The Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL): Expected Mission Contributions

One of the candidate missions in the evolution of the Copernicus Space Component (CSC) is the Copernicus polaR Ice and Snow Topography ALtimeter (CRISTAL). The aim of this mission is to obtain high-resolution sea-ice thickness and land ice elevation measurements and includes the capability to determine the properties of snow cover on ice to serve Copernicus’ operational products and services of direct relevance to the Polar Regions. The evolution of the CSC is foreseen in the mid-2020s to meet priority user needs not addressed by the existing infrastructure, and to reinforce the Copernicus 30 services by expanding the monitoring capability in the thematic domains of anthropogenic emissions (CO2), polar and agriculture/forestry/emergency. This evolution will be synergetic with the enhanced continuity of services foreseen with the next generation of the existing Copernicus Sentinels. New high-priority candidate satellite missions have been identified by the European Commission (EC) for implementation in the coming years to address gaps in current capability and emerging user needs. This paper describes the CRISTAL mission objectives, main mission requirements driving its design, the payload 35 complement currently under development and its expected contributions to the monitoring of important components of Earth’s cryosphere. https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
Earth's cryosphere plays a critical role in our planet's radiation and sea level budgets. Loss of Arctic sea ice is exacerbating planetary warming owing to the ice-albedo feedback (e.g. Budyko, 1969;Serreze and Francis, 2006;Screen and Simmonds 40 2010), and loss of land ice is the principal source of global sea level rise, see Chen et al (2013). The rates and magnitudes of depletion of Earth's marine and terrestrial ice fields are among the most important elements of future climate projections (IPCC/ SROCC, 2019, Meredith et al, 2019. The Arctic provides fundamental ecosystem services (including fisheries management and other resources), sustains numerous indigenous communities, and due to sea-ice loss is emerging as a key area for economic exploitation but the fragile ecosystems are subject to pressures from a growing number of maritime and 45 commercial activities. The potentially devastating contribution of the Antarctic ice sheet to global sea level rise is also subject to large uncertainties in ice mass loss, with high-end estimates of sea-level contribution exceeding a metre of global mean sealevel rise by 2100 (DeConto and Pollard, 2016).
A long-term programme to monitor the Earth's polar ice, ocean and snow topography is important to both operational and 50 scientific communities with interests in the Arctic and Antarctic. Europe has a direct interest in the Arctic due to its proximity.
Changes in the Arctic environment affect strategic areas including politics, economics (e.g. exploitation of natural resources including minerals, oil and gas, fish) and security. It also has an indirect interest in the Antarctic due to the Antarctic Treaty, which permits international access in support of science. Besides economic impacts of Antarctic and Arctic changes (Whiteman et al, 2013), Europe's interest in both Polar Regions is due to their influence on patterns and variability in global 55 climate change, thermohaline circulation and the planetary energy balance. Last but not least, changes in the Arctic system have potential impacts on European weather, with consequences for extreme events (Francis et al., 2017). The Copernicus polaR Ice and Snow Topography Altimeter (CRISTAL) mission, described in this paper, addresses the data and information requirements of these user communities.

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In the following section, we provide an overview of satellite missions and developments that are being prepared by the European Space Agency (ESA) in partnership with the European Union (EU) in response to user needs expressed by the Copernicus user community. In Section 3, we describe the objectives of the CRISTAL mission and its relation to the Copernicus services. In Section 4, an overview of CRISTAL's technical systems is described highlighting the use of heritage technology and needs driving technical advancements to improve observational capabilities beyond current missions. We then 65 discuss the key contributions from the CRISTAL mission, both in terms of specific mission objectives as well as expected scientific contributions towards improved knowledge of various state variables in Section 5. Conclusions and a current mission status statement is provided in Section 6. https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License.

