**5. Cassini mission experience**

Cassini-Huygens is a "Class A" Flagship mission, which requires that it be configured as a low risk, high robustness design with all practical measures taken to assure mission success. Numerous analyses and test programs were required before launch approval could be obtained for Cassini by NASA, in order to assure the mission's technical worthiness. These programs were also needed to fulfill mission requirements, which consisted of spacecraft loads analyses to demonstrate that all structural margins met expected safety standards, including a modal test program that yielded experimental data to verify the spacecraft and instruments via a finite element model arranged in the launch configuration. Dynamic tests of the spacecraft and instruments were also performed, as well as acoustic/vibration tests [7–10]. Thermal analyses provided environmental verification, proving the functionality of all components. Also, verified were the heater power and the radiator area for engineering, as well as transducer performance [11, 12].

For Cassini, JPL's "conceptual life cycle strategy" was implemented. This consisted of splitting the development effort into several phases [13]:

**137**

*Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example*

Once in Phase E, Cassini's operations phase was also divided into phases. Each phase was executed by way of several uplinked command sequences which were stored and executed onboard the spacecraft (sequences were designated as "C" for

The Launch Phase spanned from launch (L) to L + 30 days, during which time launch activities and essential engineering checkouts and calibrations were required to prepare for the first main engine maneuver at L + 25 days. The Inner Cruise Phase encompassed the trajectory interior to Earth's orbit, included two Venus flybys and an Earth flyby. In this phase, the two close flybys of Venus and Earth were required to gain the needed velocity boost through gravity-assist maneuvers, to allow Cassini's trajectory to continue on to Saturn (via the next flyby at Jupiter). The science activities during this period were limited to instrument checkout exercises, with limited science performed during the Venus and Earth flybys. Since Cassini was in close proximity to the sun, the HGA was used to shield the spacecraft to prevent overheating. During the Outer Cruise Phase, the HGA was used for data transmission (instead of the LGA) since the relative distance between Cassini and the sun was now increasing rapidly and overheating was no longer an issue. Instrument checkout activities continued during this phase, as well as checkout of the Huygens probe. Also included was the final gravity-assist flyby of Jupiter, where extensive science activities began. The Science Cruise Phase began 2 years prior to arrival at Saturn, in order to prepare for Cassini's arrival. Science activities increased during this time,

The Saturn Approach Phase included a one-time opportunity flyby of the Phoebe moon and the SOI deceleration burn. After launch, the SOI burn was the most crucial activity of the entire mission since it not only allowed Cassini to be captured into Saturn's orbit, but also was an opportunity to view the planet at the closest range of the entire Prime Mission. The Probe Mission Phase was completed on the third encounter (flyby) with the moon Titan. The Tour Phase began at SOI and continued for 13 years (including the two extended missions, Equinox and Solstice). The moon Titan was massive enough to offer gravity-assist capability, and

*DOI: http://dx.doi.org/10.5772/intechopen.82161*

• Launch and Deployment (C1–C4 sequences)

• Science Cruise Phase (C33–C41 sequences)

• Saturn Approach Science Phase (C42–C44 sequences)

• Inner Cruise (C5–C16 sequences)

• Outer Cruise (C17–C32 sequences)

• Saturn Tour (S01–S06 sequences)

• Huygens Probe Mission (S07 sequence)

• Tour (all three tours; S08–S101 sequences)

and final instrument calibrations were completed.

• Phase C: Detailed design

• Phase D: Build & test

• Phase E: Operations

cruise or "S" for science):


*Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example DOI: http://dx.doi.org/10.5772/intechopen.82161*


*Aerospace Engineering*

**4.6 Meeting challenging problems through FP & FSW uploads**

health and fault condition to support fault recovery).

visibility added to the downlinked telemetry stream.

