ROSETTA GROUND SEGMENT AND MISSION OPERATIONS
M. WARHAUT∗ , P. FERRI and E. MONTAGNON
European Space Agency/European Space Operations Centre,
Darmstadt, Germany
(∗ Author for correspondence: E-mail: Manfred.Warhaut@esa.int)
(Received 15 September 2006; Accepted in final form 17 November 2006)
Abstract. At the European Space Operations Centre in Darmstadt (Germany) the activities for ground
segment development and mission operations preparation for Rosetta started in 1997. Many of the
characteristics of this mission were new to ESOC and have therefore required an early effort in identifying all the necessary facilities and functions. The ground segment required entirely new elements
to be developed, such as the large deep-space antenna built in New Norcia (Western Australia). The
long duration of the journey to the comet, of about 10 years, required an effort in the operations
concept definition to reduce the cost of routine monitoring and control. The new approaches adopted
for the Rosetta mission include full transfer of on-board software maintenance responsibility to the
operations team, and the installation of a fully functioning spacecraft engineering model at ESOC, in
support of testing and troubleshooting activities in flight, but also for training of the operations staff.
Special measures have also been taken to minimise the ground contact with the spacecraft during
cruise, to reduce cost, down to a typical frequency of one contact per week. The problem of maintaining knowledge and expertise in the long flight to comet Churyumov–Gerasimenko is also a major
challenge for the Rosetta operations team, which has been tackled early in the mission preparation
phase and evolved with the first years of flight experience.
Keywords: space missions operation, ground segment, Rosetta, interplanetary exploration
Introduction
The beginning of the third millennium coincided with the launch of three new
interplanetary missions of the European Space Agency (ESA): Rosetta, Mars Express and Venus Express. The field of deep-space exploration was relatively new
for the European Space Operations Centre (ESOC): in the history of ESA only
two missions, Giotto in the second half of the 80’s and Ulysses in the 90’s, belong
to the category of deep-space scientific missions. Rosetta (Verdant and Schwehm,
1998; Gardini et al., 1999), being a cornerstone of the ESA Horizon-2000 scientific programme, has driven the development of the new facilities and functions
required to support this type of missions. All ESOC areas of expertise and support
were involved in this process: ground stations and communications, ground data
processing, flight dynamics and mission operations.
The purpose of this paper is to give an overview of the ground segment architecture and mission operations being carried out for Rosetta. The main characteristics
Space Science Reviews (2007) 128: 189–204
DOI: 10.1007/s11214-006-9123-9
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Figure 1. Rosetta ground segment overview.
of the Rosetta ground segment are described, emphasising the new aspects and
new developments linked to the unique characteristics of this mission. The mission
operations concept is then explained through an accelerated travel through all the
phases of this long duration mission. Finally the problems related to maintenance
of the ground segment and preservation of knowledge for the entire duration of the
mission are presented and the adopted solutions described.
Ground Segment
The Rosetta Mission Operations Centre (RMOC) is located at ESOC, and is responsible for monitoring and control of the complete mission (Figure 1). For the
entire mission duration, facilities and services are provided by the RMOC to the
scientific community for planning and execution of scientific data acquisition. This
includes the generation and provision of the complete raw-data sets and necessary
auxiliary data to the Principal Investigators (PIs) and the Rosetta Lander Ground
Segment.
The Rosetta Science Operations Centre (RSOC) supports scientific mission planning and quick-look scientific data monitoring for all the phases of the mission in
which payload operations are carried out. The RSOC consolidates all experiment
command request received from the Principal Investigators and submits them to the
RMOC. The RSOC will also make pre-processed scientific data and the scientific
data archive available to the scientific community. The Rosetta Lander Ground
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Segment supports operations of the Lander before and after completion of the landing and relay phase. It coordinates scientific mission planning and command request
submittal to the RSOC for integration in the overall scientific operations schedule
of Rosetta.
