INTRODUCTION

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The mandate of the European Space OperationsCentre is to conduct mission operations for ESA Satellites and to establish, operate andmaintain the necessary ground segment infrastructure.

Mission OperationsMission operations is the process involvingoperations planning, satellite monitoring andcontrol, in-orbit navigation, and data processingand distribution, by which the satellite missionobjectives are achieved, be they the collectionof meteorological or scientific data or theprovision of a telecommunication service.

Monitoring and control starts as soon as thesatellite is separated from the launch vehicle, itspurpose being to activate the on-board systemsfor the tasks ahead in the challengingenvironment of space. Soon afterwards, thepayload has to be configured to enable it to play its part in exploiting the mission accordingto plans based on the wishes of the users. It is around-the-clock task performed throughout theduration of the mission.

Whenever the satellite is visible from theground, health and status are monitored, a taskinvolving the analysis of as many as fivethousand telemetry parameters every minute.Operations are effected through instructionssent to the spacecraft in the form oftelecommands to change on-board settings orto activate payload equipment. Continuousmonitoring is necessary to verify correctexecution of up to one hundred commands perminute.

Satellite nativation is the process by which asatellite is brought to and kept in the desiredorbit and by which the required bodyorientation (attitude) is acquired. It involvesdetermination, prediction and control of thesatellite orbit hand in hand with determinationand control of the satellite attitude. Changes inorbit and attitude are effected by the execution of often complex manoeuvresunder ground control, using for example theon-board thrusters.

Ground Segment InfrastructureESOC has established a comprehensive groundsegment infrastructure suitable and ready tosupport various types of missions, each havingdifferent demands, requirements andconstraints. This infrastructure encompasses allfacilities and services needed for missionoperations and includes a network of groundstations around the world, a number of controlcentres, payload data-processing facilities,spacecraft control systems, simulation systemsand communications systems. All groundfacilities have to be highly reliable andmaintainable: cost-effectiveness in operations isachieved by careful introduction of newtechnology.

The infrastructure is primarily intended for ESAmissions, but can be made available to externalAgencies and industry. Depending on the typeof agreement, ESOC can provide all types ofservice, from full mission operations toconsultancy.

Lift-off of an Ariane-5launcher

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Mission SuccessThe mission operations phase is generally thefinal and arguably the most critical phase in aspace project, during which the return oninvestment is to be realised: the return in thiscase is the quantity, quality and availabiliity ofmission products or services, which in turndepend heavily on the effectiveness of missionoperations. Mission success is then gauged bythe return of mission products, and by theability to recover from deficiencies or anomaliesin the orbiting spacecraft.

Since 1967 ESOC has successfully conductedmore than 40 satellite missions (Appendix 1),each presenting a different challenge to theoperations staff, including:– Science missions in near-Earth, highly

eccentric and interplanetary orbits,– Microgravity missions in near-Earth orbit– Earth observation missions in near-Earth

orbit– Meteorological missions in geostationary

orbit– Telecommunications missions in

geostationary orbit

Of particular note are the following missions,for which ESOC had full responsibility:– The navigation of the Giotto spacecraft to

encounter Comet Halley in 1986 was aspectacular success. After surviving theencounter, a historical gravity-assist

swing-by around the Earth in 1990 tookGiotto into an orbit in which it was toencounter comet Grigg-Skjellerup two yearslater: the operations involved in thehibernation and blind reactivation of thespacecraft are of historical significance.

– The successful execution of the Eureca(European Retrievable Carrier) mission,including the deployment and retrieval ofthe spacecraft by the US Space Shuttleproved ESOC’s ability to control from theground complex rendezvous and dockingmanoeuvres in space, in close cooperationwith another Agency (NASA).

One of the first ControlRooms for the ESRO-2Mission

Multi-mission control Areaat ESOC

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Image composition ofMOP-2 and ADC

– Between 1977 and 1995, ESOC conductedthe six European Meteosat missions involving full spacecraft monitoring andcontrol, navigation, payload control, payloaddata processing, image data extraction, datadissemination and archiving. The service provided to the meteorological community was exemplary in its quality and continuousavailability.

In total, four otherwise doomed missions havebeen recovered by ESOC, namely TD-1A, Geos-1,

Olympus and Hipparcos, the latter being ofparticular importance. In August 1989,Hipparcos, which was intended to undertake a100 000-star survey from geostationary orbit,was left stranded in transfer orbit when itsapogee boost motor failed to ignite. Withinthree months a revised mission had beendefined and the necessary modifications to theground segment and on-board software hadbeen made, enabling the mission to beconducted in the unfavourable elliptical orbit.Outliving the planned mission duration,Hipparcos operations continued for three yearsto successfully achieve all intended scientificobjectives.

The task of operations as conducted from ESOCis now a well-developed discipline involvinghighly skilled staff in intensive activities both inthe preparations phase and in the operationsexecution phase. The philosophy for the controland operation of the various types of satelliteintended to serve the needs of ESA’s differentspheres of interest is founded on the principlesestablished during more than 30 years’experience. ESOC is recognised as a leadingcentre of excellence in Space MissionOperations that is unique in Europe. Consciousof the need to vigilantly maintain its highstandards of quality, ESOC achieved ISO 9001certification in 1999.

This extensive expertise allied with thecomprehensive and technologically advancedground segment infrastructure, ensures thatESOC maintains its position as mission-operations authority for ESA satellitesand guarantees that, as such, it constitutes aninvaluable resource for the Agency’s futurespace projects.

Meteosat MOP-1

Eureca returning to theSpace Shuttle

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PREPARING THE MISSION

IntroductionIn most cases, ESOC involvement starts whenthe mission and satellite concept is beingdeveloped, especially as regards analysis of thetype of orbit required, the provisions that needto be included in the spacecraft design tofacilitate the operational tasks and the ground-segment facilities required to support themission. Such involvement continues throughoutthe assessment phase and the satellite Phase Astudies.

Once a mission has been selected, ESOC will beinvolved in providing operations requirementsapplicable to the spacecraft Phases B and C/D, inthe formulation of a thorough specification of theground segment services and facilities needed tosupport the mission and in establishing thenecessary interfaces for exchange of data andinformation between the customer and ESOC.

The various units within ESOC then embark onthe execution of the preparatory tasks thathave to be completed during the periodcovering the satellite’s development, integrationand test programme (usually about five years).The culmination of the preparatory phase is thefull readiness and availability of all facilities,services and personnel and the ‘freezing’ of all software systems, hardware anddocumentation prior to the start of missionoperations.

The principal activities to be undertaken byESOC in the mission preparations phase cover awide range of disciplines, as illustrated in theFigure below and described in the followingsections.

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Table 2: Types of orbit for different missions

Type Orbit Mission type Spacecraft/vehicle

1 Near-Earth Circular Science SpacelabMicrogravity Eureca, ISS

Near-Earth Polar Earth Observation ERS-1, ERS-2, Envisat(also sun synchronous)

2 Highly Eccentric Earth-Science HEOS, ClusterAstronomical Cos-B, Exosat, ISO, XMM,

INTEGRAL, FIRST/PLANCK

3 Geostationary Telecommunications OTS, ECS, Marecs, Olympus,Artemis

Meteorology MeteosatScientific Geos, (Hipparcos)

4 Earth-Sun Libration Point Solar Science Soho

5 Interplanetary Solar System Science Giotto, Ulysses, Rosetta

6 Planetary Planetary Science Cassini/Huygens, Mars Express

Mission AnalysisMission analysis is the term used to describethe mathematical analysis of satellite orbits,performed to determine how best to achieve themission objectives in terms of achievable orbit,launch vehicle, ground station utilisation,operational complexity and lifetime.

These very important aspects are considered byESOC early in the formative stage of the missiondesign in close cooperation with the Project.This is essential, since, later on, the results willbe given to the satellite Prime Contractor asdesign driving information, for the selected orbitand the derived operations concept will have aninfluence on many aspects of the satellitedesign, as can be seen from Table 1.

Different categories of mission are best servedby different types of orbit, as shown in Table 2.

