Their outputs are injected to two receing chains, each one composed of two TM Receivers that down-convert received signal to 10 MHz and one IF Combiner where combination of the two signals is performed. Remanent Carrier Demodulator, that provides carrier and subcarrier demodulation and bit conditioning with a 3 bit soft decision output. Telecommand system Telecommands enter the system via the communications node. The uplink modulator output is fed back via the video modem to the TC controller for telecommand verification. Two uplink chains are implemented for system redundancy.
Deep space satellites with on-board coherent transponder.
With a coherent transponder the basic measurement is one of integrated Doppler shift, which, by itself allows orbit determination. The measured Doppler shift on the received carrier can also be used for an accurate prediction of the returned range tone frequency. To achieve this, it is necessary that all the station local oscillators for both up and down link chains are coherent with the same frequency reference. In this case the carrier and the ranging tone frequency are affected by the two-way Doppler shift produced by the relative motion between the ground station and the spacecraft. Codes of sequentially increasing length are used in order to reduce the time required for the synchronization of the received code.