1、Lessons Learned Entry: 0407Lesson Info:a71 Lesson Number: 0407a71 Lesson Date: 1996-06-24a71 Submitting Organization: JPLa71 Submitted by: J.A. RobertsSubject: Helios I Communication Link Degradation Due To Interferometer Effect and Spin Modulation From Spacecraft Antenna (1972) Abstract: The compos
2、ite pattern of two Helios I low-gain antenna elements caused an “interferometer effect”, with rapidly varying phase and amplitude values occurring over a small but critical portion of the antenna pattern. Design of spacecraft antennas must include careful attention to amplitude and phase characteris
3、tics of the combined antenna system pattern, rather than to those of the individual elements alone.Description of Driving Event: (Relevant Historical Lesson(s) Learned)Prior to Helios I launch, the communication links to and from the spacecraft were expected to be degraded because of spin modulation
4、 and interferometer effects created by the low-gain antenna elements. The resultant signal, as measured using a mockup, had combined amplitude, phase and frequency modulation, which had not been considered in the link design. The amplitude and phase variations degraded the received uplink and downli
5、nk channels (command and telemetry) and created a marginal condition to carrier tracking (doppler).The Helios I low-gain antenna was made of two elements: (a) a linearly-polarized dipole, mounted on top of the spacecraft, parallel to the spacecrafts spin axis; and (b) a circularly polarized antenna
6、mounted on the bottom of the spacecraft, off-set six wavelengths from the spacecrafts spin axis. The composite pattern of the two antennas caused an “interferometer effect“, with rapidly varying phase and amplitude values occurring over a small but critical portion of the antenna pattern. The “look
7、angles“ associated with this portion of the pattern were important during some mission phases.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Reference(s):1. JPL TR 32-1526, Vol. XIV “DSN Progress reports for Jan.-Feb. 1973, pp 149-1602. JPL TR 42-15
8、26, Vol. XVIII, “DSN Progress Reports for Sept.-Oct. 1973, pp 147-1623. DSN Progress Report 42-20, Jan-Feb. 1974, pp.154-1664. DSN Progress Report 42-23, July-Aug. 1974, pp. 87-91Lesson(s) Learned: Design of spacecraft antennas must include careful attention to amplitude and phase characteristics of
9、 the combined antenna system pattern, rather than to those of the individual elements alone. Communication link analysis must use sufficiently accurate approximations to the actual expected (or measured) antenna system patterns.Recommendation(s): After the antenna pattern variations were noted durin
10、g the mockup test, the Communications Ground Systems section developed a mathematical model which simulated the observed data closely. This model was used prior to launch, in conjunction with Deep Space Network station (CTA 21) to model the anticipated system degradation. It was also used to devise
11、tracking methods to minimize the disturbing effects of the interferometer and spin modulation.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): a71 Communication Systemsa71 Flight Equipmenta71 SpacecraftAdditional In
12、fo: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 1995-10-05a71 Approval Name: Carol Dumaina71 Approval Organization: 125-204a71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