G ROUND STATIONS
Whilst the Kourou station makes use of the standard ESA 15 m antenna already
in place, a new deep-space ground station with a 35 m antenna was developed
and built at New Norcia, Western Australia (Figure 2). This site, located at 130
Km north of Perth, is particularly favourable for coverage of ecliptic trajectories
in the southern celestial hemisphere, as it is the case of comet P67/ChuryumovGerasimenko at the time of the rendezvous with Rosetta. The location far away from
big cities minimises the problem of radio-frequency interference. The large size of
the antenna is required to support interplanetary missions flying at distances of
several Astronomical Units (AU) from Earth. The New Norcia deep-space ground
station supports as prime station all the phases of the mission. Kourou was used
as additional support at launch and for the first Earth swing-by phase, when the
spacecraft was close enough to Earth for its weak radio frequency signal to be
received by the station’s 15 m antenna. As from 2005 a second ESA deep space
antenna was built at Cebreros, near Madrid in Spain. This 35 m antenna, twin of
the New Norcia one, will be used as additional support to Rosetta in the most
critical months of the early comet phases. Both the new Norcia and the Kourou
ground stations are capable of supporting uplink and downlink in the deep-space
S-band and X-band. Cebreros supports X-band only, as the deep space S-band is no
longer supported in Europe, to avoid interferences with ground mobile telephone
systems. Taking advantage of the deep space antennas, the capability to perform
“delta-DOR” measurements, that is simultaneous interferometry on the spacecraft
radio signal via two stations, has been also developed and installed at the deep space
stations in 2005. The technique was successfully validated with Rosetta and Mars
Express in flight, and operationally used for the Venus Express arrival at Venus in
springtime 2006.
The Rosetta spacecraft is capable to receive telecommands at a selectable rate
ranging from 7.8 to 2000 bits per second (bps). Telemetry transmission will be
possible at bit rates ranging from 16 to 22800 bps, depending on the distances
spacecraft-Earth, which will vary from about 500 Km at the third Earth swing-by
to 6.3 AU (about 1 billion Km) in the last cruise phase.
In all phases of the mission the New Norcia deep-space ground station will
be primarily dedicated to Rosetta. During the long quiet Rosetta cruise the station is shared with other missions like Smart-1, Mars Express, Venus Express
and Herschel-Planck. Additional support from the NASA Deep Space Network
is foreseen for all critical phases of the mission, where additional tracking and
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Figure 2. ESA deep-space 35 m antenna at New Norcia (Western Australia).
back-up support is provided. Emergency support from the Deep Space Network 70
m antennas will be available in case the spacecraft is not able to communicate to
Earth with the on-board High or Medium gain antenna. In such cases the Rosetta
low-gain antennas will be used, but for those antennas the 35 m ESA deep-space
antennas are not sufficient to reach the spacecraft at the maximum possible distance.
C ONTROL C ENTRE
The heart of the mission control centre is the Rosetta Mission Control System (Ferri
et al., 1998; Sørensen et al., 1999). This ground data processing software system
was developed based on the already existing ESOC infrastructure system called
SCOS-2000. Many new functions and capabilities were added to the system to
ensure that the new characteristics of the Rosetta mission are adequately supported.
This includes, e.g., long signal travel times, parallel on-line and off-line command
verification, autonomous operations on-board and on-ground. The same control
system has been adapted to Mars Express, and later to Venus Express. In 2005, in
preparation for the launch of Venus Express, the three systems have been merged
into a single one, of which the mission-specific elements are treated as configuration
items. The new Mission Control System version, based on the Solaris 8 operating
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system and SCOS-2000 version 3.1 (“Evolution”) is successfully supporting Venus
Express since launch. The migration of Rosetta and Mars Express to this version
is planned for the second half of 2006.
The Flight Dynamics System is also based on existing infrastructure, called ORATOS. New software modules and subsystems were developed to support generic
deep-space missions features, like interplanetary orbit determination, navigation,
trajectory and manoeuvres optimisation. An example of new development is the
“Delta-DOR” tracking data analysis, for the first time applied at ESOC to data taken
via the NASA Deep Space Network, and later to measurements from the ESA stations of new Norcia and Cebreros. Rosetta-specific flight dynamics functions, such
as optical navigation support, models of the comet environment, tools for Lander
delivery strategy planning, will be developed during the long cruise to the comet.
