The Copernicus Polar Ice and Snow Topography Altimeter ...

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The Copernicus Polar Ice and Snow TopographyAltimeter (CRISTAL) high-priority candidate mission

Michael Kern Robert Cullen Bruno Berruti Jerome Bouffard Tania CasalMark Drinkwater Antonio Gabriele Arnaud Lecuyot Michael Ludwig Rolv

Midthassel et al

To cite this versionMichael Kern Robert Cullen Bruno Berruti Jerome Bouffard Tania Casal et al The Coper-nicus Polar Ice and Snow Topography Altimeter (CRISTAL) high-priority candidate mission TheCryosphere Copernicus 2020 14 pp2235 - 2251 105194tc-14-2235-2020 hal-02997093

The Cryosphere 14 2235ndash2251 2020httpsdoiorg105194tc-14-2235-2020copy Author(s) 2020 This work is distributed underthe Creative Commons Attribution 40 License

The Copernicus Polar Ice and Snow Topography Altimeter(CRISTAL) high-priority candidate missionMichael Kern1 Robert Cullen1 Bruno Berruti1 Jerome Bouffard2 Tania Casal1 Mark R Drinkwater1Antonio Gabriele1 Arnaud Lecuyot1 Michael Ludwig1 Rolv Midthassel1 Ignacio Navas Traver1Tommaso Parrinello2 Gerhard Ressler1 Erik Andersson3 Cristina Martin-Puig4 Ole Andersen5 Annett Bartsch6Sinead Farrell7 Sara Fleury8 Simon Gascoin9 Amandine Guillot10 Angelika Humbert11 Eero Rinne12Andrew Shepherd13 Michiel R van den Broeke14 and John Yackel15

1European Space Agency (ESA-ESTEC) Keplerlaan 1 2201 AZ Noordwijk the Netherlands2European Space Agency (ESA-ESRIN) Via Galileo Galilei Casella Postale 64 00044 Frascati Italy3European Commission BREY 09154 1049 Brussels Belgium4EUMETSAT Eumetsat Allee 1 64295 Darmstadt Germany5DTU Space Elektrovej 28 2800 Lyngby Denmark6bgeos Industriestrasse 1 2100 Korneuburg Austria7University of Maryland 5825 University Research Court 20740 College Park MD USA8CTOHLEGOSCNRS 14 avenue Edouard Belin 31400 Toulouse France9CESBIO Universiteacute de Toulouse CNRSCNESIRDUPS 31400 Toulouse France10CNES 18 avenue Edouard Belin 31400 Toulouse France11Alfred-Wegner-Institute Helmholtz Centre for Polar and Marine Research Am Alten Hafen 262758 Bremerhaven Germany12Finnish Meteorological Institute PO Box 503 00101 Helsinki Finland13Centre for Polar Observation and Modelling University of Leeds LS2 9JT UK14Utrecht University Princetonplein 5 3584 CC Utrecht the Netherlands15University of Calgary 2500 University Drive NW Earth Sciences 356 Calgary Alberta Canada

Correspondence Michael Kern (michaelkernesaint)

Received 6 January 2020 ndash Discussion started 21 January 2020Revised 12 June 2020 ndash Accepted 18 June 2020 ndash Published 16 July 2020

Abstract The Copernicus Polar Ice and Snow TopographyAltimeter (CRISTAL) mission is one of six high-priority can-didate missions (HPCMs) under consideration by the Euro-pean Commission to enlarge the Copernicus Space Com-ponent Together the high-priority candidate missions fillgaps in the measurement capability of the existing Coper-nicus Space Component to address emerging and urgent userrequirements in relation to monitoring anthropogenic CO2emissions polar environments and land surfaces The am-bition is to enlarge the Copernicus Space Component withthe high-priority candidate missions in the mid-2020s to pro-vide enhanced continuity of services in synergy with thenext generation of the existing Copernicus Sentinel missionsCRISTAL will carry a dual-frequency synthetic-aperture

radar altimeter as its primary payload for measuring sur-face height and a passive microwave radiometer to sup-port atmospheric corrections and surface-type classificationThe altimeter will have interferometric capabilities at Ku-band for improved ground resolution and a second (non-interferometric) Ka-band frequency to provide informationon snow layer properties This paper outlines the user con-sultations that have supported expansion of the Coperni-cus Space Component to include the high-priority candidatemissions describes the primary and secondary objectives ofthe CRISTAL mission identifies the key contributions theCRISTAL mission will make and presents a concept ndash as faras it is already defined ndash for the mission payload

Published by Copernicus Publications on behalf of the European Geosciences Union

2236 M Kern et al CRISTAL high-priority candidate mission

1 Introduction

Earthrsquos cryosphere plays a critical role in our planetrsquos radia-tion and sea level budgets Loss of Arctic sea ice is exacer-bating planetary warming owing to the ice albedo feedback(eg Budyko 1969 Serreze and Francis 2006 Screen andSimmonds 2010) and loss of land ice is the principal sourceof global sea level rise (see Intergovernmental Panel on Cli-mate Change IPCC SROCC 2019) The rates and magni-tudes of depletion of Earthrsquos marine and terrestrial ice fieldsare among the most significant elements of future climateprojections (Meredith et al 2019) The Arctic provides fun-damental ecosystem services (including fishery managementand other resources) sustains numerous indigenous commu-nities and due to sea ice loss is emerging as a key area foreconomic exploitation The fragile ecosystems are subject topressures from a growing number of maritime and commer-cial activities The potentially devastating contribution of theAntarctic ice sheet to global sea level rise is also subject tolarge uncertainties in ice mass loss with high-end estimatesof sea level contribution exceeding a metre of global meansea level rise by 2100 (Edwards et al 2019)

A long-term programme to monitor the Earthrsquos po-lar ice ocean and snow topography is important tostakeholders with interests in the Arctic and AntarcticWhile Europe has a direct interest in the Arctic dueto its proximity (see httpseceuropaeuenvironmentefenewsintegrated-eu-policy-arctic-2016-12-08_en last ac-cess 10 July 2020) the Arctic is also of interest to othercountries and international communities Changes in theArctic environment affect strategic areas including politicseconomics (eg exploitation of natural resources includingminerals oil and gas and fish) and security Besides eco-nomic impacts of Antarctic and Arctic changes (Whitemanet al 2013) Europersquos interest in both polar regions is due totheir influence on patterns and variability in global climatechange thermohaline circulation and the planetary energybalance Last but not least changes in the Arctic system havepotential impacts on weather with consequences for extremeevents (Francis et al 2017) The Copernicus Polar Ice andSnow Topography Altimeter (CRISTAL) mission describedin this paper addresses the data and information requirementsof these user communities with a particular focus on address-ing Copernicus service requirements

In the following section we provide a background of theCopernicus programme and candidate missions that are be-ing prepared by the European Space Agency (ESA) in part-nership with the European Union (EU) in response to Coper-nicus user needs In Sect 3 we describe the objectives of theCRISTAL mission and its relation to the Copernicus servicesWe then discuss the key contributions from the CRISTALmission in terms of both specific mission objectives and ex-pected scientific contributions towards improved knowledgein Sect 4 In Sect 5 an overview of CRISTALrsquos current sys-tem concept and mode of operation is described This sec-

tion also highlights the use of heritage technology and needsdriving technical advancements to improve observational ca-pabilities beyond current missions Conclusions and a currentmission status statement are provided in Sect 6

2 Expansion and evolution of the Copernicus SpaceComponent

Copernicus was established to fulfil the growing needamongst European policymakers to access accurate andtimely information services to better manage the environ-ment understand and mitigate the effects of climate changeand ensure civil security To ensure the operational provi-sion of Earth observation data the Copernicus Space Com-ponent (CSC) includes a series of seven space missionscalled ldquoCopernicus Sentinelsrdquo which are being developedby the ESA specifically for Global Monitoring for Environ-ment and Security (GMES) and Copernicus The Coperni-cus programme is coordinated and managed by the Euro-pean Commission (EC) It includes Earth observation satel-lites ground-based measurements and services to processdata to provide users with reliable and up-to-date informationthrough a set of Copernicus services related to environmentaland security issues

The intense use of Copernicus has generated high expec-tations for an evolved Copernicus system There is now alarge set of defined needs and requirements With respect tothe polar regions user and observation requirements havebeen identified structured and prioritized in a process ledby the EC (Duchossois et al 2018a b) Two distinct sets ofexpectations have emerged from this user consultation pro-cess Firstly stability and continuity while increasing thequantity and quality of Copernicus products and servicesled to one set of requirements They are distinctly addressedin the considerations for the next generation of the currentSentinel-1 to Sentinel-6 series (see eg European Commis-sion 2017) Emerging and urgent needs for new types ofobservations constitute a second distinct set of requirementsthat are mainly addressed through the evolution of the Coper-nicus Space Segment service This evolution corresponds tothe enlargement of the present space-based measurement ca-pabilities through the introduction of new missions to answerthese emerging and urgent user requirements After extensiveconsultation six potential high-priority candidate missions(HPCMs) have been identified (ESA 2019b) the Coper-nicus Hyperspectral Imaging Mission for the Environment(CHIME) the Copernicus Imaging Microwave Radiome-ter (CIMR) the Copernicus Anthropogenic Carbon Diox-ide Monitoring (CO2M) mission the Copernicus Polar Iceand Snow Topography Altimeter (CRISTAL) the Coperni-cus Land Surface Temperature Monitoring (LSTM) missionand the L-band Synthetic Aperture Radar (ROSE-L)

