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The Cassini Titan Probe's Adaptive Descent Control
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THE CASSINI TITAN PROBE'S ADAPTIVE DESCENT CONTROL K. Schilling* and H. Lehra** *Donlin GlllbH, Pustlr/ch 1-1 20, D-7990 Frinlric!i.,llillfII, FlU; ** Donlin Llljij(lIlrt GlllbH, Pustj(ll"il 1-1 20, D-7990 Frinlrichslliljfll , FRG
Abstract, The aim of the CASSINI mission is the exploration of the Saturnian system, The spacecraft is composed of an Orbiter and a Probe, The Probe is planned to land on Titan, Saturn-s largest moon_ While the Probe descends through Titan-s atmosphere the Orbiter flying by serves as a relay for the transmission of scientific data towards Earth, Due to the large Earth-Saturn distance, ground control of the Probe- s descent is impossible, Further, the knowledge about Titan-s atmosphere still includes major uncertainties, preventing an accurate prediction of the planetary entry characteristics and of the subsequent descent via parachute, The descent profile has to provide appropriate conditions for the operation of the scientific instruments and to guarantee the transmission of all gathered scientific data_ Thus, according to in-situ measurements adaptive control actions, related to the timing of parachute deployment and parachute exchange, have to be initiated, By these means it seems possible to satisfy the requirements despite uncertainties, Keywords_ Adaptive Control, Aerospace Trajectories, Entry Vehic les, Mission Analysis, Parameter Estimation, Interplanetary Spacecraft, System Ana1ysis_
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INTRODUCTION
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The knowledge about the outer planets of our solar system increased tremendously during the brief f1ybys of the Voyager and Pioneer spacecraft. Many surprising and sti ll enigmatic facts have been observed, raising the interest in successor missions including long duration observations and probe descents to the surfaces_ This is the task of "Gal i 1eo" for Jupiter and "Cassini" for Saturn respectively.
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In the framework of the Cassini-mission Titan, Saturn- s largest moon, was selected as the target for the probe-mission. Titan is the only moon in solar system possessing a significant our atmosphere (cf. Fi g. 1 ) . Although Voyager 1 its approached Titan closer than 5000 km, instruments could hardly penetrate its opaqu~ dust layers covering the surface. The atmosphere consists, li ke Earth, mainly from nitrogen (ca. 90 % Nz) and i nc 1udes even organi c mo 1ecu 1es. The complex organic chemistry, stimulated by the methane ci rcu 1at ion in the atmosphere, mi ght provide clues to the prebiotic atmosphere of the juvenile Earth.
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The Cassini-mission,appropriate1y named after Gian Domenico Cassini (1625 - 1712, discoverer of four Saturn moons and the division in Saturn's ring) is planned as a joint ESA/NASA collaboration. While NASA wi 11 provi de a Mari ner Mark I I spacecraft as the Saturn Orbiter (cf_ Fig. 2), the European Space Agency (ESA) wi 11 contri bute the Ti tan Atmosphere called "Huygens" to honour Titan's Probe, discoverer Christiaan Huygens (1629 - 1695).
Foto: ESA
Characteristics of Titan (after Lindal et al.) 2575. 0 ± 0.5 km 1 881 ± 0002 g/cm)
Surface radius Mean denSity Rotation periOd (Davle5 et at , 1980)
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At the surface: Atmosphere pressure Atmospnerlc temperature
Thi s paper descri bes the control scheme for the Probe's atmospheri c descent, to meet the mi ss ion requ i rements despite the uncerta i nt i es re 1ated to Titan's atmosphere.
Accelerallon 01 gravity
Fig. 1: Titan's
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PROBE MISSION OVERVIEW
Plasma/Radio-waV8 Experiment _
Outboard magnel:ometer _ High-gain antenna (VGR)
The transfer to Saturn is based upon a Titan IV / Centaur 1aunch in Apri 1 1996 and swi ngby manoeuvres at Earth and Jupi ter (for detai 1s cf. ESA / NASA (1988)) . Approximately 6.5 years after launch the spacecraft will be injected into an orbit around Saturn, which leads at the end of its first revolution to a rendezvous with Titan.
Circa 12 days before encounter the Probe is aligned for its Titan aim point (cf. Table la), spun up and separated. Two days 1at er the Orbi ter performs a deflection manoeuvre with a two-fold objective: o to serve as a relay for the data transmission from the Probe towards Earth o to achieve suitable flyby conditions at Titan for the 4 year long tour through the Saturnian system. Titan Probe _ Meanwhile the spin stabilized Probe approaches Titan in an almost dormant state - except for a RCS thruster timer -, without any means to influence its course. High-performance engine Before entry into Titan's atmosphere the Probe's Propulsion module Foto: ESA experiments and subsystems have to be warmed up to Fig. 2: The Cassini spacecraft in cruise configuration become operational in time. As the trajectory is so far not influenced by major uncertainties, the (Saturn Orbiter + Titan Probe) timer is an appropriate means to control the warm-ups . The Probe is designed to allow an entry velocity between 5.8 km/s and 7.2 km/s and a fl ight path The actual entry angle between -90 and -60 parameters will be optimized within this range according to the scientific requirements and engineering properties ( e. g. acceleration loads, heat fl ux profi 1es ) . The Probe des i gn i nc 1udes a decelerator ring of ca . 3. 10 m diameter to achieve within ca . 3 min a velocity of Mach 1.5 (ca. 400 m/ s) (cf. Fig. 3). Then by firing a mortar, a pilot chute is deployed which within 3 sec extracts the first main parachute and is then detached. After i nfl at i on of the mai n chute the instrument covers, in particular the nose cap, will be jettisoned and the operation of the scientific experiments is initiated. Depending upon the measured descent behavi our, after ca. 72 mi n, the 0
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to allow an appropri ate descent profi 1e for the scientific experiments with sufficient time in higher altitudes, in consideration of a total descent time close to 165 min which is of importance for the radio relay link (RRL) towards the Orbiter. After the impact on the surface (which Im-0'ITIl might be liquid or solid) the instruments, still operational, continue to gather data, which will be transmitted as long as the Orbiter is withi n the fi e 1d of vi ew of the Probe' s antenna and whi 1e power resources and thermal conditi ons allow operations.
