CRYOSAT:

Response to the ESA Explorer = Opportunity Mission AO.

by

Prof. Duncan = Wingham, University College London, UK

Prof. Ola Johannessen, = Nansen Centre, Norway

Prof. Peter Lemke, Institute of Marine = Research, Germany

Prof. Christian Tchurning, University of = Copenhagen, Denmark

Prof. Giovanni Picardi, University of Rome `La = Sapienza', Italy

Dr. Stein Sandven, Nansen Centre, Norway

Dr. = David Vaughan, British Antarctic Survey, UK

Dr. Seymour Laxon, = University College London, UK

Dr. Remko Scharroo, Delft Institute = for Earth Oriented Research, Netherlands

&

Uwe = Mallow, Dornier Satellitesysteme GmbH

Giovanni Angino, Alenia = Aerospazio

Dr. Alan Haskell, Defence Research Agency

Nick = Veck, National Remote Sensing Centre Ltd

30 September = 1998

A. Introduction. For the past 100,000 years, the = Earth's climate has shown coupled fluctuations in temperature, carbon = dioxide, land ice mass and sea-level. In today's climate, the albedo = of the cryosphere is the dominant control on the polar radiation = budget. The transport of sea ice, and its moderation of salinity and = heat exchanges between the atmosphere and ocean, are centrally = implicated in the deep overturning of ocean circulation in the North At= lantic and Southern Ocean, and together with the melting of Antarctic = Ice Shelves contribute to the production of the dense water underlying = the world's oceans. Fluctuations in the land ice are implicated in and = may dominant the current observed rise in global sea level. = Fundamental to an improved understanding of the cryosphere in the = present, past and future climate are observations of its mass fluxes = and exchange with the ocean. The mean sea ice mass flux transport is = only established to a factor of two, and little is known of its = variability, and impact on ocean vertical exchanges. The mean exchange = between the land ice and the ocean is established to perhaps 20%, but = this uncertainty is too large for any conclusion to be drawn as to the = sources of the observed rise this century in eustatic sea-level.

<= p> Fluctuations in ice mass at the Earth's surface result in volume = changes that can be measured by satellite. The European Space Agency = ERS satellites were the first high-latitude satellites with = sufficiently-accurate orbit knowledge for this task. They have = demonstrated the potential of modern geodetic satellites to make = measurements of the required sensitivity. Uncertainty in the mass imbal= ance of the Antarctic Ice Sheet interior has been reduced by a factor of = three; and the first-ever determinations of sea-ice thickness by = satellite show that ice thickness in the Canadian Arctic is = substantially thinner than indicated either sparse ground observations = or models. Nonetheless, due to the use of pulse-limited altimeter = technology, measurements are missing of the dynamic marginal regions = of the Ice Sheets, smaller ice caps and glaciers throughout the world, = and sea-ice floes smaller than 10 km. In the US, laser altimetry will = be tested in the GLAS mission in 2001. However, the polar regions = experience between 50% and 90% cloud cover; a radar technology is essen= tial to the global investigation (and later monitoring) of cryosphere m= ass.

We propose a 3-year radar mission to determine the = global variation in thickness of the Earth's land and marine ice = cover. Combined with existing knowledge of density, these will = provide, uniquely, the spatial and temporal variation of ice mass. = These data are an essential prerequisite to forecast assimilation = models of horizontal ice flux. The objectives are to demonstrate satell= ite measurement at 1[[ring]] by 1[[ring]] of changes in thickness of ice = sheets, caps and glaciers to 1 cm/yr and sea-ice floes of 1 km or = greater to 10 cm. A radar with 25 cm single-shot accuracy at 500 m = resolution (a 10-fold improvement on pulse-limited altimetry) will = provide coverage of these surfaces. The mission combines existing = ENVISAT altimeter hardware with advanced synthetic aperture and = interferometric processing, the GRACE platform heritage, and existing = operations and control facilities, to achieve the Opportunity Mission = cost envelope. The mission is timed to ensure contemporary coverage = with the US GLAS mission, allowing the radar's global capability to be = assessed using the very-high resolution laser altimetry where and when = cloud-cover permits.

