satellites use on-board processing, where the signal is demodulated, decoded, re-encoded and modulated aboard the satellite. This type, called a "regenerative" transponder, is more complex, but has many advantages, such as improving the signal to noise ratio as the signal is regenerated from the digital domain, and also permits selective processing of the data in the digital domain. With data compression and multiplexing, several video (including digital video) and audio channels may travel through a single transponder on a single wideband carrier. Original analog video only had one channel per transponder, with subcarriers for audio and automatic transmission-identification service ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder or using landlines to send it to an Earth station for multiplexing with other stations. NASA distinguishes between a "transceiver" and "transponder". A transceiver has an independent transmitter and receiver packaged in the same unit. In a transponder the transmit carrier frequency is derived from the received signal. The frequency linkage allows an interrogating ground station to recover the Doppler shift and thus infer range and speed from a communication signal without allocating power to a separate ranging signal. (en)
O transponder de um satélite de comunicações é um conjunto de unidades interligadas que formam um canal de comunicação entre o receptor e as antenas de transmissão. É usado principalmente em comunicações por satélites para transferir sinais recebidos. Um transponder é tipicamente composto por: * Um dispositivo de limitação de banda de entrada (um filtro passa-faixa) * Uma entrada de amplificador de baixo ruído (LNA), para amplificar o sinais recebidos a partir da estação terrestre (normalmente fraca devida às grandes distâncias) * Um conversor de frequência (normalmente composto por um oscilador e um misturador de frequência) utilizado para converter a frequência do sinal recebido para a frequência necessária para que o sinal seja transmitido * Uma saída do filtro passa-faixa * Um amplificador de potência (pode ser um tubo de deslocação de onda ou um amplificador de estado sólido) (pt)