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TIME (IN MINUTES)
Cassini Titan Probe's Adaptive Descent Control
DESCENT SYSTEM DESIGN
PILOT CHUTE
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The mission requirements concerning path through Titan" s atmosphere are Tables lb and lc. Duri ng the entry phase a ri gi d dece 1erator ri ng aerobrakes the probe from its actual entry speed down to Mach 1.5. For the subsequent descent phase several alternatives, such as descent in the entry configuration, inflatable decelerator (ballute, ba 11 oon), fo 1dab 1e rotor, different types of parachutes. Among these candidates a sequentially operating two parachute system was preferred, based on the following advantages : . o very high efficiency, characterlzed by a low ratio of decelerator weight to drag area, o high reliability, o good stability of the two-body system, o possibility of trajectory control. The individual components of the descent system are shown in Figure 4. The jettisonable rigid decelerator ri ng is made of 2D Carbon, a 1i ght wei ght material able to withstand the high thermal load during the entry phase. The main parts of the descent subsystem are.a mortar, a pi lot chute, a 1arge and a sma 11. ma~ n parachute. The mortar for pi lot chute eJectlon 1 s integrated in the aft cover structure. The houslngs of the two mai n parachutes are i nsta 11 ed on the antenna platform under the aft cover. The 1o~d clevises for the three bridle legs of each maln parachute are pos it i oned at the ci rcumference ?f the antenna platform. Sizing of the two maln parachutes is based on the scienti~ic requiremen~s and a nominal descent time of 165 mlnutes. The maln dimensions of the parachutes are given in Table 2. The parachutes are made of KEVLAR, a mate~ial which is assumed to withstand the extreme enVl ronmenta 1 conditions of Titan" s atmosphere. The material
TABLE1' Probe Mission Requirements a) Probe Targetting Requirements 1. Entry and descent on the dayside of Titan
2. Atmospheric descent within the 1at itude range from 60 N to 60 S 3. Suitable trajectory properties for zonal wind experiments 0
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b) Entry Requirements 1. Probe design appropriate for:
Entry velocities between 5.8 km/s and 7.16 km/s Entry angles between -90 and -60 2. Deceleration to Mach 1.5 in an altitude hi9her than 170 km 0
0
c) Descent Requirements 1. Operational conditions for chemical samp lin9
experiments at minimal altitude of 170 km 2. Total descent duration between 120 - 180 min 3. Minimal time to be spent between 170 km and 100 km is 15 min (Gas Chromatograph/Mass Spectrometer) 4. Spinning Probe (ca. 1 rpm) below 10 km altitude (for good quality images) 5. Reliability and attitude stability of descent system 6. Surface impact before Joss of signal
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Fig. 4: Titan Probe exploded view TABLE2
Main Dimensions of the Parachutes Pilot Chute
Desi gn Type Nominal Diameter Length of Suspension Lines Length of Riser
1st Mai~el 2nd Main Parachut ParachutE 20 degree conical ribbon 1.84 m 4.98 m 2. 31 m
2.20 m 11 .50 m
6.00 m 7.70 m
2.80 m 10.90 m
properties at very low temperatures as well as the ageing of the material for a storage time of up to 7 years are the subject of several ongoing investigations. In order to achieve good stability during the entry and descent phases most of the instruments are mounted on the experiment platform which results in a low center of gravity position for the Probe, with a wei ght of 192.3 kg, in the entry configuration. Fig. 5 illustrates the parachute deployment sequences. For the eXChange of main parachutes the three load clevises for the bridle legs of the first main parachute will be released. Simultaneously one of the bridle legs will be used for extraction of the second small main parachute from its hous i ng with re 1ease of the fi rst parachute and of the dep 1oyment bag of the second parachute. During the descent with the parachute, spin devices should keep the Probe spinning at a low rate of about 1 - 2 rpm at low altitudes . Thi s requi res a swivel in the parachute riser to decouple the spin of the Probe from the parachute system. Probe spi nni ng wi 11 be generated by aerodynami ca 11 y shaped instrument cut-outs at exposed locations, (cf. Fig. 4), after the release of covers and subsequent to parachute dep 1oyment . Fi g. 6 shows that independent of the uncertain initial spin rate an appropriate behaviour is always achieved at low altitudes by this approach.
218
K. Schilling and H. Le hra
Taking as an example the critical altitude to in it i ate the descent phase the i nterre 1at ion between mission and design parameters is displayed in Table 4 . Thus gain in height could be achieved by a shallower entry angle, by a higher deploY!:1ent velocity or by a large decelerator area.
PI LOT PA RA C HUT E
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TABLE3
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Baseline Descent Parameters
R ISE R
atmospheric density: atmospheric dynamics: mass at entry decelerator diameter: Probe diameter entry angle entry velocity parachute exchange descent duration
SMA LL PARACHUTE HOU SING "\ \
LA RG E PA RAC HUTE R ISER
_SWIVE L
nominal Lellouch / Hunten-model nominal Flasar et al. - model 192.3 kg 3.10 m 1.65 m - 65 deg 7 km/ s after 72 mi n 165 min
LARGE PARACH UT E BRIDL E LEGS
TABLE4 Effects of design parameter variation on the altitude at which the Descent Phase starts . For the baseline data (cf . Table 3) after 170 sec Mach 1.5 is reached in an altitude of 191.67 km. Variation of
Fig. 5: Two parachute system deployment sequences
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Influences of shallow angles upon RRL-geometry and day side landing site, as well as peak deceleration and peak heat flux for steep entry angles have to be checked
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A smaller diameter leads to mass savings and eases configuration layout for launch envelope
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Deployment above Mach 1.5 can cause mater i a1 prob 1ems; be low Mach 1 the dynami c pressure at deployment is very low
- 14.49 km 15.88 km
Unavoidable dispersion due to model uncertainties
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Cassini Titan Probe 's ,-\dapti\'e Desce nt COlltrol
UNCERTAINTIES AFFECTING THE DESCENT
219
Atmospheric Dynamics
Some Titan properties, still to be explored in detail by the Probe, influence the atmospheric trajectory. Models developed for the purpose of mission design include a broad range for potential variations. A minor source for uncertainties is re 1ated to the Probe" s phys i ca 1 propert i es after the long i nterp 1anetary crui se phase. All numbers given in the following to quantify deviations refer to the baseline as defined in Table 3. Density Profile For Titan " s middle atmospheric layer (200 km to 1270 km) density measurements are missing. Lellouch and Hunten (1987) developed the actual density model (LH-model) which includes, in addition to the nominal profile, a maximal and minimal profile representing bounds for the expected density distribution (cf. Fig . 7a). These variations have major i nfl uence on Probe desi gn parameters 1i ke peak deceleration (cf. Fig. 7b), peak heat flux and the altitude of Mach 1.5, but don "t affect much the time to decelerate to Mach 1.5. In the altitude range below 200 km the density profile satisfie s higher accuracy requirements. But the accumulation of deviations during the relatively long duration of the descent phase (165 min) causes considerable effects amounting to a dispersion of ± 7.5 min. The sink velocity before impact ranges from 5.3 m/s (minimal LH-model) to 4.6 m/ s (maximal LH-model).
