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THE METEOSAT SYSTEM EUM TD 05
THE
METEOSAT
SYSTEM
EUM TD 05
Published by: EUMETSAT(European Organisation for theExploitation of MeteorologicalSatellites)©1998 EUMETSATDesign: Grigat und Neu
THEMETEOSATSYSTEMDecember 1998
..............................................Performance monitoring
Telecommunications
System frequencies
User frequencies
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PREFACE
1 OVERVIEW
2 OPERATIONAL ASPECTS
3 THE SATELLITES
The spacecraft
Structure
Power supply
Attitude and orbit control
The radiometer
The telescope
Scanning concept
Detectors
Passive cooler
4 THE GROUND SEGMENT
Ground segment facilities
Primary Ground Station
Overview
Antennas
Monitoring and control
Equipment
DCP Retransmission System
Back-up Satellite Control Centre
The Mission Control Centre
Overview
Core Facility
Monitoring and control
Image processing
Dissemination
User Station Display Facility
Data Collection System
Meteorological products extraction
Archive and retrieval facility
Other facilities
The main communications links
Back-up Ground Station
Foreign satellite data relay station
MDD up-links
Land-Based Transponder
User systems
5 IMAGE PROCESSING
Overview
Data pre-processing
Rectification
6 METEOROLOGICAL PRODUCTS
Segmentation
Segment processing
Meteorological products
System planning
Satellite orbits
Routine operations
Eclipse operations
Contingency planning
Satellites
Ground segment facilities
Joint contingency plans
Support to INDOEX
Support to MAP
Introduction
Objectives
Meteorological satellites
Future programmes
Meteosat history
The programmes
Meteosat services
Earth imaging
Image dissemination
Data collection and distribution
Meteorological Data Distribution
Meteorological and climatological
products
Archiving and retrieval
The MOSAIC concept
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TABLE OF CONTENTS
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10 IMAGE AND PRODUCTARCHIVE
System description
Historical data
Retrieval services
11 GLOBAL COORDINATION
CGMS
Coordinated services
Mutual support
Geostationary satellite systems
Polar satellite systems
12 METEOSAT INTO ORBIT
Satellite launches
Launchers
Launch site
Launch services
Launch activities
13 THE EUMETSAT USER SERVICE
Background
Addresses and points of contact
14 EUMETSAT PUBLICATIONS
List of FiguresList of TablesGlossary
Cloud motion winds
Sea surface temperatures
Cloud analysis
Upper tropospheric humidity
Clear sky radiances
Cloud top heights
Climate products
Climate data set
ISCCP
GPCP
Product quality
Quality control
Calibration
Quality monitoring
Distribution and archiving
EUMETSAT data policy
7 IMAGE DISSEMINATION
Dissemination system
High Resolution Image dissemination
Description
Primary Data User Stations
WEFAX dissemination
Description
Secondary Data User Stations
Operational aspects
Registration
Operational information
8 DATA COLLECTION SYSTEM
Data Collection Platforms
Meteosat Data Collection System
The International DCS
DCP Retransmission System
9 METEOROLOGICAL DATADISTRIBUTION
The requirement
The MDD system
Up-link sites
User stations
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Figure 1 Colour enhanced Meteosat image. Full earth disc images are generated each half-hour, day and night.
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PREFACE
This document serves as a general introduction to the facilities and services
provided by EUMETSAT through its Meteosat satellite system. It is a
publication in the Technical Documentation (TD) series and provides an
overview of the complete system, including both the ground segment and
space segment, as well as operational aspects and the services and products
of the system.
The basic service of Meteosat is the provision of images at 30 minute intervals
of an area covering all of Europe, the Middle East, the entire continent of
Africa, most of the North and South Atlantic oceans and some portions of
South America. In addition there are many other supporting services and
functionalities which this document seeks to introduce.
An earlier document with a similar name was published by the European
Space Agency and describes the Meteosat system in its initial configuration.
That original system served the user community well during the years since
the launch of the first Meteosat satellite in 1977, but after nearly two decades
of successful operations the time had come to completely update the ground
segment. The present document therefore describes the EUMETSAT ground
segment as brought into operation in December 1995.
It should be emphasised that, from the point of view of the user, there is a
strong degree of continuity with the earlier system. There is of course no
change to the satellites themselves and the basic products and services
formerly provided through ESOC are also continued, with enhancements
where appropriate.
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Figure 1.1 The EUMETSAT headquarters, which was inaugurated in 1995 and contains the Mission Control Centre
This chapter recalls the reasons whymeteorology and climatology are soimportant to the global economy and thecontribution of meteorological satellites tothose disciplines. The history of Meteosatis described briefly, and this is followed bya summary of the various Meteosatsatellite programmes and an introductionto the Meteosat services.
Introduction
Meteorological satellites have becomeessential for both meteorology and
climatology, continuing the two funda-mental concepts of data exchange andinternational cooperation which have beentraditional for more than 150 years. Theyprovide vital data at frequent intervals overwide areas, in the context of theinternational cooperation needed toensure adequate worldwide datacoverage. Cooperation exists at two levels.First at the European level through thosecountries which have come together toestablish EUMETSAT. This ensures the
continuity of the Meteosat system and theavailability of data over nearly one quarterof the planet. The second level ofcooperation is on a global scale, whichensures the availability of satellite dataover the entire Earth.
Meteosat is a European contribution to theglobal observing system required for bothmeteorology and climatology. Thefollowing chapters describe both thesystem itself and its place in the larger,worldwide context.
1OVERVIEW
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Objectives
The primary objective of the Meteosatsystem is to provide cost-effective satellitedata and related services which meet therequirements of the EUMETSAT MemberStates. To the greatest extent possible thesystem also addresses the requirementsexpressed by the World MeteorologicalOrganization. It offers services to allcountries of the world able to receive thedata and is therefore truly international inscope.
The data and services are mainly focusedon the requirements of operationalmeteorology, with the emphasis on supportto operational weather forecasting.However, the data are of use for all areasof this discipline, including marine,agricultural and aviation meteorology, aswell as, for example, climatology and themonitoring of planet Earth.
Precise and accurate weather forecastsare of much greater importance than theiruse for merely predicting if it will rain ornot during the next hours or days. Theyhave become essential for the transportindustry to ensure efficient and reliableoperations, for the construction andagricultural industries to scheduleactivities which may be affected byweather, and by the retail industry to planstocks of food and clothing for which thedemand varies according to the weather.The energy industries also vary theavailable capacity of their plants accordingto weather-dependent predictions ofdemands. Accurate weather forecasts aretherefore a strong contributor to theefficiency of the way in which manyindustries work and therefore a strongcontributor to national economies.
The need for climate data also has astrong economic justification. If weatherpatterns change, then agriculture will alsochange and this may have a profoundeffect, both on individuals and on theeconomies of nations. If sea levelschange, expensive coastal defences maybecome necessary, or populations mayhave to migrate. Where changes are
found to be due to human activity, majoractions may have to be taken to reversethe trends.
The variations in weather and climate haveenormous economic consequences whichare increasing as the world populationgrows and becomes more industrialised.The need to understand, monitor andpredict the weather and the changingclimate is becoming increasinglyimportant. The next section describes howsatellites contribute to this necessaryunderstanding.
Meteorological satellites
Meteorology and climatology are twodisciplines which require frequentobservational data at closely spacedlocations over the entire globe. Con-ventional, surface-based systems canobserve the atmosphere with greataccuracy but could not, under any con-ceivable circumstances, provide globalcoverage. By contrast, meteorologicalsatellites can and do provide thenecessary global coverage at the requiredintervals.
This became immediately evident with thefirst experimental weather satellite,launched by the USA into a low earth orbitin April 1960. For the first timemeteorologists could actually see thedistribution of weather systems over thesurface of the Earth and no longer had torely on inferences from widely scatteredconventional observations. Within adecade the USA had established the twoclasses of meteorological satellitesforming the basis of the systems whichhave been in operation since the late1970s. The first weather satellite was in alow earth orbit, the so-called polar orbit.Today these satellites fly at an altitude ofabout 850 km, circling the Earth every 100minutes. They provide detailed obser-vations of the atmosphere, the clouds andthe surface of the Earth itself, covering theentire globe twice during each twenty-fourhour period.
The polar satellites are complemented by
an equatorial ring of geostationarymeteorological satellites which fly at analtitude of 36,000 km and circle the Earthat the same speed as the Earth itselfrotates. These satellites are effectivelystationary over one point of the Earth andprovide images and other data on analmost continuous basis, each satellitecovering about one quarter of the Earth�ssurface. Meteosat is the Europeangeostationary meteorological satellitesystem.
