SPACE FOR EUROPE BULLETIN/bulletin121.pdf · Felix Toran et al. 28 Frequency Management for ESA’s...

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ESA bulletin 121 - february 2005

SPACE FOR EUROPE

number 121 - february 2005

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w.esa.int

COVER BUL-121 4/19/05 10:25 AM Page 1

european space agency

The European Space Agency was formed out of and took over the rights and obligations of, the two earlier European Space Organisations – theEuropean Space Research Organisation (ESRO) and the European Organisation for the Development and Construction of Space Vehicle Launchers(ELDO). The Member States are Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain,Sweden, Switzerland and the United Kingdom. Canada is a Cooperating State.

In the words of its Convention: the purpose of the Agency shall be to provide for and to promote for exclusively peaceful purposes, co-operation amongEuropean States in space research and technology and their space applications, with a view to their being used for scientific purposes and for operationalspace applications systems:

(a) by elaborating and implementing a long-term European space policy, by recommending space objectives to the Member States, and by concertingthe policies of the Member States with respect to other national and international organisations and institutions;

(b) by elaborating and implementing activities and programmes in the space field;(c) by co-ordinating the European space programme and national programmes, and by integrating the latter progressively and as completely as

possible into the European space programme, in particular as regards the development of applications satellites;(d) by elaborating and implementing the industrial policy appropriate to its programme and by recommending a coherent industrial policy to the

Member States.

The Agency is directed by a Council composed of representatives of the Member States. The Director General is the chief executive of the Agency andits legal representative.

The ESA HEADQUARTERS are in Paris.

The major establishments of ESA are:

THE EUROPEAN SPACE RESEARCH AND TECHNOLOGY CENTRE (ESTEC), Noordwijk, Netherlands.

THE EUROPEAN SPACE OPERATIONS CENTRE (ESOC), Darmstadt, Germany

ESRIN, Frascati, Italy.

Chairman of the Council: P. Tegnér

Director General: J.-J. Dordain

agence spatiale européenne

L’Agence Spatiale Européenne est issue des deux Organisations spatiales européennes qui l’ont précédée – l’Organisation européenne de recherchesspatiales (CERS) et l’Organisation européenne pour la mise au point et la construction de lanceurs d’engins spatiaux (CECLES) – dont elle a repris lesdroits et obligations. Les Etats membres en sont: l’Allemagne, l’Autriche, la Belgique, le Danemark, l’Espagne, la Finlande, la France, l’Irlande, l’Italie,la Norvège, les Pays-Bas, le Portugal, le Royaumi-Uni, la Suède et la Suisse. Le Canada bénéficie d’un statut d’Etat coopérant.

Selon les termes de la Convention: l’Agence a pour mission d’assurer et de développer, à des fins exclusivement pacifiques, la coopération entre Etatseuropéens dans les domaines de la recherche et de la technologie spatiales et de leurs applications spatiales, en vue de leur utilisation à des finsscientifiques et pour des systèmes spatiaux opérationnels d’applications:

(a) en élaborant et en mettant en oeuvre une politique spatiale européenne à long terme, en recommandant aux Etats membres des objectifs enmatière spatiale et en concertant les politiques des Etats membres à l’égard d’autres organisations et institutions nationales et internationales;

(b) en élaborant et en mettant en oeuvre des activités et des programmes dans le domaine spatial;(c) en coordonnant le programme spatial européen et les programmes nationaux, et en intégrant ces derniers progressivement et aussi

complètement que possible dans le programme spatial européen, notamment en ce qui concerne le développement de satellites d’applications;(d) en élaborant et en mettant en oeuvre la politique industrielle appropriée à son programme et en recommandant aux Etats membres une politique

industrielle cohérente.

L’Agence est dirigée par un Conseil, composé de représentants des Etats membres. Le Directeur général est le fonctionnaire exécutif supérieur del’Agence et la représente dans tous ses actes.

Le SIEGE de l’Agence est à Paris.

Les principaux Etablissements de l’Agence sont:

LE CENTRE EUROPEEN DE RECHERCHE ET DE TECHNOLOGIE SPATIALES (ESTEC), Noordwijk, Pays-Bas.

LE CENTRE EUROPEEN D’OPERATIONS SPATIALES (ESOC), Darmstadt, Allemagne.

ESRIN, Frascati, Italy

Président du Conseil: P. Tegnér

Directeur général: J.-J. Dordain

Editorial/Circulation OfficeESA Publications DivisionESTEC, PO Box 299, Noordwijk2200 AG The NetherlandsTel.: (31) 71.5653400

EditorsBruce BattrickBarbara Warmbein

Design & LayoutIsabel Kenny

AdvertisingBarbara Warmbein

The ESA Bulletin is published by the European SpaceAgency. Individual articles may be reprinted providedthe credit line reads ‘Reprinted from ESA Bulletin’, plusdate of issue. Signed articles reprinted must bear theauthor’s name. Advertisements are accepted in goodfaith; the Agency accepts no responsibility for theircontent or claims.

Copyright © 2005 European Space AgencyPrinted in the Netherlands ISSN 0376-4265

www.esa.int

Cover: The surface of Titan, as seen for the first time byESA’s Huygens probe. See article on page 6.

INSIDE COVER-B121 4/19/05 9:56 AM Page 2

Cluster –A microscope and a telescope

for studying space plasmas

Europe Arrives at the NewFrontier – The Huygens landing on Titan

Europe Arrives at the New Frontier – The Huygens landing on Titan 6

Cluster– A microscope and a telescope for studying space plasmasHarri Laakso et al. 11

AmerHis:The First Switchboard in SpaceManfred Wittig et al. 21

Satellite Navigation, Wireless Networks and the InternetFelix Toran et al. 28

Frequency Management for ESA’s MissionsEdoardo Marelli & Enrico Vassallo 36

www.esa.int esa bulletin 121 - february 2005 1

Columbus: Ready for the International Space StationBernado Patti et al. 46

Software Engineering: Are we getting better at it?Michael Jones 52

‘Maxwell’– A New State-of-the-Art EMC Test FacilityJean-Luc Suchail, Alexandre Popovitch & Philippe Laget 59

Programmes in Progress 64

News – In Brief 80

Publications 88

AmerHis:The FirstSwitchboard in Space

bulletin 121 - february 2005 Contents

‘Maxwell’– A New State-of-the-Art EMC TestFacility

Frequency Managementfor ESA’s Missions

Satellite Navigation, WirelessNetworks and the Internet

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6 11 21

CONTENTS-B121 3/3/05 10:05 AM Page 3

Europe Arrives at theNew Frontier– The Huygens Landing on Titan

Lebreton 3/3/05 10:08 AM Page 6

esa bulletin 121 - february 2005www.esa.int 7

Europe at the New Frontier

O n 14 January 2005, after a marathon seven-year journey through the Solar System aboard the Cassini spacecraft, ESA’s Huygensprobe successfully descended through the atmosphere of Titan, Saturn’s largest moon, and landed safely on its surface. It wasmankind’s first successful attempt to land a probe on another world in the outer Solar System.Following its release from the Cassini mothership on 25 December, Huygens reached Titan’s outer atmosphere after 20 days and a 4 millionkilometre cruise. The probe started its descent through Titan’s hazy cloud layers from an altitude of about 1270 km at 09:06 UTC. Duringthe following three minutes, Huygens had to decelerate from 18 000 to 1400 km per hour.

A sequence of parachutes then slowed it to less than 300 km per hour. At a height of about 160 km, the probe’s scientific instruments wereexposed to Titan’s atmosphere for the first time and Huygens started to transmit its radio signal to Cassini at 09:12 UTC (spacecraft eventtime). The Huygens radio signals also arrived on Earth, but 67 min later as a faint tone that was detectable by large radio telescopes. Atabout 120 km altitude, the main parachute was jettisoned and replaced by a smaller one to complete the descent. The Huygens radio signalwas detected on Earth at about 11:20 CET by the 110-m Robert Byrd Green Bank Telescope in West Virginia. About 2 hours later, the probe’ssignal was picked-up by telescopes in Australia, which indicated that Huygens had landed and continued to transmit after landing. Cassinilistened to Huygens for 4h 36 min and then transmitted the Huygens data to Earth via NASA’s Deep Space Network once the Huygensmission was over. The first scientific data arrived at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany at 17:19 CET, having also taken 67 min to travel across space. An hour later, the data indicated that Huygens had landed safely at 12:39 CETand had transmitted data for 72 min from Titan’s surface.

Thirty-image composite of Titan’s surface fromaltitudes varying between 13 and 8 kilometres(Credit: ESA/NASA/JPL/University of Arizona)


Science

8 esa bulletin 121 - february 2005 www.esa.int

The Descent Huygens was expected to provide the firstdirect and detailed sampling of Titan’satmospheric chemistry and the firstphotographs of its hidden surface, and tosupply a detailed ‘weather report’. One ofthe main reasons for sending Huygens toTitan was that its nitrogen atmosphere, richin methane, and its surface may containmany chemicals of the kind that existed onthe young Earth. Combined with theCassini observations, Huygens couldtherefore deliver an unprecedented view ofSaturn’s mysterious moon.