.,
According to the wind model of Flasar et al. (1981) the wind velocity decreases rather linear from a velocity of 100 m/ s at 200 km altitude to 0 m/ s at the surface . But the uncertainties are expected to allow a multiplication / division of the values by the factor two . Although wi nds in the wes t -east direction are expected, also the opposite direction is possible . Thus, during the descent phase, with the Probe hangi ng beneath the parachute, wi nds cou 1d shift the surface impact location by up to 700 km. While horizontal winds don"t affect the height profile of the descent trajectory, an impact displacement would influence the radio relay 1ink geometry and could cause an undesirable surface impact on Titan "s night side. Topography Analogies with the surface relief of other planets recommend to include a deviation of ± 2 km for the mean Titan radius. At the low sink velocity just before impact this leads to a variation of the descent duration of up to ± 7.2 min. Parachute Drag Coefficient Each production lot of parachutes exhibits a certain variation in parachute performance properties, in particular for the ballistic coefficient C . As the Probe Descent Module acts as forebody for Phe parachute, additional wake effects have to be included in drag calculations. The related dispersion of the parachute drag coefficient was assumed to be simi lar to former planetary missions, leading to a deviation in the descent duration from + 5.9 min to - 6.3 min.
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The interactions between spin and aerodynamic effects prevent predictions about the spin rate at the beginning of the descent phase. Thus any spin rate between the value of the coast phase (10 rpm) and zero spin seems possible. Other sources of uncertainty like the ephemeris data for Titan and the spacecraft, the precision of the separation manoeuvre, the Orbiter trajectory after the deflection manoeuvre, atmospheric precipitation, etc. are expected to cause only minor effects.
200
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Fig . 7: The Titan density profile as proposed by Lellouch and Hunten (1987) and effects on the deceleration profile
REQUIREMENTS FOR AUTONOMOUS PROBE CONTROL As the signal propagation delay from Saturn to Earth amounts to 90 mi nutes, there is no i ntervention by ground control possible during the Probe descent. Therefore no faci 1i ti es are foreseen to allow command access after separation. Thus the Probe has to react autonomously in real-time if defects occur or if measurements of the Titan environment deviate from predictions . To influence the descent profile in particular, the timing for o jettison of decelerator o deployment of the first parachute o exchange of parachutes has to be optimized with respect to scientific and engineering requirements. For the worst case descent situations there are two potentially scarce resources related to data transmission and power. The admissible data transmission rate increases wi th di mi ni shi ng Orbiter/ Probe distance (512 bps at 100 000 km, ... , 16 384 bps below 19 200 km) . Data acquired in excess of this rate are stored for later transmission. It is planned that the Orbit er follows the Probe wi th a
K. Schilling and H. Lehra
fixed delay, thus data rate switching will be performed by the Probe' s timer based upon the expected arbiter trajectory, If the descent is quicker than planned there might be the danger that not all acquired data can be transmitted before surface impact. In the case of a slower descent, the arbiter cou 1d 1eave the Probe antenna's fi e 1d of view before surface impact. Furthermore the power contingency could become critical, in particular if complementary electrical heating should become necessary for thermal control purposes. The optimal operation modes of most experiments are functi ons of the di stance to Ti tan's surface and shou 1d be adapted to actua 1 altitude measurements. Thus adaptive control processes influencing the descent profi 1e, the therma 1 housekeepi ng and the operational modes of scientific experiments should contribute to mission success. A further advantage of this autonomy concept is the potential to update mission parameters according to the progress in scientific knowledge and objectives, which will possibly occur during the 12 years from the begi nni ng of satellite development unti 1 its arrival at the Saturnian system.
interval can be realized . The descent trajectories for different timing options are plotted in Fig. 8.
H (KM) 200
PARACHUTE DEPLOYMENT AT M = 1.5 . Ho :: 191.75 KM ,GAMMA O
= -60.74
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RECOMMENDED LELLOUCH ATMOSPHERE 180
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EFFECTS OF DESCENT CONTROL ACTIONS ' 0
In the following quantative effects resulting from the available descent control actions are evaluated .