B. Mission Characteristics. Spatial = coverage is determined by the distribution of ice on Earth. = Land ice consists of the Antarctic and Greenland ice sheets, the = ice caps and glaciers of the US, Canadian, Norwegian and Russian High = Arctic, and temperate glaciers distributed over the Earth's continents.= The marine cryosphere occupies the Arctic and Southern Oceans and seas= onal ice occupies the Labrador and Greenland Seas. At its maximum = southern extent it may reach 45[[ring]] N in the North Hemisphere. = These considerations lead to a desired spatial coverage of all = latitudes.

Spatial sampling, and thickness change = accuracy, are determined by the distribution of thickness change = uncertainty in space. The Antarctic and Greenland Ice Sheets are in = balance to within 400 Gt/yr and 150 Gt/yr, and the remaining ice caps = and glaciers are thought to be losing in total 150 Gt/yr to the ocean, = with an uncertainty of perhaps 50 Gt/yr. These numbers correspond to an= average ice thickness rate uncertainty of 5 cm/yr, distributed over the = area of land ice of 14 x 106 km2. In the Northern hemisphere, marine = ice may range from 0 to 20 m in thickness through ridging; in the = Southern Hemisphere, where mass flux divergence is usually positive, 2 = metres is a more typical maximum. In both hemispheres the spatial = distribution of thickness is very poorly known. Observing variation in = sea ice thickness with an accuracy of 50 cm would greatly increase = understanding of the mass budget. Thin sea ice, however, has a vital = role in moderating heat and water exchanges between the polar atmosphere = and ocean; a tighter accuracy on sea ice thickness is desirable. Spatial = sampling at 1[[ring]] by 1[[ring]] (106 km2) of the thickness variations = of sea ice to an accuracy of 10 cm, and land ice masses to 1 cm/yr is = therefore sufficient.

Orbit repeat period determines = the spatial sampling of an altimeter satellite. A very-long repeat = orbit, with an ~30-day orbit, achieves 1[[ring]] by 1[[ring]] = sampling at monthly scales, whilst ensuring full spatial sampling of = ice bodies of smaller dimensions. An orbit inclination of e.g= . 86[[ring]] allows cross-over analysis, and includes all dynamic e= lements of the West Antarctic Ice Sheet.

Spatial = resolution of the sensor is determined by the need to resolve spati= al variations in thickness (which is a necessary precursor to = determining their changes, even at much larger scales). 75% of the = surface of the Ice Sheets, 10% of marine ice, and little or none of = the smaller land ice bodies, is observable at 10 km resolution. At 1 = km resolution, 95% of the Ice Sheets, 70% of sea ice floes, and the = Arctic Ice Caps are observable. It is likely that the complete ice = cryosphere is observable at 100 m resolution.

Mission = duration is determined by the interannual variability. Antarctic an= d Greenland mass accumulation variability is such that two decades of c= overage of the ice sheets are required to close their mass budgets. In = the Arctic, sea ice thickness is monitored at a few sites, but its = seasonal and interannual variation is otherwise poorly known. It is = even less well known in the Southern Ocean. It is not known if any = secular trend in sea ice thickness exists. Therefore we aim for a = 3-year mission, which will extend the ERS-ENVISAT time-series for the = ice sheets, while providing for the first time observations of the = interannual variability of sea ice thickness. In the polar regions, = the seasonal cycle is the dominant fluctuation. Temporal sampling at monthly intervals is sufficient.