The Cryosphere 14 2235ndash2251 2020 httpsdoiorg105194tc-14-2235-2020

M Kern et al CRISTAL high-priority candidate mission 2237

3 Objectives of the CRISTAL mission

The strategic environmental and socio-economic im-portance of the Arctic region has been emphasizedby the European Union in their integrated policy forthe Arctic (httpseceuropaeuenvironmentefenewsintegrated-eu-policy-arctic-2016-12-08_en last access10 July 2020) including the Arctic Ocean and its adjacentseas Considering the sparse population and the lack oftransport links a capacity for continuous monitoring of theArctic environment with satellites is considered essential Inlight of this and of the importance of the polar regions morewidely guiding documents have been prepared in an EC-leduser consultation process the Polar Expert Group (PEG)User Requirements for a Copernicus Polar Mission Phase 1report (Duchossois et al 2018a) hereafter referred to as thePEG 1 report and the Phase 2 report on usersrsquo requirements(Duchossois et al 2018b) hereafter referred to as the PEG2 report

The required geophysical parameters for the polar re-gions are summarized and prioritized in the PEG 1 reportwhich addresses objectives as defined in the EU Arctic policycommunication namely climate change environmental safe-guarding sustainable development and support to indige-nous populations and local communities Floating ice param-eters were listed as the top priority for a polar mission con-sidering the availability of existing Copernicus products andservices their needs for improvement (eg in terms of spatialresolution and accuracy) and the current level of their tech-nical andor scientific maturity The specific parameters in-clude sea ice extent concentration thickness type drift andvelocity as well as thin ice distribution iceberg detectiondrift and volume change and ice shelf (the floating exten-sion of the ice sheets) thickness and extent These parameterswere given top priority by the European Commission due totheir key position in operational services such as navigationand marine operations meteorological and seasonal predic-tion and climate model validation The PEG 1 report alsostresses the importance of a measuring capability for moun-tain glaciers and ice caps seasonal snow ice sheets oceansfresh water and permafrost

The Global Climate Observing System (GCOS 2011) hasstated that actions should be taken to ensure continuation ofaltimeter missions over sea ice They suggested continuationof satellite synthetic-aperture radar (SAR) altimeter missionswith enhanced techniques for monitoring sea ice thickness toachieve capabilities to produce time series of monthly 25 kmsea ice thickness with 01 m accuracy for polar regions It wasmentioned that near-coincident data would help resolve un-certainties in sea ice thickness retrieval Such measurementscould be achieved for example through close coordinationbetween radar and laser altimeter missions In addition to seaice thickness other sea ice parameters retrievable from SARaltimetry such as ice drift shear and deformation leads and

ice ridging were pointed to as observable for future improve-ment

While the Copernicus Sentinel-3 mission provides partialaltimetric measurements of the polar oceans the satellitesrsquoinclination limits the coverage to latitudes between 815 Nand 815 S With the expected ongoing loss of Arctic seaice these satellites will monitor only a small amount of theArctic ice cover during summer periods by the mid-2020s(see eg Quartly et al 2019) Currently the ESArsquos CryoSat-2 (Drinkwater et al 2004 Wingham et al 2006 Parrinelloet al 2018) is the only European satellite to provide monitor-ing of the oldest thickest multi-year ice However continuedmonitoring of the polar regions ndash and the Arctic Ocean northof 815 N in particular ndash is at risk since CryoSat-2 has beenoperating in its extended mission scenario since its nom-inal end-of-mission lifetime of October 2013 (see Fig 1)This risk has widely been recognized by the polar and oceansurface topography community For example at the 2019Ocean Surface Topography Science Team (OSTST) meeting(Chicago IL USA 21ndash25 October 2019) a recommendationwas recorded (in view of the preparations for CRISTAL andother missions currently in operation) ldquoto minimize likeli-hood of a gap in polar ocean and ice monitoring the OSTSTencourages Agencies to strive to launch a high-resolution po-lar altimeter in the early 2020s (such as the proposed HPCMCRISTAL) and to maintain operation of CryoSat-2 ICESat-2 and SARALAltiKa as long as possiblerdquo

Based on the user requirements and priorities outlined inthe PEG 1 report a set of high-priority mission parame-ters were defined by the ESArsquos CRISTAL Mission AdvisoryGroup (MAG) and the ESA which led to the CRISTAL mis-sion objectives (Table 1) The primary objectives drive thedesign and performance specifications of the CRISTAL mis-sion whereas the secondary objectives reflect the opportu-nity to support a wider range of users and services

By addressing these objectives the mission responds to anumber of required parameters of interest and applicationsin Copernicus services A mapping of the services to the pa-rameters of interest and applications is listed in Table 2

4 Key contributions of the CRISTAL mission

The following sections describe the key contributions of themission in more detail including the key requirements thatguide the implementation of the mission

41 Sea ice freeboard and thickness

Sea ice plays a critical role in Earthrsquos climate system sinceit provides a barrier between the ocean and atmosphere re-stricting the transfer of heat between the two Due to its highalbedo the presence of sea ice reduces the amount of solarenergy absorbed by the ocean Arctic sea ice rejects brineduring formation and fresh water during melting and it is

httpsdoiorg105194tc-14-2235-2020 The Cryosphere 14 2235ndash2251 2020

Figure 1 Past operating approved and proposed polar topography altimeter missions By the mid-2020s CRISTAL will fill the gapacquiring climate-critical data over polar ice north and south of 815 latitude (image EOGBESA)

therefore a driving force of the global thermohaline circu-lation as well as the stratification of the upper layer of theocean The sea ice provides a critical habitat for marine mam-mals and for biological activity (eg Tynan et al 2009) andit is a platform that enables subsistence hunting and travel forindigenous coastal communities

The sea ice cover of the Arctic Ocean is waning rapidlyBy 2019 the decline in September Arctic sea ice extent wasabout 13 per decade relative to the 1981ndash2010 averageand the older thicker multi-year ice cover comprisedsim 20 of the winter ice pack compared tosim 45 in the 1980s (Per-ovich et al 2017 IPCCSROCC 2019) In the SouthernOcean sea ice is undergoing regional changes with a declineobserved in the Amundsen and Bellingshausen seas (Shep-herd et al 2018) These losses are having a profound impacton the climate environment and ecosystems of both polarregions Monitoring the polar oceans is therefore of regionaland global importance and the long-term continuity of seaice measurements is essential to extending both climate andoperational data services

Global warming and its Arctic amplification continue tocontribute to the decrease in multi-year ice in the central Arc-tic Ocean (north of 815 N) It is therefore critical to obtain

continuous pan-Arctic observations of sea ice thickness ex-tending as close as possible to the North Pole Continuousmonitoring of Arctic Ocean sea ice conditions is necessaryfor safe navigation through ice-covered waters When linkedto previous measurements from Envisat ICESat CryoSat-2 and ICESat-2 the CRISTAL mission will deliver obser-vations that provide a long-term record of sea ice thicknessvariability and trends that are critical to supporting climateservices Since sea ice thickness is an essential climate vari-able (see GCOS 2011) its continuous measurement is re-quired to understand the Arctic system and how ice loss isimpacting global climate

Shipping in ice-covered Arctic waters has increased sig-nificantly in recent years and is expected to continue to doso over the coming decades (IPCCSROCC 2019) In ad-dition to traditional maritime operations and fishing in thehigh Arctic several polar-class cruise liners are under con-struction This means an increase in the need and scope ofoperational ice information services A primary data sourcefor national ice services is currently synthetic-aperture radar(SAR) imagery specifically data acquired by Sentinel-1Aand Sentinel-1B RADARSAT-2 and the RADARSAT Con-stellation Mission Thus independent measurements of sea

Table 1 CRISTAL mission objectives

Nature Target Objective

Primary Sea ice To measure and monitor variability of Arctic and Southern Ocean sea ice thickness and itssnow depth Seasonal sea ice cycles are important for both human activities and biologicalhabitats The seasonal to inter-annual variability of sea ice is a sensitive climate indicator it isalso essential for long-term planning of any kind of activity in the polar regions Knowledgeof snow depth will lead to improved accuracy in measurements of sea ice thickness and is alsoa valuable input for coupled atmospherendashicendashocean forecast models On shorter timescalesmeasurements of sea ice thickness and information about Arctic Ocean sea state are essentialsupport to maritime operations over polar oceans