20
Dece1erator Jettison
.. 0
The dece1erator jettison initiates the descent phase and thus the operation of most payload instruments. Constrai nts for thi s control acti on result from the minimal admissible height in order to explore the atmosphere at about 170 km altitude. Due to the dece1erator jettison the ballistic coefficient is increased causing as a consequence a quicker Probe descent. The descent duration from Mach 1.5 unti 1 impact for the options with / without dece1erator are summarized below: Descent with Dece1erator (Z 3.10 m) Descent with Dece1erator (g 2. 80 m) Descent without Dece1erator
2.919 h 2.635 h 1.620 h
Parachute Deployment A descent without dece1erator is the quickest way to reach Titan ' s surface, but it is quite unstable . Thus at least the pilot chute should be deployed . Parachute deployment is only recommended with a velocity above Mach 1.0, where the dynamic pressure is hi gh enough to obta in su i tab 1e parachute i nf1 ati on characteri sti cs. The parachute materi a1 demands a deployment velocity below Mach 1.6. The descent times related to these parachute options are summarized below: Descent with pilot chute Descent with small main chute Descent with large main chute
2. 095 h 2. 328 h 3.968 h
Fig. 8: Descent profiles with variation of the timing for parachute exchange Conclusions The combination of the three options for adaptive descent control offers the means to adapt the descent durat ion wi thi n the range of 1.620 h to 3.968 h. ADAPTIVE DESCENT CONTROL The aim of the adaptive control scheme is o to enable an appropriate descent profile to gather the scientific data o to correlate the mission progress of arbiter and Probe to achieve a suitable RRL-geometry. The Probe on 1y has i nformat i on about the arbiter's position from a prestored trajectory profile. The arbiter delay has to be previ ous 1y opt i mi zed wi th respect to the selected descent duration. Thus the main target for adaptive control is to reach Titan's surf ace at some fi xed time. An adapt i ve control scheme leads to a major reduction of the margin which must be included in the arbiter delay timing due to Probe descent uncertainties. This implies an increase in the available data transmission budget. The overall control scheme is shown in Fig . 9. In the following sections criteria for the initiation of the control actions will be discussed .
Parachute exchange Parameter Identification and Prediction Some fine tuning to achieve a particular time to the impact can be done by parachute exchange. After parachute deployment, the time from a velocity of Mach 1. 5 unti 1 surface impact ranges between 2.328 h (second main chute all the time) and 3.968 h (first main chute all the time). By an appropriate timing of the exchange action in principle all descent time options within this
Several crucial parameters like velocity, altitude, actual aerodynamic properties of the Probe, cannot be measured directly during all mission phases but have to be inferred. Whi 1e duri ng the entry phase only the accelerometers are operational, in the descent phase most scientific instruments and the radar altimeter start operations. Thus as the
Cassini Titan Probc's Adapti\'e Dcst:c llI Control
mission progresses the knowledge of the Titan envi ronment increases. Thi s allows a steady improvement of trajectory pred i ct ions. As an example, it will be shown how crucial factors like parachute drag coefficient and atmospheric density profiles can be inferred from accelerator measurements. The acceleration due to drag a, measured by the accelerometer, obeys the fo11owiR g relationship: a O = - 0.5·C O· ~'A' vZ / m with Cn - drag coefficient, g - atmospheric density, A effective cross section area, m - mass, v - velocity.
Oece1erator Jettison The beginning of the descent phase is constrained by suitable parachute deployment velocities (between Mach 1 and Mach 1.6) and a sufficient altitude to start scientific measurements at 170 km. Thus a velocity of Mach 1.5 was selected as the criterion to initiate the dece1erator jettison . Ouri ng 'che entry phase most sensors have sti 11 to be shi e 1ded by covers, therefore thi s event can only be triggered from inputs of the timer and / or the accelerometers, presenting the following alternatives : a) a timer value fixed before separation, b) a fixed period after measurement of peak acceleration, c) numerical integration of acce1erations (from a gravity model and accelerometer measurements) departing from initial values at separation, d) prestored tables connecting dece1erations (measured by the accelerometer ) wi t h the velocity profile.
An acceptable approximation of the density profile is provided by the exponential atmosphere model:
~
(h) = Cl . e C2h
sti 11 dependi ng upon the parameters Cl' C2 and altitude h. The constants CO . C"C? are then inferred from a set of actual measurements for a . These are the inputs for the trajectory predictioRs at lower altitudes, derived from integration of the Probe"s equations of motion
x=
(
FO (h,v)
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FG (h )
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FW (h) ) / m
(including forces due to drag - FO, gravity - FG and wind effects - FW).
Solution d) was recommended as the dece1erator / velocity rela t ion in the velocity range near Mach 1.5 is rather independent from i nf1 uences of the uncertain parameter (e. g. density profile, entry velocity etc.; cf. Schilling and F1ury (1988)) •
DEPLOYMENT OF FIRST PARACHUTE
DECELERATOR JETTISON
PAYLOAD OPERATIONS
DEPLOYMENT OF SECOND PARACHUTE
TRAJECTORY ADAPTION
- CONTROL ACTIONS
ADAPTIVE DESCENT CONTROL
- PREDICTIONS
ENERGY CONSUMPTION
PROBE'S ACTUAL POSITION/ VELOCITY
MEASUREMENTS
Fig. 9: Adaptive descent control scheme
-
PARA~TER
IDENTIFICATION
222
K. Schillillg alld H. Le hra
Dep 1oyment of Fi rst r·iai n Parachute
ACKNOWLEDGEMENTS
In case the descent has to be accelerated, after jettison of the decelerator ring, delays could be introduced either for mortar firing or, after pilot chute deployment, for ext,action of the first main chute. As the instrument covers cannot be jettisoned without the fu ', ly ' dlated first main chute, the minimum altitude for the beginning of payload operations limits this option.
The authors would like to thank the ESA study team members for stimulating discussions within the Phase A study "Cassini Titan Atmosphere Probe" .
Deployment of Second Main Parachute
Flasar, F.M., R.E. Samuel son and B.J. Conrath (1981) . Titan's atmosphere: temperature and dynamics. Nature 292, p. 693 - 698.
The exchange of the 1arge mai n parachute for the second smaller one is the preferred option to adjust the descent duration. Periodic comparison of the reference trajectory should be carried out with the measured time / altitude profile. In case of major deviations the trajectory predictions should be updated. Subsequently the timing of the parachute exchange has to be optimized in order to achieve a suitable total descent time . The altitude inputs are provided preferably by radar altimeter measurements, but as back ups inferred data from pressure and temperature sensors coul d be used, too. CONCLUSIONS On the bas is of cont i nuous 1y revi sed atmospheri c mode 1s accordi ng to measurements, the autonomous prediction of the trajectory will be steadily improved, enab 1i ng an appropri ate t i mi ng for the initiation of the available control actions . According to the updated mission profiles it could also be considered necessary to adapt and optimise the uti 1i sati on of power resources and the instrument operation profiles. The proposed adapt i ve descent control wou 1d be a significant contribution to spacecraft autonomy by introducing some simple on board mission management. But it has to be carefully assessed, whether the benefits justify the increased design complexity. The proposed control scheme might well contribute to maximisation of the gain and thus the mission success. Similar strategies should also contribute to future interplanetary missions, demanding the autonomous adaptat i on of spacecraft operat ions to an uncertain environment .
