To Photo of the High-Gain AntennaOn April 11, Galileo's High-Gain Antenna (HGA) should have deployed. However, signals received from the spacecraft indicated that the antenna only partially deployed, leading to the current assumption that three of the antenna's ribs are stuck in the stowed (or closed) position, while the others are partially open. The Project has performed three spacecraft turns, the first one to warm, then the next two to cool the antenna. The antenna remains partially deployed. The last activity was a cooling turn performed August 13 through 15.
The main difficulty in accurately deducing the actual configuration of the antenna lies in the sparse amount of data available from the spacecraft. The three primary pieces of information available are derived from the motor current, from wobble identification, which tells if the antenna is deployed evenly, and from the Sun gate sensor, which is currently obscured by one of the antenna's ribs.
An HGA Deployment Anomaly Team was formed on April 11. It was chartered to develop likely failure scenarios and credible explanations for the partial deployment. Also, the Team was to assess the risks and develop supporting requirements and plans for recommended actions. The Anomaly Team consists of mechanical, electrical, thermal and materials design, reliability, and flight operations personnel from JPL and contractor personnel from Harris Corporation, the builder of the HGA.
A primary consideration in any attempt to unfurl the antenna is the Project's requirement that any spacecraft action must be safe and must not preclude subsequent deployment.
The HGA deploy sequence was executed on April 11, as planned, by issuing commands to turn on the deployment motors. The motors ran at higher than predicted power levels for nearly the full sequenced 8-minute period. The microswitch-controlled motor shutdown expected about 2.5 minutes after theÊmotors were powered on did not occur. Instead, in the first 58 seconds, motor current increased to a value consistent with full available stall torque.
Preliminary analysis of the data stemming from the attempted HGA deployment, including power consumption and attitude dynamics information, suggested that the HGA was partially and asymmetrically deployed. The Sun gate sensor became partially obscured during the deployment attempt and, based on spacecraft-Sun orbital geometry, it could be concluded that antenna rib #2 was deployed 35 deg from the stowed position. Attitude and Articulation Control Subsystem (AACS) wobble data indicated an asymmetric condition with the position of rib #2 being at, or very near, the maximum rib deploy position. This inferred antenna geometry was corroborated by analysis of the motor drive system configuration at stall.
Subsequent to the HGA warming maneuver, AACS data and radio frequency and antenna pattern data were reviewed. Analysis of the AACS wobble test data indicated that the wobble angles before and after the HGA heating turn were nearly identical--3.47 and 3.52 milliradians, respectively. Wobble changes as small as 0.1 milliradian can be discerned. If the HGA were symmetrically deployed, the expected post-turn wobble angle would have been near 1 milliradian.
By April 30, the Anomaly Team initially speculated that "one or more ribs are probably restrained in the stow position, resulting in an asymmetrical partial deployment." At that time, this was the most probable scenario, but the cause was unclear.
To verify this scenario, the Team began a comprehensive test and analysis program, including reviewing all videos and still pictures of the Galileo deployment from the Shuttle to ascertain the status of the antenna and the tip shade, continuing detailed analysis of the telemetry data, and reviewing all procedures associated with the Central Release Mechanism. In addition, the Team was evaluating the thermal differential expansion of the entire antenna tower and the feasibility of a rib tip restraint pin getting stuck, reviewing the effects of shock and vibration loads on the ribs, and testing the ball screw in a thermal vacuum.
Some of the first corrective actions considered were commanding another release of the central release mechanism (CRM), altering the thermal environment of the antenna (heating and cooling and thermal cycling), shaking the spacecraft to create a force, and turning the deployment motor on again to try to get some back drive. However, since the deployment motor would increase the friction load on the alignment pins of stuck ribs, the Team agreed that such a turn-on should not be attempted until all other efforts to free the ribs have been exhausted.