A challenge for the Flight Dynamics support is represented by the modeling of
the comet environment, which is largely unknown until the spacecraft reaches it,
and even then it will be variable and unpredictable. The comet nucleus dynamics
characterisation in terms of gravitational potential, rotation, surface features, will
be undertaken during the final approach of the spacecraft to the nucleus itself, as
well as in the initial phase around it and in the global mapping phase. Models of
the dust and gas medium which surrounds the comet have already been developed
by the Flight Dynamics team and will be maintained and upgraded with the data
collected by the spacecraft. This model will be vital for the mission, since the
spacecraft trajectories around the nucleus and all operations in the proximity of the
comet will have to be based on predictions from the model. The objective will be
to minimise disturbance to the optical sensors by the dust or disturbance torques
on the spacecraft body.
Essential for the Rosetta flight dynamics activities will be the use of the onboard camera images to perform optical navigation. This is necessary to achieve
the accuracy necessary to achieve precise and safe orbits around the nucleus.
Further elements of the ground segment were newly developed for Rosetta,
such as the System Simulator. For reasons of cost-effectiveness and thanks to the
commonality aspects with Mars Express, a single development has been undertaken.
The two missions also share the dedicated control room, support room and part of
the computer facilities for the entire duration of the mission preparation activities
and until the end of the Mars Express mission.
A very important element of the ground segment is the Rosetta System Database
(Sørensen et al., 2000). This system has been specified by experts of all areas of
the project, from the spacecraft developers to the ground test and flight operations
teams. A single database system was implemented to support all spacecraft activities, from software development to testing, integration and operation. This is the
most important element of the commonality between ground check-out and flight
operations which has been established for this mission (Ferri et al., 1999b). Thanks
to the Rosetta System Database significant cost and risk reduction was achieved in
the mission preparation activities. The spacecraft manufacturer also benefited from
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this approach, which allowed the ESOC pre-launch tests to be integrated with the
spacecraft system tests (Montagnon and Ferri, 2004). The database was populated
with spacecraft and payload telemetry and telecommands details in a joint activity
by all users of the system.
All telemetry data generated by the spacecraft are processed and archived at the
control centre by the Rosetta Mission Control System. These data are made available
to the scientific community on-line through a back-end computer system called
“Rosetta Data Disposition System”. This system provides authorised access to all
mission data ensuring at the same time security of the operational system. Requests
for scientific operations of the payload instruments are also passed electronically
by the RSOC to this computer system.
A Mission Planning System will be subject to a post-launch development since
it is not required until the arrival of Rosetta at the comet in 2014.
Mission Operations
From the point of view of mission operations the main characteristics of Rosetta
can be summarised as follows:
r Long distances from Earth. This has two main impacts:
– Long signal travel times, up to 100 minutes two-way. This requires a completely off-line operations concept. Real-time interaction with the spacecraft is practically excluded, and also emergencies have to be treated such
that the spacecraft does not depend on an immediate intervention from
ground to survive, under all foreseeable failure cases.
– Limited bit rates for both telemetry and telecommands: whilst on the command link this is not a severe constraint, the generation and downlink of
telemetry data has to be carefully planned to avoid data losses due to the
limited bandwidth.
r Long distances to Sun. This is the first interplanetary mission to carry a large
payload at distances up to 5.3 AU from the Sun and at the same time relying
as power source on solar arrays. There is a phase of the mission in which,
notwithstanding the large surface of the solar panels (64 m2 ), the energy coming from the distant Sun is not sufficient to keep the spacecraft active, thus
forcing long periods of “hibernation” of the spacecraft.
r Long mission duration, and in particular the fact that its prime scientific mission phase is at the end of the mission. This affects the planning of mission support and requires solutions for the maintenance of a competent and motivated
flight control team without provoking an explosion of mission operation cost.
