CryoSat Ice Baseline-D validation and evolutions
Authors: Meloni Marco , Bouffard Jerome , Parrinello Tommaso , Dawson Geoffrey , Garnier Florent , Helm Veit , Di Bella Alessandro , Hendricks Stefan , Ricker Robert , Webb Erica , Wright Ben , Nielsen Karina , Lee Sanggyun , Passaro Marcello , Scagliola Michele , Simonsen Sebastian Bjerregaard , Sandberg Sørensen Louise , Brockley David , Baker Steven , Fleury Sara , Bamber Jonathan , Maestri Luca , Skourup Henriette , Forsberg René , Mizzi Loretta 
Published: June 2020

Abstract. The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 to
monitor the precise changes in the thickness of terrestrial ice sheets and
marine floating ice. To do that, CryoSat orbits the planet at an altitude of
around 720 km with a retrograde orbit inclination of 92∘ and a
quasi repeat cycle of 369 d (30 d subcycle). To reach the mission
goals, the CryoSat products have to meet the highest quality standards to
date, achieved through continual improvements of the operational processing
chains. The new CryoSat Ice Baseline-D, in operation since 27 May 2019, represents a major processor upgrade with respect to the previous Ice
Baseline-C. Over land ice the new Baseline-D provides better results with
respect to the previous baseline when comparing the data to a reference
elevation model over the Austfonna ice cap region, improving the ascending
and descending crossover statistics from 1.9 to 0.1 m. The improved
processing of the star tracker measurements implemented in Baseline-D has
led to a reduction in the standard deviation of the point-to-point
comparison with the previous star tracker processing method implemented in
Baseline-C from 3.8 to 3.7 m. Over sea ice, Baseline-D improves the
quality of the retrieved heights inside and at the boundaries of the
synthetic aperture radar interferometric (SARIn or SIN) acquisition mask,
removing the negative freeboard pattern which is beneficial not only for
freeboard retrieval but also for any application that exploits the phase
information from SARIn Level 1B (L1B) products. In addition, scatter
comparisons with the Beaufort Gyre Exploration Project (BGEP; https://www.whoi.edu/beaufortgyre, last access: October 2019) and Operation IceBridge (OIB; Kurtz et
al., 2013) in situ measurements confirm the improvements in the Baseline-D
freeboard product quality. Relative to OIB, the Baseline-D freeboard mean
bias is reduced by about 8 cm, which roughly corresponds to a 60 %
decrease with respect to Baseline-C. The BGEP data indicate a similar
tendency with a mean draft bias lowered from 0.85 to −0.14 m. For the two
in situ datasets, the root mean square deviation (RMSD) is also well reduced
from 14 to 11 cm for OIB and by a factor of 2 for the BGEP. Observations over
inland waters show a slight increase in the percentage of good
observations in Baseline-D, generally around 5 %–10 % for most lakes.
This paper provides an overview of the new Level 1 and Level 2 (L2) CryoSat
Ice Baseline-D evolutions and related data quality assessment, based on
results obtained from analyzing the 6-month Baseline-D test dataset released
to CryoSat expert users prior to the final transfer to operations.