CryoSat User Workshop 3

We assess the accuracy of CryoSat-2 synthetic aperture radar interferometric (SARIn) mode data over Antarctica using data from Geoscience Laser Altimeter System onboard ICESat and also compare coverage with conventional altimetry. We used the ESA Baseline B product, corrected for various processing issues identified in this data release, with one of the objectives being to assess the ESA retracker behaviour. Biases, exceeding 4 m, were ubiquitous in areas with surface slopes above about 0.9 degree. Over the ice shelves and the interior of the ice sheet, the bias was less than 50 cm but appeared to be sensitive to snowpack density. We find that the accuracy and coverage of CryoSat-2 SARIn mode data is significantly better than for previous satellite radar altimeter missions for slopes up to 1 degree.

We investigated, specifically, biases and system performance over the ice shelves, and found a qualitative inverse relationship between surface firn density and elevation bias for the four largest ice shelves where relatively consistent values of both bias and density were derivable (Fang et al, 2015). The relationship is consistent with the idea that it is related to variations in extinction coefficient for differences in snowpack properties: a larger positive bias (ICESat-CryoSat) is obtained for lower mean densities. Surface firn density in Antarctica is not stationary in time and it is likely, therefore, that the depth of the radar mean surface will vary in time. It may be possible to eliminate or reduce this variability by using a heuristic backscatter correction, a different retracker or a correction related to waveform shape: approaches that have been used in the past for conventional radar altimetry over ice sheets.

Wang, F., J. L. Bamber, and X. Cheng (2015), Accuracy and Performance of CryoSat-2 SARIn Mode Data Over Antarctica, Geoscience and Remote Sensing Letters, IEEE, PP(99), 1-5.

The main objective of the ESA funded CryoVal-LI project has been to identify and quantify the error sources for the CryoSat-2 mission over land ice. This has been undertaken through the careful documentation of the possible error sources, the identification of suitable validation sites and the creation of a public database containing validation data for these areas from recent ground and airborne field campaigns.  These sites also offer considerable potential as sites on which to focus future validation efforts.

Through the utilization of these observational datasets, an extensive comparative analysis has been carried out in which Cryosat-2 data has been evaluated against airborne and in-situ data. A number of different Cryosat-2 data sets have been included in this analysis with the goal of testing the performance of different processing and retracking algorithms. A similar, but smaller validation analysis has also been carried out for ICESat data.

Here, we present the results of these analyses and outline the conclusions reached. Based on the findings from the project, a set of recommendations for the design of future land-ice/satellite validation campaigns will be given.  Furthermore, the outcome of the re-tracker inter-comparison will be used to advocate a set of optimal re-tracking algorithms for Cryosat-2 in the next generation of L2 products.

ESA's CryoSat-2 mission with its radar altimeter system SIRAL has been designed in order to determine fluctuations in the mass of the Greenland and Antarctic ice sheets and of sea ice floating in the polar oceans.

To fulfill the main goals of the CryoSat-2 mission it is necessary to validate the different CryoSat-2 products against independent measurements. The objective of the CryoSat Validation Experiments (CryoVEx) is to collect and analyse ground-based and airborne measurements in order to get such independent data sets.

In Antarctica, blue-ice areas are particularly suited for calibration/validation activities related to airborne/spaceborne radar systems due to the fact that altimetric returns at blue ice are dominated by surface reflection. Such an area exists about 70 km south-east of Schirmacher Oasis, Dronning Maud Land, East Antarctica.

Since 1991, surface-height changes and surface velocities along traverses crossing the blue-ice area have been repeatedly observed by the Institut für Planetare Geodäsie of TU Dresden in order to determine long-term variations. For that purpose kinematic GNSS measurements were carried out. Over the last years several Antarctic CryoVEx field campaigns in the blue-ice area were realised whereby the extent of the kinematic GNSS observations was increased substantially. These campaigns provide an excellent continuation of the long-term height observations.

Up to 2008 the repeated measurements yielded a surface-height decrease of up to -20cm/a in this area. In contrast to this, the observed rates of the surface-height change between 2008/09 and 2010/11 show a positive trend in the same order of magnitude. In order to classify the anomalous trend change and to continue the long-term observation we carried out an additional CryoVEx campaign during the 2014/15 Antarctic season.

We will discuss the final outcome of the latest CryoVEx campaign with respect to former ground-based measurements. We will compare our ground-based results with coordinated airborne measurements carried out by AWI Bremerhaven including a collection of radar (ASIRAS) and laser altimeter data. Finally, comparative investigations of ground-based determined surface heights, airborne data and most recent CryoSat-2 products will be presented.

A main goal of the CryoSat-2 mission is to observe the variations in the elevation of the polar ice sheets within a few centimeters accuracy. To fulfill this challenging objective the CryoSat-2 Level 2 products need to be validated against independent measurements.

Kinematic GNSS is a well established method to derive ground based surface elevation profiles with high accuracy, thus providing independent data to investigate the reliability of the radar altimetry observations.

During the last decade a variety of such measurements could be realized in cooperation with the Russian Antarctic Expedition (RAE). Since 2007 we have realized the installation of several geodetic GNSS equipments on vehicles of the scientific and logistic convoys. These profiles between the Russian Antarctic research station Vostok (78° 28' S, 106° 50' E) and the coastal stations Mirny and Progress have a length of about 1,600 km each. Over several years the repetition of these profiles show that the elevation change is negligibly small in this region which forms an important precondition when comparing older GNSS profiles with recent CryoSat-2 data.

Thus, the GNSS profiles give us the unique opportunity to validate both CryoSat-2 LRM-Mode data in the flat interior of the Antarctic Ice Sheet and SARIn-Mode data in the steep and rough coastal area. Validation results of the crossover analysis with Baseline B data are presented. We show how the new Baseline C processor version improves the observation quality. Furthermore, the alternate AWI processing version is used for comparison. Additionally, we will test the performance of the advanced swath processing technique in areas covered by the SARIn mode.

Besides the Level 2 products we furthermore present the results of the validation of different digital elevation models (DEM). The probably most popular DEM for Antarctica, Bedmap2, is based mainly on ICESat, ERS-1 and small-scale local datasets. We show that a DEM based on CryoSat-2 data only has the advantage of a much higher consistency and of less interpolation errors due to the dense satellite ground tracks.

Since its launch in 2010, the CryoSat-2 satellite radar altimeter has been acquiring data over Earth’s polar regions. Across Antarctica and Greenland, these measurements have been used to produce digital elevation models of the ice sheets and to develop maps of ice sheet elevation change. Here we present a high-level assessment of the mission performance over ice sheets to date, based upon evaluations of ESA’s L2 elevation product and derived elevation rate measurements across Antarctica and Greenland. For this assessment, we will use contemporaneous elevation measurements acquired by airborne laser altimetry, flown by repeated Operation IceBridge campaigns over the last 5 years. We will assess the performance of data acquired by CryoSat-2 in both LRM and SARIn modes of operation. This analysis will provide a quantitative high-level assessment of the CryoSat-2 mission performance to date.