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Table 1: Influence of orbit on satellite design

Mission Type will drive:– Launch energy vs. satellite mass– Selection of launch vehicle and orbit injection strategy– Satellite Dimensions

Orbit Injection and Control requirements will determine:– Orbit Control concepts (mono/bi-propellant/solid fuel)– Number and type of manoeuvres needed– Fuel needed on board satellite

Actual Orbit will determine:– Orbital lifetime and stability– Ground station coverage afforded by available stations– Types and positions of attitude sensors– Range of solar input for power generation and thermal control– Eclipse durations and battery requirements– Downlink frequency and RF link margins

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The principal characteristics for each type areas follows:– Near-Earth orbits (Type 1) have low launch

energy requirements and result in shortcommunication distances: they can bereached directly by Space Shuttle, allowingpotential for retrieval and repair. They are,however, subjected to air-drag perturbationsand frequent eclipses and periods of directground contact are very short.

– Highly eccentric orbits (Type 2) haverelatively low launch energy requirementsand for most of the time keep the spacecraftaway from the influence of the Earth. Orbitstability is a potential problem, as is thefrequent passage through the Earth’sradiation belts.

– Geostationary orbits (Type 3) are particularlysuited for communications satellites andcertain scientific and meteorologicalapplications, but have high launch energyrequirements.

– Orbits near the Earth/Sun libration point(Type 4) can be suited to solar or stellarobservation: launch energy requirements arehigh and communication distances are large.

– Interplanetary orbits (Type 5) have highlaunch energy requirements (alleviated bygravity-assisted transfer orbits) and tend toresult in long-duration missions.Commuications distances are very large, ascan be the distance from the Sun (whichleads to reduced energy from the Sun forpower generation).

– Planetary orbits (Type 6) are similar to Type 5: ground-station visibility isinterrupted by occulation and overallmission complexity is increased.

The illustration below shows a schematic of thedifferent types of orbit.

By no means restricted to selection of theoptimum orbit, mission analysis continuesthroughout the mission-preparation phase,addressing all orbit-related topics, including:– calculation of seasonal and daily launch

windows– calculation of mission constraints, including

ground-station visibility, eclipse periods, skyvisibility and occulations

– definition of injection strategy andoptimisation of orbit manoeuvres

– performance analysis of navigation systemand estimation of propellant budget.

These mission anslysis tasks are performed with the aid of advanced methods of celestialmechanics, applied mathematics and controland estimation theory. Powerful workstationsand a suite of sophisticated software tools andutilities allow in depth analysis of all aspects oforbits and trajectories. Animated graphicalvisualisations of particular regions of space,such as the magnetosphere or the radiationbelts, or of 3-D relationships between thespacecraft, the Earth, the Sun and the planets,are also used to support the analysis and forpublic relations and educational purposes.

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The Huygens Control Room

Preparing the Ground SegmentThe term ground segment covers all elements,facilities and services needed in support of themission operations activities on the ground anddistinguishes them from the space segment,which comprises the launch vehicle andsatellite.

The major elements of the ground segment tobe established include:– The ground station network to provide the

telemetry, tracking and telecommandinterface between the control centre and thesatellite

– Software systems for satellite monitoringand control, flight dynamics, missionplanning and data distribution

– Computer facilities to host the softwaresystems and facilities for the maintenanceand validation of satellite on-board software

– The satellite and ground segment simulator– Control centre facilities, including control

rooms from which mission operations areconducted

– Communications systems linking all thevarious elements together

– The trained Flight Control Team– Operations plans and procedures and

associated operations databases.

The various components of a typical groundsegment are shown in the table below.

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Operations PreparationsThe process of defining the ground segmentmust go hand in hand with the design anddevelopment of the satellite: each missionrequires different satellite design characteristics,each of which must have correspondingprovisions in the ground segment.

The first step in the process is to formulate theOperations Concept. This defines the overallscenario for operation and control of thesatellite, the payload and the different elementsof the ground segment. The detailedspecifications for the ground segment will laterbe based on this work. The different missionphases need to be assessed, as each will placedifferent requirements on the ground segment.

Satellite and payload design information fromwhich ground segment specifications arederived is acquired through the Project Team.The most important source of information inaddition to the design specifications, is theSatellite User’s Manual, delivered by thesatellite Prime Contractor: it provides athorough definition of the satellite and itssubsystems, and defines what has to be done ineach mission phase viewed from the operationsperspective. It contains sufficient information toenable the operations staff to gain insight intoall the internal functions of the satellite and isused to develop the plans and step-by-stepprocedures to be used by the satellitecontrollers in flight.

The Ground Station NetworkThe ground station provides the link between thesatellite in orbit and the Operations ControlCentre (OCC) on the ground. ESOC has establisheda network of ground stations (11 antennas)around the world to support ESA missions,referred to as ESTRACK, comprised of thefollowing locations: Kourou (French Guiana),Malindi (Kenya), Maspalomas (Canary Islands,Spain), Villafranca (Spain, 3 terminals) Redu(Belgium), Kiruna (Sweden, 2 terminals), Perth(Australia) and New Norcia (Australia, 35 m underconstruction). Each mission has different groundstation needs, and some special or collaborativemissions call for the services of additional non-ESA stations, for example those belonging toother European agencies or to the NASA DeepSpace Network.

Stations at various locations around the world areneeded for the different phases of each type ofmission; for all missions, it is necessary to havenearly continuous contact with the spacecraft inthe first few days of its life (the Launch and EarlyOrbit Phase: LEOP), so that all the initial criticaloperations can be performed reliably and, in somecases, so that the spacecraft can be transferredfrom the injection orbit to operational orbit.

The ESA LEOP stations at Villafranca, Spain,Perth (Australia) and Kourou (French Guiana) areideally placed to provide good coverage,particularly for satellites starting life in ageostationary transfer orbit.

ESA Maspalomas groundstation

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Satellites inserted into polar orbits are bestserved by stations near the poles, as they seethe spacecraft more often than stations near theequator: the ESA station at Salmijärvi (Kiruna,Sweden) is particularly suitable. To provide themaximum coverage in the LEOP for polarmissions, several stations along the satellitetrack are needed and additional stations on loanfrom other agencies are enlisted to join forceswith the other ESA stations at Perth, Kourouand Villafranca (Spain).

Ground stations for spacecraft in other orbitsare selected with the aim of maximising theperiods when the spacecraft is visible from thestation: spacecraft in geostationary orbit, beingvisible 24 hours a day from points on aparticular meridian, pose the least problem, asthe station need only be located at or near theappropriate longitude. The ESA station at Redu(Belgium) has been used to serve a number ofgeostationary missions.

Any station selected to support missionoperations will need to be tailored to suit theindividual characteristics of the spacecraft inquestion, and various parts of the stationequipment will be engineered to suit the up- anddownlink frequencies, data types and othermission characteristics.

Table 3 shows the characteristics of a typicalESA S and X Band ground station. Theillustration below shows the locations of thedifferent ESA stations around the world.

Flight Control SystemsThe facilities employed within the ControlCentre for processing satellite telemetry andpreparation of commands needed for theconduct of mission operations are broadlytermed flight-control systems. They comprisethe software and hardware systems set up tosuit the needs of each mission. Many elementsof these systems may be applicable to a rangeof missions: the process of defining how best toserve the needs of any new project includesconsideration of the re-use of parts of existing

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Ground station at Perth,Australia

Table 3: Characteristics of a typical ESA ground station

Radio Frequency:Antenna Diameter 15 mTransmit frequency 2025 - 2120 MHzReceive frequency S-Band 2200 - 2300 MHzReceive frequency X-Band 8400 - 8500 MHzEIRP 74 dBWG/T S-Band 29 dB/KG/T X-Band 39 dB/K

Telemetry:Nominal Data Rate up to 1 Mb/sMaximum Data Rate up to 105 Mb/sStandards supported PCM and CCSDS Packet Telemetry

Telecommand:Nominal Uplink Rate 2 kb/sStandards supported PCM and CCSDS Packet Telecommand

Tracking:Range measurement accuracy 1 mRange rate measurement accuracy 0.1 mm/s

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flight-control systems and the extent to whichnew features are required. It is especially herethat the application of standards in operationalrequirements for spacecraft design can result insavings in the cost of developing flight controlsystems.