The mission operations concept and the overall architecture of the ground segment are driven by these important factors.
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Figure 3. Mission phases overview.
MISSION PHASES
The phases of the mission are summarised in Figure 3 (Ferri, 2006). After launch
on 2nd March 2004 the spacecraft was immediately injected on an heliocentric
trajectory that brought it back to Earth almost exactly one year later (4th March
2005) for the first gravity assist swing-by.
The trajectory correction manoeuvre planned for the second day of flight to
correct eventual launcher dispersion errors, was skipped due to the very high precision of the trajectory injection provided by Ariane-5G. The error of about 3 m/s
was negligible, and it was decided to include the necessary corrections in the first
deterministic deep space manoeuvre planned for May 2004.
In the first three months after launch all spacecraft subsystems and instruments
were checked-out and commissioned. Daily contact with the spacecraft through the
New Norcia deep-space ground station allowed real-time operations to be conducted
on the instruments. The first deterministic manoeuvre of 152 m/s was successfully
carried out in two legs on 10th and 16th May 2006. After this manoeuvre, the
preparation for a full on-board software upload for the Avionics subsystems started,
and was completed at the end of July 2004 (Ferri and Warhaut, 2004).
After a short period of quiet cruise the second part of the in-flight commissioning
was executed in September and October 2004. In this phase the spacecraft executed
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complex pointing profiles to allow special calibration targets to be observed by
the remote sensing instruments (Alice, Miro, Osiris, Virtis). Also a 2-days crossinstruments interference campaign was carried out.
The first Earth swing-by phase started with minor trajectory correction manoeuvres in December 2004 and February 2005. The spacecraft’s closest approach to
Earth was on the evening of 4th March 2005, at a distance from the surface of about
1950 Km. The extreme precision of the spacecraft trajectory correction manoeuvres and of the orbit determination process resulted in a very smooth swing-by,
without the need of post-swing-by correction manoeuvres. The favourable geometrical conditions of the Earth swing-by allowed the mission control team to plan
and execute, a few hours after closest approach to Earth, the first in-flight test of
the Asteroid Flyby Mode. In this mode the spacecraft attitude control subsystem
uses the Navigation Camera to track the asteroid image (in this case the Moon) and
controls the attitude of the spacecraft such that the tracked object remains in the
centre of the Navigation Camera field of view. Various scientific instruments were
operated in the weeks around the Earth swing-by. This allowed calibrations for the
remote sensing and the plasma instruments to be carried out using the well known
Earth environment.
The start of the quiet cruise was delayed compared to the original mission plan,
due to the approval of a previously unplanned “target of opportunity” scientific
activity. This resulted from a NASA request to use Rosetta’s remote sensing instruments to support the observations of the collision of NASA’s Deep Impact probe
with the nucleus of comet Tempel-1. The impact took place on 4th July 2005, and
for about three weeks Rosetta pointed its instruments to the comet to observe the
nucleus’ activity before and after the event.
Finally in August 2005 the first period of quiet cruise could start. The spacecraft
was put into the so-called “Near Sun Hibernation Mode”, an attitude control mode
based on thrusters and star trackers, which relaxes the attitude deadband at the
expenses of communications with ground. This mode is specifically designed to
save lifetime of the gyroscopes and the reaction wheels, at the same time minimising
the use of fuel for attitude control. The mode was however exited earlier than
planned due to an unexplained transient anomalous thruster behaviour. The quiet
cruise continued with the spacecraft in normal mode, with weekly ground station
contacts and the payload instruments switched off, with the exception of occasional
testing and maintenance activities (Montagnon and Ferri, 2005a).
In March 2006 the first solar conjunction took place. The spacecraft spent about
two months at an angular distance from the Sun, seen from Earth, below 5 degrees,
down to a minimum of 0.3 degrees on 13th April. This causes disturbance to the
radio signal from and to Earth, and for this reason the activities on board the
spacecraft were reduced even further.
