High Resolution Wide Swath SAR imaging
SAR technology has provided terrain structural information to geologists for mineral exploration,[1] oil spill[2] boundaries on water to environmentalists, sea state and ice hazard maps to navigators,[3] and intelligence, surveillance, reconnaissance and detecting information to military operations.
[4] Conventional SAR systems are limited in that a wide swath can only be achieved at the expense of a degraded azimuth resolution.
Since wide coverage swaths and high resolution output are both important, this poses challenges and contradicting requirements on the design of space-borne SAR systems and related new algorithms.
Its major payload is an X-band (3.1 cm) radar sensor, with different modes of operation, which allows it to provide multiple imaging modes for recording images with different swath width, resolution and polarizations, see the figure for more details.
In stripmap mode (spatial resolution of 3m), it needs 10 weeks to map global Earth's landmass.
Flying in close formation only a few hundred metres apart, the two satellites are imaging the terrain below them simultaneously but from different angles.
It requires one year to achieve one global interferometric acquisition of the Earth's landmass for TanDEM-X.
Longer wavelength allows it to fulfills the requirements for a tomographic measurement of the three-dimensional structure of vegetation and ice regions, also for large scale surveying of deformations with millimeter accuracy.
The future SAR missions may require a mapping capability one or even two orders of magnitude better than that of Tandem-L, whose goal is the investigation of dynamic processes on the Earth's surface.
Given a single satellite, frequent and seamless coverage can only be achieved if a wide swath is imaged.
For small bandwidth SARs, the usual linear relation between azimuth frequency and angle with wavelength
In order to optimize performance and control the range of ambiguities, the PRI must be larger than the time that it takes to collect returns from the entire illuminated swath.
[6] One example is the combination of displaced phase centers in azimuth with the low resolution ScanSAR or terrain observation by progressive scans (TOPS) mode.
A multichannel SAR in azimuth can be interpreted as a linear system of filter functions which characterize the individual apertures’ impulse responses in amplitude and phase in dependence on the Doppler frequency
Assuming a single transmitter and several receiver channels, the physical along-track distance between Rx
A possible drawback of multichannel ScanSAR or TOPS approaches is the rather high Doppler centroid,[9] which is one of the most important parameters need to be estimated in computing SAR images.
Moreover, by controlling a highly directive receiver beam following the radar pulse as it travels on the ground, multiple channels in elevation can improve the SNR (signal noise ratio) without reducing the swath width.
Then, the digitally recorded sub-aperture signals are combined in a spatiotemporal processor to simultaneously form multiple independent beams and to gather additional information about the direction of the scattered radar echoes.
An alternative to a planar array is a reflector antenna in combination with a digital feed array, which is of special interest for low frequency radar systems operating in L- and P-band (1 m),[11] combines the capabilities of digital beamforming with the high directivity of a large reflector antenna.
The reflector based architecture offers the potential to use all array elements simultaneously for the transmission of a broad beam without spill-over as desired for wide swath illumination.
However, this method also has its drawback that is the presence of blind ranges across the swath, as the radar cannot receive while it is transmitting.
A parabolic reflector focuses an arriving plane wave on one or a small subset of feed elements.
A drawback of the multi-beam mode is the presence of blind ranges across the swath, as the radar cannot receive while it is transmitting.
[12][13] Several innovative techniques using multiple receive apertures (‘Rx’) have been suggested to overcome the inherent limitations of conventional SAR to perform HRWS imaging.
sub-apertures has to result in equally spaced effective phase centers thus leading to a uniform sampling of the received signal.
For this, the individual aperture signals are regarded as independent Rx channels (See lower figure, A/D stands for Analog to Digital Converter).
As stated in the previous section, that for multi-beam modes, it has a disadvantage which is the presence of blind ranges across the swath, as the radar cannot receive while it is transmitting.
This works because in satellite SAR imaging, antenna length and required azimuth resolution set an upper bound to the selected PRI.
The PRI, in turn, will limit the maximum continuous swath width in slant range, which is only slightly influenced by the uncompressed transmitted pulse length
So when the overall synthetic aperture is taken into consideration, it turns out that at each slant range, only some of the transmitted pulses are missing, thus it is possible to obtain a SAR image over a wide continuous swath.