The systems used for the interpretation oftelemetry and the generation of telecommandmessages are database driven, and theinformation needed to establish the operationsdatabases (ODB) is delivered by the satellitePrime Contractor to ESOC in electronic form.Error-free creation, validation and maintenanceof this information are of paramountimportance for reliable spacecraft operations.

The flight control systems are designed toperform a multitude of tasks, examples of whichare shown in Table 4. Of particular importance isthe need for high reliability in a real-timeenvironment, quick system response,guaranteed availability of data and clarity ofinformation. Special facilities are included toensure all commands are correct before uplinkand to watch for errors in the telemetry. Theneed to process large volumes of data and tomanage complex autonomous on-boardsystems has placed heavy demands on theflight control systems. These are normally runon high availability Unix-based client/serverconfigurations comprising prime and back-upelements.

Many satellite functions, particularly forAttitude and Orbit Control (AOCS) and On BoardData Handling (OBDH), are implemented insoftware resident in microprocessors on boardthe satellite. If any difficulties arise in thefunctioning of the satellite, it is often necessaryto make modifications to the on-board software,for example to compensate for degradation orfailure of on-board equipment (gyros or otherhardware) later in the satellite life. Havingresponsibility for mission operations means thatESOC must make provision not only formaintenance of the on-board software, but alsofor operational validation of the modifications.Changes to the on-board software maynecessitate changes to elements of the groundsegment: these too must be correspondinglymodified and operationally validated in anenvironment of the strictest configurationcontrol to prevent later errors in operations.

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Cluster Dedicated ControlRoom

ESA ground station atVillafranca, Spain

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Table 4: Flight Control System Components and their Functions

Operations Preparation and Maintenance of thePreparation – Flight Operations Plan (FOP)

– Operations Database (ODB)– Mission Planning Database.

Mission Planning Processing of external planning inputs.Generation and validation of satellite operations timelines.

Network Control Link control between Operations Control Centre and the ground stations for– Telemetry data– Telecommands– Satellite tracking information.

Spacecraft Telemetry reception from ground station.Monitoring Space to ground time correlation.

Conversion of satellite telemetry to engineering values.Limit checking and status monitoring of satellite telemetry.Display of satellite data in graphical or numerical form in real time or fastforward / backward mode.On-line access to historical satellite data.

Spacecraft Generation of command messages for all types of satellite commands.Commanding On-line or time-scheduled release of commands to the ground station for uplink

to the satellite.Pre-transmission checking of command contents.Post-transmission verification of commanded actions.Logging and display of command history.

User Facilities Control of user access using a system of privileges.

On Board S/W Maintenance of on-board software for the satellite processors.Validation of the software changes before installation on board.

Data Distribution Distribution of on-line and off-line data to external users (other Control Centres, DataCentres, Research Institutes).

Performance Access and retrieval of all historical satellite data.Analysis Off-line and periodic analysis and visualisation of user-defined algorithms.

Generation of operations reports.

Computer Room at ESOC

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Flight Dynamics SystemsAn important aspect of mission operations issatellite navigation, this being thedetermination, prediction and control of thesatellite’s orbit or trajectory in space and thedetermination and control of its orientation inspace, i.e. its attitude. These activities are thedomain of ESOC Flight Dynamics specialists andinvolve the use of dedicated Flight Dynamicssystems.

In order to achieve the objectives of its mission,each satellite is intended to fly in a predefinedorbit and it is the task of Flight Dynamicspersonnel to refine and put into practice theearlier mission analysis work. One of their firstoperational tasks in any mission is to determinethe characteristics of the initial or injection orbitafter the satellite has separated from the launchvehicle and to prepare the information neededto bring the satellite into its final orbit.

The first step is to process tracking data fromthe ground station and calculate the orbitalelements of the initial orbit: detailed knowledgeof both ground station systems and spacecraftsystems is needed in this process. It is thennecessary to identify how much the achieved orbit differs from the desired orbit andto make the necessary changes to the satellitevelocity by performing orbit manoeuvres. Thesatellite will be equipped with the necessaryboost motors and thrusters according to theinjection strategy and mission needs.Interplanetary missions may require theimplementation of suites of orbit manoeuvres

over several years to take them on their journeythrough deep space to a particular region inspace, perhaps swinging by other planets onthe way, to give the extra boost needed.Missions involving in-orbit docking of onespacecraft to another, place high demands onthe Flight Dynamics team and system.

Orbiting spacecraft are subject to minuteperturbations, caused for example by solar-radiation pressure or resulting fromgravitational effects of the Earth, the Moon andthe planets. These orbital perturbations add upover time and must be counteracted by regularexecution of small orbit-correction manoeuvresthroughout the mission. For a geostationaryspacecraft, this is known as ‘station-keeping’and is necessary to keep the satellite within aspecified latitude and longitude ‘box’ havingdimensions of some small fraction of a degree.

The orbit of the European Remote Sensingsatellite, ERS, is such as to take thespacecraft over the same locations onEarth at regular intervals. At its altitude of800 km, the atmospheric drag slowlyreduces the height of the orbit and upsetsground track repetition: as the extent ofthe drag depends on the density of theatmosphere, it is necessary to takeforecasts of solar activity into account inthe planning of orbit manoeuvres.

Flight Dynamics Room atESOC

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While the satellite trajectory in space must bemonitored, it is also necessary to ensure thatthe satellite body is correctly oriented. Sensitiveinstruments on scientific satellites must bepointed towards selected stars or nebulae butkept averted from the strong radiationemanating from the Sun or the Earth. It may benecessary to make hundreds of observations ofdifferent objects, the satellite pointing directionmoving from one to the next in rapidsuccession. On the other hand, antenna disheson telecommunications satellites need to bepointed constantly towards the Earth forseveral years.

Information on satellite orientation is providedby attitude sensors mounted on board thespacecraft. Some are designed to identify therelative direction of the Sun or the Earth, someto identify a pattern of stars in the sky (startrackers) and others to measure the rate atwhich the attitude is changing. Using advancednumerical estimation techniques to analyse this information, the Flight Dynamics system isable to determine the satellite’s attitude to ahigh degree of accuracy, often within tens ofseconds of arc, and to pass the results to usersof the satellite data in support of their analysis.

Changes to the attitude are effected by meansof actuators or thrusters on board thespacecraft: the information the spacecraft needsin order to execute each manoeuvre is preparedwith the aid of the Flight Dynamics system andmay include thruster firing data, data to adjustthe attitude sensors and data to control on-board safety measures, such as Sunavoidance. This information is passed to thespacecraft in the form of telecommandsconstructed by the Flight Control system andtransmitted by the ground station. The progressof executing manoeuvres is carefully observedin real time to monitor the correct operation ofthe on-board systems and to ensure spacecraftsafety at all times.

Like all intricate systems, the satelliteequipment used for attitude and orbitmeasurement and control must be calibrated:estimates of sensor alignment, drift and biaserrors are needed in addition to estimates of

individual thruster performance. As thesecalibration parameters are likely to vary withtime, it is necessary to perform regularcalibration activities throughout the mission.Calibration data are also needed to enablerecords of on-board fuel expenditure to be keptso that the remaining useful life of the satellitecan be predicted.

ESOC’s experience in Flight Dynamics isunsurpassed in Europe and is dependent on theskills of highly trained staff and thedevelopment of sophisticated Flight Dynamicssystems. Like other ground-segment facilities,the Flight Dynamics system needs carefulpreparation and has to be tailored to therequirements of each new mission, a task madeeasier by the introduction of a mission-independent software infrastructure ORATOS(Orbit and Attitude Operations System). Themajor components and functions of a typicalFlight Dynamics system are set out in Table 5.