Copernicus evolution
Copernicus was established to fulfil the growing need amongst European policy-makers to access accurate and timely 70 information services to better manage the environment, understand and mitigate the effects of climate change and ensure civil security. To ensure the operational provision of Earth-observation data, the Copernicus Space Component (CSC) includes a series of seven space missions called 'Copernicus Sentinels', which are being developed by ESA specifically for GMES/Copernicus. Some of these missions have already entered their operational life, some are being commissioned, and the remaining ones are targeted for launch in the coming years. 75 The Copernicus programme is coordinated and managed by the European Commission (EC). It includes Earth observation satellites, ground-based measurements, and services to process data to provide users with reliable and up-to-date information through a set of Copernicus Services related to environmental and security issues. Services provide critical information to support a wide range of applications, including environmental protection, management of urban areas, regional and local 80 planning, agriculture, forestry, fisheries, health, transport, tourism, climate change, sustainable development, emergency management response to disasters, risk management and civil protection.
The intense use and increased awareness for the potential of Copernicus have generated high expectations for an evolved Copernicus system. There is now a large set of defined needs and requirements for the future. With respect to the Polar Regions, 85 user and observation requirements have been identified, structured and prioritised in a process led by the EC (Duchossois et al.,2018a;2018b). Two distinct sets of expectations have emerged from this user consultation process. Firstly, stability and continuity, while increasing the quantity and quality of Copernicus products and services, led to one set of requirements. They are distinctly addressed in the considerations for the next generation of the current Sentinels 1 to 6 series. Emerging and urgent needs for new types of observations constitute a second distinct set of requirements. They are mainly addressed in the 90 considerations for the timely Evolution of the Copernicus space segment service. This evolution corresponds to the enlargement of the present space-based measurement capabilities through the introduction of new missions to answer these emerging and urgent user requirements. The so-called Long-Term Scenario, is a multi-annual implementation plan describing the main elements of this architecture, which has been generated through close consultation with the EC and EUMETSAT (see, ESA 2019). In order to identify synergies, complementarities and the evolution paths for the missions in response to EC requirements, an integrated end-to-105 end system approach is adopted and four observational capability families are defined: Microwave Imaging Family, Optical Imaging Family, Topographic Ocean and Ice Measurement Family and Spectroscopic Atmosphere Measurement Family. It is emphasised that CRISTAL is an essential part of the Topographic Ocean and Ice Measurement Family, and the evolution in Copernicus capabilities to address polar user needs.

CRISTAL mission objectives and contributions to Copernicus services 110
"An integrated European Union policy for the Arctic" (https://ec.europa.eu/environment/efe/news/integrated-eu-policy-arctic-2016-12-08_en) emphasises the strategic, environmental and socio-economic importance of the Arctic region, including the Arctic Ocean and adjacent seas. Continuously monitoring the vast and inhospitable Arctic environment with satellites (considering the sparse population and the lack of transport links) is considered essential. Following this, several guiding documents have been prepared in a European Commission-led user consultation process: Polar Expert Group (PEG) User 115 Requirements for a Copernicus Polar Mission Phase-I report (12th June 2017), Duchossois et al. (2018a), hereafter referred to as PEG-1 report, and the Phase 2 report on Users' requirements (31st July 2017), Duchossois et al. (2018b), hereafter referred to as PEG-2 report.
The PEG-1 report has summarised and prioritised the required geophysical parameters addressing objectives as defined in the 120 EU Artic Policy Communication, namely: climate change, environmental safeguarding, sustainable development, support to indigenous populations and local communities (see PEG-1 report). Floating ice parameters were listed as the top priority for the polar mission user requirements by a collective of polar experts. These parameters were selected considering the availability of existing Copernicus products and services of direct relevance to the Artic as well as their needs for improvement (e.g. in terms of spatial resolution, accuracy, etc) and the current level of technical and/or scientific maturity for some candidate 125 parameters. The specific parameters include sea ice extent/concentration/thickness/type/drift/velocity, thin sea-ice distribution, iceberg detection/drift and volume change as well as ice shelf thickness and extent. These parameters are given a top priority by the European Commission due to their key position in operational services such as navigation and marine operations, and in meteorological and seasonal prediction and climate model validation. suggested continuation of satellite SAR altimeter missions, with enhanced techniques for monitoring sea ice thickness, to achieve capabilities to produce time series of monthly, 25 km sea ice thickness with 0.1 m accuracy for north and south Polar 135 Regions. It was mentioned that near-coincident data, achieved for example, through close coordination between radar and laser altimeter missions, would help resolve uncertainties in sea ice thickness retrieval. In addition to sea ice thickness, other seaice parameters retrievable from SAR, such as ice drift, shear and deformation, leads and ice ridging, were pointed to as variables for future improvement.