REFERENCES ESA / NASA (1988). Cassini-Report on the Phase A Study. ESA SCI(88)5.
Lellouch , E. and D.M . Hunten (1987) . Titan atmosphere engineering model. ESA Report ESLAB 87 / 199. Schilling, K. and W. Flury (1988) . Autonomy and onboard mission management aspects for the Cassini Titan Probe . IAF 88-388, to appear in acta astronautica. Schilling, K. and G. E.N Scoon (1989). Mit Cassini zum Saturn, Kapsel landet auf Titan. Luft- und Raumfahrt 1/ 89 Scoon, G. E. N. and W. Flu ry ( 1987) . Cassini mission - the Titan Probe. IAF 87-445. Sergeyevsky, A.B., S. J . Kerridge, and D. S. Stetson (1987). Cassini - a mission to the saturnian system . AAS 87-423 .
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THE CASSINI TITAN PROBE'S ADAPTIVE DESCENT CONTROL K. Schilling* and H. Lehra** *Donlin GlllbH, Pustlr/ch 1-1 20, D-7990 Frinlric!i.,llillfII, FlU; ** Donlin Llljij(lIlrt GlllbH, Pustj(ll"il 1-1 20, D-7990 Frinlrichslliljfll , FRG
Abstract, The aim of the CASSINI mission is the exploration of the Saturnian system, The spacecraft is composed of an Orbiter and a Probe, The Probe is planned to land on Titan, Saturn-s largest moon_ While the Probe descends through Titan-s atmosphere the Orbiter flying by serves as a relay for the transmission of scientific data towards Earth, Due to the large Earth-Saturn distance, ground control of the Probe- s descent is impossible, Further, the knowledge about Titan-s atmosphere still includes major uncertainties, preventing an accurate prediction of the planetary entry characteristics and of the subsequent descent via parachute, The descent profile has to provide appropriate conditions for the operation of the scientific instruments and to guarantee the transmission of all gathered scientific data_ Thus, according to in-situ measurements adaptive control actions, related to the timing of parachute deployment and parachute exchange, have to be initiated, By these means it seems possible to satisfy the requirements despite uncertainties, Keywords_ Adaptive Control, Aerospace Trajectories, Entry Vehic les, Mission Analysis, Parameter Estimation, Interplanetary Spacecraft, System Ana1ysis_
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INTRODUCTION
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The knowledge about the outer planets of our solar system increased tremendously during the brief f1ybys of the Voyager and Pioneer spacecraft. Many surprising and sti ll enigmatic facts have been observed, raising the interest in successor missions including long duration observations and probe descents to the surfaces_ This is the task of "Gal i 1eo" for Jupiter and "Cassini" for Saturn respectively.
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In the framework of the Cassini-mission Titan, Saturn- s largest moon, was selected as the target for the probe-mission. Titan is the only moon in solar system possessing a significant our atmosphere (cf. Fi g. 1 ) . Although Voyager 1 its approached Titan closer than 5000 km, instruments could hardly penetrate its opaqu~ dust layers covering the surface. The atmosphere consists, li ke Earth, mainly from nitrogen (ca. 90 % Nz) and i nc 1udes even organi c mo 1ecu 1es. The complex organic chemistry, stimulated by the methane ci rcu 1at ion in the atmosphere, mi ght provide clues to the prebiotic atmosphere of the juvenile Earth.
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The Cassini-mission,appropriate1y named after Gian Domenico Cassini (1625 - 1712, discoverer of four Saturn moons and the division in Saturn's ring) is planned as a joint ESA/NASA collaboration. While NASA wi 11 provi de a Mari ner Mark I I spacecraft as the Saturn Orbiter (cf_ Fig. 2), the European Space Agency (ESA) wi 11 contri bute the Ti tan Atmosphere called "Huygens" to honour Titan's Probe, discoverer Christiaan Huygens (1629 - 1695).
Foto: ESA
Characteristics of Titan (after Lindal et al.) 2575. 0 ± 0.5 km 1 881 ± 0002 g/cm)
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Thi s paper descri bes the control scheme for the Probe's atmospheri c descent, to meet the mi ss ion requ i rements despite the uncerta i nt i es re 1ated to Titan's atmosphere.
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PROBE MISSION OVERVIEW
Plasma/Radio-waV8 Experiment _
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The transfer to Saturn is based upon a Titan IV / Centaur 1aunch in Apri 1 1996 and swi ngby manoeuvres at Earth and Jupi ter (for detai 1s cf. ESA / NASA (1988)) . Approximately 6.5 years after launch the spacecraft will be injected into an orbit around Saturn, which leads at the end of its first revolution to a rendezvous with Titan.
