REG NASA-LLIS-0418-1996 Lessons Learned Command Feedthrough Problem on Voyager X-Band Downlink (~1975).pdf
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1、Lessons Learned Entry: 0418Lesson Info:a71 Lesson Number: 0418a71 Lesson Date: 1996-07-08a71 Submitting Organization: JPLa71 Submitted by: J.A. RobertsSubject: Command Feedthrough Problem on Voyager X-Band Downlink (1975) Abstract: Since Galileo used inherited Voyager transponders, the mission also
2、inherited the problem that an uplink carrier suppression that is optimal for command purposes tends to degrade X-band telemetry. The lesson describes link accommodations successfully implemented by both Voyager and Galileo.Description of Driving Event: (Relevant Historical Lesson(s) Learned)The desi
3、gn of phase-locked loop transponders for Deep Space missions is such that the loop is sensitive to uplink modulation components within its bandwidth. In the two-way coherent mode of operation, the frequency reference for the downlink carriers is the loop Voltage-Controlled Oscillator (VCO). On Voyag
4、er, the command subcarrier at 512 Hz and the command data at 16 bps modulated on the subcarrier produce a residual phase jitter in the loop VCO when optimum Deep Space Network command carrier suppression factors are used. The downlink carrier jitter degrades the telemetry demodulation process; this
5、effect was imperceptible for S-band downlinks in the past. Command feedthrough was first observed on the experimental X-band transmitter (XTX) downlink on Mariner Venus-Mercury 1973 (MVM73); however, Voyager was the first mission with the primary telemetry link at X-band. Voyager found that optimum
6、(for command) uplink carrier suppression destroyed X-band telemetry.The Galileo telecommunications system includes inherited Voyager transponders; therefore, Galileo can expect the same command feedthrough problems as have been experienced on Voyager. Galileo does not plan to “fix“ the problem becau
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