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Artist’s impression of thefour Cluster II spacecraft(photo ESA)

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Flight Dynamics Room

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Table 5: Flight Dynamics System Components and their Functions

Attitude Determination and Processing of telemetry dataControl Attitude estimation

Attitude manoeuvre preparationAngular momentum and fuel budgeting

Calibration Actuator and thruster performance and alignment calibrationOptical sensor performance and alignment calibrationGyro calibration

Orbit Determination Processing of tracking dataand Control Calculation of orbital elements

Prediction of ground station visibility, eclipse times, etc.Preparation of station antenna pointing estimateOrbit manoeuvre preparationFuel budgeting

Mission Planning Support Generation of orbit-related planning informationPreparation of attitude and orbit related inputs into mission plansPrediction of angular momentum and fuel budgets

Auxiliary Data Generation Generation of attitude history dataGeneration of orbit related dataGeneration of spacecraft-related calibration data

Quality Assurance Independent system validationSupervision of all Flight Dynamics tasks and functionsVerification of Flight Dynamics performance and quality.

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Main Control Room in theOCC at ESOC

Control Centre FacilitiesIn addition to the software and hardwaresystems established for the performance ofmission operations, it is necessary to provide arange of support facilities, including the controlrooms in the Control Centre andcommunications networks linking the variouscomputer systems, both within and outside theControl Centre. The various control roomfacilities at ESOC include:– The Main Control Room, used for the conduct

of operations in the early phases (LEOP) of amission (as shown below)

– The Flight Dynamics Room– The Project Support Room, in which staff

from the Project and industry are accommodated while they provide on-site consultancy during LEOP

– The Dedicated Control Room, from which operations are conducted in the later routinephases of the mission

– The Ground Configuration Control Room,where operators oversee and configure thelinks and systems for data and commandrouting between the ground stations and theControl Centre.

ESOC has also established further specialisedcontrol centre facilities at the following ESA sites:– The Science Operations Centre at the

Villafranca ground station near Madrid– The Telecommunications Operations Centre

at the Redu ground station in Belgium.

One further important aspect is theestablishment of facilities to allow reliable round-the-clock communication between eachstation in the ground network and the ControlCentre for telemetry, telecommands and voiceand data traffic. ESOC has set up and maintainsa communications network to complement theground station network and provides facilities for other communications needs, covering forexample, links between the different ESA sitesand their working partners in industry, atresearch institutes and at other Agency sites.

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Ground Segment Integrationand ValidationIn order for the whole ground segment to workcorrectly, providing ready access to the satelliteat all required times and fulfilling the intendedmission requirements, a thorough andsystematic series of tests must be carried out atall stages of ground-segment integration,moving from testing of units, to testing ofsubsystems to testing of large systems, to finaloperational testing of the whole groundsegment.

This activity extends over a period of manymonths towards the end of the preparationsphase and requires the services of a team ofexperts in ground segment integration andtesting, who first define a comprehensive testplan and then execute the plan step by step.

Table 6 shows some of the more significanttests performed during this phase.

Satellite SimulatorsThe single most important tool for thevalidation of the ground segment systems,operational databases and flight controlprocedures is the so-called satellite simulator.The simulator is itself a sophisticated softwaresystem modelling the satellite in such a waythat when connected to the flight-controlsystems, it enables operations staff to exerciseall satellite operations in a highly realisticfashion.

The simulators are developed by the SimulatorGroup at ESOC: the sources of information forsimulator developers are the satellitespecifications, the detailed design documentsand the requirements documents for on-boardsoftware. The simulator is built up aroundreusable modules, with additional modules torepresent the specific features of the mission inquestion

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Dedicated Control Roomfor Earth Observation

Table 6: Integration and validation tests

Test Activity Scope

Software System Acceptance To validate all functions of delivered softwaresystems

Database Validation To confirm the correctness of all entries in theoperations databases

Procedure Validation To validate all operations procedures

Station Acceptance Tests To validate all ground station functions

RF Compatability Test To test the interface between the satellite andthe ground station RF systems

System Validation Test (SVT) To validate all flight control and flightdynamics systems against the Flight ModelSatellite on-line tests

Operations Validation Tests (OVT) To validate the ground segment operationally(end to end)

Mission Readiness Tests (MRT) To validate readiness to commence missionoperations

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The simulations programme is designed totransform a number of trained individuals intoan integrated mission operations team. This iseffected by undertaking a selected set ofmission operations cases in a realisticoperational manner, creating a trainingenvironment hardly distinguishable from reallife in which the Flight Control team interactwith the satellite simulator as if it were the realspacecraft.

Typical cases to be exercised include:

– Pre-launch phase interactions with CSG,Kourou, through to lift-off and spacecraft separation

– Spacecraft separation through toestablishment of 3-axis spacecraft control

– Spacecraft attitude and orbit manoeuvresand related calibration

– Spacecraft subsystem check-out– Instrument switch-on and check-out– Routine operations, including perigee exit,

start of observations, eclipse, perigee entryetc.

Strong emphasis is placed on the execution ofboth nominal and contingency operations.Anomalies defined by the Simulations Officerare not known to those taking part, but areinjected into the simulation to create thedesired realism and to train operations staff inanomaly identification and recoveryprocedures. Simulations are further a means ofproving the mission documentation andoperational procedures, and ensuring that theground segment performs as required.

Such simulations have in the past brought tolight satellite design problems that had nothitherto been identified in the satelliteintegration and test programme.

At the termination of the simulationsprogramme and after several years of missionpreparations, the Flight Control Team is readyand eager to take on its responsibilities in theexecution and conduct of mission operations.

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Table 7 summarises the features of a typicalscientific satellite simulator.

The Simulations ProgrammeThe simulations programme is conducted at theend of the preparations phase, its purposebeing to enable all planned operations to beexercised in a realistic environment and theoperations staff to be trained to work togetheras an efficient flight-control team well versed inboth nominal and contingency operations.

Table 7: Major features of a scientific satellite simulator

Element Modelling

Satellite Subsystems Fully representative hardware, software,mechanisms, pyros, motors, appendages etc.Realistic telemetry in all modesCorrect command responses

On-Board Software Systems Hardware emulationExecutable on-board software

Satellite Dynamics Satellite mass propertiesAttitude manoeuvre executionAttitude sensor fields of view

Satellite Environment Orbital motion and orbit manoeuvresEclipse and thermal inputsGround station contact periodsStar catalogues (including planets)

Network Interfaces Ground stationsCommunications

Simulator Control Faster than real timeOperator-injected anomaliesSimulator monitoringCommand/telemetry replay facility

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CONDUCTING MISSIONOPERATIONS

Establishing Contact with theSpacecraftContact with the spacecraft is necessary for theday-to-day execution of the mission operationsand is established when the spacecraft passesover a suitable ground station. A suitablestation is one which has been tailored tointeract with the spacecraft in question.

Unless it is in geostationary orbit, where it willappear to stay in a fixed position in the sky, the spacecraft will generally rise over thehorizon and become visible from the station. Inanticipation of the upward path, the station will have moved its antenna in azimuth andelevation towards a direction predicted on thebasis of the determination of the orbit. As soonas the spacecraft appears, the station will lockon to its radio signal and start to track itsmovement in the sky automatically: should thesignal be disturbed, the antenna will continue totrack according to a predicted satellite path (alsoderived from the known orbit). Soon after locking on to the radio signal, the ground station will be able todemodulate and decode the telemetry and routeit directly to the Control Centre giving theoperations staff the data they need to assess thehealth of the returning spacecraft. At this point,the ground station transmits a radio carriersignal to the spacecraft: in order to ensure thatthe on-board receiver locks on to the uplinkedcarrier and to compensate for the Doppler effectcaused by the rapid motion of the spacecraft,the uplink frequency is made to sweep aroundthe nominal value. After the end of this sweepand with confirmation in the telemetry data thatthe on-board receiver is in uplink lock, theground station is ready to uplink anycommands received from the Control Centre.

All telecommand requestmessages received fromthe Control Centre arechecked by the stationtelecommand equipmentand transmitted to thesatellite by modulating theRF carrier. Whilecommanding and receivingtelemetry, the station maymake range and range-rate measurements, tobe passed to the Control Centre for orbitdetermination.