Future programmes
With developments in the accuracy ofnumerical weather prediction, the need formore frequent and comprehensive datafrom space has evolved. This has led tothe present work on the Meteosat SecondGeneration (MSG) system. The newsatellites will be spin-stabilised like thecurrent generation, but with many designimprovements including a new radiometerwhich will produce images every fifteenminutes, in twelve spectral channels. TheMSG data will help in the swift recognitionand prediction of dangerous weatherphenomena such as thunderstorms, fogand explosive development of small butintense depressions.
In cooperation with EUMETSAT, theEuropean Space Agency (ESA) isresponsible for the development of the firstMSG satellite, planned for launch in theyear 2000.
The lack of observational coverage in partsof the globe such as the Pacific Oceanand southern continents has increased theimportance of polar-orbiting satellites innumerical weather prediction and climatemonitoring. The EUMETSAT Polar System(EPS), now in preparation, is the Europeancomponent of a joint European/US polarsatellite system. The Metop-1 satellite,developed in cooperation with ESA andplanned for launch in 2003, will carryEUMETSAT instruments. Later satellitesin the series will provide a service well intothe second decade of the 21st century.
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Figure 1.2aMeteosat visible image - The Meteosat visible channel measures solar radiation reflected from the Earth's surface. The oceans appear as darkareas, while land surfaces are grey and clouds white.
Meteosat-7 20. NOV. 1998 12:00 UTC VISS, VISN NOMINALSCAN (c) 1998 EUMETSATRect: R.T. Splines Area: AREA x1 y1 15000 p5000
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Figure 1.2bMeteosat infrared image - The Meteosat infrared channel measures thermal radiation emitted from surfaces. Dark regions represent warm areassuch as the oceans, land surfaces and low clouds. The white areas are cold and correspond to regions of high cloud or ice and snow.
Meteosat-7 20. NOV. 1998 12:00 UTC IR NOMINALSCAN (c) 1998 EUMETSATRect: R.T. Splines Area: AREA x1 y1 12500 p2500
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Figure 1.2cMeteosat water vapour image - The Meteosat water vapour channel measures thermal radiation from water vapour in the middle atmosphere. Thedark areas on the image correspond to areas of low atmospheric humidity.
Meteosat-7 20. NOV. 1998 12:00 UTC WV NOMINALSCAN (c) 1998 EUMETSATRect: R.T. Splines Area: AREA x1 y1 12500 p2500
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Meteosat history
The importance of satellites for bothmeteorology and more generalenvironmental issues was quickly realisedthroughout the world, leading to thedevelopment of satellite systems by thosecountries already in possession of, orwishing to establish, a space capability.Europe was no exception and thedevelopment of Meteosat followed aninitiative by France, which performed thefirst feasibility studies and undertook thepre-development of the radiometer for anew geostationary satellite. By 1972 thishad been established as a Europeanprogramme undertaken by eight par-ticipating States of the former EuropeanSpace Research Organisation (ESRO),which later became the European SpaceAgency (ESA). That programme led to thelaunch of the first Meteosat satellite on23 November 1977 and so to the start ofthe successful Meteosat system.
The first satellite provided a faultlessservice for two complete years until theradiometer failed in November 1979. Bythis time it had become an established partof the meteorological observing systemand major efforts were made, not only tolaunch a replacement satellite as soon aspossible, but also to establish amechanism for the operational long-termcontinuation of the satellite series.
The second satellite was launched in1981, the same year in which anIntergovernmental Conference of 17European countries was convened toconsider the matter of long-term continuity.The Conference decided that a newspecialised operational organisation wasneeded and, in a second session in 1983,agreed on the Convention for the futureEUMETSAT organisation. At the sametime, the Member States of the EuropeanSpace Agency agreed to initiate theconstruction of three further satelliteswhich could, in due course, be handedover to EUMETSAT.
By 1986 the EUMETSAT Convention hadbeen ratified and entered into force, initiallywith 16 Member States, the 17th Member,
Austria, joining in late 1993. Under theterms of an agreement signed with ESA inJanuary 1987, EUMETSAT assumedoverall financial responsibility for theMeteosat system while ESA continued tomanage the programme and operate thesatellites on behalf of EUMETSAT.
The agreement between ESA andEUMETSAT covered the period until theend of November 1995, and as that dateapproached it became clear that thefunding community should take a moredirect responsibility for its satellite systems.In 1991 the EUMETSAT Council took thedecision to establish its own independentground segment to replace the systemestablished by ESA in 1977. This newsystem was completed during 1995 andinstalled in the new EUMETSATheadquarters building, in Darmstadt,Germany, coming into operation inDecember 1995, as planned.
Meteosat services have continued withoutinterruption since 1981, the operationaltask having been assigned successivelyto Meteosat-3, an old prototype satellitelaunched in 1988 in order to ensurecontinuity, and then to Meteosat-4 and -5.From February 1997 until June 1998Meteosat-6 was used operationally beforeMeteosat-7 took over this responsibility.
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Figure 1.3 Schematic of the Meteosat system showing main services
The programmes
The Meteosat system is defined by anumber of overlapping programmes whichestablish the respective legal and financialframeworks during particular periods.These programmatic arrangements donot, in general, affect the user communityto any great extent, but are often referredto and are recalled here for the sake ofcompleteness.
The development and early operation ofMeteosat was covered by a series of ESAprogrammes until 1983. These ensuredthe development of the two original flightmodels, and of the prototype which waslater refurbished and flown as Meteosat-3.
When EUMETSAT was defined in 1983,ESA initiated the Meteosat Operational
since been agreed to extend MTPOperations until 2003 to provide anoverlap with the next generation ofsatellites (MSG). The current system asdefined in this document is thereforeoperated within the MTP framework.
Meteosat services
The main service provided by theMeteosat system is the generation ofimages of the Earth, showing its cloudsystems both by day and by night, andthe transmission of these images to theusers in the shortest practical time. Thereare several other important supportingservices summarised in the followingsections and described in more detail inlater chapters.
Programme (MOP) and from 1987 this wasconducted as a joint programme, underthe overall authority of EUMETSAT. Thisprogramme provided the framework for theconstruction and launch of three furthersatellites of a slightly modified design,Meteosat-4, -5 and -6, as well as theoperation of the complete system from1983 until the end of November 1995.
Since it had been decided that a newgeneration of satellites would not beimmediately available by the end of theMeteosat Operational Programme,EUMETSAT implemented the MeteosatTransition Programme (MTP), whichincludes provision and launch of a furthersatellite of the same design (Meteosat-7),the development of a new ground system,and routine operations from December1995 until the end of the year 2000. It has
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Earth imaging
The Meteosat radiometer is the principalpayload of the satellite. It provides thebasic data of the Meteosat system, in theform of radiances from the visible andinfrared parts of the electromagneticspectrum. These form images of the fullearth disc, as seen from geostationaryorbit. The radiometer operates in threespectral bands:
0.45 to 1.0 µm the visible band (VIS),used for imaging duringdaylight,
5.7 to 7.1 µm the water vapourabsorption band (WV),used for determining theamount of water vapourin the middleatmosphere,
10.5 to 12.5 µm the thermal infrared(window) band (IR),used for imaging by dayand by night and alsofor determining thetemperature of cloudtops and of the ocean'ssurface.
Earth images are generated at 30 minuteintervals. Each image is transmitted fromthe satellite to the Primary Ground Stationin Italy and relayed to the central facilitiesin Germany for further processing,distribution and archiving. As can be seenfrom the illustrations in this document,each image covers a substantial portionof the Earth, centred at the sub-satellitepoint, which is over the equator andnormally at 0o longitude. Using theseimages, meteorological features can beidentified and weather patterns tracked outto nearly 70o of great circle arc from thesub-satellite point. The distortedperspective introduced by the Earth'scurvature makes quantitative use of thedata less satisfactory at large distancesfrom the sub-satellite point, but quanti-tative products are generated routinely fordistances of at least 60o great circle arc.
anywhere in the world and are supportedthrough the International Data CollectionSystem (IDCS), which is coordinated byall of the geostationary meteorologicaloperators.
The DCP data are distributed by a varietyof means. The Meteosat DCP Retrans-mission System (DRS) broadcasts DCPdata directly to small user terminals, whilemeteorological data from many DCPs arealso transmitted over the GlobalTelecommunication System (GTS) of theWorld Meteorological Organization(WMO).
