[Insert Table 1 here]
Both two data archives cover a certain number of lakes in the Tibetan
Plateau (Table 2). The long-term water level change information of 16
lakes was obtained based on Hydroweb and DAHITI data archives. However,
the number of lakes cannot fully meet the needs of our lake level
monitoring (refer back to Fig. 1). Therefore, in this study, SARAL and
Sentinel-3 altimetry data were used to supplement the missing
observations from the LEGOS Hydroweb and DAHITI data archives. We
interpreted and combined SARAL and Sentinel-3 altimetry data to obtain a
total of 12 lake level time series during the period 2013-2018. Six of
them, covering the same lakes as Hydroweb and DAHITI data archives, were
used to verify the reliability of SARAL and Sentinel-3 altimetry data.
Others were used as the data supplement.
2.2.2 Supplementary altimetry
data
2.2.2.1 SARAL/AltiKa data
SARAL is a common mission of the Indian Space Research Organization
(ISRO) and the France space agency (Project 2010). The mission, launched
on 25 February 2013, is a continuation of the ERS series and Envisat
missions.
The
orbit and revisit cycle of SARAL satellite were the same as Envisat, but
after July 2016,
SARAL
satellite was then transformed to drifting ground track mode. Compared
to the previous altimetry satellites, the improvement of SARAL/AltiKa
was that it was the first altimeter to measure in the Ka band, with
higher spatial resolution. In addition, AltiKa’s bandwidth (500 MHz) had
been enhanced, which resulted in a higher vertical resolution and higher
pulse repetition frequency (Project 2010).
We
downloaded the SARAL data (spanning from March 2013 to June 2016) from
ftp://avisoftp.cnes.fr.
2.2.2.2 Sentinel-3/SRAL data
The
Surface Topography Mission uses the Synthetic Aperture Radar Altimeter
(SRAL) instrument, which transmits pulses occasionally at Ku band for
altimeter range measurements, supplemented by a C-band frequency to
correct range delay errors. The SRAL instrument acquires data in two
measurement modes, the Low-Resolution Mode (LRM) which is useful over
homogeneous open-ocean surfaces, and the Synthetic Aperture Radar (SAR)
mode which achieves high along-track resolution over relatively flat
surfaces. Sentinel-3 satellite has multiple new functions and modes with
the dual-frequency SAR radar altimeter (SRAL) (Donlon et al. 2012).
Among them, the tracking mode is the most critical characteristic(Jiang
et al. 2019). We downloaded the needed data from March 2016 to December
2018 through
https://sentinel.esa.int/web/sentinel/
sentinel-data-access.