Primary Land ice To measure and monitor the surface elevation and changes therein of polar glaciers ice capsand the Antarctic and Greenland ice sheets The two ice sheets of Antarctica and Greenlandstore a significant proportion of global fresh water volume and are important for climate changeand contributions to sea level Monitoring grounding-line migration and elevation changes infloating and grounded ice sheet margins is important for identifying and tracking emerginginstabilities These instabilities can negatively impact the stability of the ice sheets leading toice mass loss and accelerated sea level rise

Secondary Ocean To contribute to the observation of global ocean topography as a continuum up to the polarseas Polar altimetry will contribute to the observation system for global observation of meansea level mesoscale and sub-mesoscale currents wind speed and significant wave height In-formation from this mission serves as critical input to operational oceanography and marineforecasting services in the polar oceans

Secondary Inland water To support applications related to coastal and inland waters Observations of water level atArctic coasts as well as rivers and lakes are key quantities in hydrological research Riversand lakes not only supply fresh water for human use including agriculture but also maintainnatural processes and ecosystems The monitoring of global river discharge and its long-termtrend contributes to the evaluation of global freshwater flux that is critical for understandingthe mechanism of global climate change (Prowse et al 2011 Zakharova et al 2020) Changesto seasonal freezing of Arctic rivers and lakes in the context of climate change will also beimportant to study and understand Their observation could help forecast their evolution andorganize alternative modes of transport

Secondary Snow To support applications related to snow cover and permafrost in Arctic regions Snowmelt tim-ing is a key parameter for hydrological research since it modulates the river discharge of Arcticbasins (Shiklomanov et al 2007) Surface state change in permafrost regions indicates the ini-tiation of ground thaw and soil microbial activities in the seasonally unfrozen upper soil (active)layer The rapid evolution of the permafrost also has important impacts on human activities andinfrastructures

ice thickness distribution at reasonable latencies provided byCRISTAL will complement existing SAR measurements andbenefit operational ice charting Furthermore observed seaice thickness or freeboard distributions can be assimilatedinto sea ice models to generate ice forecasts needed for icenavigation and offshore operations

Historically satellite observations had primarily been usedto monitor ice extent until Laxon et al (2003) producedthe first Arctic-wide sea ice thickness estimates from Euro-pean Remote Sensing (ERS) satellite radar altimetry Sincethen various methods for converting the received signal tophysical variables have been established (Giles et al 2008aLaxon et al 2013 Kurtz et al 2014 Ricker et al 2014Price et al 2015 Tilling et al 2018 Hendricks et al 2018)The capability to obtain an estimate of sea ice freeboard and

thickness and convert it to estimates of ice volume has en-abled scientists to better understand the changing Arctic icecover Most recently sea ice freeboard has been estimatedfrom both Ka- and Ku-band measurements (Armitage andRidout 2015 Guerreiro et al 2016 Lawrence et al 2018)

Most sea ice thickness products are currently provided ona 25 km grid (see eg Sallila et al 2019 for an overviewof different products currently available) which correspondsto the GCOS user requirements (GCOS 2011) but does notmeet the specified accuracy requirements of 01 m The resid-ual systematic uncertainty in sea ice thickness is estimatedto be 056ndash061 m for ICESat (Connor et al 2013) and itis 06 m for CryoSat-2 observations over first-year ice and12 m for those over multi-year ice (Ricker et al 2014) Theuncertainty in ice thickness derived from CryoSat-2 obser-

Table 2 Copernicus services addressed by CRISTAL

Copernicus service Relevant geophysical parameters of interest Core information service addressed or affected(forecasting monitoring or projections)

Copernicus MarineEnvironmental Monitoring Ser-vice (CMEMS)

ndash Sea ice thickness and snow depthndash Sea level anomaly and geostrophic oceancurrents in polar oceansndash Significant wave height in polar oceansndash Global sea levelndash Global sea surface wind and waves

Maritime safety coastal and marine environ-ment marine resources and weather seasonalforecasting and climate activities

Copernicus ClimateChange Service (C3S)

ndash Ice sheet topographyndash Sea ice thickness and volumesndash Global sea levelndash Snow depth over sea ice

Observations climate reanalysis seasonalforecasts and climate projections

Copernicus LandMonitoring Service(CLMS)

ndash Ice sheet and glacier topography Biophysical monitoring land cover and landuse mapping thematic hotspot mapping refer-ence data and ground motion service

Copernicus Atmospheric Moni-toring Service (CAMS)

ndash Snow depth over sea ice Meteorology and climatology seasonal fore-casts and climate projections

Copernicus EmergencyManagement Service(CEMS)

ndash Lake and river level and stage Flood awareness forecast and emergency man-agement system mapping

vations is driven mainly by the unknown penetration of theradar pulse into the snow layer as a result of variable snowproperties (Nandan et al 2017 2020) as well as the choiceof retracker (Ricker et al 2014) Reference is also made toMallett et al (2020) who find that assumptions concerningthe time evolution of overlying snow density can lead to un-derestimates of sea ice thickness from radar altimetry

While the focus of the Copernicus programme is on theArctic comprising all areas north of the southernmost tip ofGreenland (sim 60 N) the parameters specified for polar re-gions should equally be provided for its southern counterpartthe Antarctic as well as all non-polar snow- and ice-coveredsurfaces

The requirements for CRISTAL are currently stated to pro-vide sea ice freeboard with an accuracy of 003 m along or-bit segments of less than or equal to 25 km during wintermonths and to provide meaningful freeboard measurementsduring summer months Winter months are months from Oc-tober to April in the Northern Hemisphere and from May toOctober in the Southern Hemisphere The system shall becapable of delivering sea ice thickness measurements with avertical uncertainty of less than 015 m along orbit segmentsle 25 km in winter months and of providing meaningful seaice thickness estimates during summer months The along-track resolution of sea ice thickness measurements shall beat least 80 m The uncertainty requirement for sea ice thick-ness comes with a caveat as the thickness uncertainty de-pends on the uncertainty of auxiliary products In the caseof CRISTAL snow thickness will be measured by the sys-

tem but snow and ice densities will still have to be estimatedby other means In light of the current 02 m sea ice thick-ness uncertainty from CryoSat-2 data assessed by Tillinget al (2018) for a gridded monthly product and the antici-pated improvement from the dual-altimetry technology espe-cially in the snow depth and propagation estimates a highervertical uncertainty would seem reachable but requires fur-ther study Currently the retrieval accuracy of sea ice free-board is limited by the range resolution of a radar altimeterThe large bandwidth of 500 MHz is an important driver forthe CRISTAL instrument concept generation A bandwidthof 500 MHz will improve the range resolution from 50 cm(as for CryoSat-2 with 320 MHz bandwidth) to sim 30 cm forCRISTAL A radiometer will help in activendashpassive synergyto classify sea ice type (see eg Tran et al 2009 for furtherjustification)

42 Snow depth over sea ice

An accurate estimate of snow depth over Arctic sea ice isneeded for signal propagation speed correction to convertradar freeboard to sea ice freeboard and freeboard to seaice thickness (Laxon et al 2003 2013) The penetrationaspects of a dual-frequency snow depth retrieval algorithmover Antarctica are complex (Giles et al 2008b Shepherdet al 2018) and are not further elaborated here In additionto uncertainty reduction for ice thickness and freeboard com-putation the variation in snow depth is a parameter that ishighly relevant for climate modelling ice navigation andpolar ocean research The snow climatology of Warren et

al (1999) is still the single most used estimate of snow depthin sea ice thickness processing (Sallila et al 2019) The un-certainty in the original Warren et al (1999) snow depth es-timates is halved over first-year ice (Kurtz and Farrell 2011Zhou et al 2020) but snow still represents the single mostimportant contribution to uncertainty in the estimation of seaice thickness and volume (Tilling et al 2018) The studiesof Lawrence et al (2018) and Guerreiro et al (2016) showthe possibility of using Ku- and Ka-bands in mitigating thesnow depth uncertainty Dual-frequency methods improvethe ability to reduce and estimate the uncertainties relatedto snow depth and sea ice thickness retrieval The modellingcommunity is particularly interested in the uncertainty infor-mation according to the user requirement study in the PEG1 report Having better abilities to estimate the related un-certainties improves prediction quality assessment of annualsnowmelt over Arctic sea ice (Blockley and Peterson 2018)The stratigraphy and electromagnetic properties of the snowlayer contrast with those of the underlying ice and can be ex-ploited to retrieve information on the snow layer propertiesif contemporaneous measurements are acquired from mul-tiple scattering horizons (for details see Giles et al 2007who demonstrated the propagating uncertainties associatedwith snow depth and other geophysical parameters) A dual-frequency satellite altimeter as proposed for the CRISTALmission will address this need CRISTAL aims to provide anuncertainty in snow depth retrieval over sea ice of less than orequal to 005 m The additional Ka-band measurements witha 500 MHz bandwidth support the discrimination betweenthe ice and snow interfaces