Throughout the period of contact (the pass), thestation will continue to track the spacecraft inthe sky and provide the telemetry andcommand services to the Control Centre. Forlocal safety reasons, the uplink service will be

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Envisat

Conducting satelliteoperations at ESOC

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Ground configurationControl Room

terminated when the elevation of the groundstation antenna falls to 5 degrees above thehorizon on the downward path: as thespacecraft finally approaches and falls belowthe horizon, telemetry contact will be lost,marking the end of the pass.

The Launch and Early OrbitPhaseThe launch vehicle authority is responsible forthe actual launch and management of thelaunch vehicle carrying the spacecraft intoorbit. ESOC takes over responsibility for mission operations at the moment the satelliteseparates from the launch vehicle in the phasereferred to as the Launch and Early Orbit Phase (LEOP), when the first ground contactwith the satellite is established.

This phase is critical, as activities must beperformed under strict time constraints for thepurpose of setting up the spacecraft for fulloperations in orbit and include deployingmechanical parts such as solar panels andantennas, which are normally held in a foldedposition during the launch. It is also necessaryto move the satellite from the launch vehicleinjection orbit into the orbit selected for routine

operations. Other time-critical operationsinvolve setting up the on-board conditionsneeded to determine and control satelliteattitude and to configure it for the tasks tocome.

The LEOP activities are conducted by anextended Flight Control Team headed by adesignated Flight Operations Director, who isresponsible for direction, coordination andconduct of the mission operations. The FlightControl Team is supported by teams from theProject responsible for satellite developmentand the specialists from industry: suchmeasures ensure that all necessary expertise ison hand during execution of the LEOPoperations.

The Flight Control Team is made up principallyof specialists in satellite operations and in flightdynamics who perform their tasks with the aidof workstation facilities in the Main ControlRoom (MCR) of the Control Centre, providinground-the-clock support for the duration of theLEOP. The key players in the Flight Control Team include the Spacecraft OperationsManager, the Ground Operations Manager andthe Flight Dynamics Coordinator.

Each manages the activities of a specialisedteam of fully trained staff in such a way as toconduct all planned operations and to recoverfrom any unforeseen situations arising with thenewly operating spacecraft and the associatedground segment.

The Flight Control Team is augmented by arange of service support personnel responsiblefor computer hardware and software,

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communications and ground stationequipment, Control Centre facilities, telex andtelefax operators and general services staff allproviding round-the-clock support in thisphase.

The table below shows the breakdown of atypical Flight Control Team in the LEOP.Once the milestones of the LEOP have been

passed (i.e. achievement of the required orbit,deployment of satellite appendages andinitiation of payload operations), the LEOP isterminated and a reduced operations teammoves to the Dedicated Control Room (DCR) forcontinuation of operations in the subsequentphases of the mission, in which satellite andpayload commissioning or even deep spacetravel may be undertaken.

Main Control Room

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ERS Mission Planning Office

Planning Routine OperationsOnce the early orbit phases have beencompleted and the satellite has been fullychecked out in orbit, the routine operationsphase commences, in which the satellite isoperated in the way intended to provide theservices or products required. This phase is onein which all activities have to be planned andexecuted in an orderly and dependable fashion.

The planning process needed depends on thetype of mission and the way satellite control isto be performed, but will often involve theformulation of operations plans some weeksahead of schedule and covering several days ata time. These operations plans must not onlytake account of the requests for execution ofspecific tasks, for example radar imaging of aparticular region of the Earth’s surface, theymust also take account of the constraintsassociated with operation of the satellite. Theseconstraints may take the form of limitations inavailable power under certain conditions,restrictions in available on-board storage oftelemetry data or restrictions about thedirections in which the satellite is permitted topoint as it moves around the orbit. Anyconflicts identified in the planning process mustbe resolved by rescheduling and regeneration ofthe plans.

The planning inputs are derived from requestsmade by members of the user community, byexternal bodies or by the operations staffthemselves. In addition to the user-orientedactivities, it is necessary to plan for conduct ofall satellite operations that are needed for orbitand attitude control, satellite health and safetymaintenance and any other related operationsactivities. These must all be incorporated into

the overall plan for the whole satellite and forthe period in question.

Once the operations plans are established, theymust be converted into the appropriatetelecommands for uplink to the spacecraft atthe appropriate time in order to execute theplanned satellite operations.

Routine Satellite OperationsRoutine satellite operations are conducted bytelemetry and telecommand interactionsbetween the Control Centre and the satellite,according to the operations concept devisedduring the satellite and ground segmentdevelopment phase. The extent of theseinteractions is dependent on the following:– the type of mission (telecommunications,

observatory, deep space etc.)– the type of orbit (geostationary, highly

elliptical etc.)– the extent to which ground contact is

available (number of stations and visibilityafforded)

– the provisions made on board thespacecraft for command execution andtelemetry delivery to the ground

– the complexity of the activities to beperformed.

An Earth-resources satellite (such as ERS-2) in alow polar orbit is typically only visible for a ten-minute period in each of ten out of fourteen orbits (of 100 minutes each). When out of contact with the ground, such a satellitemust be able to execute complex operationsand to store the telemetry data on board. Insuch a case, the Control Centre must define allinstructions for the upcoming orbits inadvance, in the form of a schedule: the

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schedule is then loaded on board the satellitefor later timed execution. Each time the satellite passes over a suitable ground station,the Control Centre loads more instructions intothe schedule and commands the satellite todownlink the data it has stored since the lastpass. The operations concept for such amission can be characterised as requiring:– a high degree of operations planning– production, uplink and execution of an

on-board schedule– storage and dump of satellite telemetry,

and– a high degree of autonomous on-board

safety monitoring and control.

A satellite in deep space, on the other hand,may be visible from the ground for up to tenhours per day on some days and perhaps notcontactable for days or weeks in between. Afurther complication is the time taken forcommands to reach the spacecraft and for thetelemetry to return to Earth; potentially up toone or two hours, travelling at the speed oflight. In such a case, the operations conceptwould be analogous so that for ERS, employingan on-board schedule and on-board datastorage. Such a satellite would also be able totake care of its own safety during the months ofdeep space flight: this may entail theautonomous switching of heaters to maintainequipment temperatures, it may entail thedetection of faulty equipment and switchingover to spare units or it may even involve anautonomously controlled series of manoeuvresto re-establish contact with the ground aftersome on-board or ground failure. The ControlCentre’s role in this mission would be to

prepare all necessary instructions for these andother on-board autonomous functions,involving often complex analysis andmodelling. In comparison with the more exoticmissions, a satellite in geostationary orbitoffering twenty-four hour coverage, permits amuch simpler operations concept.

Whatever the concept for satellite operations,the Control Centre’s task will be to perform allnecessary operations according to themission’s needs: typical routine operations for a scientific observatory mission (XMM) areshown in Table 8.

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ERS Dedicated ControlRoom

Table 8: Outline of Routine Operations for the XMM Spacecraft

Start of Pass Activities:– Acquire satellite telemetry and confirm satellite health and status– Start the uplink and commence ranging measurements– Determine spacecraft attitude with the aid of star tracker data– Adjust satellite temperatures and monitor telescope thermal situtation– Monitor on-board power situation and charge batteries if necessary– Set up instruments for start of observations (processor loads)– Calibrate attitude sensors– Load instructions for satellite and instrument safety monitoring and control

Science Observations:– Start sequence of attitude manoeuvres for planned observations

(new pointing direction every few hours)– Change instrument settings for each new pointing– Monitor satellite attitude and angular momentum– Perform instrument calibrations– Switch from one on-board antenna to another in order to keep contact

(if necessary)– Route science data to Science Centre

Before end of Pass:– Switch instruments into safe mode (to avoid saturation in Earth

radiation belts)– Manoeuvre satellite to safe attitude for perigee passage (to avoid Earth)– Set up power system for eclipse operation (battery discharge)

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Mission Services and ProductsThe services and products to be provided duringa space mission are defined by the type ofmission being flown. For a telecommunicationsmission, the goal is to provide the users with acontinuous point-to-point telecommunicationsservice.