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While Copernicus Sentinel-3A and B missions provide partial altimetric measurements of the polar oceans, their inclination limits the coverage of the Arctic Ocean to only 81.5°N. With the expected on-going loss of Arctic sea ice, these satellites will monitor only a small amount of the Arctic ice cover during summer periods by mid-2020, see e.g. Quartly et al (2019).
Currently, CryoSat-2 (originally a three year experiment as an ESA Earth Explorer mission, e.g. Drinkwater et al., 2004;Wingham et al., 2006, Parrinello et al. 2018) is the only European satellite to provide monitoring of the oldest, thickest 145 multiyear ice based on mission extension and availability following October 2013 end of nominal mission. However, continual monitoring of the Arctic Ocean north of 81.5°N is at risk, since CryoSat-2 has been operating in its extended mission scenario since October 2013, see Figure 1. This risk has widely been recognised by the polar and ocean surface topography community at large. For example, at the recent OSTST 2019 meeting (held in Chicago, IL, US on 21-25 October 2019) a recommendation was recorded in view of the 150 preparations for CRISTAL and other missions currently in operation: "To minimise likelihood of a gap in polar ocean and ice monitoring, the OSTST encourages Agencies to strive to launch CRISTAL in the early 2020s and to maintain operation of CryoSat-2, ICESat-2, and SARAL/AltiKa as long as possible." Based on the user requirements and priorities outlined in the PEG-1 report, a set of high-priority mission parameters was 155 defined by ESA's CRISTAL Mission Advisory Group (MAG) and ESA, which led to the CRISTAL mission objectives. The primary objectives of the mission are: • To measure and monitor variability of Arctic and Southern Ocean sea-ice thickness and its snow depth. Seasonal sea ice cycles are important for both human activities and biological habitats. The seasonal to inter-annual variability of sea ice is a sensitive climate indicator; it is also essential for long term planning of any kind of activity in the Polar 160 Regions. Knowledge of snow depth will lead to improved accuracy in measurements of sea-ice thickness and is also a valuable input for coupled atmosphere-ice-ocean forecast models. On shorter timescales, measurements of sea-ice thickness and information about Arctic Ocean sea state are essential support to maritime operations over polar oceans.
• To measure and monitor the surface elevation and changes therein of polar glaciers, ice caps and the Antarctic and Greenland ice sheets. The two ice sheets of Antarctica and Greenland store a significant proportion of global fresh 165 water volume and are important for climate change and contributions to sea level. Monitoring grounding line migration and elevation changes of floating and grounded ice sheet margins is important to identify and track https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License. emerging instabilities, which can negatively impact the stability of the ice sheets, leading to ice mass loss and ultimately result in accelerated future sea-level rise.
Secondary objectives of the CRISTAL mission include: 170 • To contribute to the observation of global ocean topography as a continuum up to the polar seas. This will contribute to the observation system for global observation of mean sea level, mesoscale and sub-mesoscale currents, wind speed and significant wave height. Information from this mission serves as critical input to operational oceanography and marine forecasting services in the polar oceans.
• To support applications related to coastal and inland waters. Observation of water level at Arctic coasts as well as of 175 rivers and lakes is a key quantity in hydrological research. Rivers and lakes not only supply freshwater for human use including agriculture but also maintain natural processes and ecosystems. The monitoring of global river discharge and its long-term trend contributes to the evaluation of global freshwater flux, which is critical for understanding the mechanism of global climate change. The frozen rivers and lakes are important circulation routes in the Arctic regions, which encountered dramatic changes in the context of global warming. Their observation could help forecasting their 180 evolutions and organizing alternative modes of transport.
• To support applications related to snow cover and permafrost in Arctic regions. Snowmelt timing is a key parameter for hydrological research, since it modulates the river discharge of Arctic basins, see e.g. Shiklomanov et al (2007).
Surface state change in permafrost regions indicates the initiation of ground thaw and soil microbial activities in the seasonally unfrozen upper soil (active) layer. The rapid evolution of the permafrost has also important impacts on 185 human activities and infrastructures.
The primary objectives drive the design and main performance specifications of the CRISTAL mission.
By addressing these objectives, the mission responds to a number of requisite parameters of interests and applications in Copernicus Services. A mapping of the services to the parameters of interest and applications is listed in Table 1. 190