The view from thirteen kilometres highThe picture on the previous page is acomposite of 30 images taken by Huygensfrom an altitude varying from 13kilometres down to 8 kilometres as theprobe was descending towards its landingsite. At that stage of its descent, Huygenswas dropping almost vertically at a speedof about 5 metres per second and driftinghorizontally at about 1.5 metres persecond, just a leisurely walking pace. Thecomposite image covers an area extendingout about 30 kilometres around the probe.

The final descent and the landing siteAs soon as Huygens had touched down onTitan's surface, some 15 scientists in theDescent Trajectory Working Group werehard at work to determine the location ofthe landing site. Apart from being neededto understand where exactly the probe hadlanded and to provide a reference traject-ory to analyse and interpret the Huygensdata set, having such a profile available fora probe entering the atmosphere of anotherSolar System body is extremely importantfor future space missions.

"The ride was bumpier than we thought itwould be," said Martin Tomasko, PrincipalInvestigator for the Descent Imager/Spectral Radiometer (DISR), the instrumentthat provided Huygens' stunning, imagesamong other data. The probe rocked morethan expected in the upper atmosphere.During its descent through high-altitudehaze, it tilted at least 10 to 20 degrees, whileonce below the haze layer it was morestable, tilting less than 3 degrees. TheHuygens scientists are investigating theprobe’s atmospheric environment during itsdescent in order to explain the bumpy ride.

The bumpy ride was not the onlysurprise during the descent. Scientists hadtheorised that the probe would drop out ofthe haze at between 70 and 50 kilometresaltitude. In fact, Huygens began to emergefrom the haze when only 30 kilometresabove the surface.

When the probe landed, it was not with athud or a splash, but a 'splat' – it had landedin Titanian 'mud'. "I think the biggestsurprise is that we survived landing andthat we lasted so long," said DISR teammember Charles See. "There wasn't even aglitch at impact. That landing was a lotfriendlier than we anticipated."

When the mission was designed, it wasdecided that the DISR's 20-Watt landinglamp should turn on 700 metres above thesurface and illuminate the landing site foras long as 15 minutes after touchdown. "Infact, not only did the landing lamp turn onat exactly 700 metres, but also it was stillshining more than an hour later, whenCassini moved beyond Titan's horizon forits ongoing exploratory tour of the giantmoon and the Saturnian system," saidMartin Tomasko.

“This is a great achievement for Europe and its US partners in this ambitiousinternational endeavour to explore the Saturnian system,” said Jean-Jacques Dordain,ESA’s Director General. “The teamwork in Europe and the USA, between scientists,industry and agencies has been extraordinary and has set the foundation for thisenormous success”.

“Titan was always the target in the Saturn system where the need for ‘ground truth’from a probe was critical. It is a fascinating world and we are now eagerly awaitingthe scientific results,” said Professor David Southwood, Director of ESA’s ScientificProgramme.

“The Huygens scientists are all delighted. This was worth the long wait,” said Dr Jean-Pierre Lebreton, ESA Huygens Mission Manager and Project Scientist.

“Descending through Titan was a once-in-a-lifetime opportunity and today’sachievement proves that our partnership with ESA was an excellent one,” saidAlphonso Diaz, NASA Associate Administrator of Science.

Panorama of Titan’s surface taken from a height of 8 kilometresduring Huygens’ descent (Credit: ESA/NASA/JPL/University ofArizona)



The First Scientific ResultsMore than 474 megabits of data werereceived in 3 hours 44 minutes fromHuygens, including some 350 picturescollected during the descent and on theground, which revealed a landscapeapparently modelled by erosion, withdrainage channels, shoreline-like features,and even pebble-shaped objects on thesurface.

The atmosphere was probed and sampledfor analysis at altitudes from 160 km toground level, revealing a uniform mix ofmethane with nitrogen in the stratosphere.The methane concentration increasedsteadily in the troposphere and down to thesurface. Clouds of methane were detected atabout 20 km altitude, and methane or ethanefog near the surface. During the descent,sounds were recorded in order to detectpossible distant thunder from lightning,providing an exciting acoustic backdrop toHuygens’ descent.

As the probe touched down, itsinstruments provided a large amount of dataon the texture of the surface, whichresembles wet sand or clay with a thin solidcrust, its composition, mainly a mix of dirtywater ice and hydrocarbon ice, resulting in adarker ‘soil’ than expected. The temperaturemeasured at ground level was about minus180 degrees Celsius.

Spectacular images captured by the DISRreveal that Titan has extraordinarily Earth-like meteorology and geology. Images haveshown a complex network of narrowdrainage channels running from brighterhighlands to lower, flatter, dark regions.These channels merge into river systemsrunning into lakebeds featuring offshore'islands' and 'shoals' remarkably similar tothose on Earth.

"We now have the key to understandingwhat shapes Titan's landscape," said DrTomasko, adding: "Geological evidence forprecipitation, erosion, mechanical abrasionand other fluvial activity says that thephysical processes shaping Titan are muchthe same as those shaping Earth."

Data provided in part by the GasChromatograph and Mass Spectrometer(GCMS) and Surface Science Package(SSP) support Dr Tomasko's conclusions.Huygens' data show strong evidence forliquids flowing on Titan. However, the fluidinvolved is methane, a simple organiccompound that can exist as a liquid or gas atTitan's very cold temperatures, rather thanwater as on Earth. Titan's rivers and lakesappear dry at the moment, but rain mayhave occurred not long ago.

Deceleration and penetration dataprovided by the SSP indicate that thematerial beneath the surface's crust has theconsistency of loose sand, possibly theresult of methane rain falling on the surfaceover eons, or the wicking of liquids frombelow towards the surface.

Heat generated by Huygens warmed thesoil beneath the probe and both the GCMSand SSP instruments detected bursts ofmethane gas boiled out of surface material,reinforcing the evidence for methane'sprincipal role in Titan's geology andmeteorology.

In addition, DISR surface images showsmall rounded pebbles in a dry riverbed.Spectra measurements (colour) areconsistent with a composition of dirty waterice rather than silicate rocks. However, theseform a rock-like solid at Titan'stemperatures.

Titan's soil appears to consist at least inpart of precipitated deposits of the organichaze that shrouds the planet. This darkmaterial settles out of the atmosphere.When washed off high elevations bymethane rain, it concentrates at the bottomof the drainage channels and riverbedscontributing to the dark areas seen in DISRimages.

Stunning new evidence based on findingatmospheric argon 40 strongly suggests thatTitan has experienced volcanic activitygenerating not lava, as on Earth, but waterice and ammonia.

Thus, while many of Earth's familiargeophysical processes occur on Titan, thechemistry involved is quite different.Instead of liquid water, Titan has liquidmethane. Instead of silicate rocks, Titan hasfrozen water ice. Instead of dirt, Titan hashydrocarbon particles settling out of theatmosphere, and instead of lava, Titanianvolcanoes spew very cold ice. Titan istherefore an extraordinary world, havingEarth-like geophysical processes operatingon exotic materials in very alien conditions.

“This is only the beginning; these datawill live for many years to come and theywill keep the scientists very very busy", saysJean-Pierre Lebreton, ESA's HuygensProject Scientist and Mission Manager.

The Cassini-Huygens mission is acooperative endeavour by NASA, ESA andASI, the Italian space agency. The JetPropulsion Laboratory (JPL), a division ofthe California Institute of Technology inPasadena, designed, developed, assembledand is operating the Cassini orbiter, whileESA was responsible for the Huygensatmospheric probe. r

Europe at the New Frontier

The first colour image of Titan’s surface, with centimetrescales superimposed to characterise surface features(Credit: ESA/NASA/JPL/University of Arizona)


Courtesy of AGU/D.N. Baker, Univ. of Colorado

Laakso 3/3/05 10:13 AM Page 10

esa bulletin 121 - february 2005 11

Cluster

T he four-satellite Cluster mission serves as both a‘microscope’ and a ‘telescope’ for magnetosphericscientists. Using its suite of state-of-the-artinstruments, it is providing a close-up view of complex small-scale physical processes occurring around the Earth. Theseprocesses are often reflections of other, sometimes violentprocesses that are taking place much further away from ourspacecraft, which means that Cluster also serves as a‘telescope’ for observing those more distant processes.

IntroductionWe have been investigating the Earth’s magnetospherewith space probes for nearly 50 years, allowing us todraw a general picture of the space environment thatsurrounds our planet. The origin of the magnetospherelies in the Earth’s internal magnetic dynamo, andwithout the solar wind the picture would be quitesimple. However, with the solar wind – a magnetisedsupersonic stream of electrons and ions continuouslyescaping from the Sun, which interacts with the Earth’smagnetic field – the picture becomes highly complex.Under the solar wind’s influence, the shape of theEarth’s natural magnetic field lines is transformed froma dipolar form into a large tail-like structure which is

Cluster– A microscopeand a telescopefor studyingspace plasmasHarri Laakso, Philippe Escoubet, Hermann OpgenoorthDirectorate of Scientific Programmes, ESTEC, Noordwijk,The Netherlands

Juergen Volpp, Siegmar PallaschkeDirectorate of Operations and Infrastructure, ESOC,Darmstadt, Germany

Mike HapgoodRutherford Appleton Laboratory, Didcot, United Kingdom


Science


called ‘the magnetosphere’ (see adjacentsketch).