Main events of the remaining cruise are the upcoming Mars swing-by on 25th
February 2007, followed by two more Earth swing-bys (November 2007 and 2009)
and two Asteroid fly-bys, Steins on 5th September 2008 and Lutetia on 10th July
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2010. In the period of quiet cruise in between the planet or asteroid encounters
periodic payload checkout windows will be executed, typically once every six
months or in proximity of each active phase.
The most critical part of the cruise will begin after the Lutetia swing-by. The
spacecraft in this period will travel at distances to the Sun of about 3.5 AUs. At such
distances the limited power availability and the different thermal conditions will
force a different configuration and possibly a new way of operation. The distance
to Earth will also increase, adding to the power limitations also the difficulties of an
extremely long propagation delay and reduced telemetry and telecommand bitrates.
In January 2011, at a distance from Earth of 4 AU, and of 3.9 AU from the Sun,
the first part of the comet rendezvous manoeuvre will be performed. This is a large
trajectory correction manoeuvre, imposing on the spacecraft a delta-V of about 788
m/s. Given the communications and power constraints at these large distances from
Earth and Sun, it will take a few days to execute the manoeuvre, split in a few burns,
each of several hours duration.
After completion of the manoeuvre, the distance to the Sun will continue to
increase to a point, around 4.5 AU, at which it will be necessary to deactivate
as many as possible of the on-board subsystems and to put the spacecraft into a
“hibernation mode”. All electronics systems will be switched off, including the
attitude control system and the transponders for communications to Earth. The
only exception will be the data management computer, in charge of monitoring of a
few survival heater lines. The critical activities will begin after a solar conjunction
that takes place at the end of April 2011. The spacecraft will be spun-up around an
axis that will keep the solar arrays pointed to the Sun over the entire hibernation
period within an angle of a few tens of degrees. The spin-up and configuration of
the spacecraft for the “deep space hibernation” must be completed by 21st July
2011. At this point the distance of 4.6 AU is reached, beyond which the spacecraft
cannot be operated in its nominal configuration.
In the deep space hibernation phase, of the duration of about 2.5 years, the
contact with the ground will not be possible. Rosetta will have to survive on its own
and to autonomously “wake-up” only at the end of the phase, when its distance to
the Sun will have decreased again to about 4.6 AU.
The wake-up date is currently planned for 23rd January 2014. Shortly after
re-activation and check-out, the second leg of the comet rendezvous manoeuvre
will be carried out, starting on 22nd May 2014. This is another large trajectory
correction manoeuvre, with a resulting delta-V of about 794 m/s. At this point the
spacecraft will be at a distance of 4 AU from the Sun and at a more “comfortable”
distance of 3.3 AU from Earth, which will at least relax partially the communications
constraints. Also, the nucleus of comet Churyumov–Gerasimenko will be probably
visible in the navigation and scientific cameras on-board the spacecraft, allowing
the start of the optical navigation measurements.
After completion of the rendezvous manoeuvre the payload will be gradually
reactivated and checked-out, compatibly with the very limited available power.
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Insertion into the comet orbit is currently planned for 5th September 2014, at a
distance from Earth of 2.8 AU, and from Sun of about 3.4 AU.
After having achieved an orbit around the nucleus, Rosetta will start to globally
observe and characterise the nucleus. This phase of a few weeks will be followed
by a close observation and mapping of the nucleus, during which the spacecraft will
fly down to distances of a few Km from the surface. The purpose of this phase is to
select the best landing site for the Lander. The landing on the surface of the comet
is planned for the 10th November 2014, when the comet will be at a distance from
the Sun of 3 AUs. Immediately after Lander delivery Rosetta will be manoeuvred
to an orbit where it will be acting as data relay for all the scientific data collected
by the Lander on the surface and for the Lander monitor and control by ground.
After five days the main mission of the Lander will be completed. Both orbiter and
Lander will enter the routine scientific phase, escorting the comet to perihelion (12
August 2015, Sun distance 1.25 AU) and beyond. The nominal mission will end on
30th December 2015.