43 Ice sheets glaciers and ice caps

Earthrsquos land ice responds rapidly to global climate changeFor example melting of glaciers ice caps and ice sheets overrecent decades has altered regional and local hydrologicalsystems and has impacted sea levels and patterns of globalocean circulation The Antarctic and Greenland ice sheets areEarthrsquos primary freshwater reservoirs and due to their pro-gressive imbalance have made an accelerating contributionto global sea level rise during the satellite era (Shepherd etal 2018 2019) Glaciers outside of the ice sheets constitutednearly one-third of all sea level rise over the past 2 decades(Gardner et al 2013 Wouters et al 2019) Although ice dy-namical models have improved future losses from the po-lar ice sheets remain the largest uncertainty in sea level pro-jections Due to their continental scale remote location andhostile climatic environment satellite measurements are theonly practical solution for spatially and temporally completemonitoring of the polar ice sheets

Estimates of ice sheet surface elevation change provide awealth of geophysical information They are used as the basisfor computing the mass balance and sea level contribution ofice sheets of both Greenland and Antarctica (McMillan et al2014 2016 Shepherd et al 2012) for identifying emerging

signals of mass imbalance (Flament and Reacutemy 2012 Wing-ham et al 2009) and for determining the loci of rapid iceloss (Hurkmans et al 2014 Soslashrensen et al 2015) Throughcombination with regional climate and firn models of surfaceprocesses surface elevation change can be used to isolate icedynamical changes at the scale of individual glacier catch-ments (McMillan et al 2016)

A unique and continuous record of elevation measure-ments is provided by radar altimeters dating back to 1992The maps are typically delivered in (1) high-resolution (5ndash10 km) rates of surface elevation change (for single or mul-tiple missions typically computed as a linear rate of changeover a period of several years to decades) and (2) frequently(monthly quarterly) sampled time series of the cumulativechange averaged across individual glacier basins In addi-tion to being used to quantify rates of mass balance and sealevel rise they also have a range of other applications suchas detection of subglacial lake drainage (Siegert et al 2016)and investigations of the initiation and speed of inland prop-agation of dynamic imbalance (Konrad et al 2017) that pro-vide valuable information relating to the underlying physicalprocesses that drive dynamical ice loss

CRISTAL will extend the decades-long record of the gen-eration of elevation measurements provided by radar altime-ters It will produce maps of ice surface elevation with anuncertainty of 2 m (the vertical accuracy threshold is 2 man absolute accuracy of 05 m can be assumed and thereis a relative accuracy goal of 02 m) The system shall becapable of delivering surface elevation with an along-trackresolution of at least 100 m and a monthly temporal sam-pling CRISTAL will be capable of tracking steep terrainwith slopes less than 15 using its SARIn (interferometricsynthetic-aperture radar) mode High-resolution swath pro-cessing over ice sheets (about 5 km wide) can reveal com-plex surface elevation changes related to climate variabilityand ice dynamics as well as subglacial geothermal and mag-matic processes (see eg Foresta et al 2016) Elevation mea-surements of regions with smaller glaciers are often missingin CryoSat-2 data Indeed tracking algorithms are not effi-cient when rough terrain is encountered Improvement in thetracking over glaciers is thus a key element in the instrumentconcept generation

44 Sea level and coastal and inland water

Over the years and through constant improvement of the dataquality satellite altimetry has been used in a growing num-ber of applications in Earth sciences The altimeter measure-ments are helping us to understand and monitor the oceanits topography dynamics and variability at different scalesSatellite observations for studying understanding and mon-itoring the ocean are more than essential over polar areaswhere in situ data networks are very sparse and where pro-found and dramatic changes occur This has also been ex-pressed and emphasized by the Copernicus Marine Environ-

mental Monitoring Service (CMEMS) as ldquoensuring continu-ity (with improvements) of the CryoSat-2 mission for sealevel monitoring in polar regionsrdquo (CMEMS 2017) ldquoReli-able retrieval of sea level in the sea ice leads to reach theretrieval accuracy required to monitor climate changerdquo is an-other CMEMS recommendation for polar and sea ice moni-toring (see CMEMS 2017)

Current data from the CMEMS catalogue do not allowa satisfactory sampling north of 815 N It is of prime im-portance that the CRISTAL orbit configuration allows mea-surement coverage of the central Arctic Ocean with an omis-sion not exceeding 2 of latitude around the poles Sea levelanomalies (SLAs) over frozen seas can only be provided bymeasurements in the leads CRISTAL will contribute to theobservation system for global observation of mean sea level(sub-)mesoscale currents wind speed and significant waveheight as a critical input to operational oceanography andmarine forecasting services and it will support sea ice thick-ness retrieval in the Arctic

The high-inclination orbit of CRISTAL associated withhigh-resolution SAR and SARIn bi-band altimetry measure-ments would considerably extend our monitoring capabil-ity over the polar oceans The development of tailored pro-cessing algorithms should not only have to track the low-frequency sea level trend in the presence of sea ice and tocharacterize large-scale and mesoscale ocean variations overregions not covered by conventional ocean altimeters Be-yond the observations of ice elevation variations CRISTALwould offer the unique opportunity to improve our knowl-edge of the mutual oceanndashcryosphere interactions over short-and long-term timescales for both poles Southern Ocean cir-culation plays a key role in shaping the Antarctic cryosphereenvironment First it regulates sea ice production as sea iceforms and ejects brine into the ocean the ocean destabilizesand warms submerged waters that reach the ocean surfacelimiting further ice production Second it impacts Antarcticice sheet melt when warm and salty ocean currents accessthe base of floating glaciers through bathymetric troughs ofthe Antarctic continental shelf These ocean currents melt theice shelves from below and are the main causes of the currentdecline in floating ice shelves (Shepherd et al 2019 Smithet al 2020) Thus melting of ice shelves represents one ofthe largest uncertainties in the current prediction of globalsea level change (Edwards et al 2019) creating a major gapin our ability to respond and adapt to future climate changeTightly linked with glacier melt polar shelf circulation andits interaction with large-scale circulation also control therate of bottom water production and deep-ocean ventilationwhich impact the worldrsquos oceans on a timescale rangingfrom decades to millennia Therefore with a designed oper-ational lifetime of at least 75 years (including in-orbit com-missioning) the observation from the same sensor of eachcomponent of these multi-scale icendashocean interactions wouldmake CRISTAL unique in its capability to address climateissues of regional and global relevance Over oceans a sec-

ondary objective for the mission the satellite will be ableto measure sea surface height with an uncertainty of lessthan 3 cm The main advantages and drawbacks of the Ka-band over the oceanic surface have been reviewed in Bon-nefond et al (2018) Given its high along-track resolutionof less than 10 km and high temporal resolution of sea levelanomalies the mission can further contribute a suite of sealevel products including sea surface height and mean sea sur-face (vertical accuracy in sea level anomaly retrieval of lessthan 2 cm is requested) The radiometer on board CRISTALcorrects the satellite altimeter data for the excess path de-lay resulting from tropospheric humidity The microwave ra-diometer measurements will complement wet troposphericcorrections derived from numerical weather prediction andnon-collocated atmospheric data from other satellite instru-ments to help meet the range accuracy requirement (Picardet al 2015 Legeais et al 2014 Vieira et al 2019)

Observation of water level at the (Arctic) coast as wellas of rivers and lakes is a key quantity in hydrological re-search (eg Jiang et al 2017) Rivers and lakes not onlysupply fresh water for human use including agriculture butalso maintain natural processes and ecosystems The moni-toring of global river discharge and its long-term trend con-tributes to the monitoring of global freshwater flux which iscritical for understanding the mechanism of global climatechange Satellite radar altimetry is a promising technology todo this on a regional to global scale Satellite radar altime-try data have been used successfully to observe water lev-els in lakes and (large) rivers and have also been combinedwith hydrologic and hydrodynamic models Combined withgravity-based missions like the NASA and Deutsches Zen-trum fuumlr Luft- und Raumfahrt (DLR) GRACE and GRACE-FO missions the joint use of the data will give informationfor ground water monitoring in the future

45 Icebergs

Iceberg detection volume change and drift have been listedas a priority user requirement (Duchossois et al 2018a b)

Icebergs present a significant hazard to marine operationsDetection of icebergs in open water and in sea ice generallyplaces a priority on wider satellite swaths to obtain greatergeographic coverage There is a need for automatic detec-tion of icebergs for the safety of navigation and chart produc-tion Iceberg concentration is given in CMEMSrsquo catalogue at10 km resolution covering Greenland waters SAR imageryis the core input for iceberg detection However iceberg de-tection (in particular small icebergs) is also possible usinghigh-resolution altimeter waveforms Tournadre et al (2018)demonstrated detection of icebergs from CryoSat-2 altimeterdata using several modes and mention promising results withthe Sentinel-3 data which would be fed into a comprehen-sive dataset already built as part of the ALTIBERG project(Tournadre et al 2016) The volume of an iceberg is valuableinformation for operational services and climate monitoring

For climate studies the freshwater flux from the volume ofice transported by icebergs is a key parameter with large un-certainties related to the volume of the icebergs Measuringvolume is currently only possible with altimetry by providingthe iceberg freeboard elevation from the ocean surface Ice-berg volume has been calculated with altimetry with EnvisatJason-1 and Jason-2 (eg Tournadre et al 2015)

