The NASA Spacecraft Transponding Modem

A new deep space transponder is being developed by the Jet Propulsion Laboratory for NASA. The Spacecraft Transponding Modem (STM) implements standard transponder functions and some of the command and telemetry channel service functions that have resided in spacecraft Command/Data Subsystems. The STM tracks an X-band uplink signal and provides both X-band and Ka-band downlinks, either coherent or noncoherent with the uplink. A command detector is integrated into the STM, along with a codeblock processor and a hardware command decoder. The STM provides Reed-Solomon coding, convolutional coding, and Turbo coding for downlink telemetry. The downlink symbol rates can be linearly ramped to match the G/T curve of the receiving station. Data rates range from 5 bits per second (bps) to 24 Mbps, with three modulation modes provided: subcarrier, biphase-l direct on carrier, and Offset-QPSK. Standard turn around ranging, regenerative pseudo-noise ranging capability, and Differential One-way Ranging (DOR) tones are provided.