Uplink--Downlink by Progressive Management

Author:Progressive Management
Language: eng
Format: epub
Tags: space, nasa, viking, spaceflight, cassini, voyager, deep space network
Publisher: Progressive Management

By 1989, the Goldstone antenna had been in continuous operation for 23 years, and the overseas antennas had been in operation for 16 years. The specified design life for parts subject to wear was 10 years. The antennas had been specified and designed more than 27 years earlier. They had performed remarkably well, but with a resurgence in the number of planetary missions planned for the Galileo Era, even more was to be demanded of them.

To this end, a DSN Antenna Rehabilitation Team36 was established in March 1989 to identify the resources needed to maintain the new 70-m antennas past the year 2000. The team recommended five high-priority tasks it believed would, if promptly implemented, extend the useful operating life of the antennas for the next twenty-five years.

The tasks were to develop a trend analysis program to provide early detection of incipient failures; to provide a means of lifting the antenna in the event that a pad or ball joint needed to be replaced, as had occurred in Spain and Australia in 1976; to repair or replace aging structural and mechanical elements of the antenna before further deterioration took place; to rehabilitate the elevation and azimuth gearboxes, since the gear drive assemblies ranked next to the hydrostatic bearing in the number of problems experienced over the years; and to replace the subreflector positioner and controls because these had been a longstanding source of trouble.

The hydrostatic bearings at Spain and Australia were believed to be in good condition and were not expected to require any special attention in the foreseeable future. However, there was less confidence in the future of the hydrostatic bearing at Goldstone. It was hoped that existing procedures to inhibit rust formation would be successful and that major rework would not be necessary. Only time would tell.

It was the opinion of the team that the basic design was sound and that, under normal operating conditions, the structure was not subjected to high stress levels. At the time, cyclic stresses were considered to be infrequent and of a low level, so fatigue or catastrophic failure was thought to be unlikely.

Elevation Bearing Failure

About the time that the 70-m Rehabilitation Team was publishing these findings (November 1989), the Galileo spacecraft was reaching the point where it became dependent on continuous support from the 70-m antennas to sustain the downlink telemetry data rate of 1,200 bps needed for its rapidly approaching Encounter with Venus on 9 February 1990. Closest approach would occur during the DSS 43 overlap with DSS 63, and full 70-m coverage was essential to the success of this first gravitational-assist event in the Galileo mission to Jupiter. On 13 December 1989, in the midst of this intense 70-m activity, the right-side inboard elevation bearing on the DSS 63 antenna failed, and the antenna was immobilized.

The failure occurred about 9:00 a.m. (local time) while the antenna was being used to investigate some anomalous Doppler problems at low elevation angles. It appeared that one or more of the rollers in the roller bearing race that supported the right inboard elevation bearing had cracked.

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