CryoSat-2 tracking over icebergs is operational but ice-bergs with high freeboards may be missed in the currentrange window The range window definition for CRISTAL isdefined in order to ensure that echoes from icebergs are cor-rectly acquired In-flight performances for the measurementof the angle of arrival from CryoSat-2 are around 25 arcsecAn equivalent performance is necessary to retrieve across-track slopes and elevations The CRISTAL design of theinstrument and the calibration strategy will be designed tocomply with the specification of 20 arcsec CRISTAL willprovide the unprecedented capability to detect icebergs at ahorizontal resolution (gridded product) of at least 25 m Theproducts will be produced every 24 h in synergy with otherhigh-resolution data such as SAR imagery Iceberg distribu-tion and volume products will be produced at 50 km resolu-tion (gridded) on a monthly basis

46 Snow on land and permafrost

CRISTAL may support and contribute to studies and ser-vices in relation to seasonal snow cover and permafrost ap-plications over land These are considered a secondary ob-jective for the mission The ability to retrieve snow depthwith Ku- and Ka-band altimetry is limited over land (Rottet al 2018) Snow studies over land area are so far largelylimited to scatterometer when the Ku-band is used examplesof such retrievals are reviewed in Bartsch (2010) Measure-ments as provided by CRISTAL may however be useful inretrieving internal properties of the snowpack such as the ex-istence of ice layers (eg due to rain on snow Bartsch et al2010) The relevant properties of an upper snow layer con-trast with those of an underlying ice layer (see also Sect 42)Further snow structure is reflected in differences observedin radar observations using different frequencies (Lemmetyi-nen et al 2016) Snow structure anomalies as well as landsurface state (freeze and thaw) are expected to be identifiedby time series analyses as such processes alter penetrationdepth Altimeter data are also rarely used for permafrost stud-ies Such data can also be applied for monitoring lake levelas a proxy for permafrost change (Zakharova et al 2017)Surface status is closely interlinked with ground temperature(eg Kroisleitner et al 2018) but usage of satellite altimetryin this context remains unexplored Signal interaction withvegetation limits the applicability of Ku- and Ka-bands forsoil observations regarding freeze and thaw status (Park etal 2011) as well as surface height Wider use of altimetryfor snow and permafrost applications requires higher spatialresolution and temporal coverage than what is available to

Figure 2 Illustration of the CRISTAL observation concept over seaice employing a twin-frequency twin-antenna SAR radar altimeterwith interferometric capabilities at Ku-band (image credits CLS)

date An improvement regarding the latter issues is expectedwith CRISTAL which will expand the utility of altimeter ob-servations for permafrost and snow monitoring over land

5 CRISTAL mission concept

This section summarizes the envisaged primary payloadcomponents to address the CRISTAL mission objectives Thedesign draws from the experience of several in-orbit missionsin addition to the ongoing developments within the Sentinel-6 and MetOp-SG programmes and has a 75-year lifetimeCRISTALrsquos primary payload complement consist of the fol-lowing

ndash A synthetic-aperture radar (SAR) altimeter operat-ing at Ku-band and Ka-band centre frequencies isused for global elevation and topographic retrievalsover land and marine ice ocean and terrestrial sur-faces (see Figs 2 and 3) In Ku-band (135 GHz) theSAR altimeter can also be operated in interferomet-ric (SARIn) mode to determine across-track echo lo-cation The Ka-band channel (3575 GHz) has been in-troduced to improve snow depth retrievals over sea ice(see eg Guerreiro et al 2016) A (vertical) range res-olution of about 31 cm will be achieved to enhancefreeboard measurement accuracy Furthermore a highalong-track resolution of about 20 m is envisaged toimprove ice floe discrimination Heritage missions in-clude CryoSat-2 (SARInterferometric Radar AltimeterSIRAL) Sentinel-6 (Poseidon-4) and SARAL (Satel-lite with ARgos and ALtiKa) The CRISTAL altime-ter (IRIS) is based on Poseidon-4 (Sentinel-6) andSIRAL (CryoSat-2) together with the addition of a

Ka-band channel (analogous to AltiKa) and a band-width of 500 MHz (at both frequencies) to meet the im-proved range resolution requirement in comparison toheritage altimeters It has the capability for fully fo-cused SAR processing for enhanced along-track reso-lution by means of resolving full scatterer phase his-tory (Egido and Smith 2017) Digital processing will beimplemented including matched filter range compres-sion and on-board range cell migration (RCM) com-pensation by means of a range migration compensation(RMC) mode for on-board data reduction (heritage fromPoseidon-4) reducing downlink load With respect tothe dual-frequency antenna (Ku- and Ka-band) an en-hanced antenna mounting baseplate for improved base-line stability over CryoSat-2 will be required (20 arcsecvs sim 30 arcsec for CryoSat-2)

ndash A high-resolution passive microwave radiometer is in-cluded with the capability to provide data allowing re-trievals of total column water vapour over the globalocean and up to 10 km from the coast (by means of im-proving the measurement system with high-frequencychannels) The radiometer may also support cryosphereapplications such as sea ice type classifications (Tran etal 2009) Concerning the microwave instrument selec-tion potential options include a US Custom FurnishedItem based on the National Aeronautics and Space Ad-ministration (NASA) Jet Propulsion Laboratory (JPL)AMR-C (Advanced Microwave Radiometer ndash ClimateQuality) development of an EU high-resolution ra-diometer solution and a two-channel solution derivedfrom the Sentinel-3 microwave radiometer The feasi-bility of each of these options will be further evaluatedin the next mission phase (Phase B2 at the time of thesystem preliminary design review expected late 2021)

ndash A global navigation satellite system (GNSS) receivercompatible with both Galileo and Global PositioningSystem (GPS) constellations provides on-board timingnavigation and provision of data for on-ground pre-cise orbit determination Heritage GNSS solutions ex-ist such as those based upon the GPS- and Galileo-compatible Sentinel-1 Sentinel-2 Sentinel -3 CD andSentinel-6 AB receivers Precise Orbit Determinationproducts will be provided by the Copernicus Precise Or-bit Determination service

ndash A Laser Retroreflector Array (LRA) for use by theSatellite Laser Ranging network and by the Interna-tional Laser Ranging Service for short-arc validation ofthe orbit Heritage concepts suitable for CRISTAL in-clude CryoSat-2 and Sentinel-3 LRAs

Three modes of radar operation are envisaged which areautomatically selected depending on the geographic locationover the Earthrsquos surface (see Table 3 and Fig 3) prioritizingthe retrieval of relevant geophysical parameters of interest

Figure 3 Indicative mission geographic operating mode maskused in CRISTAL altimeter data volume sizing Magenta land iceclosed-burst SARIn mode also including smaller ice caps orangesea ice and icebergs open-burst SARIn mode (maximum cover-age in Northern and Southern Hemisphere) green open and coastalocean SARIn reduced window mode purple inland water (this isnot anticipated as a mode but may be derived from one of the threekey modes) Note the wedge type feature in some of the images isan artefact of the display software

ndash Sea ice and iceberg mode in Fig 3 the proposed cov-erage is shown in orange It is proposed that this modemakes a step forward in ice thickness retrieval by oper-ating the instrument with the SAR interferometer con-figuration in Ku-band ie a two-antenna cross-track in-terferometric principle The measurement mode will bein an open-burst or interleaved arrangement in whichreceptions occur after each transmitted pulse This re-sults in an along-track resolution by ground processingto up to a few metres which enables small sea ice sheetsto be distinguished and narrow leads between them tobe detected The disadvantages of the open-burst trans-mission versus a closed-burst operation mode includea larger data volume and the power demand as wellas variations in the pulse repetition frequency aroundthe orbit The interferometric operation allows the lo-cation of across-track sea ice leads whilst open-bursttiming allows full scatterer phase history reconstructionfor fully focused processing (Egido and Smith 2017)This improves sea ice lead discrimination (by meansof improvement in sampling and resolution) and henceretrievals of elevation and polar SLAs by a significantfactor (Armitage and Davidson 2014) Open-burst Ka-

Table 3 Key altimeter characteristics in the different modes of operation (credits Thales Alenia Space France) na not applicable

Sea ice and icebergs Land ice

Open and Ice sheet interiorcoastal ocean Sea ice Icebergs (ice sheetice caps) Ice margin Glaciers

