Satellite and airborne altimetry have transformed our capacity to study Earth’s ice sheets, ice shelves, sea ice, and polar oceans. Altimeter measurements are now central to our awareness and understanding of Arctic and Antarctic environmental change, for the nemefit of science and in support of operational activities. A case in point is the marine ice sheet instability that is underway in West Antarctica, widely understood to be among the greatest contemporary imbalances in the climate system; the evolution of this signal has been charted in altimeter data since its onset, and has major implications for global sea level rise. Another example is the ability to chart sea ice and icebergs in the polar oceans. This presentation introduces a series of recent studies that have allowed both long-standing and unanticipated scientific problems in cryospheric research to be solved over the past five years. It also reports on the landmark studies in altimeter retrievals that underpin these discoveries. We discuss the techniques that have been developed to allow multi-decadal measurements of land ice, sea ice, and polar ocean elevation change, paying attention to new waveform re-tracking, interferometric processing, geophysical corrections, and approaches to time-series formation. This includes work on waveform deconvolution, and dual-frequency radar altimetry. We also present case studies on the use of these data to record multi-decadal trends in ice sheet mass balance, sea ice volume, and ocean circulation. Finally, we focus on a collection of very recent discoveries, including developing and utilising full swath-interferometric processing for mapping ice sheet elevation and elevation change, exploiting Altika and Sentinel-3 for land and sea ice studies in both hemispheres, tracking grounding line migration, verification of global sea level projections, partitioning snow and ice mass trends, and detecting the fingerprint of the seasonal melting cycle. In all cases, we explain how satellite and airborne altimetry have informed thinking across the wider scientific community.

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Satellite Altimetry has shown that the West Antarctic Ice Sheet has been persistently losing mass since 1992 while the East Antarctic Ice Sheet was relatively close to equilibrium. However, when going into detail, significant differences between the elevation change rates over different periods, observed by different satellites, become evident. We developed an approach to combine different satellite altimetry missions, which allows us to form a seamless time series of monthly surface elevation changes. A consistent reprocessing of the different pulse-limited radar altimeter measurements is the prerequisite for this combination. Beyond combining the pulse-limited data of ERS-1, ERS-2, Envisat and CryoSat-2 LRM, we also integrate high precision observations of ICESat and CryoSat-2 SARIn-Mode. By including also the Geosat and the Seasat mission, the time series range back until 1978 in the coastal regions of East Antarctica.
We will show that our reprocessing improved the measurements precision by about 50%, compared to the standard products. We will discuss the different types of inter-mission offsets, which need to be applied in order to combine the different missions, and compare the resulting elevation change grids to the output of the IMAU-Firn Model, driven by RACMO2.3p2. The high temporal and spatial resolution of our results allows to identify the precise timing of the onset of changes in the long-term trend, which will be demonstrated at several selected key regions. After the conversion to mass changes, we obtain an overall mass loss of -2068±377 Gt between 1992 and 2017 for the Antarctic Ice Sheet north of 81.5°S. The basin scale mass time series reveal how the losses in several basins accelerated in the last decade and show a remarkable agreement with independent datasets as RACMO or GRACE.

Reference and repeat-observations of ice sheet margin topography is critical to identify changes in ice thickness, provide estimates of mass gain or loss and thus quantify the contribution of the cryosphere to sea level change. Over the last 25 years, radar altimetry has been instrumental in monitoring ice sheets and their contribution to sea level change. The European Space Agency Altimetry mission CryoSat-2 is the first radar altimetry mission with a SAR/Interferometry radar altimeter payload. The aim of this new technology is to gain a better insight into the evolution of the cryosphere, in particular over the steep slopes typically found along ice sheet margins where the majority of the mass loss is taking place. CryoSat’s revolutionary design features a Synthetic Interferometric Radar Altimeter (SIRAL), with two antennas for interferometry, the corresponding SAR Interferometer (SARIn) mode of operation increases spatial resolution while resolving the angular origin of off-nadir echoes occurring over sloping terrain. The SARIn mode is activated over ice sheet margins and the elevation for the Point Of Closest Approach (POCA), or level-2, is a standard product of the CryoSat-2 mission.

The Interferometric mode of CryoSat-2 provides the ability to resolve substantially more than just the elevation at the POCA. If the ground terrain slope is only a few degrees, the CryoSat-2 altimeter operates in a manner such that the interferometric phase of the altimeter echoes may be unwrapped to produce a wide swath of elevation measurements across the satellite ground track beyond the POCA. This technique provides the opportunity to increase spatial resolution and to recover elevation over regions where conventional radar altimetry fails.

Here we will discuss recent results generated with swath altimetry over several components of the Cryosphere, including the Greenland and Antarctic Ice Sheets, ice shelves, ice caps and mountain glaciers. We will also discuss a series of thematic products derived from swath processing of CryoSat available to the community. Finally, we will discuss remaining challenges in SARIn altimetry which the Polar Ice Topography (PICE), a candidate mission for the Copernicus Sentinel expansion programme, aims at addressing.