System concept
The primary topography payload envisaged for this mission comprises an Interferometric Radar altimeter for Ice and Snow (IRIS) and a microwave radiometer (MWR). The mission draws from the experience of several in-orbit missions in addition 195 to the ongoing developments within the Sentinel-6 and MetOp-SG programmes. CRISTAL's primary payload complement consist of: • A synthetic aperture radar (SAR) altimeter operating at Ku-band and Ka-band centre frequencies for global elevation and topographic retrievals over land and marine ice, ocean and terrestrial surfaces (see Figure 2 and Figure  200 3). In Ku-band (13.5 GHz), the SAR altimeter can also be operated in interferometric (SARIn) mode to determine across-track echo location. Compared to heritage missions, the Ka-band channel (35.75 GHz) is added for snow depth measurements to distinguish between surface snow and ice layers, see e.g. Guerreiro et al (2016). A range (vertical) resolution of about 31 cm will be achieved to enhance freeboard measurement accuracy. Also, a high along-track resolution of about 20 m is envisaged to improve ice floe discrimination. Heritage missions include CryoSat-2 205 (SIRAL), Sentinel-6 (Poseidon-4) and SARAL (AltiKa). The CRISTAL Altimeter (IRIS) is based on Poseidon-4 (Sentinel-6) and SIRAL (CryoSat-2) together with the addition of a Ka-band channel (analogous to AltiKa) and a bandwidth of 500MHz to meet the improved range resolution requirement in comparison to heritage altimeters. It has the capability for fully focused SAR processing for enhanced along track resolution by means of resolving full scatterer phase history. Digital processing will be implemented including matched filter range compression and on-210 board Rang Cell Migration (RCM) compensation by means of a Range Migration Compensation (RMC) mode for on-board data reduction (heritage from Poseidon-4) reducing downlink load. With respect to the dual frequency https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License.
• A high-resolution passive microwave radiometer is included with the capability to provide data allowing global 215 ocean retrievals of Total Column Water Vapour up to 10 km from the coast (by means of improving the measurement system with high frequency channels and with potential cryosphere applications as sea ice type classifications. Concerning the Microwave Instrument selection, potential options include: a potential US Custom Furnished Item (CFI) based on the NASA-JPL AMR-C (Advanced Microwave Radiometer -Climate quality); development of an EU High Resolution radiometer solution; or a two-channel solution derived from the Sentinel-3 microwave 220 radiometer (MWR). The feasibility of each of these options will be further evaluated in the next mission phase (Phase B2 at the time of the system Preliminary Design Review, expected late 2021).
• A Global Navigation Satellite System (GNSS) receiver compatible with both Galileo and GPS constellations providing on-board timing, navigation and provision of data for on-ground precise orbit determination. Heritage GNSS solutions exist such as those based upon the GPS and Galileo compatible Sentinel-1,-2,-3 C/D, Sentinel-6 A/B 225 receivers. Precise Orbit Determination (POD) products will be provided by the Copernicus POD service (CPOD).
• A Laser Retro-reflector Array (LRA) for use by the Satellite Laser Ranging (SLR) network and by the International Laser Ranging Service for short arc validation of the orbit. Heritage concepts suitable for CRISTAL include CryoSat-2/Sentinel-3 LRAs.

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Three modes of radar operation are envisaged, which are automatically selected depending on the geographic location over the Earth's surface (see Table 2 (Egido and Smith, 2017) to improve sea-ice lead discrimination (by means of improvement in 240 sampling and resolution), and hence retrievals of elevation, and thus polar Sea Level Anomalies (SLA) by a significant factor. Open-burst Ka-band SAR is also provided to also allow for improving retrieval of snow depth. processing is applied, first implemented in the frame of Sentinel-6, which provides a considerable gain in instrument data rate reduction. In addition, data will be collected over inland water regions on a best effort basis using one of the above modes. 255 CRISTAL follows the requirements expressed in the PEG-1 and PEG-2 reports and provides products with different latency requirements for each of the respective application areas. The data latency requirements shown in Table 3 indicate the time 260 interval from data acquisition by the instrument to delivery as Level 1B data product to the user. https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License.