σ0 range in Ku-band 6 to 25 dB 0 to 55 dB 0 to +40 dB minus10 to +40 dB minus10 to +40 dBσ0 range in Ka-band +8 to +27 dB +2 to +57 dB 2 to +42 dB minus8 to +42 dB minus8 to +42 dBMeasurement mode SAR closed-burst SARIn interleaved SARIn closed-burstin Ku-bandMeasurement mode SAR closed-burst SAR interleaved SAR closed-burstin Ka-bandRange window size 256 points 256 points 256 points 1024 points 1024 points 1024 pointsTracking window size 256 points 256 points 256 points 2048 points na naRange window size 64 m 64 m 64 m 256 m 256 m 256 mTracking window size 64 m 64 m 64 m 512 m na naTracking mode Closed-loop Closed-loop Closed-loop Closed-loop Open-loop Open-loopOn-board processing RMC RMC na na na naOptional on-board Yes na na na na naprocessing

band SAR is also provided to allow for improved re-trieval of snow depth over sea ice

ndash Land ice mode in Fig 3 the proposed coverage isshown in magenta Land ice elevation is retrieved bymeans of improved surface tracking based on the largerange window The accuracy of elevation retrievals islikely improved by a factor of 2 by means of increas-ing the number of echoes per unit time by a factor of 4over the CryoSat-2 heritage design The Ku-band SARinterferometer is used to retrieve the across-track pointof closest approach supplemented with Ka-band SARClosed-burst operation (see eg Raney 1998) is usedover this surface type in which the reflections arriv-ing back at the radar are received after each transmittedburst has finished

ndash Open and coastal ocean mode in Fig 3 the pro-posed coverage shown in magenta provides Arctic andsouthern polar ocean retrieval of SLAs and precisionSAR altimetry to complement other ocean topogra-phy missions including Sentinel-3 Sentinel-6 and next-generation topographic missions In the case of openocean closed-burst SAR operation at Ku-band and Ka-band is used and the RMC on-board processing isapplied This was first implemented in the frame ofSentinel-6 which provides a considerable gain in instru-ment data rate reduction In addition data will be col-lected over inland water regions using one of the abovemodes

The latency of CRISTAL data products follows the re-quirements expressed in the PEG 1 and PEG 2 reports andprovides measurements of different latencies according tothe application need The product latencies range from 3 h

(some ocean Level 2 products) to 6 h (sea ice freeboard prod-ucts) 24 h (sea ice thickness sea ice snow depth and icebergdetection products) 48 h (some ocean Level 1 and Level 2products) and up to 30 d (surface elevation and some oceanLevel 1 products) These data latencies indicate the time in-terval from data acquisition by the instrument to delivery asa Level 1B data product to the user

6 Conclusions and CRISTAL mission status

CRISTAL directly addresses the EU Arctic policy and pri-mary user requirements collected by the European Commis-sion and provides sustained long-term monitoring of seaice thickness and land ice elevations It thereby respondsto needs for continuous pan-Arctic altimetric monitoring in-cluding the region of the Arctic Ocean north of 815 NAntarctica will be equally well covered The mission servesseveral key Copernicus operational services in particular theClimate Change Service and Marine Environmental Moni-toring Service and makes contributions to the Land Moni-toring Service Atmospheric Monitoring Service and Emer-gency Management Service

CRISTAL will cover the polar regions with a Ku-bandinterferometric synthetic-aperture radar altimeter with sup-porting Ka-band channel In addition the payload containsa high- and low-frequency passive microwave radiometer toperform wet troposphere delay correction and surface-typeclassification over sea ice and ice sheets The mission is de-signed for a 75-year design lifetime and will fly in an op-timized orbit covering polar regions (omissionle 2 weeklyand monthly sub-cycles) A key element is the high along-track resolution (by ground processing up to a few metreswhen the novel interleaved SAR operation mode is used) to

distinguish open ocean from sea ice surfaces Thanks to thedual-frequency SAR altimetry capability a snow depth prod-uct will be produced over sea ice with high accuracy in re-sponse to long-standing user needs

CRISTAL has undergone and completed parallel prepara-tory (Phase A and B1) system studies in which missionand system requirements have been investigated and consoli-dated The intermediate system requirement review has beencompleted with parallel industrial consortia compliant withthe mission and system requirements Next steps include thefull definition implementation and in-orbit commissioningof CRISTAL (Phases B2 CD and E1) where a prototypeand recurrent satellite will be developed

Appendix A List of abbreviations

AltiKa Altimeter Ka-bandAMR-C Advanced Microwave Radiometer ndash Climate QualityC3S Copernicus Climate Change ServiceCalVal Calibration and validationCAMS Copernicus Atmospheric Monitoring ServiceCEMS Copernicus Emergency Management ServiceCGLS Copernicus Global Land ServiceCIMR Copernicus Polar Passive Microwave Imaging MissionCLS Collecte Localisation SatellitesCMEMS Copernicus Marine Environmental Monitoring ServiceCOP21 United Nations Framework Convention on Climate Change 21st Conference of the PartiesCRISTAL Copernicus Polar Ice and Snow Topography AltimeterCSC Copernicus Space ComponentdB DecibelEC European CommissionEO Earth observationESA European Space AgencyEU European UnionEUMETSAT European Organisation for the Exploitation of Meteorological SatellitesFMI Finnish Meteorological InstituteGCOS Global Climate Observing SystemGMES Global Monitoring for Environment and SecurityGNSS Global navigation satellite systemGPS Global Positioning SystemIPCC Intergovernmental Panel on Climate ChangeIRIS Interferometric Radar Altimeter for Ice and SnowJPL Jet Propulsion LaboratoryLRA Laser Retroreflector ArrayMetOp-SG Meteorological Operational Satellite ndash Second GenerationNASA National Aeronautics and Space AdministrationOCO Open and coastal oceanOSTST2019 Ocean Surface Topography Science Team Meeting 2019PEG Polar Expert GroupRADAR Radio detection and rangingRCM Range cell migrationRMC Range migration compensationSAR Synthetic-aperture radarSARIn Interferometric SARSARAL Satellite with ARgos and ALtiKaSIRAL SARInterferometric Radar AltimeterSLA Sea level anomalySTC Short-time critical

Data availability The work described in this paper is the resultof consultations with Copernicus users and services as well as theCRISTAL Mission Advisory Group No specific datasets have beenused

Author contributions MK as ESA mission scientist is responsi-ble for the mission requirements for the CRISTAL mission and wasresponsible for the overall conceptualization and structure of the pa-per He drafted the manuscript and completed revisions based on co-author contributions and review RC led the CRISTAL technical ac-tivities and contributed to the system concept description in Sect 5BB JB TC MRD AG AL ML RM INT TP and GR were in-volved in the supporting scientific and campaign activities or in thetechnical activities with industry They contributed to Sects 5 and 6of this paper and to the overall prepublication critical review of thework EA and CMP described and provided input and critical reviewof Sects 1 and 2 which pertain mostly to the European Commis-sion and EUMETSATrsquos involvement in the mission preparation andset-up OA AB SaF SiF SG AG AH ER AS MRvdB and JYwere members of the ESArsquos Mission Advisory Group in Phase Aand B1 and provided input critical review and assistance with themanuscript

Competing interests The authors declare that they have no conflictof interest

Acknowledgements The authors would like to acknowledge the in-dustrial and scientific teams involved in the Phase A and B1 study ofthe CRISTAL mission significantly contributing to the success ofthe mission preparation in this feasibility phase The authors wouldlike to thank the anonymous reviewers Alex Gardner and the editorfor their comments

Review statement This paper was edited by Chris Derksen and re-viewed by Alex Gardner and two anonymous referees

References

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Bartsch A Ten Years of SeaWinds on QuikSCAT for Snow Appli-cations Remote Sens 2 1142ndash1156 2010

Bartsch A Kumpula T Forbes B C and Stammler F De-tection of snow surface thawing and refreezing in the EurasianArctic with QuikSCAT implications for reindeer herding EcolAppl 20 2346ndash2358 httpsdoiorg10189009-19271 2010

Blockley E W and Peterson K A Improving Met Of-fice seasonal predictions of Arctic sea ice using assimila-tion of CryoSat-2 thickness The Cryosphere 12 3419ndash3438httpsdoiorg105194tc-12-3419-2018 2018

Bonnefond P Verron J Aublanc J Babu KN Bergeacute-NguyenM Cancet M Chaudhary A Creacutetaux J-F Frappart FHaines B J Laurain O Ollivier A Poisson J-C PrandiP Sharma R Thibaut P and Watson C The Benefits of theKa-Band as Evidenced from the SARALAltiKa Altimetric Mis-sion Quality Assessment and Unique Characteristics of AltiKaData Remote Sens 10 83 httpsdoiorg103390rs100100832018

Budyko M I The effect of solar radiation variationson the climate of the Earth Tellus 21 611ndash619httpsdoiorg103402tellusav21i510109 1969

Chen J L Wilson C R and Tapley B D Contribution of icesheet and mountain glacier melt to recent sea level rise NatGeosci 6 549ndash552 httpsdoiorg101038ngeo1829 2013

CMEMS-1 Copernicus Marine Environmental Monitor-ing System (CMEMS) requirements for the Evolutionof the Copernicus Satellite Component available athttpmarinecopernicuseuwp-contentuploads201901CMEMS-requirements-satellitespdf (last access 10 July 2020)2017

Connor L N Farrell S L McAdoo D C Krabill W B andManizade S Validating ICESat Over Thick Sea Ice in the North-ern Canada Basin IEEE T Geosci Remote 51 2188ndash2200httpsdoiorg101109TGRS20122211603 2013

Copernicus Marine Environmental Monitoring Service (CMEMS)httpmarinecopernicuseu last access 10 July 2020