At the same time as the solar wind isdistorting the magnetosphere’s outerboundaries, the ultraviolet and X-raysemanating from the Sun are ionising theEarth’s upper atmosphere, making it highlyelectrically conductive. This region, knownas ‘the ionosphere’, is coupled to themagnetosphere via the Earth’s magneticfield lines. Above 1000 km altitude fromthe ground, the neutral and plasma particledensities are sufficiently low for themedium to be fully conductive. At higheraltitudes in the magnetosphere, theresistance in the plasma can suddenlyincrease in certain localised regions,giving rise to some of the most fascinatingprocesses that occur in space.

The Sun-Earth ConnectionMany details of how our terrestrialenvironment responds to variations in solarradiation and solar wind, and theirimplications for humans and man-madetechnologies in space and on the ground,remain unresolved. Understanding andpredicting such conditions in spacerequires multi-point measurements atcritical locations in the magnetosphere andsolar wind.

Basically all of the unresolved Sun-Earth connection issues can be groupedunder three fundamental questions:

(i) How and why does the Sun vary?(ii) How does the Earth’s environment

respond to such variations? (iii) What are the consequences for

humanity?

The Cluster mission objectives belongprimarily to the second category.

The four Cluster satellites have beenorbiting the Earth since August 2000, andtheir observations have improved and ofteneven fundamentally changed our previousunderstanding of near-Earth space. Theprimary aim of the mission is tocharacterise and model the near-Earthplasma regions and boundaries, as well asto understand the physical processes takingplace there. Cluster cannot solve all of the

mysteries of the Sun-Earth connection, butit is providing us with the first real 3-Dmeasurements in space that allow us toseparate the temporal and spatial features.Some of the key issues are related to theexistence and characteristics of magneticreconnection, plasma turbulence, andcharged-particle acceleration. These arethe main processes that control the transferof energy, momentum and particles fromone region of space to another – forinstance, between the solar wind and themagnetosphere, or between the magneto-sphere and the Earth’s atmosphere.

Electric CurrentsOne of the most difficult measurements inspace is the determination of the electriccurrents that extend over vast distances inthree dimensions. The magnitudes of thecurrents are very large, typically in therange of 1-10 million Amperes, but as thecross-sections of the current systems arealso large, Cluster needs to be able todetect very weak current densities,typically not more than a few 100 nanoAmperes per square metre.

In principle, the currents can be

estimated by calculating the fluxes ofelectrons and ions, but in fact the onlyreliable method in space is to use Ampere’sLaw. This technique is based on accuratemagnetic-field measurements at fourpoints, from which one can calculate theelectric current crossing the Clusterconstellation. The Cluster spacecraft carryvery stable and sensitive magnetometersthat can provide the requiredmeasurements. Unfortunately, this doesnot always work in practice, either becausethe physics turns out to be more complexthan described by Ampere’s Law, orbecause the separations between the fourspacecraft are not optimal with respect tothe size of the current region being studied.By varying the spacecraft separationsduring the mission, we have been able todetermine the strengths and directions ofthe electric currents in many regions, andthese data are of fundamental importancefor magnetospheric modelling.

Electric FieldsAnother difficult task is the measurement of electric fields, which are needed tounderstand how the charged particles flow

A sketch of the Earth’s magnetosphere; the Sun is to the left. The distance of the magnetopause on the dayside is approximately 10 Re(where Re is the Earth’s radius, equal to 6370 km), while the bow shock is at 14 Re. The long tail of the magnetosphere in the nightsidecan continue for more than 100 Re. Cluster orbits the Earth at between 4 and 20 Re distance, during which the four satellites encountermost of the key magnetospheric regions. The orbit shown in red is for February-March, and half a year later the apogee is in themagnetotail



Cluster

Comparative Magnetospheres

By understanding how the Earth’s magnetosphere functions and how it is driven by the energy from the solar wind, we can alsobetter model other magnetosphere-like systems that are common not only in our own Solar System but throughout the Universein stars, galaxies, etc. More than 99% of material in space appears as plasma, or an electrically charged gaseous medium. Thereare a multitude of complex physical processes occurring in such a medium, and our geospace is the only place where we canmonitor and investigate them in-situ. The observational problem, however, is that the processes occur in spatially limited regions,their locations are constantly moving, and they occur for only short periods. One therefore has to be in the right place at the righttime, which is very difficult, and so one needs to collect large data sets over various time and spatial scales in many locationsin order to fully understand, model and predict the occurrence of the processes.

Having some knowledge of how the Earth’s magnetosphere behaves, it is fascinating to visit other magnetospheres to observesimilarities or dissimilarities between them and the Earth. Venus and Mars (and perhaps Pluto) are the only planets in the Solar

System that have no magnetospheredue to the lack of an internal magneticdipole. In such cases, the solar windinteracts directly with the atmosphere.Single space probes have been used toget a view of the magnetospheres ofthe other planets, and theinterpretation of such data is mademuch easier by observing theprocesses in detail first at the Earth,especially using multiple satelliteslike Cluster.

The Sun also has its own magneto-sphere, called the ‘heliosphere’. Thesolar wind expands to very largedistances of the order of 100–1000 AUin the Solar System (1 AU is thedistance between the Sun and theEarth), and this expansion iseventually stopped by the pressure ofthe interstellar plasma. Currently afew satellites launched in the early1970’s are approaching the edge of theheliosphere and will soon be able tostudy the coupling between it and theinterstellar wind, which is somewhatsimilar to the interaction between theEarth’s magnetosphere and the solarwind.

Magnetospheres, or magnetized plasma regions, are common in our Universe. These examples show simple models for Mercury, Earth, pulsar, and Jupiter, whose sizes are within a few orders ofmagnitude. However, similar systems also occur on larger scales, such as the heliosphere formed by the solar magnetic field as well as the galaxy NGC 1265 (courtesy of C. O’Dea & F. Owen,NRAO/AUI).


Science


The Cluster Mission and Its Instruments

Cluster is the first space physics mission to be made up of several identical spacecraft. Each of them carries a complete suite ofinstruments to measure particles, magnetic fields, electric fields, and electromagnetic waves. In addition a unique spacecraftpotential-control instrument (which keeps the electrical potential of the spacecraft themselves at typically 5-7 Volts) allows thescientists to monitor low-energy electrons and ions in the plasma, which would otherwise not be possible.

In any region of the magnetosphere the spatial scales (i.e. the distances over which the plasma characteristics change) appear to behighly variable. This complexity is increased by varying time scales, which makes sound interpretations highly challenging. HavingCluster measurements at four different points is fundamentally important here, although ideally many more than four satellites areneeded to measure different scales simultaneously. So, having only four satellites, it is essential to change their separationsregularly, because the most important scale length for one region can be completely wrong for another. For changing the sidelengths of the tetrahedron-shaped formation in which the Cluster satellites fly, each satellite originally had 63 kg of fuel onboardto perform the necessary manoeuvres; at the moment, approximately half of that fuel is left. A change of constellation from onetetrahedron to another requires a complicated set of over 40 individual manoeuvres that last approximately 6 weeks. During thefirst two years of the mission, two constellation changes were performed annually. Since 2003, only one constellation change peryear has been made.

The triangles show schematically the size of separation distances of the four Cluster satellites, with the actual separation distances in kilometres given below the triangles. The masses show how muchfuel was consumed during the manoeuvres conducted for each new tetrahedron constellation (Courtesy of D. Sieg, ESOC)

The Eleven Instruments on Each of the Four Cluster SpacecraftInstrument Principal Investigator

ASPOC Spacecraft potential control K. Torkar (IWF, A)

CIS Ion composition, 0


within the magnetosphere and how theyare accelerated. Although electric potentialdifferences across many regions can be ofthe order of 10 - 100 kilovolts, the actualelectric fields appear to be small – just 0.1-10 millivolts – as the regions are verywide. Their detection therefore requireshighly sophisticated instruments. Each ofthe Cluster satellites carry two state-of-artinstruments that can monitor such weakfields. One measures potential differencesbetween electric sensors located at the tipsof 44 metre-long booms, and each satellite

carries four such booms. The otherinstrument measures the drift of a weakbeam of 1 keV electrons emitted by theinstrument, from which the electric fieldscan be determined.

Scientific HighlightsBow shockShocks are a common and importantphenomenon in the Universe as theyaccelerate particles to very high speeds.Close to the Earth, shocks appear, forinstance, at the fronts of massive coronalmass ejections released by the Sun. Theseevents cause magnetic storms at the Earth

and produce plenty of high-energyparticles, which are eventually trapped inthe radiation belts that surround it. At adistance of approximately 14 Earth radiifrom our planet towards the Sun, there is apermanent shock region, known as the‘bow shock’. It is formed by the interactionof the Earth’s magnetosphere with thestreaming supersonic solar wind. At thebow shock, the solar wind is rapidlydecelerated and the interplanetarymagnetic field is compressed.

For a detailed understanding of the physicsof shocks, one must have measurements onboth sides of the shock as well as in theshock itself. The Cluster mission is thereforehelping us to model shocks not only in frontof the Earth’s magnetosphere, but alsoelsewhere in the Universe. The position ofthe bow shock is subject to a continuousback-and-forth motion due to pressurevariations in the solar wind, which helpsCluster in its task because it means that thespacecraft usually make several crossingsonce they approach the shock. As the foursatellites are far apart, one gets simultaneousobservations from all sides of the shock.

