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
7、se the resources to do so are deemed excessive relative to the inconvenience of accommodating to it. The required accommodations (for both Voyager and Galileo) are described below.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Additional Keyword(s):
8、 Communication Link, Signal Degradation, Spacecraft Commanding, Tracking Loop FilterReference(s): JPL IOM 3395-79-013, “Project Galileo Strategy to Accommodate Command Feedthrough Effects,“ R. L. Horttor, 25 January 1979Lesson(s) Learned: 1. With a strong uplink, the command performance requirements
9、 can be met with much less than optimum uplink carrier suppression.2. Some phases of a mission may not require a two-way coherent link.3. Changes in link configuration require verification through end-to-end performance tests.Recommendation(s): 1. Voyager uses a carrier suppression of 0.54 dB for th
10、e 64-meter stations compared to 3.0 dB for the 26-meter stations. The requirement for two different modulation levels does complicate mission operations.2. In noncoherent modes, the downlink frequency reference is derived from an on-board oscillator. Phase jitter in the loop VCO is acceptable under
11、these conditions. Future projects, including Galileo, need to develop careful mission design to accommodate conditions such as command feedthrough where hardware modification is not considered feasible.3. Voyager command feedthrough parameters were established for the subcarrier frequency and bit ra
12、te used. Galileo will require similar tests for its bit rate of 32 bps, even though transponder design and command subcarrier frequency remain unchanged. 1995 Update: Missions presently in design at JPL use a 16 kHz command subcarrier to accommodate command feedthrough effects.Evidence of Recurrence
13、 Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Communication Systemsa71 Test & VerificationAdditional Info: Approval Info: a71 Approval Date: 1995-11-02a71 Approval Name: Carol Dumaina71 Approval Organization: JPLa71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