Copernicus Land Monitoring Service (CLMS) httplandcopernicuseu] last access 10 July 2020

Copernicus Atmospheric Monitoring Service (CAMS) httpsatmospherecopernicuseu last access 10 July 2020

Copernicus Emergency Management Service (CEMS) httpemergencycopernicuseu last access 10 July 2020

Copernicus Climate Change Service (C3S) httpclimatecopernicuseu last access 10 July 2020

DeConto R M and Pollard D Contribution of Antarctica to pastand future sea-level rise Nature 531 591ndash597 2016

Drinkwater M Francis R Ratier G and Wingham D The Eu-ropean Space Agencyrsquos Earth Explorer Mission CryoSat Mea-suring variability in the cryosphere Ann Glaciol 39 313ndash320httpsdoiorg103189172756404781814663 2004

Duchossois G Strobl P Toumazou V Antunes S Bartsch ADiehl T Dinessen F Eriksson P Garric G Houssais M-N Jindrova M Muntildeoz-Sabater J Nagler T and NordbeckO User Requirements for a Copernicus Polar Mission ndash Phase1 Report EUR 29144 EN Publications Office of the EuropeanUnion Luxembourg httpsdoiorg10276022832 2018a

Duchossois G Strobl P Toumazou V Antunes S Bartsch ADiehl T Dinessen F Eriksson P Garric G Holmlund KHoussais M-N Jindrova M Kern M Muntildeoz-Sabater J Na-gler T Nordbeck O and de Witte E User Requirementsfor a Copernicus Polar Mission ndash Phase 2 Report EUR 29144EN Publications Office of the European Union Luxembourghttpsdoiorg10276044170 2018b

Edwards T L Brandon M A Durand G Edwards NR Golledge N R Holden P B Nias I J PayneA J Ritz C and Wernecke A Revisiting Antarctic iceloss due to marine ice-cliff instability Nature 566 58ndash64httpsdoiorg101038s41586-019-0901-4 2019

Egido A and Smith W H F Fully Focused SAR Altimetry The-ory and Applications IEEE T Geosci Remote 55 392ndash4062017

ESA Long-term Scenario available at httpswwwcopernicuseusitesdefaultfiles2019-01Copernicus_Work_Programme_2019pdf (last access 10 July 2020) 2019a

ESA Copernicus High Priority Candidates Mission Require-ments Documents for the sixe candidate missions avail-able at httpswwwesaintApplicationsObserving_the_EarthCopernicusCopernicus_High_Priority_Candidates (last access10 July 2020) 2019b

European Commission Copernicus evolution Guidance Doc-ument for Horizon 2020 Work Programme 2018ndash2020Copernicus evolution LC-SPACE-02-EO-2018 LC-SPACE-03-EO-2018 available at httpseceuropaeuresearchparticipantsdatarefh2020otherguides_for_applicantsh2020-supp-info-space-2-3-18-20_enpdf (last access10 July 2020) 2017

Flament T and Reacutemy F Dynamic thinning of Antarctic glaciersfrom along-track repeat radar altimetry J Glaciol 58 830ndash840httpsdoiorg1031892012JoG11J118 2012

Foresta L Gourmelen N Palsson F Nienow P Bjoerns-son H and Shepherd A Surface elevation change andmass balance of Icelandic ice caps derived from swathmode CryoSat-2 altimetry Geophys Res Lett 43 12ndash138httpsdoiorg1010022016GL071485 2016

Francis J A Vavrus S J and Cohen J Amplified Arc-tic warming and mid-latitude weather new perspectiveson emerging connections WIRES Clim Change 8 e474httpsdoiorg101002wcc474 2017

Gardner A S Moholdt G Cogley J G Wouters B Arendt AA Wahr J A Berthier E Hock R Pfeffer W T Kaser GLigtenberg S R M Bolch M J Sharp M J Hagen J Ovan den Broeke M R and Paul F A Reconciled Estimate ofGlacier Contributions to Sea Level Rise 2003 to 2009 Science340 852ndash857 httpsdoiorg101126science1234532 2013

GCOS Systematic observation requirements for satellite-basedproducts for climate 2011 update Supplemental details to thesatellite-based component of the ldquoImplementation plan for theglobal observing system for climate in support of the UNFCCC(2010 update) GCOS Rep 154 available at httpslibrarywmointindexphplvl=notice_displayampid=449XwhdGefLj-g (lastaccess 10 July 2020) 2011

Giles K A Laxon S W Wingham D J Wallis D W Kra-bill W B Leuschen C J McAdoo D Manizade S S andRaney R K Combined airborne laser and radar altimeter mea-surements over the Fram Strait in May 2002 Remote Sens Env-iron 111 182ndash194 2007

Giles K A Laxon S W and Ridout A L Circumpo-lar thinning of Arctic sea ice following the 2007 recordice extent minimum Geophys Res Lett 35 L22502httpsdoiorg1010292008GL035710 2008a

Giles K A Laxon S W and Worby A P Antarctic sea ice el-evation from satellite radar altimetry Geophys Res Lett 35L03503 httpsdoiorg1010292007GL031572 2008b

Gourmelen N Escorihuela M J Shepherd A Foresta L MuirA Garcia-Mondejar A Roca M Baker S G and DrinkwaterM R CryoSat-2 swath interferometric altimetry for mapping ice

elevation and elevation change Adv Space Res 62 1226ndash1242httpsdoiorg101016jasr201711014 2017

Guerreiro K Fleury S Zakharova E Reacutemy F and KouraevA Potential for estimation of snow depth on Arctic sea ice fromCryoSat-2 and SARALAltiKa missions Remote Sens Environ186 339ndash349 httpsdoiorg101016jrse201607013 2016

Hendricks S Ricker R and Helm V User Guide ndash AWICryoSat-2 Sea Ice Thickness Data Product (v12) AWIhdl10013epic48201d001 2016

Hurkmans R T W L Bamber J L Davis C H JoughinI R Khvorostovsky K S Smith B S and SchoenN Time-evolving mass loss of the Greenland Ice Sheetfrom satellite altimetry The Cryosphere 8 1725ndash1740httpsdoiorg105194tc-8-1725-2014 2014

IPCC IPCC Summary for Policymakers in IPCC Special Re-port on the Ocean and Cryosphere in a Changing Climateedited by Poumlrtner H-O Roberts D C Masson-Delmotte VZhai P Tignor M Poloczanska E Mintenbeck K Nico-lai M Okem A Petzold J Rama B and Weyer N avail-able at httpswwwipccchsiteassetsuploadssites320191103_SROCC_SPM_FINALpdf (last access 10 July 2020) 2019

Jiang L Schneider R Andersen O B and Bauer-Gottwein PCryoSat-2 Altimetry Applications over Rivers and Lakes Water9 211 httpsdoiorg103390w9030211 2017

Konrad H Gilbert L Cornford S L Payne A HoggA Muir A and Shepherd A Uneven onset and paceof ice-dynamical imbalance in the Amundsen Sea Embay-ment West Antarctica Geophys Res Lett 44 910ndash918httpsdoiorg1010022016GL070733 2017

Kroisleitner C Bartsch A and Bergstedt H Circumpolarpatterns of potential mean annual ground temperature basedon surface state obtained from microwave satellite data TheCryosphere 12 2349ndash2370 httpsdoiorg105194tc-12-2349-2018 2018

Kurtz N T Galin N and Studinger M An improvedCryoSat-2 sea ice freeboard retrieval algorithm through theuse of waveform fitting The Cryosphere 8 1217ndash1237httpsdoiorg105194tc-8-1217-2014 2014

Lawrence I R Tsamados M C Stroeve J C Armitage TW K and Ridout A L Estimating snow depth over Arc-tic sea ice from calibrated dual-frequency radar freeboards TheCryosphere 12 3551ndash3564 httpsdoiorg105194tc-12-3551-2018 2018

Laxon S W Peacock N and Smith D High interannual vari-ability of sea ice thickness in the Arctic region Nature 425 947ndash950 2003

Laxon S W Giles K A Ridout A L Wingham D JWillatt R Cullen R Kwok R Schweiger A ZhangJ Haas C Hendricks S Krishfield R Kurtz N Far-rell S and Davidson M CryoSat-2 estimates of Arctic seaice thickness and volume Geophys Res Lett 40 732ndash737httpsdoiorg101002grl50193 2013

Legeais J-F Ablain M and Thao S Evaluation of wet tropo-sphere path delays from atmospheric reanalyses and radiometersand their impact on the altimeter sea level Ocean Sci 10 893ndash905 httpsdoiorg105194os-10-893-2014 2014

Lemmetyinen J Kontu A Pulliainen J Vehvilaumlinen J Rauti-ainen K Wiesmann A Maumltzler C Werner C Rott HNagler T Schneebeli M Proksch M Schuumlttemeyer D

Kern M and Davidson M W J Nordic Snow Radar Ex-periment Geosci Instrum Method Data Syst 5 403ndash415httpsdoiorg105194gi-5-403-2016 2016

