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2.2 Study data
2.2.1 LEGOS Hydroweb and DAHITI altimetry data archives
Observation data provided by altimetry satellites has been gradually
increased since the early 1990s, including TOPEX/POSEIDON(T/P), ERS-1,
ERS-2, Jason-1/-2/-3, Envisat and Cryosat-2(Table 1). The combination of
different altimetry satellites can increase the spatial coverage of
altimetry measurements and extend the temporal span to nearly thirty
years. At present, the long-term lake level changes reconstructed by
many institutions is based on the altimetry satellite data of different
service years. As one data source of our analyzed water levels, we
obtained lake levels from the Laboratoire d Etudes en Géophysique et
Océanographie Spatiales (LEGOS) Hydroweb altimetry data archive and the
Hydrological Time Series of Inland Waters (DAHITI) altimetry data
archive. The LEGOS Hydroweb altimetry data archive was derived from
merged Topex/Poseidon, Jason, Envisat and Sentinel data (Crétaux et al.
2009; Crétaux et al. 2011b). All background rectification have been used
to produce this data archive. Since different altimetry satellites are
used for the same lake, the water level calculation is mainly divided
into three steps. First, each satellite data was processed separately to
obtain the altimetry measurement data of different satellites, and then
Topex/Poseidon data was used as a reference standard to minimized
potential radar instrument deviations and geoid differences between
different satellites. Finally, lake levels from the different satellites
were merged. The DAHITI data archive was launched by the Deutsches
Geodätisches Forschungsinstitut (DGFI, now DGFI-TUM) in 2013. The
processing procedure of DAHITI database in detail can be referred to in
Schwatke et al. (Schwatke et al. 2015) which was based on an extended
outlier detection and a Kalman filtering of multi-source merged
altimetry measurements. In this paper, because the algorithms adopted by
the two data archives were different, we only used one of them for a
lake. When a lake was covered by the
two
data archives at the same time, Hydroweb data archive was selected as
the priority.