Launch timing is = determined by that of the complimentary ENVISAT and GLAS missions. We = propose an all-weather radar mission (section C) which provides 1 km = spatial resolution. On the other hand, sampling the complete cryosphere= requires 100 m resolution (see above), but 100 m resolution laser tech= nology will result in at least 50% to 90% of data in polar regions lost = through cloud cover, with an even greater loss of change measurements. = Therefore the optimal payload for a cryospheric mission is a combination = of laser and radar technology. With a launch date of 2002, we will = exploit the time-coincident laser coverage of the US GLAS mission to = provide optimal coverage of the Earth's cryosphere. A launch date of = 2002 also allows for cross-calibration with the ENVISAT radar = altimeter, permitting the addition of the CRYOSAT Ice Sheet = time-series to that of ERS1, ERS2 and ENVISAT.

Secondary = mission objectives include the variability of the high-latitude = polar ocean, ice and land topography, and coastal and inland water = level variability. An orbit maintenance capability is not foreseen, so = exact repeat-track sampling will not be available, and for this reason = oceanography is not a mission driver. Altimetric ice topography is now = being superceeded by that of radar interferometry, although the need = for accurate baselines and the sometimes subtle expression of ice = dynamics in the surface elevation will make the marginal topography = from this mission of importance. The drift orbit will provide almost = complete spatial sampling over the 3-year mission. Performance over = the land surface, where backscatter variations occur at very short = spatial scales, is unknown, and telemetry limitations make global, = high-resolution operation impossible. Temporal and spatial sampling con= straints make the mission of interest, but not central, to = investigations of coastal and inland water variations.

C.= Technical Outline. The measurement concept is to determine = the elevation of the ice surface by precise radar ranging from the = satellite to the surface, together with precise location of the = satellite platform. Changes in land ice thickness are determined by = observing the change in elevation with time. Sea-ice thickness is = determined by observing the difference between the elevation of the = ice and the surrounding ocean in which it floats, i.e. the ice = freeboard, and temporal variations are determined by repeating this obs= ervation with time. While individual measurements are relatively = inaccurate, the mission accuracy requirements will be met by = integration of point observations in space and time.

For = sea ice, the capability of discriminating 1 km floes is required to = monitor 70% of ice area. This will be provided increasing the = pulse-repetition frequency of existing Ku-band pulse-limited, = full-deramp compression altimeter design to permit multiple, = Doppler-sharpened beams in the along-track dimension of ~ 250 m = resolution. While the instrument remains pulse-limited across-track, 2-= dimensional pulse-limited geometry is fundamentally insensitive to = off-nadir echoes from a plane surface, and the instrument has an = effective across-track resolution of 1 km. Simulations show that = along-track, incoherent summation over the Doppler beams provides 25 = cm, single-shot accuracy. For land ice, which has large = topographic variations across-track, across-track pulse-limited = geometry introduces directional ambiguity. To overcome this, a second = altimeter, placed symmetrically across-track with respect to the first, = is used to provide an interferometric measure of the direction of the = arrival of the echo. The antennas are spaced closely to avoid = phase-ambiguity. The angle is determined by the coherent summation = across the Doppler beams of the complex product of the two altimeter = echoes. The elevation retrieval depends on surface geometry. For = across-track slopes less than the pulse-limited beam width, the = elevation of the point of closest approach is determined with an accura= cy of 25 cm. For across-track slopes greater than the beam-width, the e= levation of all points within the swath are determined, but at = considerably reduced accuracy. For secondary, ocean objectives, = and instrument cross-calibration, normal incidence, = single-channel pulse-limited operation will also be provided.

= The instrument payload will consist of a non-redundant Ku-band = 13.575 GHz interferometric altimeter with a dual, 1 m parabolic = reflector and double receiving system. Full-deramp compression and = range sampling would be performed on-board through the use of range- = and gain-tracking control loops. Mass and power are in the range 50 to = 65 kg and 180 Watts. A GPS-based 2-cm navigation and 20 arc-sec = star-tracker attitude package (10 kg and 30 W) are also required. A = microwave radiometer package (10 kg and 20 W) will be added if cost per= mits to support secondary mission objectives (atmospheric water = retrievals are not possible over ice surfaces). 50 Mbit/s worst case = is assumed for instrument compressed data rate. High resolution = operation will be limited to high latitudes due to telemetry = constraints; low resolution, pulse-limited operation will be = maintained throughout the orbit.