Mallett R D C Lawrence I R Stroeve J C Landy J C andTsamados M Brief communication Conventional assumptionsinvolving the speed of radar waves in snow introduce systematicunderestimates to sea ice thickness and seasonal growth rate esti-mates The Cryosphere 14 251ndash260 httpsdoiorg105194tc-14-251-2020 2020

McMillan M Shepherd A Sundal A Briggs K Muir ARidout A Hogg A and Wingham D Increased ice lossesfrom Antarctica detected by CryoSat-2 Geophys Res Lett 413899ndash3905 httpsdoiorg1010022014GL060111 2014

McMillan M Leeson A Shepherd A Briggs K Armitage TW K T W K Hogg A Munneke P K van den BroekeM Noel B van den Berg W J Ligtenberg S Horwarth MGroh A Muir A and Gilbert L A high-resolution record ofGreenland mass balance Geophys Res Lett 43 7002ndash7010httpsdoiorg1010022016GL069666 2016

Meredith M Sommerkorn M Cassotta S Derksen C EkaykinA Hollowed A Kofinas G Mackintosh A Melbourne-Thomas J Muelbert M M C Ottersen G Pritchard H andSchuur E A G Polar regions in IPCC Special Report on theOcean and Cryosphere in a Changing Climate edited by Poumlrt-ner H-O Roberts D C Masson-Delmotte V Zhai P Tig-nor M Poloczanska E Mintenbeck K Alegriacutea A NicolaiM Okem A Petzold J Rama B and Weyer N M IPCC2019

Nandan V Geldsetzer T Yackel J Mahmud M ScharienR Howell S King J Ricker R and Else B Effectof snow salinity on CryoSat-2 Arctic first-year sea ice free-board measurements Geophys Res Lett 44 10419ndash10426httpsdoiorg1010022017GL074506 2017

Nandan V Scharien R K Geldsetzer T Kwok R YackelJ J Mahmud M S Roesel A Tonboe R Granskog MWillatt R Stroeve J Nomura D and Frey M Snow Prop-erty Controls on Modelled Ku-band Altimeter Estimates of First-Year Sea Ice Thickness Case studies from the Canadian andNorwegian Arctic IEEE J Sel Top Appl 13 1082ndash1096httpsdoiorg101109jstars20202966432 2020

Park S-E Bartsch A Sabel D Wagner W Naeimi V andYamaguchi Y Monitoring freezethaw cycles using ENVISATASAR global mode Remote Sens Environ 115 3457ndash3467httpsdoiorg101016jrse201108009 2011

Parrinello T Shepherd A Bouffard J Badessi S Casal TDavidson M Fornari M Maestroni E and Scagliola MCryoSat ESArsquos ice mission ndash Eight years in space Adv SpaceRes 62 1178ndash1190 httpsdoiorg101016jasr2018040142018

Perovich D Meier W Tschudi M Farrell S Gerland S Hen-dricks S Krumpen T and Haas C Sea ice cover in ldquoStateof the Climate in 2016rdquo B Am Meteorol Soc 98 S131ndashS1332017

Picard B Frery M L Obligis E Eymard L Ste-unou N and Picot N SARALAltiKa Wet Tropo-spheric Correction In-Flight Calibration Retrieval Strate-gies and Performances Marine Geodesy 38 277ndash296httpsdoiorg1010800149041920151040903 2015

Price D Beckers J Ricker R Kurtz N Rack W HaasC Helm V Hendricks S Leonard G and Langhorne PJ Evaluation of CryoSat-2 derived sea-ice freeboard over fastice in McMurdo Sound Antarctica J Glaciol 61 285ndash300httpsdoiorg1031892015JoG14J157 2015

Prowse T Alfredsen K Beltaos S Bonsal B DuguayC Korhola A McNamara J Pienitz R Vincent W FVuglinsky V and Weyhenmeyer G A Past and FutureChanges in Arctic Lake and River Ice Ambio 40 53ndash62httpsdoiorg101007s13280-011-0216-7 2011

Quartly G D Rinne E Passaro M Andersen O A DinardoS Fleury S Guillot A Hendricks S Kurekin A A MuumlllerF L Ricker R Skourup H and Tsamados M RetrievingSea Level and Freeboard in the Arctic A Review of CurrentRadar Altimetry Methodologies and Future Perspectives Re-mote Sens 11 881 httpsdoiorg103390rs11070881 2019

Raney R K The DelayDoppler Radar Altimeter IEEE T GeosciRemote 36 1578ndash1588 1998

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Zakharova E A Kouraev A V Stephane G Franck GDesyatkin R V and Desyatkin A R Recent dynamics ofhydro-ecosystems in thermokarst depressions in Central Siberiafrom satellite and in situ observations Importance for agri-culture and human life Sci Total Environ 615 1290ndash1304httpsdoiorg101016jscitotenv201709059 2018

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Abstract

Introduction

Expansion and evolution of the Copernicus Space Component

Objectives of the CRISTAL mission

Key contributions of the CRISTAL mission

Sea ice freeboard and thickness

Snow depth over sea ice

Ice sheets glaciers and ice caps

Sea level and coastal and inland water

Icebergs

Snow on land and permafrost

CRISTAL mission concept

Conclusions and CRISTAL mission status


Data availability

Author contributions

Competing interests

Acknowledgements

Review statement

References

The Cryosphere 14 2235ndash2251 2020httpsdoiorg105194tc-14-2235-2020copy Author(s) 2020 This work is distributed underthe Creative Commons Attribution 40 License

The Copernicus Polar Ice and Snow Topography Altimeter(CRISTAL) high-priority candidate missionMichael Kern1 Robert Cullen1 Bruno Berruti1 Jerome Bouffard2 Tania Casal1 Mark R Drinkwater1Antonio Gabriele1 Arnaud Lecuyot1 Michael Ludwig1 Rolv Midthassel1 Ignacio Navas Traver1Tommaso Parrinello2 Gerhard Ressler1 Erik Andersson3 Cristina Martin-Puig4 Ole Andersen5 Annett Bartsch6Sinead Farrell7 Sara Fleury8 Simon Gascoin9 Amandine Guillot10 Angelika Humbert11 Eero Rinne12Andrew Shepherd13 Michiel R van den Broeke14 and John Yackel15

1European Space Agency (ESA-ESTEC) Keplerlaan 1 2201 AZ Noordwijk the Netherlands2European Space Agency (ESA-ESRIN) Via Galileo Galilei Casella Postale 64 00044 Frascati Italy3European Commission BREY 09154 1049 Brussels Belgium4EUMETSAT Eumetsat Allee 1 64295 Darmstadt Germany5DTU Space Elektrovej 28 2800 Lyngby Denmark6bgeos Industriestrasse 1 2100 Korneuburg Austria7University of Maryland 5825 University Research Court 20740 College Park MD USA8CTOHLEGOSCNRS 14 avenue Edouard Belin 31400 Toulouse France9CESBIO Universiteacute de Toulouse CNRSCNESIRDUPS 31400 Toulouse France10CNES 18 avenue Edouard Belin 31400 Toulouse France11Alfred-Wegner-Institute Helmholtz Centre for Polar and Marine Research Am Alten Hafen 262758 Bremerhaven Germany12Finnish Meteorological Institute PO Box 503 00101 Helsinki Finland13Centre for Polar Observation and Modelling University of Leeds LS2 9JT UK14Utrecht University Princetonplein 5 3584 CC Utrecht the Netherlands15University of Calgary 2500 University Drive NW Earth Sciences 356 Calgary Alberta Canada

Correspondence Michael Kern (michaelkernesaint)

Received 6 January 2020 ndash Discussion started 21 January 2020Revised 12 June 2020 ndash Accepted 18 June 2020 ndash Published 16 July 2020

Abstract The Copernicus Polar Ice and Snow TopographyAltimeter (CRISTAL) mission is one of six high-priority can-didate missions (HPCMs) under consideration by the Euro-pean Commission to enlarge the Copernicus Space Com-ponent Together the high-priority candidate missions fillgaps in the measurement capability of the existing Coper-nicus Space Component to address emerging and urgent userrequirements in relation to monitoring anthropogenic CO2emissions polar environments and land surfaces The am-bition is to enlarge the Copernicus Space Component withthe high-priority candidate missions in the mid-2020s to pro-vide enhanced continuity of services in synergy with thenext generation of the existing Copernicus Sentinel missionsCRISTAL will carry a dual-frequency synthetic-aperture

radar altimeter as its primary payload for measuring sur-face height and a passive microwave radiometer to sup-port atmospheric corrections and surface-type classificationThe altimeter will have interferometric capabilities at Ku-band for improved ground resolution and a second (non-interferometric) Ka-band frequency to provide informationon snow layer properties This paper outlines the user con-sultations that have supported expansion of the Coperni-cus Space Component to include the high-priority candidatemissions describes the primary and secondary objectives ofthe CRISTAL mission identifies the key contributions theCRISTAL mission will make and presents a concept ndash as faras it is already defined ndash for the mission payload

Published by Copernicus Publications on behalf of the European Geosciences Union

1 Introduction

45 Icebergs

References

Abstract

Introduction