Cluster has been able to measure thespeed and thickness of the Earth’s bowshock for the first time. Based on about100 bow-shock crossings, it has beenpossible to show that the shock front’sthickness is proportional to the gyro-radiusof solar wind ions*. At the shock, theelectrons and ions get separated, setting upstrong electric currents. Again for the firsttime, Cluster has observed such currents,which are typically of the order of onemillion Amperes.

CuspsThe polar cusps are the two magnetic‘funnels’ over the Earth’s north and southmagnetic poles, where particles from thesolar wind actually penetrate themagnetosphere and reach the Earth’satmosphere. Cluster has observed so-called ‘magnetic reconnection’ in theneighbourhood of a polar cusp.

Magnetic reconnection occurs whentwo regions of oppositely directedmagnetic fields interact and eventuallybecome interconnected. It is a fundamentalprocess in space and astrophysical plasmasthrough which plasmas of different originsare able to mix and become acceleratedinto energetic jets. It allows the transfer ofcharged particles between two differentmagnetised regions of space, for instancethe solar wind and the Earth’smagnetosphere. This process acceleratesions in both directions, so that theprecipitating population in the cuspproduces a ‘bright spot’ near noon, whichcan be observed on Earth at high latitudes(66 - 70 deg) in the form of the spectacular‘northern lights’.

On 18 March 2002, NASA’s IMAGEspacecraft detected such a ‘bright spot’ inthe northern polar cusp at the same time asCluster detected the typical characteristicsof polar-cusp reconnection. This was thefirst time that the reconnection process hadbeen observed in-situ together with itseffect on the Earth’s ionosphere andatmosphere.

Cluster

Density transition from downstream to upstream across the Earth’s bow shock in the solar wind. The green line is a theoretical fit; thered vertical lines show the thickness of the shock (Courtesy of S. Bale, University of California at Berkeley)

* Charged particles gyrate about magnetic field lines with a period thatdepends on the size of the magnetic field and the mass of the particle.The radius of the gyro motion depends on the gyro period and velocity ofthe gyrating particle.


Science


Kelvin-Helmholtz wavesWhen two adjacent media flow at differentspeeds, waves build up at their interfaceand a phenomenon known as a ‘Kelvin-Helmholtz instability’ occurs. The windblowing across the surface of the oceancauses waves due to this instability.Similarly in space, waves appear at theinterface between two plasma media whenthe difference in velocity is large enough,for instance at the Earth’s magnetopause.

The Cluster satellites have observedKelvin-Helmholtz waves several times, butjust recently they have discovered vorticesin the Earth’s magnetosphere caused by theinstability. Normally, Kelvin-Helmholtzwaves only distort the boundary andcannot cause particles to be transportedacross it. It has been suggested that thevortices let solar-wind particles enter themagnetosphere. Cluster’s discoverystrengthens the likelihood of this scenario,but does not yet show the precisemechanism. This is an exciting resultbecause, with the interplanetary magneticfield and the Earth’s magnetic field beingaligned, the magnetopause is presentlyassumed to be an impenetrable barrier tothe flow of solar wind, which is merelydiverted around the magnetopause.

Reconnection in the magnetotailThe tail of the magnetosphere, called ‘themagnetotail’, appears to be an explosiveregion due magnetic reconnection and asource of highly energetic particles, some ofwhich can have energies of more than 10 MeV*. There are many unsolvedmysteries associated with the reconnectionprocess in the tail, such as the actual triggerfor the process and the formation of a thincurrent sheet that Cluster has observed tooccur before the reconnection can take place.

The four Cluster spacecraft havesurrounded the reconnection region in thecentral magnetotail several times, exploringthe core region where ions and electrons getdecoupled. A change of magnetic curvatureacross the reconnection site has also beendetected. In addition, strong electric fieldsare observed near the site, which couldexplain the particle acceleration usuallyobserved at greater distances from the site.

One of the precursor processes toreconnection is the thinning of the currentsheet in the centre of the tail where thereconnection process is later going to takeplace. Recently, Cluster made its firstmeasurements of such an event, showingthat just before reconnection occurred themaximum current intensity was about 30times its usual value, and that the half-thickness of the current sheet was onlyabout 300 km, instead of the usual few1000 km. When the current sheet becomesthis thin, the motion of the ions is nolonger governed by the magnetic field, andtherefore they cannot be described as afluid. The electrons, however, do stillbehave as a fluid and are believed to carrythe main current in such a situation.

Once the reconnection process starts inthe middle of the tail, a large number ofcharged particles are energized and senttowards the Earth. Magnetic field linesguide them to precipitate into the polaratmospheres, causing intense auroraldisplays. At the same time, a huge volumeof tail plasma appears as an isolated‘plasmoid’, which is ejected down the tailfrom the Earth into the solar wind. Clusterhas already observed the formation of suchplasmoids several times.

The consequences of reconnection arealso observed in the Sun’s atmosphere.Solar flares, caused by reconnection, arethe most energetic explosions in the SolarSystem. Energetic particles accelerated inthe flares escape into interplanetary spaceand pose a danger to astronauts, the crewsof high-flying aircraft, and electronicinstruments operated in space. Similarenergy-release processes take place in othercosmic objects, such as stars, pulsars, blackholes, quasars, and galactic accretion discs.

Remote sensing and interferometry The magnetosphere is a strong source of awide range of electromagnetic emissions.Some of the waves are trapped within theEarth’s magnetosphere, but some, such asthe non-thermal continuum radiation (NTC)and auroral kilometric radiation (AKR), cantravel large distances. There are manyunsolved issues concerning the generationof such waves and Cluster is also playing animportant role here. The radio waves

In this three-dimensional view of the Earth’s magnetosphere, the curly features sketched on the boundary layer are the Kelvin-Helmholtz vortices discovered by Cluster. They originate where two adjacent flows travel with different speeds. The arrows show thedirection of the magnetic field (Courtesy of H. Hasegawa, Dartmouth College)

* 1 electron volt (eV) corresponds to a temperature of approximately 10 000 Kelvin. In the Sun’s atmosphere, the particles have energies of 10 - 100 eV. In the Earth’s upper atmosphere particle energies are of theorder of 0.1 eV. In the magnetosphere, typical energies for protons andelectrons are in the range 0.1 - 10 keV. In the radiation belts, someparticles are relativistic, so that the electron energies are 0.1 - 1 MeV andhigher and the proton energies 10 - 100 MeV and higher.



recorded by Cluster and other satellites inthe Solar System have been converted intosound waves and can be played athttp://www-pw.physics.uiowa.edu/space-audio/.

The AKR (20kHz – 2MHz) is thestrongest signal generated around theEarth and can easily be detected from verylarge distances, for instance by an aliensociety. The Earth’s magnetosphere is theonly place where we can study it in detailand Cluster’s orbit is ideally placed to lookfor its source. The wavelength of AKR is ofthe order of a kilometre, hence the name.The power of emission is of the order of abillion Watts, which greatly exceeds thepower of any radio station. However, thewaves cannot propagate through theEarth’s ionosphere and are therefore notdetectable on the ground, and hence cannotdisturb our radio transmissions.

By using simultaneous four-pointCluster wave measurements, the exactlocation of AKR’s source has beenidentified. Similar emissions are detectedat all magnetised bodies in the SolarSystem, such as Jupiter and Saturn wherethey appear with different wavelengths.For instance, Jupiter’s emission is calledJovian hectometric radiation. Measure-ments of such emissions can also be usedto detect extra-solar planets.

Future PlansCurrently the Cluster mission is in a firstextension phase that will end in December2005. The spacecraft are still workingperfectly and their payloads are in goodhealth, with 41 of the total of 44instruments still working, and expected tocontinue to do so until 2010. The sciencecommunity is therefore looking forward tothe possibility of a second extensioncovering the years 2005–2009, for whichthere is substantial scientific justification,not least to: (i) achieve full coverage of thedayside magnetosphere at large scales,(ii) start a new phase of multi-scaleobservations, (iii) visit new magneto-spheric regions, (iv) collaborate with newmissions, and (v) collect data from a largepart of a solar cycle.

The first point is obviously offundamental importance to complete theCluster mission, because observationshave not yet been collected with spacecraftseparations of 10 000 km or more. Thesecond objective is intended to collectobservations on both small and large scalesat the same time, by moving two spacecraftclose to one another while keeping theseparations between three of the satellitesat 10 000 km. Such measurements arescientifically very exciting.

Due to solar perturbations, the apogeeof the Cluster orbit drifts slowly towardsthe southern hemisphere. So far this hasbeen corrected with special spacecraftmanoeuvres, but in the future this drift willbe allowed to continue, taking the satellitesto encounters with new magnetosphericregions that have previously only beenstudied with single satellites. Plenty ofexciting scientific discoveries cantherefore be expected during this phase.

Collaboration with other missions isalso fundamentally important to Cluster asthe magnetosphere makes up a vast volumeof space. In 2004, the two satellites thatmake up the China-ESA Double Starmission were put into favourable orbitswith respect to the Cluster mission and carry similar or complementaryinstrumentation. In October 2006, NASA’sfive-satellite Themis mission will belaunched to study magnetosphericsubstorm phenomena. The Cluster andThemis apogees will be on opposite sidesof the Earth, so that when Cluster ismonitoring the solar wind or the daysidemagnetosphere, Themis will in themagnetotail, and vice versa.

