A Comparison of AltiKa and CryoSat-2 Measurements of Greenland Ice Sheet Elevation Change

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 to distances of 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.

In this study, we compare radar altimeter measurements acquired at Ka-band and Ku-band. We derive elevation change across Greenland using a plane fit method and find that the surface is lowering at a rate of -0.4 cm/yr and -3.3 cm/yr between March 2013 and April 2017 from AltiKa and CryoSat-2 respectively. To further assess the ability of waveform retracking to locate a common and stable ice sheet surface, we analyse difference between AltiKa and CryoSat-2 satellite altimetry at orbit crossovers along the EGIG line in central western Greenland. We implement three different retrackers, OCOG, TCOG and TFMRA, to assess their relative performance. Across the EGIG area, AltiKa retrieves a higher elevation than CryoSat-2 when any retracker is used, which is expected due to the lower penetration depth. The mean elevation difference between AltiKa and CryoSat-2 at orbit crossovers is 1.66 m, 1.63 m and 1.57 m for the OCOG, TCOG and TFMRA retrackers respectively, with corresponding standard deviations of 0.88 m, 0.55 m and 0.80 m. Combining Ka and Ku-band observations can provide a better understanding of the effects of penetration of the radar wave on radar altimetry derived-heights, and could help exploit the full potential of the 25 year long Ku-band altimetry record.