Circa 12 days before encounter the Probe is aligned for its Titan aim point (cf. Table la), spun up and separated. Two days 1at er the Orbi ter performs a deflection manoeuvre with a two-fold objective: o to serve as a relay for the data transmission from the Probe towards Earth o to achieve suitable flyby conditions at Titan for the 4 year long tour through the Saturnian system. Titan Probe _ Meanwhile the spin stabilized Probe approaches Titan in an almost dormant state - except for a RCS thruster timer -, without any means to influence its course. High-performance engine Before entry into Titan's atmosphere the Probe's Propulsion module Foto: ESA experiments and subsystems have to be warmed up to Fig. 2: The Cassini spacecraft in cruise configuration become operational in time. As the trajectory is so far not influenced by major uncertainties, the (Saturn Orbiter + Titan Probe) timer is an appropriate means to control the warm-ups . The Probe is designed to allow an entry velocity between 5.8 km/s and 7.2 km/s and a fl ight path The actual entry angle between -90 and -60 parameters will be optimized within this range according to the scientific requirements and engineering properties ( e. g. acceleration loads, heat fl ux profi 1es ) . The Probe des i gn i nc 1udes a decelerator ring of ca . 3. 10 m diameter to achieve within ca . 3 min a velocity of Mach 1.5 (ca. 400 m/ s) (cf. Fig. 3). Then by firing a mortar, a pilot chute is deployed which within 3 sec extracts the first main parachute and is then detached. After i nfl at i on of the mai n chute the instrument covers, in particular the nose cap, will be jettisoned and the operation of the scientific experiments is initiated. Depending upon the measured descent behavi our, after ca. 72 mi n, the 0
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TIME (IN MINUTES)
Cassini Titan Probe's Adaptive Descent Control
DESCENT SYSTEM DESIGN
PILOT CHUTE
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The mission requirements concerning path through Titan" s atmosphere are Tables lb and lc. Duri ng the entry phase a ri gi d dece 1erator ri ng aerobrakes the probe from its actual entry speed down to Mach 1.5. For the subsequent descent phase several alternatives, such as descent in the entry configuration, inflatable decelerator (ballute, ba 11 oon), fo 1dab 1e rotor, different types of parachutes. Among these candidates a sequentially operating two parachute system was preferred, based on the following advantages : . o very high efficiency, characterlzed by a low ratio of decelerator weight to drag area, o high reliability, o good stability of the two-body system, o possibility of trajectory control. The individual components of the descent system are shown in Figure 4. The jettisonable rigid decelerator ri ng is made of 2D Carbon, a 1i ght wei ght material able to withstand the high thermal load during the entry phase. The main parts of the descent subsystem are.a mortar, a pi lot chute, a 1arge and a sma 11. ma~ n parachute. The mortar for pi lot chute eJectlon 1 s integrated in the aft cover structure. The houslngs of the two mai n parachutes are i nsta 11 ed on the antenna platform under the aft cover. The 1o~d clevises for the three bridle legs of each maln parachute are pos it i oned at the ci rcumference ?f the antenna platform. Sizing of the two maln parachutes is based on the scienti~ic requiremen~s and a nominal descent time of 165 mlnutes. The maln dimensions of the parachutes are given in Table 2. The parachutes are made of KEVLAR, a mate~ial which is assumed to withstand the extreme enVl ronmenta 1 conditions of Titan" s atmosphere. The material
TABLE1' Probe Mission Requirements a) Probe Targetting Requirements 1. Entry and descent on the dayside of Titan
2. Atmospheric descent within the 1at itude range from 60 N to 60 S 3. Suitable trajectory properties for zonal wind experiments 0
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b) Entry Requirements 1. Probe design appropriate for:
Entry velocities between 5.8 km/s and 7.16 km/s Entry angles between -90 and -60 2. Deceleration to Mach 1.5 in an altitude hi9her than 170 km 0
0
c) Descent Requirements 1. Operational conditions for chemical samp lin9
experiments at minimal altitude of 170 km 2. Total descent duration between 120 - 180 min 3. Minimal time to be spent between 170 km and 100 km is 15 min (Gas Chromatograph/Mass Spectrometer) 4. Spinning Probe (ca. 1 rpm) below 10 km altitude (for good quality images) 5. Reliability and attitude stability of descent system 6. Surface impact before Joss of signal
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Fig. 4: Titan Probe exploded view TABLE2
Main Dimensions of the Parachutes Pilot Chute
Desi gn Type Nominal Diameter Length of Suspension Lines Length of Riser
1st Mai~el 2nd Main Parachut ParachutE 20 degree conical ribbon 1.84 m 4.98 m 2. 31 m
2.20 m 11 .50 m
6.00 m 7.70 m
2.80 m 10.90 m
properties at very low temperatures as well as the ageing of the material for a storage time of up to 7 years are the subject of several ongoing investigations. In order to achieve good stability during the entry and descent phases most of the instruments are mounted on the experiment platform which results in a low center of gravity position for the Probe, with a wei ght of 192.3 kg, in the entry configuration. Fig. 5 illustrates the parachute deployment sequences. For the eXChange of main parachutes the three load clevises for the bridle legs of the first main parachute will be released. Simultaneously one of the bridle legs will be used for extraction of the second small main parachute from its hous i ng with re 1ease of the fi rst parachute and of the dep 1oyment bag of the second parachute. During the descent with the parachute, spin devices should keep the Probe spinning at a low rate of about 1 - 2 rpm at low altitudes . Thi s requi res a swivel in the parachute riser to decouple the spin of the Probe from the parachute system. Probe spi nni ng wi 11 be generated by aerodynami ca 11 y shaped instrument cut-outs at exposed locations, (cf. Fig. 4), after the release of covers and subsequent to parachute dep 1oyment . Fi g. 6 shows that independent of the uncertain initial spin rate an appropriate behaviour is always achieved at low altitudes by this approach.
218
K. Schilling and H. Le hra
Taking as an example the critical altitude to in it i ate the descent phase the i nterre 1at ion between mission and design parameters is displayed in Table 4 . Thus gain in height could be achieved by a shallower entry angle, by a higher deploY!:1ent velocity or by a large decelerator area.
