3. Ida Data Return

The playback of the Ida science data recorded on the tape recorder during the August 28, 1993 encounter was a real challenge. The following issues were involved: The data return plans and resulting spacecraft sequences were necessarily complex. Because of some unforeseen events these plans/sequences were changed frequently. Ultimately, better than 98% of the high priority data was returned.

3.1 Playback Opportunities

At the outset, it was decided to return Ida data only when the telecommunications downlink supported 40 bps telemetry rate. The resulting two periods for data playback are shown on Figure 6. These periods were dictated by the unavailability of the HGA and the relative distance between the Earth and the Spacecraft. This relationship is shown on Figure 7. Approximately 1,300 hours of tracking station time, spread over the two playback opportunities, was required to return the Ida data.

Figure 6 Figure 6

Figure 7 Figure 7

3.2 Design Concept

The playback of Ida data was accomplished by spacecraft stored sequence control. Dependence on real time commanding was to be minimized. Background sequences controlled the operation of the spacecraft. In addition to other functions, each background sequence included a preview of the data on the tape recorder (Jailbar Search) so that the high priority data could be located for later return. On the basis of the jailbar search data, mini-sequences were developed (Reserve Box Sequences-RBS's). The RBS's specifically control the positioning of the tape within the tape recorder so that the desired data would be transferred from the tape recorder to the central computer and then to the ground (See Fig. 8). Commands from the background and the mini-sequences had to be integrated and issued by the spacecraft in time order. The concept of integrating background sequences and tape positioning mini-sequences provided sufficient flexibility to accommodate the issues identified earlier without an overwhelming level of work.

Figure 8 Figure 8

3.3 Phase One Return

Playback Phase 1 started on September 3, 1993. Return of the high resolution Solid State Imaging (SSI) instrument data was the objective of Phase 1. On September 5, 1993, the tracking station at Canberra, Australia experienced a power distribution transformer failure. The Phase 1 Playback had been designed to use the 70m tracking stations at Goldstone and Canberra (Canberra was the best station because it could "see" Galileo for nearly ten hours each pass). Since the playback was controlled by the background sequence already being executed by the spacecraft, the loss of a station could not be easily accommodated. The tracking station at Madrid was reassigned to Galileo initiating the complete replanning of the data retrieval to take advantage of the new tracking station allocation and the loss of Canberra. This effort resulted in a new set of mini-sequences to control the transfer of data from the tape recorder to the central computer. Fortunately, the loss of Canberra support occurred after the Jailbar search of all the high resolution imaging frames. All the information required for the design of required new mini-sequences was available. This relationship is shown in Figure 8. The Canberra transformer failure also occurred after the Reserve Box Sequence (RBS #1), which was to have controlled the tape positioning for the first part of Ida playback Phase 1, had been transmitted to the Spacecraft and had gone active. RBS #1 had to be cancelled. A total of 6 mini- sequences were generated and integrated with the background sequence. During the Phase 1, Ida data was returned for a total of 249 hours at 40 bps. On September 22, 1993, the Ida high resolution image was released to the public. It was presented at the 44th Congress of the IAF (Ref. 1).

3.4 Phase Two Playback

The second Phase of the Ida data playback started on February 16, 1994 with the Jailbar search of all the remaining high priority Ida data. The EJ-5 sequence controlled the jailbar search, the copying of data from the tape recorder to the central computer as well as all other Spacecraft operations during a three month, two week period. Early in the EJ-5 sequence it was possible to operate at 40bps for brief intervals during selected tracking passes when the Spacecraft was near zenith. The Spacecraft was commanded in real time to 40bps when that rate was supportable and then commanded back to 10bps. Three of the four jailbar searches were accomplished at 10 bps; the fourth and all the subsequent data return was accomplished at 40 bps. On March 7, 1994 when 40 bps was continuously sustainable, the playback of Ida data was resumed. During Phase 2, the Project generated 6 RBS's. The EJ-6 sequence, which controlled the remainder of the Ida data Playback, was updated shortly before it was uplinked to the spacecraft. This enabled the pre-planning of all EJ-6 data return and eliminating the need for RBS's i.e., the RBS tape positioning was built into the EJ-6 sequence.

The Ida encounter provided still another challenge. The discovery of Ida's satellite caused the rework of the previously defined priority scheme and a great deal of last minute sequence revision to make sure that all Ida satellite data was played back. This, of course, was just the kind of challenge every member of the Flight Team dreams about. Discovery is, after all, the raison d'etre of the Galileo Mission.

Ida's satellite, 1993(243)1, was first detected in the early part of the jailbar search of imaging data on February 17th; it was confirmed in the NIMS chemical map jailbar search data 8 days later. Fortunately, the Flight Team was able to adjust the science data return priorities and to generate necessary mini-sequences that retrieved exactly the desired data from the tape recorder. During Phase 2 of the Ida data playback, 988 hours of tracking was scheduled for Ida data; of those, only eight hours and twenty minutes were lost due to problems at the tracking stations-- none of this data was considered by the scientific community to be of high enough priority to preempt data still scheduled for playback.

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