CryoSat User Workshop 6

Reference and repeat-observations of Glacier, Ice Caps and Ice Sheet Margin topography are 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. The lack of such sustained observations was identified in the Integrated Global Observing Strategy (IGOS) Cryosphere Theme Report as a major shortcoming. Conventional altimetry measurements exist, but coverage has been sparse and characterized by coarse ground resolution. Additionally, and more importantly, they proved ineffective in the presence of steep slopes. Since the majority of the mass loss of the Cryosphere is occurring in sloping terrain, there is the need for more robust and dense observations in both time and space.

The CryoSat mission aims at gaining better insight into the evolution of the Cryosphere. 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 Glacier, Ice Caps and Ice Sheet Margin and the elevation for the Point Of Closest Approach (POCA) is a standard product of the CryoSat mission.However, there are significant opportunities to increase the sampling density for elevation beyond this standard product. 

Here we present, through a wide range of examples, a new approach developed through the ESA Cryosat+ Mountain Glaciers and CryoTop projects, for more comprehensively exploiting the SARIn mode of CryoSat and produce ice elevation and elevation change with enhanced spatial resolution compared to standard CryoSat-2 products. In this so-called CryoSat Swath SARIn (CSSARIn) approach, the signal beyond the POCA is analysed, leading to between 1 and 2 orders of magnitude more elevation measurements than conventional approaches, and providing elevation where conventional POCA fails. We will present the rationale, validation and results of the approach for elevation, elevation change and volume change over various Cryospheric environments e.g. ice sheet margins, ice caps, ice streams and glaciers, sub-glacial lakes. 

Austfonna, the largest ice cap on Svalbard in the Eurasian Arctic, is currently undergoing a major dynamic instability. The surge of Basin-3, one of the ice cap’s major drainage basins, started in autumn 2012, and led to a two-orders of magnitude increase in ice-surface velocities, significantly enhanced calving rates and a~5km advance of the marine terminus.

The ice cap has been selected as one of the calibration/validation sites of the Cryosat-mission. Cryosat-2 radar altimetry has been acquired over Austfonna since 2010, using the SAR interferometer (SARIn) mode. The reliable and frequent coverage of Cryosat-2 allows for investigation of intra and interannual ice-surface height changes. Here we present results from both “point-of-closest-approach” and “swath-mode” algorithms applied to level L1b data. The former algorithm delivers height estimates preferably along ridges and summit areas, while the latter provides complementary results away from protruding topographic features. Our results indicate the need for quality control in order to exclude erroneous height estimates and/or erroneous positioning of height estimates. A new DEM from airborne photography acquired over Austfonna in 2010-11 will be used as a reference against which to compare Cryosat-2 derived ice-surface heights. The development of the surge, as well as the associated ice flux, will be discussed in light of the temporal and spatial pattern of the observed height change.

CryoSat was a planned as a 3 year mission with clear mission objectives to allow the assessment rates of change of thickness in the land and marine ice fields with reduced uncertainties with relation to other non-dedicated missions. Although CryoSat suffered a launch failure in Oct 2005, the mission was recovered with a launch in April 2010 of CryoSat-2. The nominal mission has now been completed, all mission requirements have been fulfilled and CryoSat has been shown to be most successful as a dedicated polar ice sheet measurement system.

Following the completion of the nominal mission in Oct 2013 the platform was shown to be in good health and with a scientific backing provided by the ESA Earth Science Advisory Committee (ESAC) the mission has been extended until Feb 2017 by the ESA Programme Board for Earth Observation.

Though not designed to provide data for science and operational services beyond its original mission requirements, a number of services have been developed for exploitation and these are expected to increase over the next few years. Services cover a number of aspects of land and marine ice fields in addition to complementary activities covering glacial monitoring, inland water in addition to coastal and open ocean surface topography science that CryoSat has demonstrated world leading advances with.

This paper will present the overall concept for a potential low-cost follow-on to the CryoSat mission with the objective to provide both continuity of the existing CryoSat based data sets, i.e., longer term science and operational services that cannot be provided by the existing Copernicus complement of satellites. This is, in part, due to the high inclination (92°) drifting orbit and state of the art Synthetic Aperture Interferometer Radar Altimeter (SIRAL). In addition, further improvements in performance are expected by use of the instrument timing and digital hardware developments used in the Sentinel-6/Jason-CS Poseidon-4 design that would be adapted for interferometry. It is expected that the potential mission could also provide data for global ocean services complementary to those of the other Sentinel 3 and 6 missions depending on resources.

The Iridium PRIME offer, recently initiated by the Iridium company, consists in hosting payloads on customized low cost Iridium NEXT platforms on which the Main Mission Antenna (L-band) is removed. This leaves significant resources in terms of mass, volume and power consumption to host up to three payloads on these customized platforms. The Iridium PRIME satellites will be be inserted in the Iridium-NEXT constellation to take benefit of the low cost operation service (command, control and data telemetry through the life time of the Iridium PRIME mission). Given the  synergy between schedules of the Iridium PRIME program (launch around 2020) and of the Cryosat FO mission (launch by 2022) and the adequacy of the available on-board resources for such a mission, ESA tasked TAS, as responsible for the SIRAL radar instrument of the currently in-orbit Cryosat mission, to conduct a feasibility study to implement a concept for enhanced continuity of Cryosat on an Iridium PRIME satellite.
The study aims to define a cost-effective topographic payload including not only the SIRAL radar but also the necessary sub-systems to retrieve the SIRAL antenna baseline attitude (star trackers) with high accuracy and to perform Precise Orbit Determination (POD). All these aspects are presented in this paper. In particular, possible evolutions/improvements are proposed for the SIRAL radar by adding a Ka-band channel for nadir measurements and for ensuring a swath mode. Transmission of the SIRAL science data is also discussed in the paper.

During the round table, seed questions proposed by the chairs will be discussed with the audience