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LARGE PARACH UT E BRIDL E LEGS
TABLE4 Effects of design parameter variation on the altitude at which the Descent Phase starts . For the baseline data (cf . Table 3) after 170 sec Mach 1.5 is reached in an altitude of 191.67 km. Variation of
Fig. 5: Two parachute system deployment sequences
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Cassini Titan Probe 's ,-\dapti\'e Desce nt COlltrol
UNCERTAINTIES AFFECTING THE DESCENT
219
Atmospheric Dynamics
Some Titan properties, still to be explored in detail by the Probe, influence the atmospheric trajectory. Models developed for the purpose of mission design include a broad range for potential variations. A minor source for uncertainties is re 1ated to the Probe" s phys i ca 1 propert i es after the long i nterp 1anetary crui se phase. All numbers given in the following to quantify deviations refer to the baseline as defined in Table 3. Density Profile For Titan " s middle atmospheric layer (200 km to 1270 km) density measurements are missing. Lellouch and Hunten (1987) developed the actual density model (LH-model) which includes, in addition to the nominal profile, a maximal and minimal profile representing bounds for the expected density distribution (cf. Fig . 7a). These variations have major i nfl uence on Probe desi gn parameters 1i ke peak deceleration (cf. Fig. 7b), peak heat flux and the altitude of Mach 1.5, but don "t affect much the time to decelerate to Mach 1.5. In the altitude range below 200 km the density profile satisfie s higher accuracy requirements. But the accumulation of deviations during the relatively long duration of the descent phase (165 min) causes considerable effects amounting to a dispersion of ± 7.5 min. The sink velocity before impact ranges from 5.3 m/s (minimal LH-model) to 4.6 m/ s (maximal LH-model).
.,
According to the wind model of Flasar et al. (1981) the wind velocity decreases rather linear from a velocity of 100 m/ s at 200 km altitude to 0 m/ s at the surface . But the uncertainties are expected to allow a multiplication / division of the values by the factor two . Although wi nds in the wes t -east direction are expected, also the opposite direction is possible . Thus, during the descent phase, with the Probe hangi ng beneath the parachute, wi nds cou 1d shift the surface impact location by up to 700 km. While horizontal winds don"t affect the height profile of the descent trajectory, an impact displacement would influence the radio relay 1ink geometry and could cause an undesirable surface impact on Titan "s night side. Topography Analogies with the surface relief of other planets recommend to include a deviation of ± 2 km for the mean Titan radius. At the low sink velocity just before impact this leads to a variation of the descent duration of up to ± 7.2 min. Parachute Drag Coefficient Each production lot of parachutes exhibits a certain variation in parachute performance properties, in particular for the ballistic coefficient C . As the Probe Descent Module acts as forebody for Phe parachute, additional wake effects have to be included in drag calculations. The related dispersion of the parachute drag coefficient was assumed to be simi lar to former planetary missions, leading to a deviation in the descent duration from + 5.9 min to - 6.3 min.
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REQUIREMENTS FOR AUTONOMOUS PROBE CONTROL As the signal propagation delay from Saturn to Earth amounts to 90 mi nutes, there is no i ntervention by ground control possible during the Probe descent. Therefore no faci 1i ti es are foreseen to allow command access after separation. Thus the Probe has to react autonomously in real-time if defects occur or if measurements of the Titan environment deviate from predictions . To influence the descent profile in particular, the timing for o jettison of decelerator o deployment of the first parachute o exchange of parachutes has to be optimized with respect to scientific and engineering requirements. For the worst case descent situations there are two potentially scarce resources related to data transmission and power. The admissible data transmission rate increases wi th di mi ni shi ng Orbiter/ Probe distance (512 bps at 100 000 km, ... , 16 384 bps below 19 200 km) . Data acquired in excess of this rate are stored for later transmission. It is planned that the Orbit er follows the Probe wi th a
K. Schilling and H. Lehra
fixed delay, thus data rate switching will be performed by the Probe' s timer based upon the expected arbiter trajectory, If the descent is quicker than planned there might be the danger that not all acquired data can be transmitted before surface impact. In the case of a slower descent, the arbiter cou 1d 1eave the Probe antenna's fi e 1d of view before surface impact. Furthermore the power contingency could become critical, in particular if complementary electrical heating should become necessary for thermal control purposes. The optimal operation modes of most experiments are functi ons of the di stance to Ti tan's surface and shou 1d be adapted to actua 1 altitude measurements. Thus adaptive control processes influencing the descent profi 1e, the therma 1 housekeepi ng and the operational modes of scientific experiments should contribute to mission success. A further advantage of this autonomy concept is the potential to update mission parameters according to the progress in scientific knowledge and objectives, which will possibly occur during the 12 years from the begi nni ng of satellite development unti 1 its arrival at the Saturnian system.
interval can be realized . The descent trajectories for different timing options are plotted in Fig. 8.
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In the following quantative effects resulting from the available descent control actions are evaluated .
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The dece1erator jettison initiates the descent phase and thus the operation of most payload instruments. Constrai nts for thi s control acti on result from the minimal admissible height in order to explore the atmosphere at about 170 km altitude. Due to the dece1erator jettison the ballistic coefficient is increased causing as a consequence a quicker Probe descent. The descent duration from Mach 1.5 unti 1 impact for the options with / without dece1erator are summarized below: Descent with Dece1erator (Z 3.10 m) Descent with Dece1erator (g 2. 80 m) Descent without Dece1erator
2.919 h 2.635 h 1.620 h
Parachute Deployment A descent without dece1erator is the quickest way to reach Titan ' s surface, but it is quite unstable . Thus at least the pilot chute should be deployed . Parachute deployment is only recommended with a velocity above Mach 1.0, where the dynamic pressure is hi gh enough to obta in su i tab 1e parachute i nf1 ati on characteri sti cs. The parachute materi a1 demands a deployment velocity below Mach 1.6. The descent times related to these parachute options are summarized below: Descent with pilot chute Descent with small main chute Descent with large main chute
2. 095 h 2. 328 h 3.968 h
Fig. 8: Descent profiles with variation of the timing for parachute exchange Conclusions The combination of the three options for adaptive descent control offers the means to adapt the descent durat ion wi thi n the range of 1.620 h to 3.968 h. ADAPTIVE DESCENT CONTROL The aim of the adaptive control scheme is o to enable an appropriate descent profile to gather the scientific data o to correlate the mission progress of arbiter and Probe to achieve a suitable RRL-geometry. The Probe on 1y has i nformat i on about the arbiter's position from a prestored trajectory profile. The arbiter delay has to be previ ous 1y opt i mi zed wi th respect to the selected descent duration. Thus the main target for adaptive control is to reach Titan's surf ace at some fi xed time. An adapt i ve control scheme leads to a major reduction of the margin which must be included in the arbiter delay timing due to Probe descent uncertainties. This implies an increase in the available data transmission budget. The overall control scheme is shown in Fig . 9. In the following sections criteria for the initiation of the control actions will be discussed .