Sea ice freeboard and thickness
Sea ice plays a critical role in Earth's climate system since it provides a barrier between the ocean and atmosphere, restricting the transfer of heat between the two. Due to its high albedo, the presence of sea ice reduces the amount of solar energy absorbed by the ocean. Sea ice rejects brine during formation and fresh water during melting and it is therefore a driving force of the 270 global thermohaline circulation as well as the stratification of the upper layer of the ocean. The sea-ice provides a critical habitat for marine mammals and for biological activity (see e.g. Tynan et al (2009)), and its presence limits human access and maritime activities in the ice-infested polar oceans.
The sea-ice cover of the Arctic Ocean is waning rapidly, and in the Southern Ocean sea ice is undergoing regional changes, 275 with a decline observed in the Amundsen and Bellingshausen Seas. By 2017, the decline in September Arctic sea ice extent was 13.2% per decade, relative to the 1981-2010 average, and the older, thicker, multi-year ice cover comprised ~ 20 % of the winter ice pack, compared to ~ 45 % in the 1980s (Perovich et al., 2017). These losses are triggering extensive change and are having a profound impact on the climate, environment and ecosystems of both polar regions. Monitoring the polar oceans is therefore of regional and global importance, and the long-term continuity of sea ice measurements is essential to safeguard 280 both climate and operational data services.
As global warming, and its Arctic amplification, continue to contribute to the decrease of multi-year ice in the central Arctic Ocean, (north of 81.5° N), it is critical to obtain continuous, pan-Arctic observations of sea-ice thickness, extending as close https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License. as possible to the North Pole. Continuous monitoring of Arctic Ocean sea-ice conditions is necessary for safe navigation 285 through ice-infested waters and, when linked to previous measurements from CryoSat-2, the CRISTAL mission will deliver observations that will provide a long-term record of sea-ice thickness variability and trends, which are critical to support climate services. Since sea ice thickness is an essential climate variable (ECV) (see GCOS, 2011), its continuous measurement is required to understand the Arctic system and how ice loss is impacting climate at a global scale.

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Shipping in ice-covered Arctic waters has increased significantly in recent years and is expected to increase in coming years.
In addition to traditional maritime operations and fishing in the high Arctic, several polar-class cruise liners are under construction. This means an increase in the need and scope of operational ice information services. A primary data source for national ice services is currently synthetic aperture radar (SAR) imagery, specifically data acquired by Sentinel-1A and 1B, RadarSAT-2 and RADARSAT Constellation Mission. Thus, independent measurements of sea-ice thickness distribution at 295 reasonable latencies provided by CRISTAL will complement existing SAR measurements and benefit operational ice charting.
Furthermore, observed sea-ice thickness or freeboard distributions can be assimilated into sea-ice models to generate ice forecasts for the needs of ice navigation and offshore operations.
Historically, satellite observations had primarily been used to monitor ice extent until Laxon et al. (2003) produced the first 300 Arctic-wide sea ice thickness estimates from ERS radar altimetry. Since then, various methods for converting the received signal to physical variables have been established (Giles et al., 2008, Laxon et al., 2013Kurtz et al., 2014;Ricker et al., 2014;Price et al., 2015;Tilling et al., 2018;Hendricks et al., 2018). The capability to obtain an estimate of freeboard and thickness, and converting it to volume, has enabled scientists to better understand the changing Arctic ice cover. Most recently, sea-ice freeboard has been estimated from both Ka-and Ku-band measurements (Armitage and Ridout, 2015; Guerreiro et al., 2016;305 Lawrence et al., 2018). The requirements for CRISTAL are currently stated to provide sea ice freeboard with an accuracy of 0.03 m along orbit 315 segments of less than or equal to 25 km. Furthermore, the system shall be capable of delivering sea ice thickness measurements with a vertical uncertainty less than 0.1 m. The uncertainty requirement for sea ice thickness comes with a caveat, as the thickness uncertainty depends on the uncertainty of auxiliary products. In the case of CRISTAL, snow thickness will be https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License. measured by the system, but snow and ice densities will still have to be estimated by other means. In the light of the current 0.2 m sea ice thickness uncertainty from CryoSat-2 data assessed by Tilling et al. (2018) for a gridded, monthly product and 320 the anticipated improvement from the dual-altimetry technology, especially in the snow depth and propagation estimates, reaching a higher vertical uncertainty would seem reachable but requires further studies. Currently, the retrieval accuracy of sea-ice freeboard is limited by the range resolution of a radar altimeter. The large bandwidth of 500MHz is an important driver for the instrument concept generation, as it will improve the range resolution from 50 cm (as for CryoSat-2) to ~ 30 cm for CRISTAL. 325