The operations of the spacecraft in the vicinity of the comet are extremely
challenging, due to the large uncertainty in the knowledge of even basic comet
parameters such as mass, size, rotation period. All these parameters will only be
characterised when the spacecraft is put in orbit around the nucleus. Also, the
unpredictability of the environment requires at the same time extreme care but
also flexibility in planning the spacecraft operations in the vicinity of the nucleus.
Orbit and attitude profiles will have to be re-planned and adjusted on a very short
cycle, due to the rapid variations of the comet activity level. On the other hand it
will always have to be ensured that the spacecraft trajectory is collision-free for a
safe period of at least one week in the future. This is particularly complex in an
environment where the disturbance torques, such as Sun radiation pressure are of
the same order of the comet gravitational forces.
O PERATIONS C ONCEPT
The approach to mission control depends on the mission phase and on the activities
to be carried out. At high level, it is possible to define operations modes which will
be applied to the different phases as follows.
Critical Operations Mode. All critical mission phases – Launch and Early Orbit Phase, comet
rendezvous manoeuvres, Lander delivery and relay – are executed by an extended Mission
Control Team, composed of dedicated teams in charge of spacecraft operations, ground segment operations, flight dynamics and project support (Figure 4). This team will run 24-hours
operations from the Mission Operations Centre. Ground stations will also be manned. Monitoring and control will be mainly real-time for the launch and early orbit phase, whilst for
all other critical phases all nominal commanding will be carried out via the on-board timetagged Mission Timeline. Near-real-time contingency commanding will be supported at any
time during spacecraft visibility.
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Figure 4. Main Control Room (MCR) at ESOC used during critical operations.
Active Cruise Mode. In all phases of the mission in which the spacecraft is active but not
performing specific operations, the Mission Operations Centre will operate in a low activity
mode. This consists of non-frequent New Norcia ground station passes (typically once per
week) in which mainly the content of the on-board telemetry storage is downlinked, the
on-board Mission Timeline is updated and tracking data are collected.
Comet Science Operations Mode. Routine science operations will be carried out during the
comet approach and orbiting phases, using the full Mission Planning process including the
RMOC and RSOC. Inputs from the Principal Investigators for changes to payload operations
will be coordinated by the RSOC and transmitted to the Mission Operations Centre on a
cyclic basis. All New Norcia passes will be utilised, and for the three months preceding
Lander delivery, daily coverage from the second ESA deep space station at Cebreros will be
added.
The mission is normally run completely off-line for all phases, with the exception of the launch and early orbit phase and part of the near-Earth commissioning
phase. Even critical operations, due to the long light travel time, are executed via the
Mission Timeline on-board, with basically no opportunity to quickly react to unexpected events from ground. Contingency commanding is possible during ground
station passes. The command schedules for execution of all spacecraft and ground
station operations are prepared off-line during normal working hours. Routine and
long-term evaluation of spacecraft telemetry is also carried out post-pass by the
Mission Control Team during normal working hours.
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The entire spacecraft monitoring and control approach and the related software systems are based on the ESA packet telemetry and telecommand standard
(Sørensen and Ferri, 1998). The spacecraft and the payload instruments only generate telemetry that is relevant to the on-going operations, and large use is made of the
concept of event messages. Instead of continuously repeating the same housekeeping information, as it happens on a spacecraft producing fixed-format telemetry,
Rosetta produces single-shot event telemetry packets whenever the ground has to
be informed of an on-board event. This has the advantage, especially important
for missions where the downlink bandwidth is precious, to limit the downlink of
spacecraft engineering and health information only to the strictly necessary, leaving
most of the bandwidth for science telemetry. The frame-level safe uplink protocol,
is however difficult to apply to a deep-space mission. Due to the long signal propagation delays the closure of the protocol through telemetry information would slow
down telecommanding to an unacceptable rate. For this reason a special packetlevel protocol has been defined for Rosetta, based on an adaptation of the packet
utilisation standard service 13. This service allows the safe uplink of large files of
information or of long sequences of telecommands with a final success verification
on ground taking place only at the end of the uplink. This approach eliminates
the problem of verifying each uplinked command individually and can be directly
applied to a deep-space mission, independently of the length of the signal travel
interval.