The goals of any science or application missionare to make scientific observations or othermeasurements from space and to analyse andstudy the results. Final processing of thetelemetry data may be the responsibility ofESOC, or an external data centre: it mayalternatively be the responsibility of a smallgroup of specialists, or of individual researcherswithin the science community. Whatever thefinal destination, it is ESOC’s task to provide amaximum amount of good-quality data to usersin the form of mission products.

In years gone by, data products were deliveredon magnetic tapes by mail. Nowadays, users canreceive their data on CD ROM or across local orinternational communications links: users todayare able to access product stores via the internetin order to request and download products oftheir choice along with the tools needed forproduct anaysis.

Table 9 gives examples of mission products andthe user responsible for final processing.

Quality ManagementEver conscious of the need to ensure customersatisfaction with the services it provides, ESOChas embarked upon a process of qualitymanagement and continuous qualityimprovement.

The ESOC Quality Management Systemencompasses all activities of the centre, rangingfrom early negotiation of possible operationsservices through the whole mission preparationsphase, and during the mission operationsexecution phase, including mission termination.

The procedures within the quality managementsystem address contractual activities, groundsegment development cycle, anomalyidentification and resolution and reporting.

ESOC achieved full ISO 9001 certification inNovember 1999 and is committed to aprogramme of self-evaluation and improvementin the future.

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Table 9: Examples of mission products

Mission Product User or Processing Authority

Meteosat Images ESOC (before 1996)EUMETSAT after 1996

ERS Synthetic Aperture Fast Delivery Products: Kiruna Ground StationRadar Images Final Products: P.A.F.’s (National Centres)

ERS Ozone Monitoring ESTEC and Principal Investigator InstitutesData (GOME)

GIOTTO Instrument Science Principal Investigators at ESOCData Principal Investigator Institutes, Europe/USA

Giotto Halley Multicolour Max Planck Institute, GermanyCamera Images

ISO Instrument Science ISO Science Operations Centre, Villafranca, SpainData

Cluster Instrument Science Quick Look: Joint Science Operations Centre, UKData Final: Principal Investigator Institutes, Europe/USA

Hipparcos Astrometric Data HIPPARCOS Star Catalogue Consortia (FAST inFrance, NDAC in UK, TYCHO in Denmark)

Halley Multicolour Camera composite image of the nucleus of Comet Halley© 1986 Max-Planck Institut für Aeronomie

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BEYOND THE NOMINAL MISSION

Recovery from UnexpectedEventsIt is the task of the Control Centre to maximisemission return and to ensure the safe andreliable conduct of all mission operations. Notall operations progress smoothly, however, andin the harsh environment of space, unexpectedevents sometimes occur. Considerable effort isinvested in the preparations phases to minimise possible sources of failure, both onthe satellite and in the ground segment, and toprepare procedures and plans for recovery ifthey occur during the mission.

Using automated features of the Flight ControlSystems, operations staff monitor forunexpected events in the satellite telemetry asit arrives at the Control Centre: the status or values of all important telemetry parameterswill be checked against a set of predefinedlimits, and audible alarms will sound if any out-of-limit value is found. The out-of-limit value may simply indicate a slowly increasingtemperature of little concern, but it may equallyindicate a serious on-board equipment failure,such as an error in the execution of a commandor the malfunctioning of a microprocessor.

For every alarm raised, the spacecraftcontrollers will identify the anomaly and takethe necessary recovery actions, usingprocedures defined in advance: they will havebeen regularly trained in the recovery ofanomalies (using the satellite simulator) so they will know how to spot them and what todo. If a new recovery procedure needs to bedevised for a complex unforeseen anomaly, itwill be developed and exercised on thesimulator before the operation is attempted onthe spacecraft itself.

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Hardware failures on board can result in a need to change the way the spacecraftoperates, for example, if a gyro fails to provideattitude rate information. In such instances, itmay be possible to compensate for the failureby redefining some of the on-board controlalgorithms and defining new on-board softwareto do the job: all changes in the on-boardsoftware will be fully validated – again withthe aid of the simulator – before being loadedon board.

Anomalies are not restricted to the satellite, the ground segment is also subject tooccasional errors and failures. The operationsstaff must equally be well versed in recoveryfrom failures in the ground equipment, such asthe ground station antenna motor or data linksto the ground station as well as damage caused by the weather (electric storm, wind,flood). The first concern in such circumstances is to ensure that the spacecraft is not in danger and to resume normal operations assoon as possible, by means of back-upequipment.

In rare cases, the unexpected event turns out to have a major impact on the ability tocontinue with the mission as originallyforeseen, and it becomes necessary to embarkupon a major recovery effort, identifying newways of rescuing the satellite or defining ameans of salvaging the mission. Time is of theessence here, as the spacecraft can degradefurther while the rescue plan is beingdeveloped. ESOC has been faced with and metsuch challenges several times in the past.

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Olympus

GEOS

Satellite RescueOlympus was a telecommunications spacecraftlaunched in 1989. ESOC was responsible for conduct of the LEOP but transferredresponsibility for the routine phase to theItalian Space Agency’s control centre in Fucino.As time went by, premature degradation of the spacecraft made its operationincreasingly more complicated. By May 1991,only one solar panel was able to provideelectrical power, and errors in the on-board Earth sensors made attitude controldifficult. After detecting a new on-boardanomaly, the spacecraft set itself into a back-up mode: returning to normal service wasnow no longer a straightforward task, however. During the complex operations thatthis required, the spacecraft unexpectedlybegan to tumble. The spacecraft tried to controlits own motion, but the autonomous thruster firings caused the satellite to movefrom its nominal orbital position and to driftout of contact with the ground. Within oneweek both the thruster propellant and batteryelectrolyte were frozen, there was virtually notelemetry, no telecommand capability and onlymarginal, fluctuating power. The spacecraft was slowly rotating and drifting around theworld at 5 degrees per day.

ESOC was called upon to establish a rescueplan: a team of some fifty operations andsatellite specialists was assembled to deviseand implement what was to be a very complexplan. The services of additional ground stations(including those of CNES and NASA) weredrafted in and ESOC facilities were expanded toassume mission control: after weeks ofintensive and extremely critical operations andafter the uplinking of several thousand

telecommands, basic control was regained at the end of July. The spacecraft was movedback to its nominal orbit two weeks later andresumed operations as before very soonafterwards: ESOC had been able to bring thecrippled spacecraft back to life.

Mission RedefinitionGEOS was a scientific spacecraft launched inApril 1977. It was due to be inserted into ageostationary transfer orbit by a Thor Deltalaunch vehicle and moved into a geostationaryorbit by firing the on-board boost motor. Owing to a partial failure of the launcher, thespacecraft was placed into an orbit having aperiod of only 3.8 hours and not 10.5 asexpected. The spin-rate was also unacceptablylow at 1.5 instead of 90 rpm: the satellite waswobbling badly with a cone angle of 35degrees.

First contact with the spacecraft showed thatthe spin rate was too low, and a spin-upmanoeuvre was quickly executed. After some rough estimation, it became clear thatboth the spacecraft attitude and its orbit werewell off target. An attitude correctionmanoeuvre was needed ot keep satellitetemperatures within reasonable bounds but,more importantly, the low orbit meant that the solar cells were in danger of being rapidlydegraded by the Earth’s radiation belts: it wasessential to move the spacecraft into a different orbit as quickly as possible. Insertioninto the originally planned geostationary orbit

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was no longer feasible and through extensiveanalysis a search was started for the bestpossible orbit that could be reached with theaid of the boost motor. This was not easy, asthe orbit had not only to satisfy the scientificneeds of the payload, but had to take accountof a number of stringent constraints, includingthe availability of ground-station visibility, theviewing angle of the spacecraft antenna, theorbit stability and eclipse durations.

After five days, an acceptable solution had beenfound and the satellite was placed in aninclined, elliptic orbit with a period of twelvehours. In this orbit, GEOS went on to providevery useful scientific data: as a result of theinjection failure, ESA decided to launch asecond identical spacecraft, GEOS-2, whichsuccessfully entered geostationary orbit somefifteen months later. The speed with whichESOC was able to devise a solution for GEOSwas, however, instrumental in salvaging themission.

