Challenges raised from SARAL/AltiKa for Ice Sheet studies

Aurélie Michel (CNES/CLS/LEGOS, France)

CoAuthors

Frederique Remy (Legos, France)

Event: 2014 SARAL/AltiKa workshop

Session: Land ice and Sea ice

Presentation type: Type Poster

SARAL/AltiKa was successfully launched in February 2013. With more than one-year data it is possible to study annual variations, seasonal cycles, instrumental events...
This presentation aims to show the questions that are raised about this innovative mission over land ice. First, this is the first altimeter using Ka-band. SARAL/AltiKa is the ERS1/2 and Envisat follow-on with the same repeat orbit and the same repeat cycle. We thus can compare both Ku-band and Ka-band. The new frequency implies a smaller penetration depth but also a higher sensibility to the snow grain size. Moreover, the antenna aperture is smaller than with Envisat, which leads to a more accurate measurement but also a higher rate of missing points, especially over the coasts with slopes over Antarctica . In this frame, the Peachi project was launched to study the SARAL mission for the other surfaces than oceans, using a geophysical analysis or an instrumental one to obtain a global expertise.
in this presentation, we focus on the waveform classification done for SARAL/AltiKa over the Antarctic ice sheet. We show first conclusions made on the results from the first SARAL cycles, easily noticeable features or more complicated to analyze : the waveform due to the missing points is easily identified, the brownian model used by our algorithm Ice-2 covers a large area for the Antarctic ice sheet. On the other side, we see that some waveforms cannot be retracked due to their specular characteristics. This divides the analysis in two ways : the instrumental way and the geophysical one .The instrumental part allows us to edit the measurement thanks to the classification and we obtain more accurate observations. We performed an idealized editing by keeping solely brownian waveforms in our statistics. We show that the rms is lower with this specific editing. For the geophysical part, we compared MODIS images from the MOA (Modis Mosaic of Antarctica) with a 125m resolution with the spatial results from the classification. A dense study for the whole Antarctica has been performed, using various cycles from SARAL/AltiKa in order to detect temporal variations. In conclusion, we show here all the relevant observations made and the ongoing challenges. Furthermore, a comparison between SARAL/AltiKa and the previous altimeter Envisat will also be highlighted.

Figure legend : Class 8 is specular so we may wonder if the retracking Ice-2 is relevant for the measurements corresponding to the 8th class. If we take a look at the geographical features, we notice the 8th class matches exactly with the slope signature, and the measurements are not accurate enough, due to the smaller antenna aperture.
 
Aurélie Michel
CNES/CLS/LEGOS
France
aurelie.michel@legos.obs-mip.fr