Meteorological Data Distribution
Additional Meteosat telecommunicationlinks are used for the transmission ofconventional meteorological data,including observations in meteorologicaltransmission codes and meteorologicalcharts containing both data analyses andforecasts. This is Meteosat�s uniqueMeteorological Data Distribution (MDD)service. The data are transmitted directlyto the satellite through three independentup-link sites located at meteorologicalcentres in France, Italy and the UK, andreceived by small user terminals.
Meteorological and climatologicalproducts
The Meteorological Products ExtractionFacility (MPEF), located in theEUMETSAT headquarters, makes use ofthe digital Meteosat image data to gene-rate a variety of quantitativemeteorological and climatologicalproducts. The meteorological productsinclude wind vectors obtained through theautomatic tracking of clouds as they movethrough the atmosphere. These CloudMotion Winds (CMW) are of greatimportance as inputs to the computermodels used for numerical weatherprediction, especially over tropical areaswhere there are few other observations ofatmospheric dynamics.
The facility also generates several climateproducts, including the data needed for theInternational Satellite Cloud Climatology
Image dissemination
Meteosat is equipped with high poweramplifiers used to transmit processedearth images and other meteorologicalinformation to user stations locatedanywhere within the field of view ofMeteosat (Figure 3.3).
The dissemination schedule is dominatedby the transmission of Meteosat imageryin all three spectral bands. However, theseimages are complemented by image datafrom other geostationary satellites,including the USA satellites GOES-E andGOES-W over the western Atlantic andeastern Pacific, Japan�s GMS satellite overthe western Pacific and the RussianGOMS satellite over the Indian Ocean.Images from other satellites will be addedas available, so that, with a single receiverand antenna system, the Meteosat userstation can acquire images covering mostof the globe.
The satellite carries two independentdissemination channels, used to transmitimage data with minimum delay to twoclasses of user station. Digital data aretransmitted to Primary Data User Stations(PDUS), which are intended to serve thelarger meteorological centres andresearch centres, while analogue data aretransmitted to the less complex SecondaryData User Stations (SDUS), widelyimplemented in smaller meteorologicalcentres as well as in many schools andby private individuals.
Data collection and distribution
Besides its main dissemination channels,Meteosat has a further 66 telecom-munication channels used for the relay ofenvironmental data from automatic orsemi-automatic Data Collection Platforms(DCP). Regional DCPs may be locatedanywhere within the Meteosat field of view(Figure 3.3) and are served exclusively bythe Meteosat Data Collection System(DCS), which relays the data through thesatellite to the Primary Ground Station foronward distribution. The so-called Inter-national DCPs are mobile platforms, suchas ships and aircraft. These can move
Figure 1.4 The MOSAIC Concept - an integrated user facility
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Project (ISCCP). Clouds form a vital partof the Earth's climate system; they help toinsulate the Earth from excessive solarradiation during the day and to reduce heatloss from the planet at night. The ISCCP,to which Meteosat contributes, has beensystematically storing global cloudcoverage parameters since 1983 and is amajor resource for climate studies.
These, and other important products, aredescribed in more detail in chapter 6.
Archiving and retrieval
The final component of the Meteosatsystem is the Meteorological Archive andRetrieval Facility (MARF) which is alsolocated within the EUMETSATheadquarters in Darmstadt. This facilityhas been archiving all Meteosat imagedata and derived products in digital formatsince December 1995. It provides acomprehensive data retrieval serviceincluding on-line access to the datacatalogues and other information.
The digital data are written to Digital LinearTape (DLT), each having the capacity tostore several days� images. This is adifferent medium from that used to storeimages before December 1995 and thesame retrieval mechanism cannot be usedto directly retrieve the data archived priorto that date. However, the older data,extending back to 1977, remain availableand can be retrieved using independentsystems. A project to systematicallytransfer the old data from some 40,000tapes and cartridges to the newer mediumwill take some years to complete.
The MOSAIC concept
MOSAIC (Meteosat Operational Systemsfor data Acquisition and InterChange) isnot a separate service but indicates howall of the real-time Meteosat services canbe brought together at a user site toprovide a consolidated "one-stop-shopping" service, meeting many of thedata requirements of small meteorologicalcentres.
A single antenna system may be used to
receive frequent image data fromMeteosat and from other satellites aroundthe world. The same antenna can be usedas a DRS terminal to receive environmen-tal data from data collection platformslocated within the region of interest andcan also form part of an integrated MDDterminal for the reception of othermeteorological data. Furthermore, moderncomputer workstations or personalcomputer systems can be used to display,store and print all of the data in a costefficient way. Low cost disk storagesystems may be used to provide a localimage archive. Many meteorologicalcentres have all of these facilities, whichmay be combined into a single integratedfacility.
satellite and this, in turn, was succeededby Meteosat-7 in June 1998. BeyondMeteosat-7 it is expected that a newgeneration of satellites will becomeoperational by the end of the year 2000.
Satellite orbits
Geostationary satellites fly at an altitudeof about 36,000 km over the equator. Theoperational Meteosat is normally locatedclose to 0o longitude, while the sparesatellite may be located at 10oE or W, fromwhich positions it can be used occasionallyfor test purposes. These are nominallocations but, because of the unevenshape of the Earth and the gravitationalinfluence of the moon and sun, the satellitedoes not stay precisely at the nominallocation. There are two major effects, thegradual increase in satellite inclination,which affects the north-south position, andsatellite drift, which affects the east-westposition.
The inclination of the satellite orbit isessentially the small angle between itsorbital plane and the equatorial plane ofthe Earth. This causes an apparent dailymotion of the satellite in a figure of eightpattern, centred over its nominal location.The maximum excursion north and southof the equator is the same as that of theinclination. While the inclination remainsless than 0.3o no action is taken to controlthis small movement. However, during thelifetime of the satellite the inclination tendsto increase, and at intervals it is necessaryto perform a so-called "north-southmanoeuvre" to adjust the orbital plane ofthe satellite.
North-south station-keeping is expensivein fuel and is often the limiting factor in thelifetime of the satellite. When the fuel isexhausted, the inclination increasescontinuously, at about 0.9o each year, andeventually the daily north-south movementmakes reception of line data by userstations difficult. The precise inclinationlimits for successful reception of datadepend on the location and characteristicsof the individual user stations.
A further orbital effect is caused by the
uneven shape of the Earth, in particularthe location of the deep oceans, whichcauses the gravitational field of the Earthto depart from a true spherical shape. Theeffect is as if the satellites were locatedon hills, which they may slide off, or invalleys, where they may remain stable.There are two stable locations ingeostationary orbit, one of which is overthe Indian Ocean (the other is over theeastern Pacific Ocean). Meteosat, at 0o
longitude, is on the gravitational slopeleading to this "hole" and gradually driftstowards the east. The satellite is normallymaintained within a defined box aroundits nominal location. When it reaches theeastern extremity of the permitted box, thethrusters are activated and the satellite ismoved back to the western extremity ofthe box, where the process starts again.This cycle repeats every few months(depending on the current size of thepermitted box), but is not expensive in fuelutilisation. While the satellite stays withinthis box it is compliant with systemspecification, and realignment of userantennas due to satellite movement is notnecessary.
Under the terms of an intergovernmentalagreement, spent satellites must beejected from geostationary orbit at the endof life. A small amount of station-keepingfuel has to be reserved for this purpose.Generally, the retired satellites are movedto a slightly higher orbit where they do notinterfere with the operation of othergeostationary satellites and where theymay remain indefinitely.
Routine operations
The Meteosat system is operated 24 hoursa day, every day of the year, so that ingeneral the user can expect to obtain thefull range of real-time operational serviceson a continuous basis. There is a highdegree of redundancy in the groundsystem in order to ensure reliableoperations. This includes duplicateantennas at the Primary Ground Stationin Fucino, parallel communications links,via commercial satellite, between Fucinoand the central facilities in Darmstadt, andduplicate computer networks in the central
This chapter describes some of theoperational aspects affecting theperformance of the system. It starts with ageneral description of the EUMETSATlaunch plans and policies, then brieflydescribes the factors affecting orbitaloperation. This is followed by a summarydescription of the mode of operation bothduring normal periods and during theeclipse seasons. The chapter concludeswith a section on contingency planning.
System planning
The Meteosat satellites after the pre-operational series have a specified lifetimeof five years and carry fuel reserves fororbit station-keeping sufficient for at leastsix or seven years of normal operations inorbit.
