Geoscience Reference
In-Depth Information
to estimate floodplain inundation rate, water storage and drainage rate for six sections of
the Amazon main stem in Brazil. Whilst satellite gravimetry is of lower resolution than
many of the measurements discussed so far, water storage change is relevant to a range of
hydraulic, biogeochemical and ecological processes, and therefore, such data add usefully
to our knowledge of surface water processes.
3 The Proposed SWOT Satellite Mission
Section
2
demonstrates convincingly that no current satellite system can capture the detail
of surface flows in rivers, floodplains and wetlands (e.g., Alsdorf et al.
2000
,
2007a
; Bates
et al.
2006
) and that we lack a comprehensive and consistent view of global surface water
dynamics at a scale commensurate with known process variability. In the absence of
reliable observations, it is also impossible to build, calibrate and validate models that can
be applied with confidence to river, floodplain and wetland systems. Whilst progress can be
made by carefully employing the data derived from topographic mapping, oceanographic,
cryospheric and geodetic satellites, the lack of a dedicated surface water observing mission
fundamentally limits our ability to map, model and understand surface water dynamics.
Against this background, NASA and CNES are currently developing a new satellite
mission to address this gap in the global observing system: the proposed Surface Water
Ocean Topography mission or SWOT (see
http://swot.jpl.nasa.gov/
).
SWOT is being designed as a small version of SRTM. Both are ''interferometers'' that
construct radar interferometric phase using one pass of two SAR antennae that are permanently
connected by a fixed baseline. The interferometric phase is a measurement of surface elevations,
i.e. topography of land and elevations of water. Because SWOT would use a Ka-band wave-
length, which is shorter than the C-band and X-band wavelengths used by SRTM, the SWOT
boom separating the two SAR antennae would be 10 m compared to the 60-m boom used by
SRTM. SWOT would use a near-nadir viewing geometry with look angles of well less than 10.
In contrast, SRTM used *30-58 look angles. This near-vertical geometry results in height
accuracies that would be at least an order of magnitude better than those of SRTM. The
proposed SWOT mission is expected to produce ±50 cm height accuracies per sampling
element (e.g., pixel). However, this viewing geometry would also result in a greater amount of
layover for SWOT compared to SRTM (layover results when higher elevations are mapped by
the SAR geometry into spatial locations closer to the radar). Also because of this viewing
geometry, the spatial samples would vary in size from potentially as small as 2.5 m 9 10 m to
as large as 10 m 9 70 m. The height error is normally distributed, so averaging samples
improves the height accuracy by 1
=
p
ð
m
Þ
, where m is the number of samples. For example, a
250 m 9 250 m lake sampled entirely by the finest spatial resolution would have a ±1cm
height accuracy after averaging, whereas when sampled entirely by the coarsest spatial
resolution, the height accuracy is reduced to about ±5 cm.
The proposed SWOT mission is presently in ''Phase A'' of the NASA and CNES
mission development life cycle. An international science definition team is working with
SWOT project engineers and planners to define the required spatial, temporal and height
accuracies. These requirements are expected to allow sampling of rivers at least as small
as 100 m in channel width and perhaps smaller. Lakes and other water bodies
250 m 9 250 m in size and perhaps even smaller are also under consideration for the
mission design. To further help in defining the mission, an airborne version of SWOT has
been created. AirSWOT has initiated test flights and is expected to sample rivers, lake and
wetlands during 2013 and thereafter. Amongst the AirSWOT goals is to demonstrate the