NASA’s ICESat-2 satellite was launched in September 2018 and carries the Advanced Topographic Laser Altimeter System (ATLAS) instrument, a photon-counting laser altimeter. Its primary mission objective is to detect elevation changes over the Earth’s land ice (glaciers and ice sheets). On behalf of the Land Ice subgroup of the ICESat-2 Science Team, I will present an evaluation of ICESat-2 data that demonstrates the ability of the Land Ice Along-Track Height Product (ATL06) to determine precise elevations and along-track slopes from the geolocated photon clouds provided by higher-level data products. This ensures ATLAS will fulfill its requirement to continuously observe the elevation of the Antarctic and Greenland ice sheets at unprecedented precision and detail. Early scientific applications include small-scale observations of ice-shelf rifts, crevasses, and ice margins. We demonstrate the potential of continuous ICESat-2 measurements, repeating every 91 days, to extend the 25-year record of laser-altimetry data that includes ICESat (2003-2009) and airborne observations from the Advanced Topographic Mapper (ATM; 1993-present), including Operation IceBridge (since 2010). This provides opportunities to determine time-varying mass changes of grounded ice and the floating ice shelves since 1993. Additional fields included in ATL06, derived from the distribution of returned photons, contain valuable information on the surface roughness of the ice sheet and the locations of data corrupted by cloud cover.

Radar altimeters have been used for more than two decades to measure changes in ice sheet elevation and volume. Most satellite radar altimeters - such as CryoSat-2 - transmit signals at Ku-band (13.6 GHz), which can penetrate distances between 5 to 10 m below the actual ice sheet surface depending on the snowpack properties. This penetration complicates determining the precise location of radar altimeter scattering horizon, and waveform retracking is performed to mitigate this effect by locating the first scattering surface using properties of the full radar echo. Picking this location can, however, be complicated by secondary (volume) scattering from within the snowpack itself. AltiKa, launched in February 2013, is the first space-borne altimeter to operate at Ka-band (36 GHz). At this frequency, the degree of radar wave penetration into typical snowpacks is considerably shallower - in the range 0.1 to 0.3 m.
Here, we produce the first assessment of Ka-band (37 GHz) satellite radar altimetry to track changes in ice sheet elevation. We use measurements recorded by the AltiKa mission to track changes in the elevation of the Amundsen Sea Sector in West Antarctica between March 2013 and April 2017. During this period, the observed region (82.9 %) thinned by -9.12 ± 0.02 cm yr-1, on average, and the fast-flowing sections of the Pine Island and Thwaites Glaciers thinned by -124.0 ± 2.0 cm yr-1 and -127.0 ± 4.0 cm yr-1. When compared to contemporaneous measurements recorded by Operation IceBridge (OIB) airborne laser altimeter and CryoSat-2 Ku-band satellite, AltiKa elevation rates differ by 0.9 cm yr-1 and 0.02 cm yr-1, respectively. Although rates of elevation change do not differ by much, elevation measurements do (+ 3.12 m compared to OIB), presumably due to slope correction. Despite the fact that the degree of radar penetration into ice sheet surfaces differs considerably at Ka- and Ku-band, this effect is not apparent in measurements of retracked elevation change computed from either AltiKa or CryoSat-2 in this sector of Antarctica. Our study highlights the capability of AltiKa for measuring ice sheet elevation change.

The CryoSat Plus for Oceans (CP4O) project, under the ESA STSE program, aims to develop and evaluate new ocean products from CryoSat-2 data. After focusing on the additional measurement capabilities offered by the SAR mode of the SIRAL altimeter, further work was carried out to develop improved geophysical corrections. In particular, the Arctide2017 regional tidal atlas in the Arctic Ocean was implemented, based on hydrodynamic modelling with ensemble assimilation of tidal harmonic constituents derived from tide gauge and satellite altimetry observations.
This work highlighted the challenge that the Arctic Ocean represents for tidal modelling, because of its complex and not well-documented bathymetry, together combined with the intermittent presence of sea ice and the fact that the in situ tidal observations are scarce at such high latitudes.
Subsequently, the project progressed in two ways to use Cryosat-2 data and improve ocean tide modelling in the Arctic Ocean. On the one hand, a comparison between several bathymetry datasets was performed (RTopo1.0.5, RTopo-2, LEGOS, IBCAOv3…). Then, 7 years of Cryosat-2 high quality and high resolution "geodetic" SAR altimetry data up to 88°N were used to improve the deep ocean bathymetry. On the other hand, improved Cryosat-2 derived harmonic tidal constituents computed in the open ocean and in the sea ice leads were used for assimilation in the regional tidal model.
This presentation will highlight the main results of this work. It will include the evaluation of the Arctic Ocean bathymetry field derived from the high resolution DTU17 marine gravity field based on Cryosat-2 data. This new bathymetry resolves detailed features that are not given or are wrongly mapped in IBCAOv3. The presentation will also show the performance assessment of the new release of the regional tidal model in the Arctic Ocean, compared to regional (AOTIM) and global (GOT4.8, TPXO7.2, FES2014, etc.…) tidal models in the region.