*5.1.7 Tour operations*

*Aerospace Engineering*

**Figure 8.**

*Cassini-Huygens Saturn approach.*

Probe data's full Doppler shift.

onboard data storage capability, the data had to be transmitted to the Cassini orbiter directly during Titan entry, and then relayed to Earth. In this way, the spacecraft would provide the bulk of the data storage needed to support the Probe Relay task, throughout the descent and landing stages of the Probe mission. In the test, the Probe's signal was delivered to the Cassini spacecraft in flight, and then delivered to the DSN station on the ground. Results from this Probe ⇒ Cassini ⇒ DSN station relay test showed insufficient margin to maintain the carrier and subcarrier lock for the duration of the upcoming Probe mission. Analysis showed that the digital circuitry that decodes the data from the subcarrier did not have sufficient bandwidth to properly process the data from the subcarrier once it was Doppler shifted by the expected 5.6 km/s (nominal) velocity difference between Cassini and the Probe. The effect of this anomaly (caused by human error) would yield an unacceptable loss of data during the upcoming Probe Descent ⇒ Titan Landing phase since the digital circuit design did not adequately account for the

*Fix:* In January 2001, a joint effort between ESA and NASA established the Huygens

*Part 1:* The mission profile was redesigned to a Probe trajectory conducive to a low Doppler shift in the Probe-Cassini spacecraft radio link. The early part of the Saturn Tour was redesigned to a higher orbiter flyby altitude of Titan (at 60,000 km). This required that the (original) first two orbital revolutions around Saturn be increased to three revolutions (the tour configuration was unchanged after this point; this extra orbit was at a moderate ΔV cost). *Part 2:* Preheating of the Probe's transmitters was necessary before its descent into Titan's atmosphere so that the transmit frequency could be

*Part 3:* The new mission design would now have a much lower Doppler shift than that of the original Probe mission. This would require that the Probe be commanded to its "Base Frequency" (referred to as "BITE Mode," a "zero Doppler" test mode that held the lockup frequency at a level equivalent to −1 m/s relative velocity). This BITE Mode of operation must be maintained constantly, even in the presence of fault occurrences and Safe Mode activations. To accomplish this goal, an empty slot within the ATC FSW (eight ATC monitors were in use; four empty placeholders were designed into FSW for

Recovery Task Force (HRTF) team to evaluate the problem and develop a solution. This effort leads to a three-part fix that allowed full recovery of the Titan data:

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optimized.

*Safe mode activation #5:* In S33, very soon after a flyby of the Iapetus moon was completed, a SSPS Trip occurred on the prime TWTA. The spacecraft interpreted the SSPS trip as a hardware failure and executed the Safe Mode three times and swapped to the redundant backup TWTA unit. The FP also commanded a TCU swap and an RFS POR.

*Fix:* The prime TWTA was powered back on (and swapped back), and FSW was updated to implement new FP for selected devices in order to avoid activating SSPS Trip FP. This FSW fix was planned in advance of this incident, based upon observed SSPS trips, but was not uplinked in time to preclude this TWTA SSPS trip. Exactly 1 year later, another SSPS trip occurred on the prime TWTA unit. FP was not activated due to the new updates.

*Loss of MIMI motor drive:* In January 2005, a motor controlling one of MIMI's three detectors suffered a mechanical failure. Although all three detectors were still fully functional, one was forced to rely on spacecraft pointing for proper orientation. The loss to MIMI science was approximately 10%.

*Loss of SSR DRAM memory:* In December 2006, a portion of memory failed within one of the SSR's DRAM memory units, in a location where science/engineering data is stored. This failure was significant because the memory was corrupted, leading to ground software decomposition problems as well as erroneous science and engineering data. No capability to remove or bypass bad areas of SSR hardware memory had been implemented into FSW.

*Fix:* New capability to bypass corrupted memory locations was uploaded to FSW.