The EUMETSAT policy is to maintain onesatellite in operation at all times and tokeep a further operable satellite in orbitas an operational back-up. This ensuresa high level of reliability in the service. Anew satellite is launched close to the dateat which the older of the two satellitesalready in orbit is expected to exhaust itson-board station-keeping fuel. Should asatellite fail before the end of its nominallifetime, then every effort would be madeto launch a replacement as soon aspossible. The actual launch date woulddepend on the nature of the failure, theincident investigation and the availabilityof a spare satellite and launch vehicle.
A satellite rarely fails completely and theremay be occasions when there is no singlesatellite able to provide all of the requiredservices but there are two or moresatellites which each have someremaining functionalities. For such casesthe ground segment has the capability tooperate a so-called split mission, using twoor more satellites simultaneously toprovide the services normally supportedby a single satellite.
When EUMETSAT took over operationsat the end of 1995 the operational satellitewas Meteosat-5, while the in-orbit sparewas Meteosat-6. In February 1997Meteosat-6 took over as operational
15...............
2OPERATIONALASPECTS
facilities. Redundant components, at thecentral facilities or at the Primary GroundStation, can easily be brought into usewhen needed, either automatically orthrough software tools operated by theMission Controller at the EUMETSATheadquarters. These capabilities lead toa very high level of reliability with few gapsin operation even for routine maintenance.
The operational satellite does requireroutine maintenance periodically whichmay lead to temporary gaps in service ifno spare satellite is immediately available.On several occasions each year, orbitstation-keeping involves the use of thethrusters and may disturb a few imagesfrom time to time until the satellite isrestabilised.
Longer periods of data loss may occuronce or twice a year, especially in the earlystages in the life of the satellite, due tocontamination of the cold optics by a filmof ice. The ice comes from water carriedaloft with the satellite from the humidtropical launch site and held in the ther-mal blanket material protecting the variouscomponents of the satellite from extremesof temperature. The ice tends to migrateover periods of time to the coldest part ofthe spacecraft. This happens to be theoptical system used for the infrared andwater vapour channels, which is usuallymaintained at a temperature close to 90 K.Initially the contamination can be offset bya gain change in the radiometer buteventually the performance is reduced toan unacceptable level and it becomesnecessary to decontaminate the system.Heating the affected optics evaporates theice film and the radiometer is then allowedto slowly cool again. The whole processtakes about three days, during whichperiod the split mission capabilities of thesystem are usually exploited and the back-up spacecraft is used for image-taking,whilst the nominal operational spacecraft(which is undergoing decontamination) isused for dissemination and the provisionof the DCS.
In all cases where any scheduledmaintenance is to be performed, everyeffort is made to limit the interruptions to
...............16
services. Users of the disseminationservices are informed in advance of suchoperations by means of administrativemessages transmitted through the twoimage dissemination systems. Relevantinformation is also placed on the Internet.
Eclipse operations
In addition to the above, Meteosatoperations are affected for the two eclipseperiods each year when the satelliteundergoes an eclipse of the sun by theEarth. The two periods occur at the springand autumn equinoxes, and last fromapproximately 1 March to 15 April and from1 September to 15 October. During theseeclipse periods the satellite is in the Earth�sshadow for up to 70 minutes at aroundmidnight. During the actual eclipse, certainsatellite functions, including earth imagingand all dissemination services, includingHRI, WEFAX, DRS and MDD, areinterrupted in order to save power. TheDCS is not affected.
A secondary effect may occur during theeclipse period close to noon, when the sunis precisely in line with the satellite as seenfrom the Primary Ground Station. On theseoccasions the reception of images andDCP messages suffers from interference.The dissemination service is suspendedduring this time, increasing the poweravailable on the satellite for the imagingand DCP services, thereby reducing theeffects of interference. A similar effect willoccur at any Meteosat user station.
Contingency planning
As an operational agency EUMETSAT isconscious of the need for reliability ofservice and the continuity of data. IfMember States make operational use ofMeteosat data then they are entitled toexpect access to Meteosat data on everyday of the year on a continuous basis.EUMETSAT also wishes to ensure that themassive investment in the space segment- a satellite and its launcher cost more thanMEUR 100 (M$ 130) - is protected and isnot wasted through inadequate provisionin the ground segment. As a consequenceof these concerns, EUMETSAT has
developed comprehensive contingencyplans which seek to ensure continuity ofdata and a proper protection of theinvestment.
Satellites
As previously described, the first level ofcontingency planning is the policy tomaintain a second operable satellite inorbit at all times. The satellite may notalways be located in the most suitableposition for immediate use but couldnormally be reactivated up to fulloperational levels within a few days andbrought to the optimum orbital locationwithin one or two weeks at most. Thisneeds to be compared with the alternativeof launching a new satellite after failure ofthe primary satellite, which might easilytake 12 - 18 months, or even more.Therefore the provision of an in-orbit spareis a major component of the EUMETSATcontingency planning.
Ground segment facilities
In order to ensure the safety of thespacecraft there is appropriateredundancy of the key ground segmentfacilities. If the links to the Primary GroundStation (PGS), or the PGS itself, shouldfail, then the Back-up Ground Station(BGS) could be brought into useimmediately. The BGS would not provideany of the user-related services but, byrelay of telecommands and telemetrybetween the Mission Control Centre andthe spacecraft, could keep the spacecraftsafe until the problem is solved.
Similarly, if the Mission Control Centre(MCC) itself or the links between the MCCand PGS should fail, there is a Back-upSatellite Control Centre co-located with thePGS which could be used to monitor andcontrol the spacecraft but, again, wouldnot provide any user-related services.
The main communications links betweenthe MCC and the PGS are also duplicated.If the primary channel through acommercial satellite link should fail, thereis an alternative commercial satellite linkcapable of supporting the complete
Meteosat data transmission requirements.
In addition to this duplication of keyfunctionalities, an appropriate level ofduplication of workstations and othersubsystems is available to enable tasksto be easily reassigned to alternativesystems in case of need.
Joint contingency plans
The value of joint contingency plansbetween satellite operators wasdemonstrated when Europe was able tomove Meteosat-3 to a new location at75oW and operate it as a temporaryreplacement for a failed USA satellitebetween 1992 and 1995. Following thisinitiative, EUMETSAT agreed with itspartner in the USA that such provisioncould be reciprocated should a problemwith Meteosat occur which EUMETSATcould not solve. In this case a GOESsatellite from the USA could be moved toaround 5oW and operated there by theUSA. EUMETSAT would make thenecessary emergency provision (forexample, through the use of an oldMeteosat, or commercial satellite databroadcast) for the relay of the GOESimage data to its user community.
This is an extreme case which hopefullywill never need implementation but servesto illustrate the commitment of EUMETSATto operational data continuity.
Support to INDOEX
During the period from January until May1998, Meteosat-5 was slowly moved from10oW to 63oE in preparation for the supportto INDOEX (Indian Ocean Experiment).This is an international atmospheric fieldexperiment with participation from theEuropean Union, France, Germany, India,the Netherlands, Sweden, the UK and theUSA. The objective is to analyse thetransport of aerosols and pollutants bytropical atmospheric dynamics, theirevolution, and their interaction with clouds,radiation and climate. The experimentstarted in February 1998 with an intensivefield phase taking place in January to April1999.
17...............
3THE SATELLITES
The Meteosat satellite system is anexample of a very successful Europeanendeavour. First designed in the early1970s, the first model was launched in1977, and the same design is expected tobe in use until at least the end of 2003.The expected 26 years of operationalservice amply justifies the initialdevelopment effort. A few relatively minordesign changes were introduced afterMeteosat-3, and it is this updated satellitespecification which is now summarised.
The spacecraft
The overall size of the satellite is 2.1metres in diameter and 3.195 metres long.Its initial mass in orbit is 322 kg. Additio-nal to this dry mass is the hydrazinepropellant used for station-keeping,amounting to approximately a further 39 kgat the beginning of life. In orbit, the satellitespins at 100 rpm around its main axis,which is aligned nearly parallel to theEarth�s north-south axis.
Structure
Meteosat is composed of a main cylin-drical body, on top of which a drum-shapedsection and two further cylinders arestacked concentrically (Figure 3.1). Themain cylindrical body contains most of thesatellite subsystems, including theradiometer. Its surface is covered with thesolar cells which provide the electricalpower.
The cylindrical surface of the smallerdrum-shaped section is covered with anarray of radiating dipole antenna elements.Electronics within the drum activate theindividual elements in sequence, inreverse order to the satellite spin sense.This subsystem constitutes anelectronically-despun antenna whosefunction is to ensure that the maintransmissions in S-band are alwaysdirected towards the Earth. The twocylinders mounted on top of the drum aretoroidal pattern antennas for S-band andlow UHF respectively.
