A prototype of a relatively inexpensive S-band transponder has been developed for use aboard a spacecraft as part of a NASA space- craft tracking and data network (STDN). The transponder design incorporates recent advances in digital processing of received radio signals. Notable features of the design include flexibility in the choice of operating modes and “smart” acquisition of signals. The transponder design could also be adapted to military ground-based satellite communications and tracking and to commercial Earth/satellite communication systems.
The overall function of the transponder is to receive an S-band signal (uplink signal) from the ground, demodulate the signal to extract command and control information, and generate a return carrier signal modulated by either ranging or telemetry data to be transmitted back to the ground (downlink signal).
The STDN waveform comprises a binary- phase-shift-keyed (BPSK) subcarrier phase-modulated onto an S-band carrier. The transponder must phaselock to the received uplink signal, determine frequency of the received carrier, and generate the return carrier signal at a frequency of exactly 240/221 times that of the received carrier signal.
The receiver portion of the transponder includes a flexible front end that enables selection of any of the STDN channels. An analog down-converter translates the S-band received signal to an intermediate frequency (IF) of ≈3 MHz in a noncoherent mode for inphase and quadrature detection. An alldigital carrier-tracking loop coherently extracts the carrier from this IF signal. A field-programmable gate array is used to implement a carrier-tracking algorithm that supports operation of a first-, second-, or third-order phase-lock loop. The order of the loop can be changed by changing digital coefficients of a filter that is part of the loop. The return signal is translated from the IF to S band via a triple conversion process that maintains coherence between the received and transmitted carrier signals. An S-band linear phase modulator supports direct modulation of the return carrier signal.
Once the carrier has been extracted from the incoming signal, it is necessary to demodulate the subcarrier signal to extract the baseband signal that contains the command and control data; this involves subcarrier-lock and bit-synchronization operations that are performed by two commercially available application-specific integrated circuits that implement an all-digital early/late integrate-and-dump technique. The design of the transponder also incorporates an option to perform frame synchronization and Viterbi decoding. A commercially available digital signal processor performs frame-synchronization and system-control functions and serves as a communication interface with the host computer. Every system variable is editable via this transponder/computer interface.
One of the main features of the design is its ability to acquire the received carrier signal in the presence of modulation. This feature makes it unnecessary to perform a tedious acquisition exercise to lock the uplink and downlink prior to transmitting data. When, as sometimes happens, lock is lost, the transponder reacquires lock automatically. The transponder can also detect whether the carrier-tracking loop has locked to a sideband and, if so, can automatically correct the operation of the affected oscillators to cause the loop to lock on the desired uplink carrier signal.
This work was done by David Sanderlin of Shason Microwave Corp. for Johnson Space Center.