Snow depth over sea ice
An accurate estimate of snow depth over sea ice is for signal propagation speed correction to convert radar freeboard to sea ice freeboard as well as conversion of freeboard to sea-ice thickness. In addition to uncertainty reduction for ice thickness/freeboard computation, the variation of snow depth is also a parameter highly relevant for both climate modelling, ice navigation and polar ocean research. The snow climatology of Warren et al., (1999) is still the single most used estimate 330 of snow depth in sea-ice thickness processing (Sallila et al., 2019). The uncertainty in the original Warren snow depth estimates are halved over first year ice (Kurtz and Farrell, 2011), but snow still represents still the single most important contribution to uncertainty in the estimation of sea ice thickness and volume (Tilling et al., 2018). The study of Lawrence et al. (2018) and Guerreiro et al (2016) show the possibility of using Ku-and Ka-bands in mitigating the snow depth uncertainty. Dual-frequency methods improve the ability to reduce and estimate the uncertainties related to snow depth and sea-ice thickness retrieval. The 335 modelling community is particularly interested in the uncertainty information, which according to the user requirement study in the PEG-1 report is required alongside the parameters and is critical when designing and setting up assimilation systems.
Better abilities to estimate the related uncertainties improves prediction quality assessment of annual snowmelt over Arctic sea ice, the stratigraphy and electromagnetic properties of the snow layer contrast with that of the underlying ice, and this can be exploited to retrieve information on the snow layer properties if contemporaneous measurements are acquired from multiple 340 scattering horizons. A dual-frequency satellite altimeter, as proposed for the CRISTAL mission, will address this need.
CRISTAL aims to provide an uncertainty of snow depth retrieval over sea ice of less than or equal to 0.05m. The additional measurements in Ka-band, with a 500MHz bandwidth, support the discrimination between the ice and snow interfaces.

Ice sheets, glaciers and ice caps
Earth's land ice responds rapidly to and in turn affects global climate change. For example, melting of glaciers, ice caps, and 345 ice sheets over recent decades has altered local hydrological systems, and has impacted sea levels and patterns of global ocean circulation. The Antarctic and Greenland ice sheets are Earth's primary freshwater reservoirs and, due to their progressive imbalance, have made an accelerating contribution to global sea level rise during the satellite era. Although ice dynamical models have improved, future losses from the polar ice sheets remain the largest uncertainty in global climate and sea level https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License.
projections. Due to their scale, remote location, and hostile climatic environment, satellite measurements are the only practical 350 solution for spatially and temporally complete monitoring of the polar ice sheets.
Estimates of ice sheet surface elevation change provide a wealth of geophysical information. They are used as the basis for computing the mass balance and sea level contribution of both ice sheets of Greenland and Antarctica (McMillan et al., 2014(McMillan et al., , 2016Shepherd et al., 2012), for identifying emerging signals of mass imbalance (Flament and Rémy, 2012;Wingham et al., 355 2009) and for determining the loci of rapid ice loss (Hurkmans et al., 2014;Sørensen et al., 2015). Through combination with regional climate and firn models of surface processes, surface elevation change can be used to isolate ice dynamical changes, at the scale of individual glacier catchments (McMillan et al., 2016).
The continuous record of elevation measurements provided by radar altimeters, dating back to 1992, provides a unique long-360 term record of surface elevation change and mass balance. The maps are typically delivered in (1) high-resolution (5-10 km) rates of surface elevation change (for single or multiple missions, typically computed as a linear rate of change over a period of several years to decades), and (2) frequently (monthly-quarterly) sampled time series of the cumulative change, averaged across individual glacier basins. In addition to being used to quantify rates of mass balance and sea level rise, they also have a range of other applications, such as detection of subglacial lake drainage (Siegert et al., 2016) investigations of the initiation 365 and speed of inland propagation of dynamic imbalance (Konrad et al., 2017), which in turn provides valuable information relating to the underlying physical processes that drive dynamical ice loss.
CRISTAL will continue the generation of elevation measurements provided by altimeters and will produce maps of ice surface elevation with an absolute uncertainty of 2 m, horizontal resolution of less or equal than 100 m and temporal sampling of at 370 least 30 days. CRISTAL will be capable of tracking steep terrain with slopes less than 1.5° using its SARIn mode. Highresolution Swath processing over ice sheets (about 5 km wide) can reveal complex surface elevation changes, related to climate variability and ice dynamics, and subglacial geothermal and magmatic processes. Elevation measurements of regions with smaller glaciers are often missing in CryoSat-2 data. Indeed, tracking algorithms are not efficient when rough terrain is encountered. Improvement in the agility of tracking over glaciers is thus a key element in the instrument concept generation. 375

Sea level, coastal and inland water
Over the years and through constant improvement of the data quality, satellite altimetry has been used in a growing number of applications in Earth sciences. The altimeter measurements are helping us understanding and monitoring the ocean: its topography, dynamics and variability at different scales. The need of satellite observations to study, understand and monitor the ocean is more than essential over polar areas, where in-situ data networks are very sparse, and where profound and dramatic 380 changes occur. This has also been expressed and emphasised by CMEMS as "Ensuring continuity (with improvements) of the Cryosat-2 mission for sea level monitoring in polar regions" (CMEMS, 2017). In addition, "Reliable retrieval of sea level in https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License. the leads to reach the retrieval accuracy required to monitor climate change" is another CMEMS recommendation for polar and sea ice monitoring, see CMEMS (2017).