HIPPARCOS was a scientific satellite dedicatedto extremely precise measurements of starpositions, distances and space motions. It, too,was intended for geostationary orbit where itwould have been in continuous contact withESA’s Odenwald ground station. Launched inAugust 1989, the transfer-orbit operationsproceeded smoothly until the moment whenthe apogee boost motor was instructed toignite: owing to a failure in both pyrotechnicchains, ignition failed to take place and thespacecraft was unable to move out of itstransfer orbit.

Using the small amount of hydrazine fuelavailable, the orbit perigee was raised toreduce air drag and the harmful effects ofatomic oxygen. In the elliptic orbit with itsperiod of 10 hours 40 minutes, coverage fromOdenwald was only about 32% of the time.Additional stations were needed and equipment had to be procured and installed atPerth and Kourou: NASA later provided thesupport of its Goldstone station in the Mojavedesert.

Contact was not continuous and longeclipses were experienced; there wasfrequent payload occultation; attitudeknowledge was hard to obtain and atti-tude control was very difficult in theperigee region because it was there thatthe spacecraft passed through the VanAllen radiation belts. A significantproportion of the Hipparcos flightoperations procedures had to be rewritten:early on, operations were extremely risky, sincethe pressure of time did not allow fullassessment of all possible eventualities ofpreparation for potential contingencies.

The implementation of the revised missionallowed scientific data collection to start lessthan four months after launch. At first, thedata recovery rate was about 50%, but thislater increased to 65% after optimisation ofoperations procedures. Despite the successivefailure of four of the five gyroscopes andnumerous other anomalies, largely attributableto the severe radiation levels, the Hipparcosmission lasted four years – 18 months longerthan had been foreseen for the original,nominal mission.

Although trapped in the wrong orbit andexperiencing damaging levels of radiation,Hipparcos accomplished all its scientific goals.An ingenious zero-gyro attitude-controlprocedure had even been developed, butradiation damage to the on-board computerstopped the mission before that could be triedout.

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Hipparcos

Hipparcos orbits

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In the case of Giotto, ESOC was faced with thechallenge of defining a second comet encounter mission for an already orbitingsatellite. Launched in July 1985, the scientificspacecraft was designed for a nine-monthinterplanetary journey to a high-speed fly-by ofcomet Halley. This mission was very successfuland, contrary to expectations, the satellitesurvived the hyper-velocity dust impactsaround closest approach to the comet,although it took a severe battering. Moreover,90% of the original fuel load was still beingcarried thanks to thrifty navigation and preciseinjection accuracies of both Ariane and the on-board boost motor.

Despite structural damage, radical changes inthermal behaviour, instability in the power-control system, a punctured star-mapper baffleand the loss of many autonomous functions,the spacecraft was still operable. No detailedextended mission plan had, at that time, beenworked out and resources were not available topermit further continuous control. To keep most options open, Giotto was placed on an Earth-return trajectory (which cost more thanhalf the remaining fuel), then configured intominimum-power mode; its attitude waschanged to make it thermally optimal and thetransmitter was switched off: it had been putto sleep.

After four years in hibernation, Giotto wasreactivated. Re-establishing communications,an operation never before attempted, had to be achieved blind, with no telemetry and withGiotto at a distance of 100 million km fromEarth. The strategy which had required man-years of effort to devise, led to six days ofcomplex operations, and resulted in successfulreactivation of the spacecraft.

Check-out of the spacecraft revealed that abouthalf the scientific payload was still working, but on-board redundancy had been furtherreduced by two failures of vital spacecraft units. Comet Grigg-Skjellerup was chosen asthe next target, being the best compromisebetween scientific interest and operationalfeasibility. Even so, the damaged satellite stillhad to operate outside its intended designenvelope in many respects.

ESOC took Giotto on to make the first-ever Earth gravity-assist swing by for a spacecraftreturning from deep space. It was put intohibernation a second time, survived extremesof high temperature and was reactivated againtwo years later, in May 1992. At the secondencounter, in July 1992, when Giottoapproached to within 200 km of the cometarynucleus, the great distance from the sun caused the power margin to be so low that it is doubtful whether even one additionalexperiment could have been switched on. TheGiotto extended mission added only a fewpercent to the cost of the original mission.

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Scientists watching Giottoencounter with CometGrigg-Skjellerup

The Giotto spacecraft

Orbit of Giotto for theExtended Mission (GEM)

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Mission TerminationA spacecraft is designed to fulfil the missionobjectives for a defined period of time, rangingtypically from two years to ten years or more.Aspects which must be taken into accountwhen designing for lifetime are principally:– the need to maintain the desired orbit

(geostationary, polar, etc.) throughout themission and the provision of sufficient fuelfor all foreseen orbit manoeuvres;

– the need to ensure that the spacecraft attitude is controlled throughout the mission and provision of sufficient fuel for all foreseen attitude manoeuvres and momentum control;

– the need to provide sufficient electricalpower throughout the mission, accountbeing taken of the time- and orbit-dependent degradation that solar cellsexperience in orbit;

– the need to provide adequate equipmentresilience to in-orbit degradation and adequate equipment redundancy to achieve the desired reliability goals over the mission.

Mission operations will continue while thespacecraft and its payload are fulfilling themission objectives. If the resources needed onboard (power, fuel) become depleted, or if failures of on-board equipment occur, makingoperations more and more difficult, then mission termination has to be considered.

In some cases, mission termination can bedramatic (sudden loss of the spacecraft), inother cases it can be extremely long andlaborious (the final months of the successfulHipparcos mission), and in yet other cases quite straightforward (termination of the GiottoExtended Mission).

If termination can be planned, then it is oftendesirable to perform a series of end-of-life tests on spacecraft mechanisms, for example. Ifpossible, it is worthwhile moving thespacecraft into a so-called graveyard orbitwhere it poses less of a risk to other orbitingsatellites or to the space environment.For the control centre, a mission is deemed tohave terminated when ground contact with thespacecraft is finally and permanently termin-ated, whatever the fate of the spacecraft, whether it is slowly drawn towards Earth to

finally burn up in the atmosphere, or whetherit remains in orbit unattended for thousands ofyears.

Space DebrisSpace debris is a term used to describe theever-increasing amount of inactive spacehardware in orbit around the Earth: it is thecollection of burnt-out launch vehicle upperstages, dead or inactive spacecraft, ejectedboost motors, fragments of satellite and rocketstage breakup, and innumerable other metallicshields, booms, covers and caps. They rangefrom items the size of an automobile or biggerdown to microscopic dust. Space debris isbecoming a serious threat to current and future missions, as the steadily growing risk ofcollision with active spacecraft, especially inhighly populated orbits like the geostationary‘ring’ or the low-Earth orbit region (altitude lessthan 2000 km), is of concern to all spaceagencies.

In order to alleviate the problem, it is nowcommon practice, certainly for ESA, to applymitigating measures. Launch-vehicle andsatellite designers are becoming ‘debrisconscious’ and are endeavouring to reduce to a minimum the debris produced once theirvehicle or satellite enters space. ESA hasadopted the policy of moving eachgeostationary spacecraft at the end of itsuseful life into a disposal orbit at least 300 kmabove the geostationary ring.

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Observable space debrispopulation based onNASA/USSpaceCom data.Objects have beenconsiderably enlarged inorder to make themvisible.

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Another concern is the danger of re-entry intothe Earth’s atmosphere: most objects are burntup on re-entry, but fragments of large massiveobjects have been known to strike the surfaceof the Earth, potentially threatening populatedareas.

The following spacecraft have re-entered andhave reached the surface of the Earth withoutcomplete disintegration:– KOSMOS 954 in January 1978: a Russian

spacecraft containing a nuclear reactor;– SKYLAB in July 1979: the 75 ton US space

laboratory;– KOSMOS 1402, one part in January 1983

and another part in February 1983: aRussian spacecraft containing a nuclearreactor;

– Salyut-7/Kosmos 1686 in February 1991:the 40 ton Russian space station.

