2. Mission Operations - Selected Topics

The most significant operations activities during the past year were Trajectory Correction Maneuver (TCM) -22, the Ida data retrieval, and the SL-9 observations. The Ida and SL-9 activities are described elsewhere in this paper.

Only two TCMs were performed this past year (See Fig. 5). TCM-22 in early October 1993 imparted 38.6 m/sec, the largest maneuver ever required of the 10N thrusters. It was the first maneuver to actually target Galileo to the Jupiter aim point.

Figure 5

The maneuver was divided into five portions each controlled by a separate mini-sequence. Starting with Portion 2, each mini-sequence was transmitted to the spacecraft after the mini-sequence for the preceding portion was completed. On October 1, 1993, the spacecraft was commanded to the maneuver attitude. TCM-22 started on October 4, 1993 at 10:25 UTC. The last portion was completed on October 9, 1993 at 04:22 UTC. Each portion included 8 lateral segments averaging approximately 962 pulsing revolutions - the L thrusters were used with each thruster firing once each revolution. TCM-22 used 35.2 kg of propellant.

TCM-22A, one of the smallest maneuvers yet, was performed on February 15, 1994 to correct the TCM-22 execution errors. It was a single portion maneuver consisting of one lateral segment with 6 pulsing revolutions using the L thrusters and one axial segment with 36 pulsing revolutions using the P1A thruster.

The Flight Team was on line for more than 12 hours each maneuver day including for TCM-22 the daily uplink of the sequence for the next portion. They did an outstanding job. The performance of the Spacecraft, in general, and the Retropropulsion Module (RPM) in particular was outstanding.

Propellant Margin is the estimate of the usable propellant that will be remaining at the end of the primary mission with 90% probability. Currently, the Propellant Margin is approximately positive 15 kg having increased about 10 kg in the past year. This improvement is primarily attributed to the following major changes:

1. Deletion of a previously planned "High Gain Antenna (HGA) cold turn" and another 10 RPM spin-up demonstration
2. Expansion of the sequence box size which allows post-satellite encounter orbit trim maneuvers to be executed 3 days after closest approach.
3. Improvements in orbit determination accuracy based upon inclusion of optical navigation data to update satellite ephemeris estimates.
4. Incorporation of an Io occulation in the tour following the E6 encounter. The Io occulation actually reduced Propellant Margin, but the reduction was more than offset by the other changes.

Over this past year, the cummulative count of commands transmitted from the ground to Galileo since launch has doubled to a total of 254,289 commands as of September 1, 1994. Spacecraft activity, however, was less than in past years. Of the 123,399 commands transmitted this year, over 90,000 were used to test every cell in half of the CDS memory and half of the AACS memory - the halves that are not in regular use. Every cell was found functional. The regular use of the prime halves and these tests give high confidence that all memory will be functional for the new flight software required at Jupiter (Ref. 1). This year's record commanding activity was accomplished without a flaw.