High Resolution Imagery will bedisseminated from Meteosat-5. Visiblechannel images will be disseminatedduring daylight with IR and WV imageryavailable day and night. Derived productswill be distributed on the GTS and alsoarchived in the MARF. The image data willalso be archived in the MARF.
Support to MAP
From its location of approximately 10oW,the stand-by satellite, Meteosat-6, will beused to support the Mesoscale AlpineProgramme (MAP) during the intensivefield phase from August until November1999. The Mesoscale Alpine Programmeis an international research initiativedevoted to the study of atmospheric andhydrological processes over mountainousterrain. It aims towards expandingknowledge of weather and climate overcomplex topography, and thereby toimprove current forecasting capabilities.
Meteosat-6 will be used to provide rapidscanning over the Alpine region duringinteresting weather features such as thebuildup of deeply convective clouds.During these episodes, up to eight mini-scans per half-hour slot will be scannedand the resulting images archived in theMARF for subsequent transfer to the MAPdatabase.
Figure 3.2a Schematic of the Meteosat radiometer
The optical chain of the Meteosat radiometer
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Figure 3.1 Meteosat
Power supply
The main cylindrical body of Meteosatfeatures more than eight thousand solarcells. These are mounted on five standardpanels, each covering an arc of the body,and one special panel containing the largeaperture for the radiometer telescope. Thesolar cells provide the main electricalpower for operating the satellite, althoughtwo rechargeable batteries are alsoavailable and are used during eclipseperiods and for peak power demands.
Attitude and orbit control
The spacecraft has four main (10 N)thrusters and two smaller thrusters ratedat 2 N. All are fed by hydrazine propellantcontained in three fully interconnectedspherical tanks. These are sized to holdenough fuel for at least six years of station-keeping under normal operationalconditions but the actual amount of fuelloaded is the maximum which the specificlaunch situation will allow. This subsystemis used to maintain Meteosat�s orientationin space, to make small adjustments to itsorbit as described on page 15 and to movethe satellite to a new nominal location ifrequired for operational reasons.
Two of the main thrusters are mounted withtheir thrust axes parallel to, but offset from,the satellite spin axis. They are used fornorth-south station-keeping for inclinationcontrol and as they can generate a torqueabout the spacecraft�s centre of gravitythey may also be used to makeadjustments to the satellite spin axis. Theother two main thrusters are radialthrusters acting with their axes at rightangles to the spin axis. They are used foreast-west station-keeping and are fired invery short bursts at the correct phase ofthe satellite spin cycle.
The small vernier motors act in a plane atabout 12o to that of the radial thrusters andare also offset from the centre of gravity inopposition to each other. The torquesthereby generated are used for spin ratecontrol.
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400 mm diameter
140 mm diameter
535 mm
VIS
0.45 - 1.0
Si photodiodes
3,650 mm
WV
5.7 - 7.1
HgCdTe
IR
10.5 - 12.5
HgCdTe
Table 3.2.1 Principal characteristics of the Meteosat radiometer
Primary aperture
Secondary aperture
Image band
Spectral range (µm)
Detector type
Focal length
Telescope
Image Channel
Spectral range (µm)
Line duration
Line recurrence
Image duration
Imaging recurrence
Transmission to ground
Table 3.2.2 Earth imaging details
Number of detectors(+ redundant)
Lines per image
Pixel samples per line
Instantaneous field ofview at sub-satellitepoint
Visible
0.45 -1.0
Water Vapour
5.7 - 7.1
Infrared
10.5 - 12.5
2.5 km 5 km 5 km
30 msec
600 msec
25 min
30 min
333 kbps (normal) 2.7 Mbps (burst mode)
5,000
5,000
2,500
2,500
2,500
2,500
1 (+1) 1 (+1)2 (+2)
In operation, the thrusters are activatedby telecommands transmitted through thePrimary Ground Station (PGS). Theinterval between manoeuvres is typicallyevery three months or so for east-weststation-keeping and about every six toeight months for inclination control.
Attitude measurement is achieved bymeans of two earth sensors whichroutinely search for the earth disc as thesatellite rotates. Each can provide a pulseat the earth-to-space transition and at thespace-to-earth transition. In addition, twosun-slit sensors each give one sun pulseat each revolution of the spacecraft. Theseattitude data are transmitted to the PGSevery 0.786 seconds to give constantinformation on attitude. They are also usedwithin the spacecraft to activate the imagesampling during each line of earth scan,and for directional control of theelectronically-despun antenna.
The radiometer
The Meteosat Visible and InfraRed Imager(MVIRI) is a high resolution radiometer(Figure 3.2a) with three spectral bands,and constitutes the main payload ofMeteosat. The instrument allows cont-inuous imaging of the Earth. Radiancedata from the full earth disc are acquiredduring a 25 minute period. This is followedby a five minute retrace and stabilisationinterval, so that one complete set of fullearth disc images is available every half-hour.
The telescope
The major feature of the radiometer opticalsystem is a scanning Ritchey-Chrétientelescope with a primary aperture of400 mm diameter and a focal length of3,650 mm for visible light. The telescopeis mounted on two flexible plate pivots andthe scanning mechanism is a highprecision jack screw driven by a step motorvia a gearbox. A series of mirrors is usedto collect the incoming visible, infrared andwater vapour radiation and to direct theradiation onto the correspondingdetectors.
Figure 3.2bScanning concept and distribution of detectors in the focal plane of the radiometer
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Scanning concept
The satellite scans the full earth disc withina 30 minute period. Scanning from east towest is achieved through the spin of thesatellite. Scanning from south to north isachieved by small incremental steps in thepointing of the telescope. At each satelliterotation during the imaging process, thespin clock delivers a signal to the scanningmotor electronics, which has the effect ofrotating the telescope through an angleof 1.25 x 10-4 radians.
By this means, with every rotation of thespacecraft, the telescope scans a new lineon the Earth approximately 5 km north ofthe previous scan line. By successive scansteps, the telescope is made to scanthrough 18 degrees in the direction fromsouth to north, generating a full earth scanof 2,500 lines in 25 minutes. The telescopethen retraces to its starting position in 2.5minutes, during which time a black bodycalibration of the infrared and water vapourchannels may be performed. A 2.5 minutestabilisation period allows for nutationdamping before the next scanning periodis initiated. Thus the radiometer generatesa new image in three spectral bands duringevery half-hour period.
The image radiance data are sampledelectronically 2,500 times as the telescopesweeps out each east-west line.Consequently, the infrared and watervapour images each comprise 2,500 linesof 2,500 picture elements (pixels). Thevisible channel is sampled 5,000 timesrather than 2,500 times, and there are twovisible detectors in operation. The visibleimage therefore comprises a total of 5,000pixels in each of 5,000 lines, the linesinterleaved between the two detectors.
Detectors
The optical visible, infrared and watervapour signals are converted intoanalogue electrical signals by the variousdetectors. These are divided into twosubsets: the visible detectors in one set,kept at ambient temperatures, and theinfrared and water vapour detectors in theother, cooled to below 90 K.
21...............
In total there are eight detectors:
a redundant pair for infrared imagery,
a redundant pair for water vapourimagery,
two redundant pairs for visible imagery.
At any given time, one infrared detector,one water vapour detector and one pairof visible detectors will be in operation.Since the detectors are distributed acrossthe focal plane of the radiometer theirrespective fields of view on the earth scenedo not coincide but are displaced relativeto each other. The misalignment due tothese displacements is corrected bycentral on-ground image processingbefore the images are distributed to users.Figure 3.2b indicates the effectivedisplacement for one Instantaneous FieldOf View (IFOV) in all available channelsof the satellite.
The size of the IFOV at the Earth�s surfaceof any detector is determined by the fieldof view of the detector and the distance tothe Earth�s surface. In the case of thevisible detectors, the field of view is 0.07mrad, giving an IFOV of about 2.5 km atthe sub-satellite point. The infrared andwater vapour detectors have a field of viewof 0.14 mrad, yielding an IFOV of about5 km at the sub-satellite point.
The output of each detector passesthrough an amplifier in which the gain maybe varied by a factor of 1.2 in 16 separatesteps. This feature, used occasionallywhen the infrared and water vapourdetectors become contaminated by ice,allows a rather coarse control to maintainthe output of the detectors.
