1、Lessons Learned Entry: 1531Lesson Info:a71 Lesson Number: 1531a71 Lesson Date: 2005-04-18a71 Submitting Organization: JPLa71 Submitted by: F.H. TaylorSubject: Indication of Post-Launch Degradation of the Downlink Signal due to Feedthrough (2003) Abstract: A feedthrough condition commonly occurs shor
2、tly after most JPL spacecraft launches during initial acquisition of the downlink signal. Caused by the extremely strong uplink signal transmitting over a relatively short distance, the indication of downlink signal degradation tends to elicit consternation from the Flight Control Team during a crit
3、ical mission phase. Modify signal level and signal-to-noise ratio displays to indicate to the FCT when these indicators may be affected by feedthrough or saturation effects.Description of Driving Event: A feedthrough condition occurs after most JPL spacecraft launches during initial acquisition of t
4、he downlink signal. The stations downlink signal performance indicators are affected by feedthrough from the uplink to the downlink. This feedthrough source may be uplink transmitter noise or, if there is ranging modulation, it may be components of the uplink ranging channel. In current link designs
5、, the condition does not noticeably degrade the telemetry or ranging signals processed from the downlink for delivery to the project.Feedthrough occurs because of the extremely strong uplink signal over a distance of just a few thousand kilometers; this has the effect of reducing the stations report
6、ed value of downlink carrier power or of telemetry signal-to-noise ratio. Saturation effects in the stations receiver software algorithms contribute to the indications of signal degradation. This saturation causes the indicated signal-to-noise ratio to top out at a particular value even when the exp
7、ected level may be tens of dB higher.One variation on this problem was experienced by both Mars Exploration Rover (MER-A) and Mars Odyssey. The initial acquisition downlinks from both missions experienced post-launch downlink Provided by IHSNot for ResaleNo reproduction or networking permitted witho
8、ut license from IHS-,-,-saturation effects that produced lower-than-predicted downlink levels. In this case, uplink ranging data power harmonics feedthrough caused a “pedestal effect,“ effectively raising the noise floor in the station receiver and degrading a key estimator (Symbol Signal-to-Noise R
9、atio).A related problem was documented in a lesson learned written on the Voyager mission. Voyager experienced command feedthrough problems because the uplink command modulation index (carrier suppression) that was optimal for commanding tended to degrade X-band telemetry. Link accommodations were s
10、uccessfully implemented by the Voyager mission. The problem described in Reference (1) has since been “designed out“ with the 16 kHz command subcarrier frequency used by modern transponders in place of the Voyager-era 512 Hz.None of these strong-signal receiver nonlinearities affect the ability of t
11、elemetry subsystems to support ongoing activities. However, the apparent downlink signal degradation tends to elicit consternation in a critical phase of the mission if these expected effects have not been thoroughly coordinated between the Flight Project and the Deep Space Network, or if the Flight
12、 Control Team (FCT) has not yet become fully familiar with this idiosyncrasy of the mission operations system.References1. “Command Feedthrough Problem on Voyager X-Band Downlink,“ NASA Lesson Learned No. 0418, March 7, 19792. Additional Key Words: Communication Link, Spacecraft Commanding; Signal A
13、cquisition; RF Compatibility; RF Interference; Downlink Degradation; RF Interference; Radio Frequency Subsystem; Communications Lesson(s) Learned: Telemetry feedthrough from the uplink and saturation of signal-to-noise indicators may cause apparent non-critical downlink signal degradation during ini
14、tial acquisition of the downlink signal. As the spacecraft moves farther from Earth, the reported signal levels tend to move closer to predicted values.Recommendation(s): 1. Provide comprehensive pre-launch coordination between the flight project and the Deep Space Network to evaluate initial acquis
15、ition signal modes and levels and their strong-signal effects on the station receiving equipment.2. Ensure that telecommunication engineers a) apprise the flight project of the likelihood of the expected post-launch indications of feedthrough- and saturation-related, downlink signal degradation; b)
16、support the project during initial downlink signal acquisition to distinguish between apparent and real degradation; and c) provide the Flight Control Team with Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-procedures for responding to this anomaly
17、.3. Reduce the likelihood of errors in the monitoring of station data and the evaluation of the associated predicts during critical mission sequences. This may be accomplished by modifying signal level and signal-to-noise ratio displays to indicate to the FCT when these indicators may be affected by
18、 feedthrough or saturation effects. A less preferred alternative to this alert is to modify the link models to mark the predict values that may reach levels subject to feedthrough and saturation.Evidence of Recurrence Control Effectiveness: JPL opened Preventive Action Notice (PAN) No. Z87142 on Aug
19、ust 2, 2005 to initiate and document appropriate Laboratory-wide action on the above recommendations.Documents Related to Lesson: “Flight Project Practices, Rev. 5,“ JPL DocID 58032, February 27, 2003, Paragraphs 6.3.12 & 6.3.14 (Mission Operations).Mission Directorate(s): a71 Exploration Systemsa71
20、 Sciencea71 Aeronautics ResearchAdditional Key Phrase(s): a71 Communication Systemsa71 Flight Operationsa71 Ground Equipmenta71 Ground Operationsa71 Hardwarea71 Range Operationsa71 Risk Management/Assessmenta71 Safety & Mission Assurancea71 SpacecraftAdditional Info: Provided by IHSNot for ResaleNo
21、reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2005-08-29a71 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-,-,-
