IEEE Geoscience and Remote Sensing Magazine - March 2013 - 33

Biosphere

Deforestation, Degradation, Fires* (REDD)
Forest Biomass Change*
Biodiversity
Geosphere

Earthquakes
Volcanic Activities
Land Slides
Sea Ice Extent*

Cryosphere
Permafrost*

Glacier and Ice Cap Dynamics*
Hydrosphere

Soil Moisture*
Flooding

*) Essential Climate
Variables

Ocean Currents*

Days

Weeks

Months

Years

Observation Interval

FIGURE 25. Requested observation intervals for the systematic monitoring of some exemplary dynamic processes on the Earth surface.

Note that some processes may require multiple SAR data acquisitions for a single measurement to provide the required accuracy. An
example is the estimation of forest biomass and its change by polarimetric SAR interferometry, where several interferometric acquisitions
with varying cross-track baselines have to be combined to achieve high accuracy for different forest types and heights.

a high PRF to avoid azimuth ambiguities in the SAR
imaging process. The high PRF limits, however, the
unambiguous swath width since the range echo from
the illuminated swath has to fit between two successive
radar pulses to avoid range ambiguities. Wide unambiguous swath coverage and high azimuth resolution pose
therefore contradicting requirements on the design of
spaceborne SAR systems.
Several SAR imaging modes have been developed
that provide different trade-offs between spatial coverage and azimuth resolution. Examples are the ScanSAR
(or TOPS) mode, which enables a wide swath at the cost
of an impaired azimuth resolution, and the Spotlight
mode, which allows for an improved azimuth resolution at the cost of a noncontiguous imaging along the
satellite track (see Section II). It is, however, up to now
not possible to combine both imaging modes simultaneously in one and the same data take. To overcome
this fundamental limitation, several innovative digital
beamforming techniques have been suggested where the
receiving antenna is split into multiple sub-apertures
that are connected to individual receiver channels. The
digitally recorded sub-aperture signals are combined
in a spatiotemporal processor to simultaneously form
multiple independent beams and to gather additional
march 2013

ieee Geoscience and remote sensing magazine

information about the
direction of the scattered
More and more users
radar echoes. This inforask for time series of
mation can be used to 1)
high-resolution radar
suppress spatially ambiguous signal returns from the
images which are
ground by null-steering, 2)
acquired in repetition
increase the receiving gain
intervals that are as
without a reduction of the
short as possible.
imaged area by switching between narrow, high
gain beams, 3) suppress
spatially localized interferences by space-time adaptive processing, and 4) gain additional information
about the dynamic behavior of the scatterers and their
surroundings.
A. High-Resolution Wide-Swath Imaging
Digital beamforming and multi-aperture signal recording
are key technologies that will boost the performance of
future SAR systems by at least one order of magnitude.
These improvements are evident, both from research
activities and demonstration campaigns [168]-[196]. A
prominent example is the high-resolution wide-swath
(HRWS) SAR [172] which is currently under development
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2013