The NASA Spacecraft Transponding Modem

A report describes the NASA Spacecraft Transponding Modem (STM) — a spacecraft transponder now under development for planned use on deep-space missions scheduled for launch in the year 2003. In comparison with a traditional deep-space transponder, the STM will be smaller and less power hungry; the reductions in size and power demand will be effected by use of custom application-specific integrated circuits. The STM will perform all of the traditional deep-space-transponder functions, plus some coding, decoding, and time-tagging functions: The STM will track an X-band uplink signal and transmit both X- and Ka-band downlinks. A command detector, a code-block processor, and hardware command decoder will be integral parts of the STM. Coding functions will include Reed-Solomon coding, convolutional coding, and turbo coding for downlink telemetry. Downlink symbol rates could be ramped linearly to match the expected gain/noise temperature of a receiving station. Data could be transmitted by any of three different phase-modulation schemes at rates from 5 b/s to 24 Mb/s. Other functions will include standard turnaround ranging, regenerative pseudonoise ranging, and differential one-way ranging.