The satellite = platform provides for a non sun-synchronous, high inclination, = long-repeat period orbit. Thermal design will allow for all solar aspec= ts. Power requirements will be met using a `digital barbecue mode' and = [[radical]]2 over-sizing of the solar array. (The across-track alignment = of the two radar antenna make over-sizing impossible to avoid). To = support the drift orbit without orbit maintenance, a 94[[ring]] = inclination, 652 km altitude has been selected, providing a 32-day = repeat with 0.5[[ring]] drift per day. (Satellite drag will range from = a near-maximum to a near-minimum from 2002 to 2004). 500 Gbit = solid-state memory will be provided. S- and X-band downlinks will = provide 100 Mbits/s baseline.

Launch vehicle. The = total satellite including payload has a maximum power demand of 375 W, = and launch mass of 380 kg. The launch window is not specific and the = inclination is not unusual. Several launchers are available; a ROCKOT = launcher from the Plesetzk site with a dog-leg manoeuvre is taken as ba= seline.

Single ground-station operation and downlink = is assumed as baseline, with the platform having sufficient = carry-over storage. Data acquisition, storage and down-link strategy = will be examined in more detail prior to the full proposal. Platform = navigation will be performed from on-board GPS tracking; the need for = laser tracking is not foreseen.

The ground = segment will provide engineering, science and distribution elements= . Engineering elements will provide engineering range and gain correcti= ons, atmospheric range corrections, along-track Doppler beam processing= , incoherent and coherent Doppler beam stacking, and angle computation.= A full simulation of these procedures over a variety of terrain has = confirmed their performance. These procedures result in high-resolution = echoes from which elevations may be extracted. The science processing = will consist of land-ice elevation estimation and sea-ice thickness = extraction. Both measurements have been verified (albeit at low = resolution) during the ERS altimeter missions. The mapping of changes = of these functions in time and space and closure of the external = calibration and verification activities are also functions of the = science ground segment. Distribution of the geophysical products to = applications scientists and other users is foreseen.

Verifi= cation on ground of point changes in polar ice observed by the sate= llite is very difficult. On ice sheets, the high spatial variability of = mass accumulation, high point variability of the radar measurements, and = the difficulty of maintaining ground-based time series make ground = observations matched to the radar very difficult. Land ice elevations = will be verified by comparison with GPS observations on the = Ronne-Filchner Ice Shelf, Antarctica, and the GRIP core site, = Greenland, although ERS experience indicates these are unlikely to = provide closure at the required accuracy. Range and orbit measurements = may be verified to the required accuracy by comparison with ENVISAT/JAS= ON observations of the global ocean. Land ice change measurements will = be cross-compared with air-borne laser altimeter observations over Gree= nland, and with GLAS laser measurements of Antarctica (where these are = available). Over sea ice, the motion of floes and their unpredictable v= ariability also introduces great difficulties. Sea ice thickness and = thickness distribution will be verified statistically by comparison = with upward-looking sonar array measurements in the Fram Strait.

<= i>

Technology maturity and heritage. The mission is = based on the extensive re-use of hardware from previous missions. The = instrument antenna structure is new but not challenging. The RF/IF = sections are common with the ENVISAT RA-2 altimeter, as are the = instrument tracking control functions. Range compression and tracking = are implemented on ERS and ENVISAT altimeters. The radiometer (if inclu= ded) is also standard ENVISAT hardware. The platform structure, harness = and thermal control are mission specific, but data handling, avionics, = communication and power are all existing systems from previous missions = (e.g. Rosetta, Oersted, ENVISAT, GRACE etc.). Baseline = down-link rates are ERS/ENVISAT/METOP compatible. Platform data = storage, precision on-board location and 10 arc-second pointing = knowledge are no longer issues today. The science and industry team = (Annex) have experience spanning all ERS and ENVISAT altimeter mission = aspects from design to science exploitation.

