Articles | Volume 11, issue 3
The Cryosphere, 11, 1041–1058, 2017
The Cryosphere, 11, 1041–1058, 2017

Research article 04 May 2017

Research article | 04 May 2017

A revised calibration of the interferometric mode of the CryoSat-2 radar altimeter improves ice height and height change measurements in western Greenland

Laurence Gray1, David Burgess2, Luke Copland1, Thorben Dunse3, Kirsty Langley4, and Geir Moholdt5 Laurence Gray et al.
  • 1Department of Geography, Environment and Geomatics, University of Ottawa, Ottawa, ON K1N 6N5, Canada
  • 2Geological Survey of Canada, Natural Resources Canada, Ottawa, ON K1A 0E8, Canada
  • 3Department of Geosciences, University of Oslo, 0316 Oslo, Norway
  • 4Asiaq, Greenland Survey, 3900 Nuuk, Greenland
  • 5Norwegian Polar Institute, 9296 Tromso, Norway

Abstract. We compare geocoded heights derived from the interferometric mode (SARIn) of CryoSat to surface heights from calibration–validation sites on Devon Ice Cap and western Greenland. Comparisons are included for both the heights derived from the first return (the point-of-closest-approach or POCA) and heights derived from delayed waveform returns (swath processing). While swath-processed heights are normally less precise than edited POCA heights, e.g. standard deviations of  ∼  3 and  ∼  1.5 m respectively for the western Greenland site, the increased coverage possible with swath data complements the POCA data and provides useful information for both system calibration and improving digital elevation models (DEMs). We show that the pre-launch interferometric baseline coupled with an additional roll correction ( ∼  0.0075° ± 0.0025°), or equivalent phase correction ( ∼  0.0435 ± 0.0145 radians), provides an improved calibration of the interferometric SARIn mode.

We extend the potential use of SARIn data by showing the influence of surface conditions, especially melt, on the return waveforms and that it is possible to detect and measure the height of summer supraglacial lakes in western Greenland. A supraglacial lake can provide a strong radar target in the waveform, stronger than the initial POCA return, if viewed at near-normal incidence. This provides an ideal situation for swath processing and we demonstrate a height precision of  ∼  0.5 m for two lake sites, one in the accumulation zone and one in the ablation zone, which were measured every year from 2010 or 2011 to 2016. Each year the lake in the ablation zone was viewed in June by ascending passes and then 5.5 days later by descending passes, which allows an approximate estimate of the filling rate. The results suggest that CryoSat waveform data and measurements of supraglacial lake height change could complement the use of optical satellite imagery and be helpful as proxy indicators for surface melt around Greenland.

Short summary
We use surface height data from west Greenland and Devon Ice Cap to check the performance of the new interferometric mode of the ESA CryoSat radar altimeter. The detailed height comparison allows an improved system calibration and processing methodology and measurement of the height of supraglacial lakes which form each summer around the periphery of the Greenland Ice Cap. The advantages of the SARIn mode suggest that future satellite radar altimeters for glacial ice should use this technology.