Passive cooler
The infrared and water vapour detectorsand associated cold optics are cooled tothe required operating temperature of 90 Kor less by a passive cooler filling thesouthern face of the satellite (the end ofthe cylinder opposite to the antennas). Thecooler consists of two concentric blackcones with the apex of each pointing
inward to the satellite body. The colddetectors and cold optics are located atthe apex of the inner cone, which is thecoldest part of the spacecraft. Anequilibrium temperature is established,corresponding to a balance betweenthermal inputs, for example from insulationdeficiencies and from thermal conductionalong electrical leads, and thermal outputs,in the form of radiation to space from thesurface of the cooler.
The inner conical reflector is protectedfrom direct radiation from the sun andEarth by the secondary conical reflector,which serves both as a sun shield and asa further radiator to space.
Fine tuning of the detector temperature to90 K is ensured by a heater and thermistorfixed on the detector plate, giving aconstant temperature which ensuresconstant spectral sensitivity. During theeclipse season the satellite passes into theEarth�s shadow once a day and the ther-mal equilibrium of the cooler takes sometime to stabilise, leading to slight variationsin effective gain during these periods.
At rare intervals, heaters are used to raisethe temperature of the detectors far abovethe normal operating temperature. This isin order to evaporate ice or othercontaminants from the cooler and colddetector area and, as mentioned earlier,results in a loss of image-taking capabilityfor about three days.
Performance monitoring
A mechanism is provided within theradiometer to enable in-flight monitoringof the infrared and water vapour detectorperformances.
The black body mechanism consists of twoblack bodies located on opposite sides ofthe main optical path, one kept at ambienttemperature of about 290 K as a coldreference and the other heated to about50 degrees higher, as a warm reference.Two mirrors are mounted on a turningbracket which can be rotated by a torquemotor, such that, in one extreme positionthe infrared and water vapour detectors
...............22
Figure 3.3 Telecommunications coverage area
Telecommunications
will look at the cold reference, and in theother extreme position both detectors willlook at the warm reference. When theturning bracket is at rest in its centrallocation, normal earth and space viewimaging operations can be carried out.
The temperatures of the two black bodiesare telemetered to the Primary GroundStation together with the correspondingblack body imaging values, to give an
indication of calibration trends. The systemcannot provide information on absolutecalibration because during calibration themain radiometer optics are not in theoptical path.
The view of cold space obtained duringnormal imaging operations serves as apractical supplement to the on-boardcalibration mechanism. This has thefurther benefit of using the entire optical
system and thus serving to help calibratethe entire radiometer, not just thedetectors.
These on-board calibration techniques aresupplemented by vicarious calibration,using surface-based measurements,performed within the MeteorologicalProducts Extraction Facility (MPEF), asdescribed in chapter 6.
System frequencies
The spacecraft has a comprehensivetelecommunications capability which canbe described under two headings: thesystem frequencies used for raw imagereception, telemetry, telecommands andfor spacecraft orbit determination,described in this section; and the useraccessible frequencies, described in thefollowing section.
S-band in the range 2098 - 2110 MHz andL-band in the range 1675 - 1690 MHz arethe frequency bands used for system-related functions.
These include:
the transmission of the raw image fromthe spacecraft to the Primary GroundStation in Fucino,
the transmission of telemetry data fromthe spacecraft and telecommands to thespacecraft,
the transmission of DCP reports from thespacecraft to the Primary GroundStation,
the up-link of image dissemination datafrom the Primary Ground Station to thespacecraft,
the up-link of MDD data from a maximumof four ground stations,
the ranging signals transmitted betweenthe Primary Ground Station in Fucino,the spacecraft and the Land-BasedTransponder (located in FrenchGuiana), used for determination of theprecise location of the satellite in orbit.
Housekeeping telemetry data is alsorepeated in S-band, within the range 2200- 2300 MHz, but this redundant link ismaintained only as a back-up foremergencies and is not usually used.
The normal data rate of the raw image datafrom the spacecraft to the Primary GroundStation is 333 kilobits per second (kbps).This is achieved through on-board
23...............
buffering of each line of image data duringthe earth scan so that it may be transmittedduring the much longer period (20 timesas long) when the radiometer is viewingspace. If this on-board buffering is disabledthen an alternative direct transmissionwould be used at a data rate of 2.7 Mbps.
All of these transmissions are intended forpoint to point transmissions between thesatellite and the EUMETSAT PGS or BGSand are not broadcast for general use. Forsecurity and copyright reasons thetransmissions are not available to the usercommunity and are strictly reserved forsystem use.
User frequencies
Data transmissions to user stations formpart of the essential service of Meteosatand the frequencies used are, of course,available to registered users.
The L-band is for user-related functions,including the following frequenciesrequired to receive the specified Meteosatservice:
1691.0 MHz WEFAX analogue imagedissemination and theDCP RetransmissionSystem (DRS),
1694.5 MHz HRI digital imagedissemination, with afew WEFAXtransmissions,
1695.605 -1695.935 MHz Meteorological Data
Distribution, with up tofour channels spacedat 30 kHz.
In addition, Data Collection Platforms maybe given access to one of the 66 up-linkchannels in the UHF band between 402.0and 402.2 MHz with 3 kHz channelseparation. A channel and precise timeslots for data transmission are assignedto each individual DCP when registrationis accepted.
The Meteosat ground segment was almosttotally renewed during 1995 and broughtinto operation under the direct control ofEUMETSAT by December 1995. Thischapter describes this system.
Ground segment facilities
The Meteosat ground segment consistsof a number of major components atdifferent locations. These include:
the Primary Ground Station (PGS), usedfor all normal communications with thespacecraft,
the Mission Control Centre (MCC),where the complete system is moni-tored and controlled, and all dataprocessing is undertaken,
the major communications facilities,which connect the PGS in Italy with theMCC in Germany,
a Back-up Ground Station (BGS), usedin emergencies for monitoring andcontrol of the spacecraft,
the Lannion facility, used to up-linkimage data from satellites other thanMeteosat,
the up-link sites for the MeteorologicalData Distribution (MDD) service, usedto broadcast meteorological datathrough Meteosat,
the Land-Based Transponder (LBT),used for orbit determination,
the user facilities, located at user sitesand under the direct control of the rele-vant users.
These components are described in thefollowing sections.
Figure 4.1 Block diagram of the Meteosat system
...............24
4THE GROUNDSEGMENT
25...............
Primary Ground Station
Figure 4.2 Meteosat antennas at the Fucino Primary Ground Station
...............26
Overview
The Meteosat Primary Ground Station(PGS) is established in Fucino, Italy. It isa facility fully owned by EUMETSAT, butlocated within a commercially operatedcentre which includes a major antennafarm serving many satellite systems. Theactual site is in a wide valley in themountains some 150 km east of Rome.
The PGS serves as the main channel ofcommunications with the Meteosatsatellites and is an essential componentof the Meteosat system. The separateBack-up Ground Station (BGS) in Weil-heim, Germany, can be used in emer-gencies for satellite control purposes, butonly the PGS has the operationalcapability to support the main userservices, handling the raw imagetransmissions from the satellite, andtransmitting the processed images backthrough the satellite to the users. The PGSalso uniquely supports many other user-related functions and has the capability toact as the Back-up Satellite Control Centre(BSCC), in the event of severe problemsat the Mission Control Centre (MCC) inthe EUMETSAT headquarters or failure ofthe main communications links betweenthe MCC and PGS.
To accomplish these vital tasks aconsiderable amount of redundancy isincorporated in the station which, to a greatextent, can function completely auto-matically. No operating staff are normallyrequired at the PGS; engineering supportis available for maintenance purposes onlyduring normal working hours, while normaloperations are supervised by the MCC inDarmstadt.
Antennas
Two fully steerable 13.2 metre diameterparabolic antennas (Figure 4.2) arelocated at the PGS and used exclusivelyto support all communications withMeteosat. Each antenna is capable ofsupporting all the transmissions and datareception required for one Meteosatspacecraft and is used for telemetry andtelecommands, raw image reception,
processed image dissemination, the DataCollection System (DCS) and formonitoring the Meteorological DataDistribution (MDD) service. In addition, inorder to support the INDOEX service (asdescribed in chapter 2) a third slightlysmaller antenna was installed. Theantennas are situated within a few tens ofmetres away from a building usedexclusively for the Meteosat equipment.
Monitoring and control
The control of the PGS is actuallyexecuted by a local monitor and controlsystem located in Fucino and interactingwith the MCC in Darmstadt. The PGS canoperate in two different modes: remotely,under the control of the MCC; or throughuse of the system consoles in Fucino. Thisflexibility ensures maximum reliability incase of problems.