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Actual data from the CMEMS catalogue does not allow a satisfactory sampling north of 81.5°N. It is of prime importance that the CRISTAL orbit configuration allows measurement coverage of the central Arctic Ocean with an omission not exceeding 2° of latitudes around the poles. The Sea Level Anomaly (SLA) over frozen seas can only be provided by measurements in the leads. With its high spatial resolution dual-frequency altimeter system, CRISTAL will contribute to the observation system for global observation of mean sea level, (sub-)mesoscale currents, wind speed and significant wave height as a critical input to 390 operational oceanography and marine forecasting services, as well as supporting ice thickness retrieval in the Arctic.
The high inclination orbit of CRISTAL associated to high -resolution SAR/SARIn bi-band altimetry measurements would extend considerably our monitoring capability over the Polar Oceans. The development of tailored processing algorithms should allow not only to track the low-frequency sea level trend in presence of sea ice but also characterize ocean large scale 395 and mesoscale variations over regions not covered by conventional ocean missions. Beyond the observations of ice elevation variations, CRISTAL would therefore offer the unique opportunity to improve our knowledge on the mutual Ocean-Cryosphere complex interactions over short and long-term time scales over both north and south poles. The Southern Ocean circulation indeed plays a key role in shaping the Antarctic cryosphere environment. First, it regulates sea-ice production: as sea-ice forms and reject brines into the ocean, the ocean destabilizes and warms underwater waters which reaches the ocean surface, hence 400 limiting further ice production. Second, it impacts Antarctic ice sheet melt, when warm and salty ocean currents access the base of floating glaciers through bathymetric troughs of the Antarctic continental shelf. These ocean currents melt the ice shelves from below, and are the main causes of the current decline of floating ice-shelves. This induced melt represents the largest uncertainty in the current prediction of global sea-level change, creating major ambiguity in our way to respond and adapt to future climate. Tightly linked with glacier melt, the polar shelf circulation and its interaction with largescale circulation 405 also control the rate of bottom water production and deep ocean ventilation, which impact the world's oceans on timescale ranging from decades to millennia. Therefore, with a designed operational lifetime of at least 7.5 years (including in-orbit commissioning), the observation from the same sensor of each components of these multi-scale ice-ocean interactions would make CRISTAL unique in its capability to address climate issues of regional and worldwide relevance. Over oceans, which represents a secondary objective for the mission, the satellite will be able to measure sea surface height with an uncertainty of 410 less than 3 cm. The main advantages and drawbacks of the Ka-band over the oceanic surface have been reviewed in Bonnefond et al (2018). Given its high along-track resolution of less than 10 km and high temporal resolution of sea level anomalies of less than 10 days, the mission can further contribute a suite of sea level products including mean dynamic topography and a sea level anomalies vertical uncertainty of less than 2 cm.
Observation of water level at the (Arctic) coast as well as of rivers and lakes is a key quantity in hydrological research, (e.g. Jiang et al 2017). Rivers and lakes not only supply freshwater for human use including agriculture but also maintain natural processes and ecosystems. The monitoring of global river discharge and its long-term trend contributes to the monitoring of global freshwater flux, which is critical for understanding the mechanism of global climate change. Satellite radar altimetry is a promising technology to do this on a regional to global scale. Satellite radar altimetry data has been used successfully to 420 observe water levels in lakes and (large) rivers, and has also been combined with hydrologic/hydrodynamic models. Combined with gravity-based missions like GRACE and GRACE-FO the joint use of the data will give fundamental information for ground water monitoring in the future.