The initial scenario involved three to five stuck ribs, but now it is believed three stuck ribs are restrained in the stow position and that additional ribs may have been involved initially. No damage has occurred to the dual-drive motor/ball screw, and full capability is likely if the ribs can be opened; nor has structural failure occurred to any hardware component in the motor drive subassembly. Efforts should continue to perform an antenna tower cooling turn in an attempt to free the ribs. These conclusions are based on the close comparison achieved between flight data and modeling data profiles and a confirming scenario demonstration deployment with the spare antenna.
Originally, the Anomaly Team thought that heating the antenna might free the stuck ribs. Therefore, a warming turn was performed. On May 20, the spacecraft was turned about 38.2 deg (the largest turn to date) to provide solar heating of the HGA tower; the spacecraft angle off-Sun after the turn was about 43 deg. Spacecraft performance during the turn was excellent and, generally, near predicted levels, achieving the desired turn attitude within 4 milliradians. This warming turn, unfortunately, did not release the stuck ribs.
The first HGA cooling turn was performed on July 10 at a solar distance of 1.84 AU. The turn pointed the spacecraft's -Z axis about 165 deg from the Sun to shade the entire HGA. Spacecraft performance throughout the turn was normal; the turn angle achieved was about 9 milliradians from the expected 165 deg.
Having reached attitude, the spacecraft was then commanded to the all-spin mode for the duration of the nominally planned 32-hour "cold soak." A "quick look" review of the actual temperatures indicated that all were within the allowable temperature limits or that the limits had been waived specifically for the turn. After 24 hours at attitude, the HGA element temperatures reached very near steady-state values. The HGA final temperatures achieved were compatible with those used in rib-release analyses.
Following the spacecraft's return to the Sun-pointed attitude after the 32-hour HGA cold-soak activity, telemetry indicated that the Probe shelf temperatures were still increasing and, based on projections, could reach or exceed the pre-turn agreed-to limit of 22.5 deg C. Immediately prior to the start of the return to Sun point, Probe shelf temperatures had reached about 16 deg C. Nearly 24 hours later, Probe temperatures had gone slightly above 21 deg C due to thermal soak back. The shelf temperatures then stabilized. By July 15, Probe shelf temperatures had dropped to near 16 deg C and were continuing a downward trend.
This cooling turn did not release the stuck ribs. "We have to get colder, and we know how to do that," said Bill O'Neil, Project Manager. "We can get it cold enough so the frictional forces can no longer hold the ribs in. This cooling turn was flawlessly executed and represents a tremendous improvement in our knowledge of cooling for the next turn."
A second HGA cooling turn occurred on August 13 at a solar distance of about 1.98 AU. This turn incorporated several changes to cool the antenna, including lowering the spacecraft bus power consumption and turning off the Plasma Wave Subsystem magnetic field sensor heater located on the antenna tower. The time at the cooling attitude was increased from 32 to 50 hours.
Gary Coyle, Galileo Antenna Task Manager said, "This cooling turn yielded an additional antenna tower contraction of 0.004 inch compared to the first attempt." This was still short of what was necessary to loosen the stuck ribs. Coyle went on to state that, "The wobble identification, on August 19, and the Sun gate sensor check, on August 20, showed no change from the previous configuration of the HGA."
For this turn, Hughes Aerospace Corporation, Ames Research Center, and JPL negotiated a Probe upper temperature limit of 33 deg C; this new limit represents a thermal constraint relaxation of about 11 deg C from the July turn. "The Probe's temperature during this cooling turn reached 31 deg C," noted Ron Reeve, Galileo Temperature Control Cognizant Engineer. "We believe the Probe remains in good shape."
The Plasma Wave Subsystem (PWS), on the other hand, endured temperatures that went, literally, off its scale. A reconstruction of the temperature readings suggests that the PWS's search coil preamplifier reached a temperature of 165 deg C.
The preamplifier is currently drawing a current, which may indicate that everything is in working order. A more detailed analysis of the effect of such cold temperatures on the instrument will be conducted at the end of September, when the PWS is next scheduled to send back data.
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