**5. Cassini mission experience**

• Prephase A: Advanced studies

• Phase B: Preliminary design

• Phase A: Mission & systems definition

To aid in many of the above challenges, onboard autonomous Fault Protection routines are implemented into the computers' FSW to monitor the spacecraft's many systems and devices to autonomously detect fault occurrences and respond to anomalous conditions. FP consists of "canned" automated responses that can swap to redundant devices (if available), command actions (like closing valves, commanding alternate targets, etc.) and/or place the spacecraft into a "safe state" using preprogrammed instructional routines. A general-purpose, "Safe Mode" fault response routine is typically executed if the fault condition interferes with the onboard running sequence (along with other corrective actions performed by FP if required), which terminates the onboard running sequence, configures the spacecraft to a lower power state by powering off all nonessential spacecraft loads, commands a thermally safe attitude and safe state for the hardware, establishes a low uplink and downlink rate for earth communications, and commands the LGA antenna (to accommodate the low rates). This safe, predictable spacecraft state allows the SOFS sufficient time to evaluate the fault causes and determine a solution [6]. On Cassini, FP was implemented early in the design phase. In general, FP responsibility is allocated to both the SOFS team and the spacecraft (which must deliver sufficient information on its

Unexpected conditions and problems can potentially exist for spacecraft missions that are exploring unknown parts of our solar system. New devices never flown in space before can experience unexpected faults due to the adverse flight environment. For these reasons (and those stated above), designers provide the SOFS team with the ability to upload FSW patches (replacing the memory locations within the onboard FSW with new data), and to replace entire CDS, AACS, or instrument FSW loads so that unknown problems can be addressed and increased

Cassini-Huygens is a "Class A" Flagship mission, which requires that it be configured as a low risk, high robustness design with all practical measures taken to assure mission success. Numerous analyses and test programs were required before launch approval could be obtained for Cassini by NASA, in order to assure the mission's technical worthiness. These programs were also needed to fulfill mission requirements, which consisted of spacecraft loads analyses to demonstrate that all structural margins met expected safety standards, including a modal test program that yielded experimental data to verify the spacecraft and instruments via a finite element model arranged in the launch configuration. Dynamic tests of the spacecraft and instruments were also performed, as well as acoustic/vibration tests [7–10]. Thermal analyses provided environmental verification, proving the functionality of all components. Also, verified were the heater power and the radiator

For Cassini, JPL's "conceptual life cycle strategy" was implemented. This con-

area for engineering, as well as transducer performance [11, 12].

sisted of splitting the development effort into several phases [13]:

**136**

• Phase E: Operations

Once in Phase E, Cassini's operations phase was also divided into phases. Each phase was executed by way of several uplinked command sequences which were stored and executed onboard the spacecraft (sequences were designated as "C" for cruise or "S" for science):


The Launch Phase spanned from launch (L) to L + 30 days, during which time launch activities and essential engineering checkouts and calibrations were required to prepare for the first main engine maneuver at L + 25 days. The Inner Cruise Phase encompassed the trajectory interior to Earth's orbit, included two Venus flybys and an Earth flyby. In this phase, the two close flybys of Venus and Earth were required to gain the needed velocity boost through gravity-assist maneuvers, to allow Cassini's trajectory to continue on to Saturn (via the next flyby at Jupiter). The science activities during this period were limited to instrument checkout exercises, with limited science performed during the Venus and Earth flybys. Since Cassini was in close proximity to the sun, the HGA was used to shield the spacecraft to prevent overheating.

During the Outer Cruise Phase, the HGA was used for data transmission (instead of the LGA) since the relative distance between Cassini and the sun was now increasing rapidly and overheating was no longer an issue. Instrument checkout activities continued during this phase, as well as checkout of the Huygens probe. Also included was the final gravity-assist flyby of Jupiter, where extensive science activities began. The Science Cruise Phase began 2 years prior to arrival at Saturn, in order to prepare for Cassini's arrival. Science activities increased during this time, and final instrument calibrations were completed.

The Saturn Approach Phase included a one-time opportunity flyby of the Phoebe moon and the SOI deceleration burn. After launch, the SOI burn was the most crucial activity of the entire mission since it not only allowed Cassini to be captured into Saturn's orbit, but also was an opportunity to view the planet at the closest range of the entire Prime Mission. The Probe Mission Phase was completed on the third encounter (flyby) with the moon Titan. The Tour Phase began at SOI and continued for 13 years (including the two extended missions, Equinox and Solstice). The moon Titan was massive enough to offer gravity-assist capability, and was used as "the tour engine" enabling orbit rotation, orbital period, and inclination changes needed to study Saturn's geometry, as well as to set up the many icy satellite encounters.