In such cases, ESOC has been involved in theanalysis and prediction of the time and locationof re-entry so that the authorities of ESAMember States can be advised in time. ESOC isalso the co-ordinator of ESA’s space debrisresearch programme, which focuses on thefollowing activities:– assessment and modelling of the terrestrial

particulate environment

– maintenance of the space debris database,DISCOS

– analysis and prediction of lifetime in orbit ofESA objects in space

– hypervelocity impact analysis and shieldingmeasures

– debris control methods and orbital analysis.

DISCOS is unique in Europe and containsinformation such as orbit parameters, size andmass of the current population of about 7500orbiting space objects larger than 10-20 cm.New additions are made to the catalogueseveral times a week as agencies start new orterminate current missions and as radar andoptical sensors of the US Space SurveillanceNetwork provide new information.

These measures make it possible to predict theevolution of the debris population and areaimed at achieving a future globalunderstanding of the problem.

An important objective of these activities is tounderstand the evolution of the debrispopulation and identify and apply mitigationmeasures in order to keep the space debrishazard within acceptable tolerances.

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Aerial view of ESOC atDarmstadt, Germany

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Appendix 1 : ESA missions supported from ESOC

Name Purpose Launch Mission Orbit Stations Remarksdate duration

(years)

ESRO-2 Science 17.05.1968 3 LEO Redu/Falkland/FBA/Spitzbergen/Tromsø

ESRO-1A Science 03.10.1968 2 LEO Redu/Falkland/FBA/TromsøHEOS-A1 Science 05.12.1968 7 HEO Redu/FBA/ESRO-1B Science 01.10.1969 2 months LEO Redu/Falkland/FBA/Tromsø Launch vehicle

underperformanceHEOS-A2 Science 31.01.1972 2 HEO Redu/FBA/SpitzbergenTD-1A Science 12.03.1972 2 LEO variousESRO-4 Science 20.11.1972 2 LEO Redu/FBA/Spitzbergen/TromsøCOS-B Science 09.08.1975 7 HEO ReduGEOS-1 Science 20.04.1977 5 GTO Redu/ODW/NASAISEE-2 Science 22.10.1977 10 HEO STDN OCC at GSFCMeteosat-1 Meteorology 23.11.1977 8 GEO ODWIUE Science 28.01.1978 18 years/ HEO Vilspa OCC at Vilspa

8 months (joint NASA mission)GEOS-2 Science 14.07.1978 6 GEO Redu/ODWOTS-2 Telecom 11.05.1978 13 GEO Fucino/ReduMeteosat-2 Meteorology 19.06.1981 10 GEO ODWMarecs-A Telecom 20.12.1981 14 years/ GEO Redu/Vilspa

9 monthsExosat Science 26.05.1983 3 HEO VilspaECS-1 Telecom 16.08.1983 13 years/ GEO Redu OCC in Redu

6 monthsECS-2 Telecom 04.08.1984 9 GEO Redu OCC in ReduMarecs-B2 Telecom 10.11.1984 ongoing GEO Redu/VilspaGiotto Science 02.07.1985 hibernation interplanetary Carnarvon/Parkes/DSN/PerthECS-4 Telecom 10.09.1987 ongoing GEO Redu OCC in ReduMeteosat-P2 Meteorology 15.06.1988 7 GEO ODWECS-5 Telecom 21.07.1988 12 GEO Redu OCC in ReduMOP-1 Meteorology 06.03.1989 6 GEO ODWOlympus Telecom 12.07.1989 4 GEO Fucino Only LEOPHipparcos Science 08.08.1989 4 GTO ODW/Perth/GoldstoneUlysses Science 06.10.1990 ongoing interplanetary DSN OCC at JPLMOP-2 Meteorology 02.03.1991 ongoing GEO ODW OCC moved to EUMETSATERS-1 Earth Observ. 17.07.1991 9 LEO KirunaEureca Microgravity 31.07.1992 1 LEO MASPAL/KRU Retrieved by ShuttleMOP-3 Meteorology 20.11.1993 ongoing GEO ODW OCC moved to EUMETSATERS-2 Earth Observ. 21.04.1995 ongoing LEO KirunaISO Science 17.11.1995 3 HEO Vilspa/Goldstone LEOP in ESOC

Routine in VilspaHuygens Science 15.10.1997 ongoing interplanetary via JPL, Pasadena Joint NASA missionTeamsat Technology 30.10.1997 5 days GTO Kourou joint with ESTECPastel Payload on Spot4 24.04.1998 ongoing LEO CNES missionXMM Science 10.12.1999 ongoing HEO Perth, KRU/Santiago (Chile)Cluster II Science 16.07.2000 ongoing HEO Vilspa

09.08.2000

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Glossary of terms

AOCS Attitude and Orbit Control Subsystem

CD ROM Compact Disc Read Only MemoryCNES Centre National d’Etudes SpatialesCSG Centre Spatial Guyana

DCR Dedicated Control Room at ESOCDISCOS Database and Information System Characterising Objects in

SpaceD/OPS ESA Directorate of Operations

EIRP Effective Isotropical Radiated PowerESA European Space AgencyESOC European Space Operations Centre

FOP Flight Operations Plan

G/T Gain/Noise Temperature

LEOP Launch and Early Orbit Phase

MCR Main Control Room at ESOC

NASA National Aeronautics and Space Administration (USA)NASDA National Aeronautics and Space Development Agency

(Japan)

OBDH On Board Data Handling SubsystemOCC Operations Control Centre at ESOCODB Operations DatabaseORATOS Orbit and Attitude Operations System

PCM Pulse Coded Modulation

RF Radio Frequency

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ESA Public Relations Contact Addresses:

ESA HQ: 8-10 rue Mario Nikis75738 Paris Cedex 15FranceTel. (33) 1 53 69 71 55Fax (33) 1 53 69 76 90

ESTEC Postbus 2992200 AG NoordwijkThe NetherlandsTel. (31) 71 565 3006Fax (31) 71 565 7400

ESOC Robert-Bosch-Str. 564293 DarmstadtGermanyTel. (49) 6151 90 2696Fax (49) 6151 90 2961

ESRIN Via Galileo Galilei00044 Frascati (Rome)ItalyTel. (39) 6 94 18 02 60Fax (39) 6 94 18 02 57

ESA Washington Office 955 L’Enfant PlazaSuite 7800Washington DC 20024Tel. (1) 20 24 88 41 58Fax (1) 20 24 88 49 30

ESA Internet sites:

ESA Home Page http://www.esa.int

ESRIN Home Page http://www.esrin.esa.int

ESOC Home Page http://www.esoc.esa.int

ESTEC Home Page http://www.estec.esa.int

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Published by ESA Publications DivisionESTEC, Noordwijk, The Netherlands

Author: Howard Nye

Additional contributions from: Kurt Debatin, Water Flury, Guy Janin,Trevor Morley, Xavier Marc, Boris Smeds and Norbert Schmitt

Coordinator: Jocelyne Landeau-Constantin

Editor: Brigitte Schürmann

Layout: Isabel Kenny

Price: 25 Dfl / 10 Euros

International Standard Book:Number: ISBN 92-9092-380-6

Copyright: © 2001 European Space Agency

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1

ABSTRACT

ESA invests heavily in the development of highly complex satellites to meet the needs of its different Programme Directorates. For manyoutside observers, the moment of launch of thesatellite is the high point and end of theirinvolvement and interest.

However, the satellites are all designed toperform specific functions in space, either in thefield of telecommunications, meteorology,observation of the Earth, microgravity, solarsystem science or space astronomy, and it is theresponsibility of ESA’s European SpaceOperations Centre (ESOC) in Darmstadt,Germany, to ensure that the objectives of each of the different missions are attained.

ESOC fulfils this resonsibility by undertaking the preparations necessary for establishingground-segment facilities and services followedby the execution and conduct of the missionoperations.

This brochure describes the process ofpreparation and execution of satellite missionoperations as exercised at ESOC to provideinsight into how such things are done.

TABLE OF CONTENTS

INTRODUCTION 2

PREPARING THE MISSION 5

CONDUCTING MISSION OPERATIONS 19

BEYOND THE NOMINAL MISSION 25

APPENDIX 1:Missions Supported from ESOC 31

APPENDIX 2: Glossary of Terms 32

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