To celebrate the 40th anniversary of theInternational Geophysical Year, theInternational Polar Year will be organizedin 2007–2008 and the InternationalHeliospheric Year in 2007. During these years, spacecraft, ground-basedobservatories and theoretical modellingwill be brought together in a determinedattempt to fully understand the Sun’seffects on the Earth’s environment. Clustercan make significant contributions to thateffort! r

Cluster

Magnetic reconnection occurs frequently in the magnetotail,creating a magnetically neutral region called the neutral line. As a result, on over-stretched field lines earthward of the neutralline, a large amount of plasma is accelerated towards the Earth,causing fantastic auroral displays in the polar regions.Simultaneously, on the other side of the neutral line a vastplasmoid is ejected down the tail into the solar wind (Courtesy of J. Slavin, NASA Goddard Space Flight Center)


Wittig 4/19/05 10:21 AM Page 20


AmerHis

Satellite telecommunications has now reached anadvanced stage of maturity with nearly 40 satelliteoperators controlling around 250 satellites ingeostationary orbit. The services that those operatorsprovide range from backbone trunk networks in thetelephony and data fields, to the distribution andbroadcasting of thousands of TV channels to hundreds ofmillions of viewers around the World.

Successful as they are today, satellite systems are on thebrink of yet another major revolution that will doubtless alsohave a significant impact on our daily lives in the near future.This revolution will be brought about by the provision of anew generation of cost-effective broadband interactiveservices, initially to larger corporations, later to small andmedium-sized enterprises, and eventually to private homes.

IntroductionThe basis for the introduction of these new services liesfirstly in the availability of precisely targetted multiplespot-beam coverage from satellites, allowing greateroptimisation of the use of the satellite’s resources interms of power and frequency spectrum, and secondlyin the consolidation of open standards based on thesuccessful DVB-S/DVB-RCS suite. Designing aroundsuch open standards guarantees that the receiving

Manfred Wittig, Felix Petz, Frank Zeppenfeldt, Stephane Pirio,Ian Davis, Jean-Pierre Balley & Jose Maria CasasTelecommunications Department, Directorate of EU andIndustrial Programmes, ESTEC, Noordwijk, The Netherlands

AmerHis: The FirstSwitchboard in Space

Wittig 4/19/05 10:21 AM Page 21

European Union and Industrial Programmes


terminals and services will be availablefrom many vendors, which is fundamentalto the creation of an open competitivemarket and the delivery of economies ofscale.

The consolidation of the DVB-basedopen-standard approach has paved the wayfor the introduction of a new satellitepayload and system architecture, wherebya number of transmitting/receivingtransponders on the same satellite can beinterconnected via an onboard digitalswitch. In this way, the network topologybecomes star-like in form, centred on thenode constituted by the satellite and its

Good wishes to Amazonas, on the fairing of the Proton launcher

Good wishes to Amazonas, on the fairing of the Proton launcher

The AmerHis switchboard in space The Amazonas coverage zones

Wittig 4/19/05 10:21 AM Page 22


transponders. This new architecture allowsoptimisation of the resources assigned toeach individual transponder in terms ofcoverage, power and bandwidth.

The ESA Telecommunications Long-Term Plan foresees greater cooperationwith telecommunications operators inorder to foster the introduction of newpayload technologies and services. As partof this process, ESA entered into anagreement with the Spanish companyHispasat, with the cooperation of Spain’sCDTI (Centro para el DesarrolloTecnológico Industrial), for theimplementation of a regenerativemultimedia system, known as the‘AmerHis System’, onboard Hispasat’s‘Amazonas’ satellite, which was launchedon 5 August 2004.

The AmerHis SystemAmerHis is an advanced satellitecommunication system based aroundAlcatel’s 9343 DVB On-Board Processor.This processor has the demodulation,decoding, switching, encoding andmodulation capabilities needed for the fourtransponders on Amazonas. Eachtransponder covers one of the fourgeographical regions served by the satellite- namely Europe, Brazil, North and SouthAmerica.

The complete AmerHis system consistsof:– The regenerative payload onboard

Amazonas.– A network management system,

containing the Network Control Centre(NCC) and associated managementcontrol, responsible for managing theonboard resources and the userterminals.

– User terminals (Return-ChannelSatellite Terminals, or RCSTs) orientedtowards the commercial demonstrationof new services.

– Gateways (RCST Satellite Gateways, orRSGWs) that provide the system withaccess to terrestrial networks.

With AmerHis, the satellite operator is ableto offer broadband connectivity via asingle hop to users anywhere within the

four geographical areas covered byAmazonas. This is a great improvementcompared with existing satellite-basednetworks, which require a double hop via aground-based hub. The AmerHis conceptputs that hub onboard the satellite, savingthe delay of about 250 milliseconds causedby the additional second hop. Theregenerative payload thereby enables real-time broadband connections between smalluser terminals.

This unique combination of onboardprocessing and full compatibility with theopen standards of DVB-S (downlink) andDVB-RCS (uplink) gives an AmerHis-equipped telecommunications satelliteunprecedented potential compared with itsrivals relying on conventional systemarchitectures.

In summary, the key advantages ofAmerHis for the user are:– Provision of direct ‘end-to-end’

connectivity between any two users indifferent regions via a single satellitehop. This allows real-time voice andvideo services, as well as reducingbandwidth usage.

– Full flexibility both for theinterconnection of coverage areas andpayload-capacity management, allowingoptimum exploitation of availableonboard resources. Thanks to thedynamic resource-allocation process,

the system supports predictablesymmetric up- and downlink traffic, aswell as intermittent ‘bursty’ trafficgenerated by a large number of users.

– The regenerative nature of the AmerHispayload, and the utilisation of DVB-Ssaturated carriers on each downlink,provide substantial improvements whenusing the AmerHis-enabled tran-sponders. These improvements arereflected both in the enhancedthroughput capacity and the reducedreceive-antenna size requirements forthe users.

These features, combined with the use ofstandard low-cost and high-performancebroadband interactive user terminals,represent a major qualitative stepforward in the successful developmentof interactive multimedia services viasatellite.

The Services Offered by AmerHisThe great flexibility for managing andselling AmerHis capacity is such that all ofthe main telecommunications players willbenefit from this advanced technology.Real- and non-real-time multimediaservices and applications can be providedon readily available DVB-S/DVB-RCScompatible terminals. The system permitsthe assignment of resources to differentsubnetworks in a very flexible manner andallows user transmission rates rangingfrom 512 kbit/s to 8 Mbit/s. The systemsupports Internet Protocol (IP) based aswell as native MPEG-based services, withefficient mechanisms for the provision ofuni- and multicast services, and thepossibility to define various quality-of-service levels to meet different user needs.

AmerHis supports mesh connectivityand star connectivity, in both cases withjust one satellite hop. Unidirectional orbidirectional point-to-point connectionsare possible in both cases. These types ofconnections can either be established ondemand by the terminals and the gateways,or by the Management System. A givenconnection can be assigned one of threepriorities within AmerHis - low priority,

AmerHis

AmerHis provides connectivity between spot-beam coverage areaswithout the need for double hops between ground and spacecraft

Wittig 4/19/05 10:21 AM Page 23



The AmerHis Elements

Space Segment – The regenerative payloadThe AmerHis payload is made up of a novel set of On-Board Processing(OBP) technologies, which are being flown for the first time on the Amazonassatellite. The key feature of AmerHis is that the payload is regenerative andprovides unique connectivity possibilities via its ‘switchboard in the sky’functionality.

The uplink format to the OBP is MF-TDMA according to the DVB-RCSstandard (MPEG-2 option), with up to 64 carriers per transponder. Data ratesof 512 kbit/s, 1, 2, 4 and 8 Mbit/s can be combined in the same transponder.The downlink format is according to the DVB-S standard, with a maximumdata rate of 55 Mbit/s per transponder. The OBP, which relies on complexdigital signal-processing functions implemented in ASICs, offers full routingflexibility between uplink and downlink channels using dynamic capacitymanagement.

The AmerHis payload installed on Amazonas consists of eight boxes: the downconverter, the onboard processor, four modulators and two Ku-band filters.

Ground SegmentThe AmerHis ground segment consists of user terminals to access the system, gateways interfacing to terrestrial services and amanagement system to configure and manage the network.

User TerminalsA Return Channel Satellite Terminal (RCST) provides access to other users, as well as external access to terrestrial networks andService Providers. The peak uplink data raw bit rates for different terminal classes are:- 512 kbit/s for Class-1- 1.036 Mbit/s for Class-2- 2.073 Mbit/s for Class-3- 4.417 Mbit/s for Class-4.

The antenna and Solid-State Power Amplifier(SSPA) sizes vary from 1.2 to 3 metres and 2 to8 Watts, respectively, depending upon the classtype and coverage area.

An RCST can support both guaranteed rate anddelay and best-effort classes of service. Thenetwork quality-of-service mechanism in theRCST performs prioritisation of IP flows andselects the most suitable transmissionparameters for the application in question. Thisprovides priority to mission-critical datatransactions or video or voice transmissions,which require faster turnaround, whileproviding lower priority to less-time-sensitivetraffic such as e-mail and web surfing. AmerHis user terminals (RCSTs) manufactured by NERA (Norway)

and EMS Technologies (Canada)

One of the AmerHis ASICs

Wittig 4/19/05 10:21 AM Page 24


AmerHis

These terminals are based on standard DVB-RCSproducts and are already available from two companies.The point-to-point connectivity provided by theAmerHis regenerative payloads requires a call-handlingprotocol, which is now being considered for DVB-RCSstandardisation. The AmerHis RCSTs from differentvendors are interoperable, i.e. both terminals can workwith the same hub, and have a call-handling procedureimplemented.