Equipment
The Fucino PGS is fully equipped tohandle two complete Meteosat spacecraft,with additional redundancy of keycomponents. The only exception to thisphilosophy is the support for the DCS,since it is envisaged that only onespacecraft would support this service.
All of the 66 DCP channels can besupported simultaneously, with primaryand back-up DCS systems.
DCP Retransmission System
Whilst most of the Meteosat dataprocessing is performed at the MCC inDarmstadt, one service is conductedentirely within the PGS, namely the DCPRetransmission System (DRS). DCPmessages received in the PGS areselected according to a pre-defined listand are then transmitted directly from thePGS to the spacecraft in the gaps betweenthe transmission of individual WEFAXimage dissemination formats. Thisnormally ensures the delivery of DCPmessages within four minutes ofobservation to any user having a DRS datareception terminal.
In addition to this DRS activity, all DCPmessages received at the PGS aretransmitted to the MCC for furtherprocessing and distribution, depending onthe requirements of each operator.
Back-up Satellite Control Centre
Also located at the PGS is a Back-upSatellite Control Centre (BSCC) estab-lished as a functional extension of theMCC in Darmstadt. In an emergency itcould be used in stand-alone mode tomonitor and control the spacecraft and thePGS, as well as to perform all essentialflight dynamics activities. It is not designedto support the Meteosat user services butdoes ensure the safety of the spacecraftuntil the problem is solved.
The BSCC can also be used in parallelwith the MCC to operate the PGS and tomonitor the spacecraft.
The Mission Control Centre
Figure 4.3a The Mission Control Centre
27...............
...............28
Overview
The Mission Control Centre (MCC) is adedicated facility incorporated in theoperations wing of the EUMETSATheadquarters building in Darmstadt,Germany. Dedicated communicationslinks connect it to the Primary GroundStation in Fucino and to the Back-upGround Station in Weilheim. The MCC isthe core of the Meteosat ground segment.The entire system is controlled from theMCC and this is also where all of thecentral processing is conducted. Facilitiesare installed for the monitoring and controlof the main components of the Meteosatsystem, including the spacecraft, thePrimary Ground Station, the maincommunications links and the MCC itself.Additional facilities are used to generatemeteorological products from theMeteosat image data, to archive theimages and products, and to monitor theend-to-end performance of the systemfrom the point of view of an end-user.
Core Facility
The Core Facility of the Mission ControlCentre provides the required monitoringand control facilities as well as basic imageprocessing, dissemination and the centraltasks of the Data Collection System. Thefacility is based on networks of powerfulcomputer workstations interconnectedthrough high speed local networks. Thereare two independent processing chains,each capable of supporting the processingload from one spacecraft. In addition thereis a development system for testingenhancements to the system and whichcan also be used as a basic back-upconfiguration. The main system incorp-orates around 26 operational workstations,able to keep two spacecraft in fulloperations and maintain a third in stand-by mode.
The total computer processing power issubstantial, actually exceeding the main-frame-based systems which it replaced. Itis arranged in a flexible way which can bereadily reconfigured in case of need butobviating the need for a permanent shiftof computer operators. The facility is
operated by a shift team of two controllers:the Satellite Controller and the GroundSegment Controller.
Monitoring and control
Meteosat monitoring and control functionsare executed within the Core Facility on aseries of linked computer workstations,each supporting standard display facilitiesused by the human operators. Theydisplay the necessary data using multipledisplay windows on one screen to providethe maximum amount of information in themost effective way. Display windows canbe selected according to the focus ofactivity at any given time and can easilybe remapped or duplicated to othermonitors to ensure the maximum flexibilityof operation.
The basis for the activity is the operationsplan which governs the routine cycle ofoperations and is monitored using theseconsoles. Routine spacecraft commandsare stored in the computer system andtransmitted automatically in accordancewith a pre-defined schedule. Non-routinecommands are transmitted in accordancewith pre-defined procedures. Thetransmission of the command sequencesand the telemetry from the spacecraft areall displayed on the consoles, colourcoded according to the status of the activityor result. Many hundreds of parametersare monitored for each spacecraft and,where any parameter exceeds pre-definedthresholds, an audible alarm is soundedand the parameter shown in distinctivecolours.
The configuration of the spacecraft and ofthe ground segment can also be shownon the monitors, with mimic displaysshowing which of the redundantcomponents are in active use and whichare available in stand-by mode. Thesystem permits reconfiguration of both thespacecraft and the ground station undersoftware control from the consoles. Thisenables remote operation and control ofthe Primary Ground Station in Fucino.
Image processing
Apart from the control function, a primarytask of the Core Facility is to process theimage data in real-time. Raw images arereceived from the Primary Ground Stationand processed line-by-line to removeimage imperfections. In particular, the datafrom the various on-board sensors arerealigned by resampling in order to makethe image from each set of detectorscoincide with the other images. At thesame time, the sampling removes theslight perturbations caused by themovement of the spacecraft, therebyrectifying the image so that it appears tocome from the nominal location of thespacecraft. Adjustments to the individualdata values are made according tocalibration information, then the image ispassed to the dissemination computers forimmediate relay to users and to themeteorological computers for furtherprocessing. Additional details are given inchapter 5.
Dissemination
The dissemination of Meteosat images isalso prepared in the Core Facility of theMission Control Centre using dedicatedworkstations. Processed images are cutinto individual formats ranging in size fromthe full earth disc to segments coveringEurope or smaller areas. These formatsare prepared according to a pre-definedschedule for the two Meteosat dis-semination channels and sent back downthe communications links to Fucino for up-link to the spacecraft and thencetransmitted to the users. The aim is tomake the processed image data availableto the users with the minimum of delay.Priority is given to the European sectors,for which the processing is completed anddissemination started within three minutesof the completion of image acquisition.Transmission of the image covering thefull earth disc starts four minutes later, andthe full earth imagery data are normallyfully available on the users� computersystems within a maximum of 20 minutesfrom the completion of image acquisition.Further details of the disseminationprocess are given in chapter 7.
Figure 4.3b A user display screen used for monitoring image reception
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User Station Display Facility
The dissemination of processed imagesis the primary function of the Meteosatsystem and is accomplished strictlyaccording to a pre-defined schedulematching user requirements. Theoperation of the central service ismonitored by the Core Facility of theMission Control Centre but there may berare occasions when the disseminatedimages may be subject to some distortionsor errors which are not readily detectedby the mission monitoring computers.
Alternatively, the system at a user site mayitself have problems and the user needs
to know if there is a problem with the localterminal or with the central facilities.Accordingly, a complete User StationDisplay Facility (USDF) is co-located withthe Core Facility. The USDF has its ownindependent antenna and receivingsystem, together with a display system(Figure 4.3b), so that it can independentlymonitor the final results of the imagedissemination system. The receivedimages from both communicationschannels are displayed on monitors andare used as a final check on quality. Thedetails of the received data are alsomonitored by the USDF computers andpassed back to the Core Facility foranalysis and comparison with the
transmitted data. By this means the systemoperators can, at any stage, be aware ofany problem in the complete system andtake immediate action to rectify thesituation.
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Data Collection System
The Data Collection System is alsooperated and monitored within the MissionControl Centre, in the Core Facility.Messages from Data Collection Platforms(DCP) are received in the Primary GroundStation from the 66 Meteosat com-munications channels and transmittedimmediately to the MCC in Darmstadt.There they are compared with the masterlist of expected DCP reports andprocessed and distributed as appropriate.This is performed entirely automatically.Malfunctions of the DCP, such as thoseDCPs which report outside the expectedfrequency range or assigned time slot, arereported to the DCP owner and themessage itself may be suppressed fromfurther dissemination. There are threemethods of onward dissemination, theprimary method of distribution being thepreviously-mentioned DCP Retrans-mission System (DRS). The other methodsare through the Global TelecommunicationSystem (GTS) of the World MeteorologicalOrganization (WMO), which is used totransmit environmental data tometeorological services throughout theworld, and via the Internet.
Meteorological products extraction
Another facility located within the MissionControl Centre is the MeteorologicalProducts Extraction Facility (MPEF). Thiscomprises another network of dedicatedworkstations which receives processedimages from the Core Facility and usesthem, together with ancillary data, toextract quantitative meteorological andclimatological products. The powerfulworkstations have a windowed userinterface similar to the other systems inthe Mission Control Centre. The operatorsuse these consoles to monitor theprogress of the automatic processing ofthe image data and the dissemination ofthe final products. Processing takes placewithin the hour following image receptionand most of the meteorological productsare distributed to the meteorologicalcommunity over the GTS. The products,including especially those of value forclimatology, are also stored indefinitely in
the Meteosat archive.