Stream-lined = management approach. Cost limitations force advanced management = concepts. A `lean' concept will be used, consisting of a lead scientist= and project manager, reporting to the agency, and a single engineering= layer with direct interaction with the hardware supplier. The engineer= ing layer will focus on operations, AIV&T, performance and external = (launcher & ground segment) interfaces. Because all components have = proven use, effort will focus on interface adaptation. For efficiency, = `as-designed' documentation will be established in phase A/B, starting = with an interface control document. Incompatibilities will be = identified through this procedure, and in this manner an integrated = and interface compatible system will be established. In this was the = B/C phases may be accelerated, and project schedule compressed.

D. Mission Elements and Assumed Funding Sources.

Re= sponsibilities are divided (Table 3) between the Science Team, = comprising (currently) scientists from the UK, Norway, Germany, = Denmark, Holland and Italy, with responsibility for science = definition, verification campaigns and data utilisation; the space = segment, comprising principally Dornier Satellitensysteme GmbH (DSS) = as platform lead and Alenia as payload lead; and the ground segment, = comprising (UK) Defence Research Agency (DERA) as mission control, = ground station and operations lead, Delft Institute for Earth-Oriented = Research providing satellite localisation, and National Remote Sensing = Centre (NRSC) as data processing and archiving lead.

 =
      Mission element                            Implementation     =
Assumed Funding   
                                                      =
                  =
Source         Science preparation          Scientific          =
SCIENCE TEAM          NATIONAL       
                             =
definition                                               
               =
              =
studies                                                  
               =
              =
Campaigns           SCIENCE TEAM          NATIONAL        System =
engineering and                            DSS/ALENIA             ESA    =
      
=
assembly integration and                                                 =
             
=
test                                                                     =
             
=
Space segment                Instrument(s)          ALENIA               =
ESA                                        Platform                 =
DSS                ESA                                        Launcher =
              =
ROCKOT               ESA           Ground segment facilities    =
Command and             DERA                ESA          
               =
              =
acquisition                                              
               =
              =
stations                                                 
               =
              =
Operations              DERA                ESA          
               =
              =
centre                                                   
               =
              =
Processing and          NRSC         ESA  (to level 2)   
               =
              =
archiving                                                 Mission =
control and data     Mission Control         DERA                ESA     =
     
=
exploitation                                                             =
             
                             =
Data utilisation    SCIENCE TEAM          NATIONAL         
<= p> Table 3: Implementation and funding source = assumptions

The mission is self-standing. Range = correction data are freely available. Cross-calibration data exchange = with the US mission has been agreed in principle with the GLAS Project = Scientist, and will be pursued formally during the development of the = mission.

Annex. Team Composition

=

Lead Investigator. Prof. D. J. Wingham. Department = of Space and Climate Physics, University College London, 17-19 Gordon = St., London WC1H 0AH. Phone: 44 (0) 171 419 3677; Fax: 44 (0) 171 419 = 3418; Email: DJW@mssl.ucl.ac.uk.