GatewaysAn RCST Satellite Gateway (RSGW) providesAmerHis users with internetworking capabilities toexternal networks (PSTN, ISDN, Internet). In effect,the RSGW is the hub in an access network with a startopology. It incorporates a standard low-cost andslightly modified RCST, which is an attractive solutionfor medium to small service providers. It has beendesigned to share its satellite and terrestrial bandwidthresources with a large number of simultaneous activesubscribers.

The gateway support commonly used interfaces to the terrestrial networks such as ISDN, Ethernet and ATM. To support the deliveryof business-class data services, the RSGW is able to provide service guarantees to subscribers based on different quality-of-servicecriteria for the different subscription levels.

The peak uplink transmission raw bit rate of the RSGW is at least 8 Mbits/s (2 x 4 Mbit/s terminals). The RSGW maximum downlinkthroughput at IP level is at least 8 Mbit/s. In order to offer low-cost RSGWs targeted specifically at small Internet Service Providers(ISPs), a standard RSGW can be simplified by not offering voice/video services and reducing the peak uplink rate to 2 Mbit/s.

Management SystemThe Management System consists of a Network Control Centre (NCC) and a Network Management Centre (NMC). The NCCcontrols the interactive network, services satellite access requests from users and manages the OBP configuration. An NCC Return-

Channel Satellite Terminal (RCST)provides terminal access to the satellite.The NMC handles the systemconfiguration and manages the AmerHisnetwork elements, supporting remoteconfiguration and monitoring of theNCC, user terminals, gateways and OBP,as well as providing fault, configuration,performance and security features for allelements.

Connection-control management func-tions both support permanent connectionset-ups and handle on-demandconnection requests, such as callestablishment, call modification and callrelease.

An AmerHis gateway

The AmerHis management system

NETWORKMANAGEMENT

SYSTEM

NETWORK CONTROL CENTRE (NCC)

Wittig 4/19/05 10:21 AM Page 25



high priority or high priority jitter-sensitive - each of which is associated witha specific set of traffic parameters. Anadmission-control function ensuresoptimal use of the available capacity andprovision of the best possible service forthe different types of application.

The physical capacity of AmerHis isdistributed over Virtual Private Networks(VPNs). Each VPN can be allocated adedicated set of logical capacity, reflectingthe service provider’s needs, or it can sharea set with other VPNs. Any VPN can alsotake advantage of the AmerHis broadcastcapability.

AmerHis is therefore creating a new erafor relationships between Internet NetworkAccess Providers (INAPs), serviceproviders and customers by offering muchgreater flexibility than any other satellite-based system so far. Being a connection-oriented system, it allows full control overthe applications crossing the network andthe amount of resources allocated to thoseapplications. In this way, over-provisioningcan be avoided and billing is triggered onlywhen applications are really using thenetwork. This opens the door for newbusiness models, reflected in turn in theService-Level Agreement (SLA) betweenthe INAP and service provider or serviceprovider and customer.

The following are some typical AmerHisapplication scenarios:

Internet Service ProvidersIn this scenario, the Internet ServiceProviders (ISP) manage their own low-costgateway and provide reliable Internetaccess to subscribers. Value-addedservices, such as Voice over IP (VoIP) orvideo conferencing based on the ITUH.323 standard, can be provided toindividual customers via a simpleconfiguration. ISPs can choose to offertheir own SLAs, be they flat-rate, volume-based or based on a certain quality-of-service profile.

The AmerHis gateways are intended to below-cost and have minimum infrastructurerequirements. The ISPs therefore have theflexibility to locate their gateways indifferent AmerHis coverage regions and

thereby offer more reliable Internet accessand/or more attractive tariffs.

A novel feature of AmerHis is thesupport it provides for multicast services,allowing streaming contents to bedelivered to a large number of subscriberssimultaneously. The AmerHis gatewayssupport all of the necessary functionalityfor implementing these services accordingto the latest IETF (Internet EngineeringTask Force) standards. The AmerHisnetwork can support all of the variousdeployement methods currently used byISPs, such as private and public IPaddressing support, NAT (NetworkAddress Translation), authentication andbilling support.

Corporate ServicesCompanies with multiple branch offices inEurope, Brazil, North and South Americacan easily set up their own Virtual PrivateNetworks (VPN) and share their allocatedcapacity between all offices.

An important feature of the AmerHissystem for corporate communications isthe high quality of service that can be

offered for business-grade video and voiceconferencing and the possibility of alwaysreaching each branch office with a singlesatellite hop. In addition, access to theISDN and POTS terrestrial networks isprovided via the AmerHis gateways.

The AmerHis system allows thescheduling of specific connectivityrequirements, such as the daily distributionof newspaper copy to printing facilities indifferent geographical regions.

Video ServicesThe AmerHis payload offers multiplexingand de-multiplexing of MPEG-2 transportstreams and is therefore not only capable ofoffering IP services over MPEG-2, but alsoallows the routing of video. Contributionscan be made from different uplink stations

The AmerHis in-orbit testing (IOT) station, antenna and shelterand (right) the ground-support equipment used for IOT

Wittig 4/19/05 10:21 AM Page 26


and, depending on the onboard switchconfiguration, duplicated and sent tomultiple destinations if necessary, using theDVB-S standard for direct-to-Home (DTH)services. Business television services,occasional-use services and videocontributions from smaller terminals canall be supported more easily by exploitingthese capabilities.

Early AmerHis OperationsThe Amazonas satellite was launched on 5 August 2004 from Baikonur inKazakhstan by a Proton launch vehicle.One of the first tasks in the Amazonas in-orbit testing (IOT), which started on the 11August, was to upload the initial settingsfor the onboard processor via thetelecommand link. The AmerHis payload’sperformance was verified from Hispasat’sArganda ground station, southeast ofMadrid, and a signal emitted by a station inBrazil was received. After extensive testingit was concluded that the AmerHisregenerative payload had survived thelaunch and was performing as expected.

All of the network elements required forthe first AmerHis pilot operations arecurrently available. The intention is thus to

verify the whole AmerHis network’soperation and performance starting in2005 and to provide the Amazonas satelliteoperator Hispasat with a pilot network forearly commercial customers.

In addition to this commercial use ofAmerHis capacity, ESA is also planningseveral projects geared to furthertechnology and application development,including:– Technology projects to look into the

characteristics of the current system,propose and perform additional tests,and address further enhancements of theground segment.

– Application projects are expected to usetypical AmerHis features such asquality-of-service, multicasting anddemonstrate that applications willbenefit from these. Such trials willexploit AmerHis’s functionality toconvince service providers of the addedvalue of onboard processing and anadvanced ground segment.

One potential pilot project is to providecommunications capabilities to hospitalsin the Amazon rainforest region. An EU-sponsored telemedicine network is being

established and some of the hospitals areseveral thousand kilometres away fromwell-populated areas in Brazil. Onlysatellite links can provide reliablecommunication capabilities. The importantfeature of AmerHis is being able to providea single-hop link, for example to a hospitalin Portugal for medical-specialistconsultation in the event of an emergencyoccurring at a hospital in the rainforest.

ConclusionThe onboard part of AmerHis has beenproven to function as expected in orbit,with a first videoconference between threesites in Europe taking place in earlyFebruary 2005. The technology offered byAmerHis provides the means to help tobridge the digital divide in developingregions of the World, with a specialemphasis on institutional applications suchas telemedicine.

In addition, the regenerative onboardpayload concept is also well-suited toserving communications scenarios forsecurity, civil protection and emergencyapplications, which typically havedemanding requirements in the areas ofmesh connectivity and quality of service.

r

AmerHis

Wittig 4/19/05 10:21 AM Page 27

Satellite Navigation,Wireless Networksand the Internet– Greater together than the sum of

the parts?

Satellite Navigation,Wireless Networksand the Internet– Greater together than the sum of

the parts?

Toran 4/19/05 10:19 AM Page 28


Satellite Navigation, Wireless Networks & Internet

O ver the last 20 years, major developments havetaken place in parallel in the key technology areas ofwireless communications and satellite navigation. Bothare gradually becoming indispensable tools in the professionaland consumer worlds. Nowadays, major synergies aredeveloping in terms of wireless mobile terminals with anavigation capability, or navigation terminals with wirelesscommunications capabilities. ESA is undertaking a series ofactivities to support the exploitation of such synergies for thebenefit of Europe’s citizens through the provision of new andupgraded services from space.

IntroductionFor the time being, satellite navigation is only known tothe public via consumer applications of the US GlobalPositioning System (GPS). The implementation of awide range of other potentially very useful GPS-basedapplications is still hampered by various institutionaland technical difficulties. The lack of guaranteedintegrity of today’s GPS data is a handicap for a widerange of critical applications, not only those involvingthe safeguarding of human lives. The limited accuracyand coverage of today’s GPS systems in towns andcities – due to what is known as the ‘urban canyon’effect – is currently tending to restrict applications tosomewhat basic guidance services.