Archive and retrieval facility
The Meteorological Archive and RetrievalFacility (MARF) is the final component ofthe MCC. It includes dedicated computersystems for receiving the image data fromthe Core Facility and from theMeteorological Products Extraction Facilityand archives all data on digital opticaldisks. Equipment is available to provideimages to users in both digital andphotographic form in a variety of formats.
Other facilities
The main communication links
The two main components of the Meteosatground segment are the Mission ControlCentre, located in Darmstadt, Germany,and the Primary Ground Station, locatedin Fucino, Italy. They are connected byhigh speed data communication links toenable the necessary transmission of databetween them. Two independent links viacommercial satellites are provided in orderto ensure system reliability. Each has acapacity of 640 kbps and a bit error ratebetter than 1 in 108. The terminals for thisservice, including the necessary antennas,are located directly at the EUMETSATheadquarters and at the Primary GroundStation.
Back-up Ground Station
In the unlikely event of a complete systemfailure at the PGS, or of a complete failureof the main communications links betweenthe MCC and PGS, it would still benecessary to control the spacecraft toensure their safety. This would beestablished through use of the Back-upGround Station (BGS) located in Weilheimin Germany. This facility is not owned byEUMETSAT and is not dedicated toMeteosat operations, but an agreement isin place for its use in an emergency tocommunicate with and control thespacecraft. Control would continue to beperformed from the MCC in Darmstadt.The BGS does not include a capability forprovision of a full operational service for
the users, which would have to besuspended in an extreme case for theduration of the emergency.
Foreign satellite data relay station
The satellite ground station facilitiesowned and operated by the Frenchmeteorological service in Lannion havebeen associated with the Meteosat systemsince the start of operations in 1977.EUMETSAT provides and maintainsfacilities at Lannion for the relay of imagedata from additional satellites, tocomplement the Meteosat imagestransmitted from the PGS. The primaryrequirement is to relay images coveringthe western part of the Atlantic and theAmericas. These images are obtainedfrom the USA geostationary satelliteknown as GOES-E, which is usuallylocated at 75oW. A large antenna atLannion receives image data directly fromGOES-E at three-hourly intervals. Theimages are then reformatted on theLannion computers into the same formatas the normal Meteosat imagery. Trans-mission then takes place from Lannion tousers through the Meteosat spacecraft,exactly as if the transmission was ofMeteosat imagery.
Images from the Japanese GMS satellite,located at 140oE, are received at Lannionat three-hourly intervals using con-ventional land-based communicationssystems, and are also up-linked toMeteosat as if they were Meteosat images.Similar arrangements have beenestablished for geostationary image dataover the Indian Ocean now that a satelliteis on station there (Meteosat-5, in supportof INDOEX, as described in chapter 2).
Lannion thus contributes an essentialfeature of the overall Meteosat system.The Meteosat user station needs only asingle antenna and receiver to receivefrequent images of most of the world. Thiscapability is vital for checking the analysedfields of global numerical predictionmodels used in operational weatherforecasts. It is also vital for aviationmeteorology, since the images can beused to brief the air crews starting the
Figure 4.4 Main system data flows
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long intercontinental flights which nowspan half the globe without stopping.
MDD up-links
The Meteorological Data Distribution(MDD) service is a contribution byEUMETSAT to the Global Telecom-munication System of the WMO. Itspurpose is to distribute vital meteorologicaldata to WMO members, particularly inthose regions of the world whereconventional communications areinadequate. MDD consists of four indepen-dent dissemination channels on Meteosat,each of which can transmit data at 2,400bps, and the corresponding up-linkstations. Three up-link stations have beenimplemented by EUMETSAT at centres
where meteorological data are readilyavailable. These stations are located inthe meteorological centres in Toulouse(France), Rome (Italy) and Bracknell (UK).
Each of the up-link stations has atransmitter and antenna system andoperates autonomously, transmittingmeteorological data on a free-flowschedule. The MCC can monitortransmissions and can also initiate testtransmissions through the Meteosat PGSbut is not otherwise responsible for theroutine operation of the MDD up-links.
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The raw images as initially transmitted fromthe satellite are not in the most convenientformat for the end-user and are subject tovariations which need to be adjusted. Thisis achieved through the on-ground imageprocessing, which is designed to ensurethat the end-user has access to the bestpossible product in an easy to use format.This aspect of the central groundprocessing is described in the followingtext.
Overview
Meteosat image processing is conductedin two stages. First, the raw image isreceived at the Primary Ground Station inFucino and is subject to preliminary pre-processing before transmission to theMCC in Darmstadt. This ensures acommon format before transmission. Thepre-processing is completed within theCore Facility of the MCC, to ensure thatthe data are in the most suitable form forfurther use, removing and compensatingfor the artifacts inherent in the radiometerand satellite characteristics.
Data pre-processing
Image processing starts when individuallines of raw image data arrive at the PGSin Fucino. The raw image might be affectedby variations in the satellite spin rate andby, in rare cases, the use of satellite datatransmission in the high speed burst modeinstead of the nominal stretched data rate.These variations are eliminated at thePGS and the image data are transmittedin a standard format to the MCC. At thisstage the data are still in the original line-by-line format as sampled by thespacecraft radiometer. Each line iscomposed of 48-bit words containinginterleaved visible, infrared and watervapour data, corresponding to one linescan of the Earth by the radiometer.Housekeeping telemetry data are includedin the line data.
Within the MCC the first task is todemultiplex (i.e. separate) the raw imagedata into the different radiometric channels.The individual data elements are adjustedto a common calibration standard and a
5IMAGEPROCESSING
compensation is made for minor imagingsystem imperfections. The amplituderesponse of the two visible channels isalso normalised to the same levels.
Rectification
No geostationary satellite stays preciselyat the nominal location. Meteosat is noexception, and in normal operation maybe allowed to deviate by up to 1o of latitudeand 1o of longitude from its nominallocation. The spacecraft attitude withrespect to the Earth's axis also varies andthese variations cause undesirablechanges in the image perspective.Variations in spin rate and the instant ofline start also affect the image. All of thesevariations make the images appeardeformed with respect to a referenceimage which would be observed from thenominal location under nominalconditions. These deformations occurbetween images and, to a lesser extent,within one image. They make it muchharder for the user to locate individualscene features in terms of latitude andlongitude and make it impossible to createsuccessful image sequences to be viewedas animations. The rectification processis the means by which such effects areremoved.
The deviation of the actual image from theideal reference image is known as thedeformation. This is calculated for specificpoints within each image by means of amathematical model describing the orbitand attitude variations of the spacecraft.It is updated by means of measurementsmade on each image as it is received,including the automatic determination ofthe horizons and the location of keylandmarks. There is a feedback processfrom this stage to the image acquisitionprocess, whereby the sampling start timesare adjusted to ensure that subsequentimages are centred in the image frame tothe maximum extent possible.
The deformation vectors are then used toresample the original image data, usingtwo-dimensional interpolation, todetermine the value of all the pixelelements corresponding to the nominal
Land-Based Transponder
The Land-Based Transponder (LBT) ispart of the system used to determine theposition of the satellite in orbit by meansof ranging operations. Ranging signalstransmitted from the PGS are transmittedby the satellite back to the PGS (this isknown as two-way ranging). The LBT canalso receive the same signals and sendthem back to the PGS via the satellite(four-way ranging). The precise timing ofthese sequences is established at the PGSand transmitted to the MCC for use in orbitdetermination. A series of suchmeasurements, normally made at three-hourly intervals, is needed to establishaccurate orbit information.
The LBT consists of a parabolic 4.6 metreantenna and an equipment cabin,designed for continuous unattendedoperation in a tropical environment. It islocated near Kourou, in French Guiana,South America.
User systems
In order to make use of the Meteosatsystems, the user has to obtain thenecessary facilities. Images can bereceived on either a Primary Data UserStation (PDUS) or a Secondary Data UserStation (SDUS). Use of the MeteorologicalData Distribution service, or the DCPRetransmission System (DRS), requiresan MDD or DRS terminal, respectively.More than 400 registered PDUS andapproximately 2000 registered SDUS arein operation. In addition, there are about115 MDD user stations in use. There areapproximately 105 DCP operatorsoperating almost 1400 platforms.
All of these facilities are operated entirelyat the responsibility of the respectiveusers. EUMETSAT does not provide userfacilities but can provide a list of the manycommercial Meteosat equipmentsuppliers.