Pro= fessor Duncan Wingham is Head of Climate Physics of the Mullard = Space Science Laboratory, Department of Space and Climate Physics, Univ= ersity College London. He has 15 years of experience in satellite obser= vation of polar ice sheets, and in particular the determination of the = contribution of ice sheets to rising sea-level. He is Coordinating = Principal Investigator of the 8-nation, 22 member `VECTRA' program = aimed at determining the velocity structure of the Antarctic & = Greenland Ice Sheets using ERS SAR interferometry, and was Programme = Coordinator of the 5-nation CEC `ESAMCA' Programme to map using = satellite altimetry the Ronne-Filchner Ice Shelf using ERS altimetry. = He was Chairman of the ESA Earth Explorer Topography Mission Science = Advisory panel and member of the ESA ENVISAT RA2 Science Advisory Group= . He is a member of the UK National Environment Research Council Marine = Sciences and Technology Board and Earth Observation Experts Group, and = of the British National Space Centre Earth Observation Programme = Board. He is also experienced in satellite projects. He was Consultant = to the ESA ERS-1 radar altimeter project, and is Leader of the ENVISAT = RA-2 Altimeter Expert Support Laboratory. His group operationally = quality control the ERS altimeter waveform product, and also supplied = to ESA the ENVISAT RA-2 System Simulator. Prof. Wingham's latest = revision of the mass imbalance of the Antarctic Ice Sheet will be = published shortly in `Science'.

Prof. Ola M. = Johannessen is Director of the Nansen Environmental and Remote = Sensing Center (NERSC). He has more that 20 years experience in satell= ite remote sensing in various fields such as coastal and deep sea ocean= ography, ice research, climate, marine biology, surface oil pollution, = natural oil seepage and operational ice monitoring and forecasting. He = has expertise in process studies of ocean fronts, eddies, vortex = pairs, ice edge upwelling, internal waves, turbulence studies in the = boundary layer above and below the ice, chimneys, deep water = formation, CO2 uptake by the ocean and CO2 injections in the ocean. = Recently, he has also worked with ambient noise, acoustic propagation = including acoustic tomography and acoustic thermometry for ocean = climate work. He is the author and co-author of more than 300 = scientific publications and reports. Presently he is elected member of = the ESA Earth Science Advisory Committee (ESAC), the Norwegian = Advisory and Coordination Committee for Earth Observation and several = other committees.

Professor Dr. Peter Lemke is = the director of the Marine Meteorology Department of the Institute of = Marine Research, Kiel, Germany, and since 1 January 1997 he is Acting = Deputy Director of the entire Insitute. He has 23 years of experience = of working with satellite data in climate and sea ice research in the = Arctic and Antarctic. He has participated in three polar expeditions = with the German research icebreaker Polarstern. On one expedition he = acted as chief scientist. During his affiliation with the Alfred-Wegene= r-Institute of Polar and Marine Research (1989-1995) in Bremerhaven, = Germany, he was the head of the Sea Ice - Remote Sensing Group. Since = 1995 he is the director of the Marine Meteorology Department of the Ins= titute of Marine Research, Kiel, Germany, and since 1 January 1997 he is = Acting Deputy Director of the entire insitute. He served on many = national and international committees. Currently he is the Chairman of = the Sea-Ice-Ocean Modelling Panel of the World Climate Research = Program (WCRP), the co-ordinator of the WCRP Antarctic Sea Ice = Thickness Project, a member of the Scientific Steering Group of the = Arctic Climate System Study (ACSYS), and a member of the Joint = Scientific Committee of the WCRP. In 1991 he received the German Polar = Meteorology Award.

Professor Carl Christian Tscherning = is Head of the Department of Geophysics, University of Copenhagen. = His Department executes glaciological research in Greenland (GRIP, = NGRIP) which forms the logistic basis for mapping changes of the = surface topography of selected areas in Greenland. This has served as = basis for comparison with satellite radar altimetry (ERS-1, ERS-2), air= borne laser altimetry and (airborne and satellite) SAR interferometry. = His ongoing projects include annual or bi-annual re-surveying of local = GPS networks on the ice as well as the comparison of the above = mentioned techniques in selected areas. Several of the activities are = expected to continue up to 2005. Professor Tscherning is Secretary = General of the International Association of Geodesy. He has been a = Principal Investigator for the ERS-1/2 project, and is currently = menber of the ESA RA-2 Science Advisory Group, and the ESA Explorer Gra= vity and Ocean Explorer Science Advisory Group. He has published more = than 150 papers and reports.