Felix Toran, Javier Ventura-Traveset, Alberto Garcia,Jean-Luc Gerner, Simon Johns & Juan De MateoNavigation Department, ESA Directorate of European Unionand Industrial Programmes, Toulouse, France

Toran 4/19/05 10:19 AM Page 29



The European Geostationary NavigationOverlay Service (EGNOS) system, the firstpan-European Global Navigation SatelliteSystem (GNSS) infrastructure that willbegin operating in 2005, is a step forward inreducing such GPS limitations. EGNOSpermanently monitors the GPS satelliteconstellation and provides the users withreal-time integrity and correction data forthe GPS satellites, thereby enablingguaranteed high-accuracy positioning.EGNOS messages are broadcast overEurope via geostationary satellites,including ESA’s Artemis satellite, and this isan excellent broadcast medium for manyapplications (e.g. civil aviation ormaritime).

For other applications such asnavigation in urban areas, in 2001 ESAdeveloped the SISNeT technology, whichimproves urban penetration by providingthe EGNOS data to the user through theInternet, thereby making them accessibleto any wireless mobile terminal. Todaymany users are already using this by nowwell-proven technology, which has alreadycontributed to the emergence of a largenumber of innovative applications.SISNeT is just one of the many servicesthat combining satellite navigation systems

and wireless networks can offer. Moregenerally, ESA is also developing anEGNOS product-dissemination conceptbased on the EGNOS Data Access System(EDAS), which will be operational at theend of 2005, making full synergy betweenmobile and navigation services possible.

The EGNOS system is being developedby the ESA EGNOS Project Office locatedin Toulouse (F), with support from the ESADirectorate of Technical and QualityManagement at ESTEC in Noordwijk (NL),which has put a team of experts in place andhas set up a European NavigationLaboratory. There is also close coordinationwith the Galileo Joint Undertaking –responsible for the European Commission’snavigation research programme and theestablishment of the Galileo Concessionaire– and with the ESA NavigationApplications and User Services Office,which coordinates all ESA navigation-related application technologies.

Satellite Navigation and Communications – Technology and Services

Technology synergiesThe integration of the communication andsatellite navigation functions has majorbenefits at the user level. The satellitenavigation function can benefit from thesupport data provided through thecommunications channel for better accessto the wide-area differential and integrityGPS corrections provided by EGNOS.This avoids the problems of decreasing

The impact of EGNOS on GPS in terms of improved accuracy. ‘Selective availability’ is the ability of the GPS service provider tointentionally degrade system performance

EGNOS service area corresponding to the current baseline, with 34 monitoring stations (Courtesy of Alcatel Space)

Toran 4/19/05 10:19 AM Page 30


GNSS performance due to ‘shadowing’ ofthe EGNOS geostationary satellites, or insome indoor environments. Such links alsoenable the user terminal to access the latestsatellite ephemeris and SBAS messages onrequest, which dramatically decreases thereceiver’s Time To First Fix (TTFF), i.e. thetime needed to start providing position.Such synergies are particularly beneficialfor users at high latitudes, who may easilyexperience a low satellite visibility, andusers in suburban and urban ‘canyons’.

On the other side, the communicationsfunction benefits from the positioninformation, allowing power adjustmentbased on distance and network resourceoptimization, in addition to the primaryuser benefit of access to localisation-basedservices for which huge range ofapplications are being developed.

The ESA SISNeT Technology EGNOS will primarily broadcast wide-area/integrity GPS corrections throughgeostationary satellites. To providecomplementary transmission links, ESAhas launched specific activities to assessand demonstrate the feasibility ofbroadcasting the EGNOS signal by othermeans, such as standard FM radio signalsor GSM mobile phones.

As early as 2001, ESA provided accessto the EGNOS Test Bed messages via theInternet. A new product was thereby borncalled SISNeT (Signal in Space throughthe Internet), which proved to be full ofpotential. In February 2002, the prototypeSISNeT service was made accessibleworldwide via the Internet, offeringimmediate advantages to the GPS land-user community. Numerous usersequipped with a GPS receiver and wirelessInternet access have exploited the SISNeTservice and benefitted from the EGNOSTest Bed augmentation signals, even insituations where the geostationary EGNOSsatellites were not visible.

Extensive market studies haveconfirmed the value of the futureconvergence and merging of satellitenavigation and wireless networks intocompact mobile devices (e.g. mobilephones including a GNSS receiver) to offerGNSS-based Location Based Services

(LBS). Such studies serve to underline thegreat potential of SISNeT technology,which is already being applied in manyindustrial developments under ESAcontract.

The EGNOS Data Access System (EDAS)In addition to providing the InternetSISNeT service since 2002, ESA has alsoperformed a number of other feasibilitystudies on dissemination of the EGNOSmessages via other non-space means, suchas DAB (Digital Audio Broadcasting) orRDS (Radio Data Service). The highdegree of success achieved in those studieshas motivated the Agency to design aprofessional evolution of the SISNeTservice to match the needs of a broadspectrum of dissemination technologies.This evolution, called the EGNOS DataAccess System (EDAS), will constitute themain interface point for multimodalService Providers supplying the EGNOSproducts in real-time, within guaranteeddelay, security, and safety performanceboundaries. Application Service Providerswill then exploit these EGNOS products tooffer a variety of superior services to endusers.

EDAS will be developed as part of theGNSS Support Programme Step 1, with aview to having an EDAS systemoperational before the end of 2005, thusopening the way for its commercialexploitation by the EGNOS CommercialOperator.

The extensive suite of services thatcould be spawned from EDAS includes theprovision of:– SISNeT services– EGNOS pseudolites– EGNOS services via the Radio Data

System (RDS)– EGNOS services via Digital Audio

Broadcasting (DAB)– Wide Area Real-Time Kinematics

(WARTK) services, allowing decimetrelevel positioning accuracies atcontinental scales

– Accurate ionospheric monitoring– EGNOS performance information in

real-time– Archiving of EGNOS messages– EGNOS corrections in standard RTCM

SC104 format, ready for use by DGPSreceivers.

Demonstration Activities

A PDA-based SISNeT ReceiverTo verify the feasibility of the SISNeTconcept, ESA placed a contract with theFinnish Geodetic Institute to develop theWorld’s first SISNeT receiver. Based on aconventional Personal Digital Assistant(PDA) pocket-PC device, it includes both alow-cost GPS card and Internet access via astandard GSM/GPRS wireless modem.Special software combines the GPSmeasurements with EGNOS correctionsobtained from SISNeT via the Internet.Almost any commercial GPS software canbe enhanced with SISNeT positioning inthis way.


SISNeT handheld receiver based on the Siemens SX45 mobilephone/Personal Digital Assistant (PDA), developed by the FinnishGeodetic Institute (FGI) under ESA contract. This was the firstdevice to demonstrate the enormous potential of integratingEGNOS, Wireless Networks and the Internet in a single unit

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During field testing in Finland, the newreceiver delivered horizontal positioningaccuracies of 1-2 metres and verticalaccuracies of 2-3 metres. In a further step,the device has been integrated into aSiemens SX45 mobile phone.

SISNeT-based Navigation for the BlindThis demonstration activity assessed thebenefits of using the SISNeT concept tohelp blind people in a city environment.The project was based on an existingpersonal navigator for the blind, known asTORMES, developed by the Spanishcompany GMV Sistemas and the Spanishorganisation for the blind (ONCE). Itincludes a Braille keyboard, a voicesynthesizer, a cartography database andnavigation software. The positioning isbased on a conventional GPS receiver, andhence suffers the typical signal-shadowingproblems in urban areas.

The improvement provided by theintegration of EGNOS data via SISNeTwas tested in the outskirts and downtownareas of Valladolid in Spain. The resultswere excellent, with a significantimprovement in both accuracy and serviceavailability, which will allow blindpedestrians to be forewarned of obstaclesin the street. Being connected to theInternet could also give TORMESimportant added-value, allowing users to

not only receive information but also sendrelevant data back to the system, e.g.notification of a blocked street orpavement.

SISNeT-based Bus-Fleet ManagementThis activity was designed to demonstratethe improvements that EGNOS canprovide for an urban bus operator. A

handheld SISNeT receiver combining aGSM/GPRS modem and a GPS receiver,developed by the French companyNavocap under ESA contract, was testedon a bus operating in the French city ofToulouse. The route included bothresidential and downtown areas, allowingthe robustness of the unit to be tested fordifferent degrees of satellite visibility. Theresults were quite promising, indicatingthat SISNeT could be a very adequatecomplement to (or even a replacement for)the differential GPS systems currentlyemployed, which need a costly infra-structure.

ShPIDER: A Professional High-Performance SISNeT ReceiverAn integrated professional device calledShPIDER, which includes a GPS receiverand a GPRS modem, has been developedby GMV (Spain) under ESA contract. Itsnavigation algorithms allow significantlyimproved positioning accuracy and

TORMES: A personal navigator for the blind, enhanced with SISNeT capabilities, developed by GMV Sistemas (Spain) and ONCE (theSpanish Organisation for the Blind). This device has highlighted the immense benefits that the synergy between Satellite Navigation andCommunications can bring to visually impaired people

SISNeT receiver based on a PDA device, integrating a GPSreceiver and a wireless link to the Internet in a single box. It hasdemonstrated the benefits of the ESA EGNOS-SISNeT technologiesfor urban bus-fleet management systems. This receiver has beendeveloped by Navocap (France) under ESA contract

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availability compared with GPS-onlysolutions. The benefits are even morepronounced for low-visibility conditions,such as in cities. The ShPIDER receiverhas also been tested in the framework ofthe bus-fleet management trial inValladolid and the ESA EGNOS TRANproject (see below) in Rome, showingpromising results in both cases.