Parachute exchange Parameter Identification and Prediction Some fine tuning to achieve a particular time to the impact can be done by parachute exchange. After parachute deployment, the time from a velocity of Mach 1. 5 unti 1 surface impact ranges between 2.328 h (second main chute all the time) and 3.968 h (first main chute all the time). By an appropriate timing of the exchange action in principle all descent time options within this
Several crucial parameters like velocity, altitude, actual aerodynamic properties of the Probe, cannot be measured directly during all mission phases but have to be inferred. Whi 1e duri ng the entry phase only the accelerometers are operational, in the descent phase most scientific instruments and the radar altimeter start operations. Thus as the
Cassini Titan Probc's Adapti\'e Dcst:c llI Control
mission progresses the knowledge of the Titan envi ronment increases. Thi s allows a steady improvement of trajectory pred i ct ions. As an example, it will be shown how crucial factors like parachute drag coefficient and atmospheric density profiles can be inferred from accelerator measurements. The acceleration due to drag a, measured by the accelerometer, obeys the fo11owiR g relationship: a O = - 0.5·C O· ~'A' vZ / m with Cn - drag coefficient, g - atmospheric density, A effective cross section area, m - mass, v - velocity.
Oece1erator Jettison The beginning of the descent phase is constrained by suitable parachute deployment velocities (between Mach 1 and Mach 1.6) and a sufficient altitude to start scientific measurements at 170 km. Thus a velocity of Mach 1.5 was selected as the criterion to initiate the dece1erator jettison . Ouri ng 'che entry phase most sensors have sti 11 to be shi e 1ded by covers, therefore thi s event can only be triggered from inputs of the timer and / or the accelerometers, presenting the following alternatives : a) a timer value fixed before separation, b) a fixed period after measurement of peak acceleration, c) numerical integration of acce1erations (from a gravity model and accelerometer measurements) departing from initial values at separation, d) prestored tables connecting dece1erations (measured by the accelerometer ) wi t h the velocity profile.
An acceptable approximation of the density profile is provided by the exponential atmosphere model:
~
(h) = Cl . e C2h
sti 11 dependi ng upon the parameters Cl' C2 and altitude h. The constants CO . C"C? are then inferred from a set of actual measurements for a . These are the inputs for the trajectory predictioRs at lower altitudes, derived from integration of the Probe"s equations of motion
x=
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Solution d) was recommended as the dece1erator / velocity rela t ion in the velocity range near Mach 1.5 is rather independent from i nf1 uences of the uncertain parameter (e. g. density profile, entry velocity etc.; cf. Schilling and F1ury (1988)) •
DEPLOYMENT OF FIRST PARACHUTE
DECELERATOR JETTISON
PAYLOAD OPERATIONS
DEPLOYMENT OF SECOND PARACHUTE
TRAJECTORY ADAPTION
- CONTROL ACTIONS
ADAPTIVE DESCENT CONTROL
- PREDICTIONS
ENERGY CONSUMPTION
PROBE'S ACTUAL POSITION/ VELOCITY
MEASUREMENTS
Fig. 9: Adaptive descent control scheme
-
PARA~TER
IDENTIFICATION
222
K. Schillillg alld H. Le hra
Dep 1oyment of Fi rst r·iai n Parachute
ACKNOWLEDGEMENTS
In case the descent has to be accelerated, after jettison of the decelerator ring, delays could be introduced either for mortar firing or, after pilot chute deployment, for ext,action of the first main chute. As the instrument covers cannot be jettisoned without the fu ', ly ' dlated first main chute, the minimum altitude for the beginning of payload operations limits this option.
The authors would like to thank the ESA study team members for stimulating discussions within the Phase A study "Cassini Titan Atmosphere Probe" .
Deployment of Second Main Parachute
Flasar, F.M., R.E. Samuel son and B.J. Conrath (1981) . Titan's atmosphere: temperature and dynamics. Nature 292, p. 693 - 698.
The exchange of the 1arge mai n parachute for the second smaller one is the preferred option to adjust the descent duration. Periodic comparison of the reference trajectory should be carried out with the measured time / altitude profile. In case of major deviations the trajectory predictions should be updated. Subsequently the timing of the parachute exchange has to be optimized in order to achieve a suitable total descent time . The altitude inputs are provided preferably by radar altimeter measurements, but as back ups inferred data from pressure and temperature sensors coul d be used, too. CONCLUSIONS On the bas is of cont i nuous 1y revi sed atmospheri c mode 1s accordi ng to measurements, the autonomous prediction of the trajectory will be steadily improved, enab 1i ng an appropri ate t i mi ng for the initiation of the available control actions . According to the updated mission profiles it could also be considered necessary to adapt and optimise the uti 1i sati on of power resources and the instrument operation profiles. The proposed adapt i ve descent control wou 1d be a significant contribution to spacecraft autonomy by introducing some simple on board mission management. But it has to be carefully assessed, whether the benefits justify the increased design complexity. The proposed control scheme might well contribute to maximisation of the gain and thus the mission success. Similar strategies should also contribute to future interplanetary missions, demanding the autonomous adaptat i on of spacecraft operat ions to an uncertain environment .
REFERENCES ESA / NASA (1988). Cassini-Report on the Phase A Study. ESA SCI(88)5.
Lellouch , E. and D.M . Hunten (1987) . Titan atmosphere engineering model. ESA Report ESLAB 87 / 199. Schilling, K. and W. Flury (1988) . Autonomy and onboard mission management aspects for the Cassini Titan Probe . IAF 88-388, to appear in acta astronautica. Schilling, K. and G. E.N Scoon (1989). Mit Cassini zum Saturn, Kapsel landet auf Titan. Luft- und Raumfahrt 1/ 89 Scoon, G. E. N. and W. Flu ry ( 1987) . Cassini mission - the Titan Probe. IAF 87-445. Sergeyevsky, A.B., S. J . Kerridge, and D. S. Stetson (1987). Cassini - a mission to the saturnian system . AAS 87-423 .