PAYLOAD O PERATIONS
For all critical payload activities (Ferri et al., 2000) and in those phases in which
near-real-time interaction is envisaged, such as the commissioning phases or the
delivery of the Lander to the surface of the comet, the presence of payload teams at
the control centre is required. Facilities are provided to allow the payload teams to
install their check-out equipment in the ESOC premises and to access the telemetry
data to support the operational decisions.
During the rest of the comet phases, the Science Operations Centre acts as the
focal point for all science planning and operations coordination activities. The baseline science plans decided by the Science Working Team and approved by the Project
Scientist are made operational, i.e. transformed into cyclic payload operations requests by the RSOC. The consolidated operations requests are then transmitted
to the Mission Operations Centre, where they are checked via the Mission Planning System and merged with the routine spacecraft activities. The consolidated
schedule of payload and subsystems operations is then converted into sequences of
telecommands for transmission to the spacecraft.
In case of unforeseen behaviour or anomaly of any payload instrument, the
Principal Investigator team is always responsible for the resolution, supported by the
Mission Control Team at ESOC, of any contingency involving its own experiment.
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Monitoring of housekeeping telemetry produced by the instrument is possible onboard the spacecraft (although limited to a few essential parameters and event
messages) and on-ground at the Mission Operations Centre. Monitoring of science
telemetry is possible at the Science Operations Centre, and for each instrument only
on specific request of the Principal Investigator.
C RUISE O PERATIONS
The long duration of the cruise phases before arrival to the comet, has dictated, for
cost reasons, an approach that minimises activities and ground contact throughout
the mission (Montagnon and Ferri, 2005b), until the actual science phase starts in
2014.
Reducing activities in cruise means first designing a spacecraft mode and configuration that does not require frequent intervention from ground. The modes defined
for the spacecraft, however, still require, based on the experience of the first two
years in flight, a typical ground station pass frequency of once per week. This frequency is mainly dictated by the need for reconstruction of telemetry history in
case of anomalies. Also, any necessary periodic activity, such as reaction wheel
momentum off-loading, has to be performed close to or during a ground station
pass, to allow the ground to intervene rapidly in case of problems.
The Rosetta mission baseline for cruise was not to operate at all the payload
instruments outside well identified windows of periodic checkouts every typically
six months. Also no science operation was foreseen during cruise, with the exception
of the periods around planet swing-bys, and of course the two asteroid fly-bys. Inflight experience has however soon demonstrated that such rigid approach is not
easily applicable. Additional slots become in fact necessary for important in-flight
tests, and science opportunities occur which may be important for the mission and
can be satisfied with a minimum additional effort. As a consequence, the level of
activities during cruise has been higher than planned, and a new set of rules has
been established on how and when to plan and execute activities outside the original
baseline (Ferri and Montagnon, 2006).
Ground Segment Maintenance and Knowledge Preservation
K NOWLEDGE
AND
E XPERTISE
Preservation of knowledge and expertise in the mission control team and all the
ground segment support teams is a major problem due to the long duration of this
mission, with the main scientific operations phase only at the end.
Experience from the first two years, however, has shown that the small teams in
charge of following the mission, in the Mission Operations Centre, in the Science
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Operations Centre and in the payload community, are continuously busy. Their
activities cover all areas of expertise, from conducting real-time operations to planning of future activities and execution of corrective and preventive maintenance
tasks. This maintains a quite high level of expertise and does not require any special training measure compared to other missions with a more constant level of
activity.
On the other hand the very reduced size of the active teams makes them extremely vulnerable to staff turnover. This makes cross training extremely important,
although at the same time very difficult to carry out due to the limited resources.
The vicious circle can only be broken by continuous attention to the problem,
discipline in documenting knowledge and experiences, and rotation of tasks and
responsibilities within each team.