Prof. Giovanni Picardi = is at the INFO-COM Department of the University of Rome `La Sapienza' = where he is Head of the Radar and Remote Sensing Group. He has a long = experience of the system analysis and signal processing of radar altime= ters, in Earth Observation and Planetary applications. Different members = of the group have been involved over the past 15 years ago in various = programs. Among these are the system analysis and development of a = suboptimum estimator for the ERS-1 altimeter; responsibility for a = feasibility study of a radar altimeter for navigation and science = (with sounding capabilities) for the Comet Approach and Landing System = of the Rosetta/ Comet Nucleus Sample Return mission; under an = ESA/ESTEC research contract a study of "beam sharpening" techniques to = improve the along-track spatial resolution of pulse-limited altimeters,= based on the application of a Doppler processing to the altimeter data= (the system was called Synthetic Aperture Sounding Radar Altimeter); = two researchers of the group are members of the Cassini Radar Science = Team with particular commitments regarding the altimeter mode of the = Cassini Radar; they studied, under an ESA/ESTEC contract, a bistatic = model of ocean scattering for bistatic altimetry applications; and = finally they submitted to ESA, having the PI-ship of the experiment = which involves also the JPL and an international team of researchers, = a Subsurface Sounding Radar Altimeter with high spatial resolution = subsurface sounding and ionospheric sounding capabilities: this instrum= ent has been selected by ESA as a scientific payload of the Mars Express = mission.

Dr. David Vaughan is Principal Investigator of the = Global Interations of the Antarctic Ice Sheet System (GIANTS) = Programme at British Antarctic Survey. He has 12 years of experience = of Antarctic glaciology, and has worked extensively on the West = Antarctic Ice Sheet investigating glacier dynamics and ice-sheet = climate interactions, in addition to the use of optical and SAR data fo= r ice-dynamic investigations. He is Coordinator of the Antarctic BEDMAP = project, which is a Scientific Council of Antarctic Research (SCAR) = project consolidating continent-wide observations of Antarctic Ice = Sheet thickness. He is leader author of the ice sheet section of the = International Panel on Climate Change (IPCC) 3rd Assessment of Climate = Change. He has recently published the most authoritative assessment of = the mass accumulation over the Antarctic Continent.

Dr. = Stein Sandven is Senior Scientist and Research Director of the Nans= en Environmental and Remote Sensing Center (NERSC). He has nearly 20 = years of experience in marine and polar remote sensing. Sea ice is = currently his main area of interest. He has published over 30 = scientific articles in refereed journals and a number of project = reports. He has been project leader of several ESA and EU-funded = remote sensing studies, primarily sea ice projects. Recently, he was = leader of a study where NERSC and MSSL carried out "Performance = analysis of an Ice Topography Mission" (ESTEC Contract no. 12124/96/NL/= CN), where the capability of a beam-limited 94 Ghz radar altimeter and = an interferometric SAR altimeter at 13 Ghz was studied. He has together = with Swedish Space Corporation and some other partners carried out a = "Mission study of an Ice Topography Observation System" using a laser = altimeter. He was member of ESA's Earth Explorer Topography Working = Group 1995-1996.

Dr. Seymour Laxon is Lecturer in = Climate Physics at the Department of Space and Climate Physics, = University College London. He is widely recognised as the worlds' = leading authority on the use of satellite radar altimetry over sea = ice, and has more than 15 years experience of working in the field. He = pioneered the use of satellite radar altimetry in mapping sea ice = extent. He produced the first-ever marine gravity fields over = ice-covered regions in the Arctic and Antarctic oceans, leading to = major advances in understanding the tectonic development of the crust = and lithosphere of these regions. More recently, he has generated the = first-ever satellite-derived maps of sea ice thickness, ocean = variability and tides in the Arctic Ocean. He was also closely involved= with the development of algorithms for the ERS altimeters, and in the = initial evaluation of the instrument performance. Dr. Laxon is currently = a member of the ESA RA-2 Science Advisory Group.

