IEEE Geoscience and Remote Sensing Magazine - March 2013 - 8

as the discovery time with a strong believe on the potential
and future of radar remote sensing. The launch of the ERS1/2 (C-band), JERS-1 (L-band) and Radarsat-1 (C-band)
satellites in the 90s represented further milestones in the
spaceborne SAR development in Europe, Japan and Canada,
respectively. SAR techniques like polarimetry for improved
parameter retrieval, interferometry for deriving the surface
topography and differential interferometry for the measurement of Earth surface displacements were developed in
the 80s and 90s [13]-[16]. The application fields of these
techniques were catapulted by the shuttle missions SIRC/X-SAR (Shuttle Imaging Radar mission with X-, C- and
L-band radars, the latter two being fully polarimetric) in
1994 and the Shuttle Radar Topography Mission (SRTM) at
X-band and C-band in 2000. A further milestone in the SAR
development was associated to differential SAR interferometry with permanent scatterers (PS) for subsidence monitoring [17], a technique that was developed using data from

(a)

(b)
FIGURE 1. Comparison of a SAR image corresponding to the state
of the art during the 90s [(a) ca. 20 m resolution, C-band, radar
illumination from the left] and the current generation of SAR satellites
available since 2007 [(b) 1 m resolution, X-band, radar illumination
from the right]. The images show the pyramids of Giza, Egypt.

ERS-1/2 and later ENVISAT/ASAR (C-band). The latter was
the first SAR satellite using the antenna technology with
transmit/receive modules for achieving greater flexibility in
the steering of the radar antenna beam and therefore in the
selection of different imaging modes. In the last 10 years,
considerable progress has been achieved with polarimetric
SAR interferometry (Pol-InSAR) [18] and tomography for
obtaining information of volume scatterers [19]. Tomography has been also used in combination with PS techniques
to solve the problem associated with the layover effect in
urban areas. Most recently, holographic tomography has
been proposed for generating a 360c imaging view of volume scatterers [20].
With the launch of the bi-static SAR satellites TerraSAR-X
and TanDEM-X (X-band), the COSMO-SkyMed satellite
constellation (X-band) as well as Radarsat-2 (C-band) a new
class of SAR satellites was introduced providing images with
resolution in the meter regime. Fig. 1 shows a comparison
of a SAR image with moderate resolution corresponding to
the state of the art in the 90s and a SAR image obtained
with the new generation of high-resolution SAR satellites.
The trend for future systems shows the need for an
increased information content in SAR images that can be
achieved by multi-channel operation (polarimetry, multifrequency), improved range and azimuth resolution, time
series (frequent revisit of the same area) as well as observation angle diversity (interferometry and tomography).
These user requirements push the development of new
technologies (e.g., digital beamforming, MIMO, bi- and
multi-static, large reflector antennas) that are shaping the
future of spaceborne SAR systems with the ultimate goal to
allow a wide-swath high-resolution monitoring of dynamic
processes on the Earth surface in a quasi-continuous way.
Table 2 provides an overview of spaceborne SAR sensors (a
recent compilation of airborne SAR sensors is given in [21]).
More than 10 SAR satellites will be launched within the next
5 years. A golden age for SAR remote sensing has started!
This paper is organized as follows. Section II provides an
introduction to the SAR principles, image formation process and SAR image properties. Sections III to VI explain
the techniques of polarimetry, interferometry, differential interferometry, polarimetric SAR interferometry and
tomography along with application examples. Section VII
provides an overview on emerging technologies for future
spaceborne SAR systems driven by the user requirements.
Section VIII concludes the paper and provides a vision
for SAR remote sensing.
II. Basic SAR Principles
A Synthetic Aperture Radar is an imaging radar mounted
on a moving platform. Similar to a conventional radar,
electromagnetic waves are sequentially transmitted
and the backscattered echoes are collected by the radar
antenna. In the case of SAR the consecutive time of transmission/reception translates into different positions due
to the platform movement. An appropriate coherent
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2013