At ESOC experience with long deep-space missions (Ulysses, Huygens) and
also with hibernation phases (Giotto) exists and all the measures adopted are based
on the ideas and lessons learned in those missions (Ferri et al., 1999a; Fertig, 1999).
Traditional training and simulations programmes which normally take place before
launch are extended for Rosetta into the mission, and complemented by proficiency
training and cross-training between experts in various disciplines. Training manuals
are produced and periodic exercising of contingency procedures for all members
of the mission control team is carried out during flight.
An important role plays the engineering model of the spacecraft, located in
ESOC since March 2003 and used throughout the flight as a major training and
familiarisation tool for all the team members.
G ROUND SEGMENT FACILITIES
The long duration of this mission imposes obvious problems in the area of maintenance of the ground segment facilities. The development of the ground segment
began about seven years before launch and the nominal mission is planned to finish at the end of 2015. It is clear that very few elements of the ground segment
will survive this long period of almost 20 years without changes, upgrades and/or
replacements. In the area of software this is even more true, considering that the
average lifetime of operating system versions and commercial off-the-shelf products nowadays is just about 18 months!
In order to cope with this problem a few basic rules are being followed both in
the design and development of Rosetta specific ground segment elements and in the
procurement of commercial elements. Any upgrades to the generic infrastructure
(ground station, network and control centre facilities, in terms of both hardware
and software) have to be generally made backwards compatible with the Rosetta
system, wherever possible.
A characteristic problem of Rosetta is that the main scientific phase will start
shortly after a long period of hibernation, during which the ground segment will not
ROSETTA GROUND SEGMENT AND MISSION OPERATIONS
203
be used for spacecraft operations over a period of about 2.5 years. This makes the
re-activation of the system particularly critical. For this reason a series of activities
will be carried out to achieve ground segment validation and readiness, as follows:
r Confidence testing of major ground segment elements/building blocks;
r Interface testing (internal/external) as appropriate;
r Validation of reactivation procedures using the system simulator and possibly
the Rosetta Engineering Model;
r Simulations of the reactivation scenario (nominal and contingency cases);
r Mission Readiness Tests with relevant ground stations concerned;
These activities will be starting in the last two years prior to the de-hibernation,
planned for beginning of 2014. In this period the small mission control team that
supported the flight up to the start of the hibernation will be gradually increased to
start the preparation for the execution of the critical comet phase of the mission.
The ground segment re-validation activities, combined with periodic proficiency
training on the spacecraft simulator and engineering model, will at the same time
form the basis for knowledge preservation and transfer in this long inactive period.
Conclusions
When the definition phase of the Rosetta ground segment started in early 1997, deep
space missions were known at ESOC only on paper, and from the old experience
of Giotto and Ulysses, far distant both in space and time. After about 9 years a
small team of operations engineers, computer and telecommunications experts and
of course the flight dynamics team control three interplanetary missions in flight.
Rosetta, since more than two years travelling to its target comet, Mars Express
orbiting the red planet since 3 years, Venus Express just arrived at Venus for its main
mission. Common ground systems have been developed, interchangeable across the
three missions. Two deep space ground stations have been built in Australia and
Spain, currently supporting a variety of missions and able to provide cross-support to
other Agencies. The expertise accumulated in these years is invaluable, and is being
used not only to continue successfully the operations of the flying missions, but also
to prepare for future ESA interplanetary scientific missions like Bepi Colombo.
More challenges are expecting the Rosetta operations team in the future. Maintaining all elements of the ground segment operational for the next ten years, well
beyond the lifetime of Mars Express and Venus Express. Ensuring health and safety
of the spacecraft flying at large interplanetary distances from Earth and Sun. Mastering the spacecraft operations at distances of the order of a few Km from the
comet nucleus when it will be at distances of the order of 3 AUs from Earth and
Sun, in an environment which is almost completely unknown.
The reward will be the unique opportunity of having contributed to one of the
most daring enterprises since human exploration of the Solar System has begun.
204
M. WARHAUT ET AL.
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