Snow on land and permafrost
While these are considered a secondary objective for the mission, with its dual-frequency altimeter payload and high-resolution 425 passive microwave radiometer, CRISTAL may support and contribute to studies and services in relation to seasonal snow cover and permafrost applications over land. The ability for the retrieval of snow depth with Ku-/Ka-band altimeter is limited over land (Rott et al. 2018). Snow studies over land area are so far largely limited to scatterometer in case of Ku-band (examples are reviewed e.g. in Bartsch, 2010). Measurements as provided by CRISTAL may, however, be useful in retrieving internal properties of the snowpack. Snow structure anomalies as well as land surface state (freeze/thaw) are expected to be identified 430 by time series analyses as such processes alter penetration depth. Information from altimeter is currently also rarely used for permafrost studies. It can be applied for monitoring lake level as proxy for permafrost change (Zakharova et al. 2017). Surface status is closely interlinked with ground temperature (e.g. Kroisleitner et al. 2018) but usage of altimeter in this context remains unexplored. Wider use of altimetry for snow and permafrost applications does require higher spatial resolution and temporal coverage than available to date. It is therefore expected that CRISTAL will expand the utility of altimeter observations for 435 permafrost and snow monitoring over land.

Icebergs
Iceberg detection, volume change and drift have been listed as a priority user requirement (Duchossois et al., 2018a;2018b).
Icebergs present a significant hazard to marine operations in those ocean areas where they occur. Detection of icebergs in open water and in sea ice generally places a priority on wider satellite swaths to obtain greater geographic coverage. There is a need 440 for automatic detection of icebergs for the safety of the navigation and chart production. Iceberg concentration is given in CMEMS' catalogue at 10 km resolution covering Greenland waters. SAR imagery is the core input for icebergs detection.
However, iceberg detection (in particular small icebergs) is also possible using high-resolution altimeter waveforms. Tournadre et al. (2018) demonstrated detection of icebergs from CryoSat-2 altimeter data using several modes, and mention promising results with the Sentinel-3 data, which would result into a comprehensive dataset, already built under ALTIBERG project 445 (Tournadre et al., 2016). The volume of an iceberg is valuable information for operational services and climate monitoring.
For climate studies, the freshwater flux from the volume of ice transported by icebergs is a key parameter, with large https://doi.org/10.5194/tc-2020-3 Preprint. Discussion started: 21 January 2020 c Author(s) 2020. CC BY 4.0 License. uncertainties related to the volume of the icebergs. Measuring volume is currently possible only with altimetry, by providing the iceberg freeboard elevation from the ocean surface. Iceberg volume has been calculated with altimetry with Envisat, Jason-1 and Jason-2, see for example by Tournadre et al. (2015). 450 CryoSat-2 tracking over icebergs is operational but icebergs with high freeboards may be missed in the current range window.
The range window definition for CRISTAL is defined in order to ensure that echoes from icebergs are correctly acquired. Inflight performances for the measurement of the Angle of Arrival from CryoSat-2 are around 25 arcsec. An equivalent performance is necessary to retrieve across-track slopes and elevations. The CRISTAL design of the instrument and the calibration strategy will be design to comply with the specification of 20 arcsec. 455 CRISTAL will provide the unprecedented capability to detect icebergs at a horizontal resolution (gridded product) of at least 25 m. The products will be produced every 24 hours in synergy with other high-resolution data such as SAR imagery. Iceberg distribution and volume products will be produced at 50 km resolution (gridded) on a monthly basis. CRISTAL will cover the Polar Regions not observed by other altimetry mission, including a Ku-band Interferometric Synthetic Aperture Radar Altimeter with supporting Ka-band channel. In addition, the payload contains a high and low frequency passive microwave radiometer to perform wet troposphere delay correction, and surface-type classification over sea ice and ice sheets.
The mission is designed for a 7.5 years design lifetime and will fly in an optimized orbit covering Polar Regions (omission <= 470 2°; sub-cycle < 10 days). A key element is the high along-track resolution to distinguish open ocean from sea-ice surfaces and a track spacing of 5 km at 50° latitude and less for higher latitudes. Thanks to the dual-frequency SAR altimetry capability, a snow depth product will be produced over sea ice with high accuracy in response to long-standing user needs.
CRISTAL has undergone and completed parallel preparatory (Phase A/B1) system studies in which mission and system 475 requirements have been investigated and consolidated. The intermediate system requirements review has been completed with parallel industrial consortia compliant with the mission and system requirements. Next steps include the full definition, implementation and in-orbit commissioning of CRISTAL (Phases B2, C/D and E1) where a prototype and recurrent satellite will be developed. Competing interests. The authors declare that they have no conflict of interest.
Acknowledgements. The authors would like to acknowledge the industrial and scientific teams involved in the Phase A/B1 495 study of the CRISTAL mission significantly contributing to the success of the mission preparation in this feasibility phase.