Dr. = Remko Scharroo is at the Delft Institute for Earth-Oriented Research = (DEOS) at the Delft University of Technology. He has six years of = experience in satellite geodesy, specialising in the modelling and = provision of geodetic satellite location using the GEODYNE software. = He has been particularly concerned to provide the most precise orbit = reconstructions oover Antarctica in support of ERS ice sheet mass = balance experiments, and is co-author of the latest mass-balance = assessment appearing in `Science'. He is responsible for the provision = to the community of precise satellite orbits for the ERS and GEOSAT-Fol= low on (GFO) missions. He was a member of the ERS-1/2 cross calibration= experiment team. He is a member of the ENVISAT RA-2 Science Advisory = Group.

Dornier Satellitesysteme GmbH (DSS). Project = Leader: U. Mallow. Dornier Satellitesysteme has more than 20 years = experience in space missions. The latest projcets, CHAMP, GRACe and = ROSETTA and the proposal for MARS EXPRESS demonstrate that DSS has = taken up the challenge of participating in the development of small = and compact satellites which are specifically designed to fulfil a = dedicated mission objective. Based on the DSS experience of these curre= nt projects, and on the quality of the hardware return proposals = received from subcontractors, DSS are convinced that mission-specific = satellite tailoring is the most cost-efficient route to `small' = mission success. In addition to the leading position of DSS in space = projects, DSS has a long record in supplying space hardware for nearly = any satellite subsystem.

Alenia Aerospazio. Project = Leader: G. Angino. Alenia Aerospazio has 20 years experience in the = field of space telecommunication and remote sensing systems. ALS has, = moreover, gained specific know-how in the design and testing of the = spaceborne altimeters ERS-1, ERS-2 and RA-2, covering the role of instr= ument prime contractor in both ERS and ENVISAT programmes. Important e= xperiences have also been gained in the field of microwave radiometry = through the design and development of the multifrequency imaging = radiometer MIMR demonstrator and MWR flight model for the ENVISAT = programme. Based on the background capabilities acquired on these = projects, as well as in other successful programmes (X-SAR / SIR-C, = CASSINI, COSMO), proposals (ROSETTA, MARS EXPRESS) and ESA funded = studies in the field of high resolution radar altimetry (TOS, HSRRA), = ALS is well-placed to provide expertise for the CRYOSAT mission.

<= p> Defence Research Agency (DERA). Project Leader: Dr. Alan = Haskell. DERA operates the 13 m X-band facilities at West Freugh. Data = collection technology at West Freugh uses direct recording to computer = to minimise costs, and allows for advances in data technology to be = exploited with the minimum of new investment. DERA TT&C = facilities, currently at Lasham, are being consolidated on the West = Freugh site to further reduce operational costs. Recent experience of = DERA TT&C includes the full operation of the DERA STRV satellites = using CCSDS TT&C standards. The operation includes the use of links= to the NASA Deep Space Network for operations out of reach of the UK. = Links have been developed to NRSC to support ERS and Radarsat operation = and exploitation.

National Remote Sensing Centre = Ltd. (NRSC). Project Leader: N. Veck. NRSC is one of the world = leading suppliers of Earth Observation imagery, products and services. = It has divisions supporting Oil, Gas and Minerals; Agriculture and = Environment, Airborne Data Applications, and Applied Information = Systems (AIS). AIS specialise in space-related data processing, ground = segment development and archiving systems. NRSC hosts the UK Processing = and Archiving Facility (UK-PAF) of the ERS Ground Segment, forming the = primary archive for ERS Kiruna data, and secondary archive for = low-bit-rate data. It is responsible for the production of the ERS = waveform product (WAP). It has developed the ENVISAT Archive Reference = Facility as part of the ENVISAT Payload Data System, and is preparing = to implement the UK Processing and Archiving Centre for ENVISAT data = products.

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