EGNOS Dissemination throughTerrestrial Networks - The EGNOS TRANProjectIn the framework of its Advanced Researchin Telecommunications Programme(ARTES), ESA launched a number ofactivities focussing on EGNOS signalbroadcasts for areas with weak space-signal reception. Several solutions weretested through the EGNOS TRAN project(EGNOS Terrestrial Regional Aug-mentations Networks), all of them relyingon the use of terrestrial networks asEGNOS augmentations.

Since most applications addressed weresafety-critical, the use of the terrestrial linkhas multiple benefits. For example, thecommunications link could be used toresend the EGNOS informationencapsulated in the appropriate format(RTCA original, GBAS, RTCM) in amanner transparent to the end user. In

other cases, the communication link wasused to help the end user to improve theirpositioning accuracy and integrity bymeans of an TRAN service centre that hasaccess to EGNOS either by direct line-of-sight or a SISNeT connection, and is ableto re-compute and send back the end-usernavigation solution.

Several demonstrations have shown theEGNOS performance via TRAN to be veryclose to that when using geostationary-satellite data, despite the extra delayintroduced by the communication link. TheProject has confirmed EGNOS as a verycost-efficient alternative to a number ofexisting local-area GPS augmentationsystems.

EGNOS Dissemination over RDSESA has also studied the feasibility ofEGNOS dissemination using FM RadioData System (RDS) signals, through acontract with TDF (France). A laboratorychain was set up in which EGNOSmessages from the ESA SISNeT servicewere broadcast via RDS signals. A keychallenge was the need to reduce theamount of information to be broadcast inorder to fit the RDS bandwidth available.This was solved by developing selectivefiltering algorithms able to extract only theinformation relevant to the user, depending


ShPIDER, a professional high-performance SISNeT-powered EGNOS receiver, developed by GMV (Spain). It integrates a GPRS wirelesslink to the Internet and a GPS receiver and has been tested in the context of both bus-fleet management in Spain and the EGNOS TRANProject in Italy

Summary of applications developed in the context of the EGNOSTRAN Project

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on his/her position. The results showedtypical horizontal accuracies of 1-3 metres,demonstrating both the feasibility andpotential of RDS EGNOS broadcasting.

EGNOS Dissemination over DABThe results with RDS encouraged ESA tothink about other possible means ofEGNOS transmission. The focus was onDigital Audio Broadcasting (DAB), atechnology that allows the listener to receiveCD-quality radio programmes withoutsignal distortion or interference. Moreover,DAB is able to carry not only audio signals,but also text, pictures, data and even videosto the receiving set, and is beingimplemented and exploited worldwide.

The concept was tested in Germany,through an activity performed by Bosch-Blaupunkt (Germany) under ESA contract.The EGNOS message was obtained via theSISNeT service and transmitted over DAB.A DAB car radio system received thesignals and extracted the EGNOS data,which was decoded and applied to a GPSreceiver through an integrated hardware-

software platform. A tour throughperipheral, residential and downtown areasrevealed a significant improvement interms of positioning accuracy with respectto GPS-only applications.

Integrated Receiver TechnologiesESA is also supporting several projects inthe receiver-technology domain. One of thefirst is to develop and demonstrate a small,low-cost EGNOS/Inmarsat terminal that is

capable of computing safe positions andcommunicating them to the web for a varietyof safety, navigation and other applicationsin the maritime domain. The terminal isbeing developed by TransCore-GlobalWave(Canada) based on a commercially availableproduct. It receives GPS/EGNOS navigationsignals and computes position which,together with other sensor and messageinformation, is communicated back to thebase station using the Inmarsat-3 satellites,and to the customer via the Internet. Theterminal’s position is monitored through theGlobalWave application server, located atthe end-user’s premises. The integratedterminal thereby provides a two-waycommunications function between themobile terminal and the web, with theservice area determined by Inmarsat-3satellite coverage.

Another project is the development and demonstration of an IntegratedNavigation/Mobile Communication UserTerminal based on the TETRAPOLsystem. TETRAPOL is a fully secure,digital mobile communications network(GPRS-like), designed to meet thegrowing needs and expectations of highly-demanding Professional Mobile Radio(PMR) users, such as public safetyservices, transport, or industry. Theterminal supplies the actual EGNOS-basedposition information via the TETRAPOLcommunication network to an Automatic

The GlobalWave system architecture

EGNOS service-coverage expansion to the MEDA region, thanks tothe addition of four monitoring stations to the EGNOS baseline.This enhancement is planned for 2006 (Courtesy of AlcatelSpace)

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EGNOS V2.1 (in 2006) EGNOS V2.2 (in 2007/8) EGNOS V2.3 (in 2008/9)

Provision of operational Provision of EGNOS time Broadcasting of EGNOSnon-SoL services service messages in L5(MT0/MT2)

MT28 provision Upgrading RIMS L1/E5Provision of EGNOS to be able to also processproducts through non- Expansion of EGNOS Galileo IPPs to improveGEO means (INSPIRE services in Africa IONO measurements (TBC)and EDAS)

Expansion of EGNOSExpansion of EGNOS services to EU accessingservices in MEDA countriesregion (North Africaand Middle East, Avail. improvement byMediterranean countries) modern RIMS RX (L2C)

SAR return link & I/FCOSPAS/SARSAT (TBC)

STEP 1 STEP 2


Vehicle Localization (AVL) server. Theend-user is able to track and guide severalterminals on an AVL display andcommunicate with them via the voice anddata channels of the TETRAPOL system.The navigation module is able to acquireassisted GPS and assisted EGNOSinformation via the communication systemwhenever the clear view of the satellites isobstructed. The main applications are thetracing and tracking of mobile targets, andsecurity and emergency locationdetermination. A demonstration phase hasverified the system’s performance.

The communication part of the projectis under the responsibility of EADSTelecom, whereas the navigation part isbeing implemented by IMST, Fraunhoferand IFEN. TeleConsult is responsible forthe demonstration phase.

Future Developments and EvolutionAt the time of writing (October 2004), thedeployment of the EGNOS infrastructureis quasi-complete, with all four MasterControl Centres (MCC) and all sixNavigation Land Earth Stations (NLES)already in place. 31 of the 34 RIMSRanging and Integrity MonitoringStations) are now installed, and all threeEGNOS geostationary satellites are

already successfully transmitting EGNOStest signals. The test transmissions startedin December 2003, and the qualificationtests are now approaching completion withthe EGNOS Operational ReadinessReview, which will open the way for initialsystem operations, scheduled for early2005.

In parallel with the start of operations,EGNOS already plans to respondpositively to today’s dynamic GNSSenvironment. Since 1998, when theoriginal EGNOS mission requirementswere set, the launch of the GalileoProgramme and the planned modernisationof the GPS and WAAS systems havebrought further opportunities for EGNOS.In this global strategic race to promote andbenefit from future GNSS applications, theCouncil of the European Union confirmedin June 2003 that, as an integral part of theEuropean Satellite Navigation policy,EGNOS services should be extendedwithin a long-term perspective to otherparts of the World. In response to thatmandate, a GNSS Support Programmewas defined by ESA and the EuropeanCommission – Step 1 covering the 2005-2006 timeframe and Step 2 the 2006-2008timeframe – to further maximise thepotential benefits of GNSS for Europe’s

citizens, and in particular to define andimplement the most appropriateevolutions of the EGNOS system to bestprepare for the Galileo services to beavailable from 2009 onwards.

ConclusionsSince 2000 and the implementation of theEGNOS Test Bed, ESA has been playing aleading role in demonstrations of thepotential benefits that European citizenscan expect from these technologies.Special attention will continue to bedevoted to the future EGNOS Data AccessSystem (EDAS), which will already beoperational at the end of 2005, facilitatingfull synergy between mobile andnavigation services. These advancedGNSS services are likely to becomeessential assets on a day-to-day basis formany Europeans from 2005 onwards, firstwith EGNOS and later on a worldwidebasis with the Galileo system. r


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FrequencyManagement forESA’s Missions

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Frequencies for ESA’s missions

P eople not familiar with the subject sometimes see the frequency management for a satellite as an activitywhereby the ‘good’ frequency is selected by applyingsome mysterious formula. In reality, as this article will try toexplain, there is much more to it. Frequency management is arather broad discipline in which international regulations,technical discussions and negotiations play a key role. Theradio-frequency spectrum is becoming an increasingly scarceresource and more and more users of all kinds are competingwith each other for their share. In a nutshell, frequencymanagement involves minimising the implications of thisproblem for the satellite users.

International Regulations The frequency bands available for use by the differentradio-frequency services, the technical/operationalconditions under which it is possible to operate, and therelevant protection criteria are specified in the RadioRegulations of the International TelecommunicationUnion (ITU), a UN international treaty signed by 190nations. These regulations are particularly importantfor satellite services, because their radio-frequencyemissions go well beyond national boundaries.

The Radio Regulations can be changed only by aWord Radiocommunication Conference (WRC), aformal meeting of the Delegates from all nations

Edoardo MarelliDirectorate of European Union and Industrial Programmes,ESTEC, Noordwijk, The Netherlands

Enrico VassalloDirectorate of Opera

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