## 4 Near-Space High-Resolution and Wide-Swath Imaging
### Potentials and Challenges  
It is well known that imaging radar applications include a wide range of sometimes conflicting requirements on coverage, revisit and access time. For example, security applications typically require a very high resolution image of a small area to be available within a very short time in the order of hours. On the other hand, agricultural applications require a relative low resolution image of a large area within a periodically repeated cycle. Near-space HRWS SAR can meet these requirements with adjustable resolution and coverage. This allows future imaging radar systems to simultaneously provide different end-users with high quality information products depending on their need. Particularly, near-space HRWS SAR is of great interest in evaluating natural disaster damage [74, 75]. An example is the catastrophic earthquake occurred in Wenchuan (Sichuan province, China) on 12 May 2008, which killed thousands of people. In this case, near-space HRWS SAR can play a particularly important role in detecting human settlements, evaluating earthquake damages, and monitoring environment changes because of its fast revisiting capability besides relative high resolution and wide swath imaging. Another potential application is ocean remote sensing. A good example is tsunami detection. Tsunami can be originated by earthquake, submarine landslides, volcanic eruptions or a combination of these factors. It has been proven that there is a link between tsunami wave amplitude and its microwave RCS, and significant (a few dB) variations of the RCS synchronous with the sea level anomaly has been found both at C and Ku band in the geophysical data record of the altimetry satellite Jason-1 [76, 77]. From the view of microwave active remote sensing, the detectable oceanographic features of a tsunami wave can be summarized as [12]: (1) tsunami wave height, (2) tsunami orbital velocities, (3) tsunami-induced RCS modulations. It has been proved the capability of microwave radars to detect internal waves [78], which are generally triggered by tides. Along with tides, tsunamis are shallow water waves and have the potential to trigger internal waves. As such, tsunami-induced RCS modulations can be used to predict future tsunami. This involves not only detection but also an estimate of the tsunami magnitude. Due to its fast revisiting frequency, relative simultaneous high resolution and wide swath, near-space HRWS SAR does indeed seem to be the obvious solution to these remote sensing applications. However, for optimum near-space HRWS SAR imaging performance the relation between sensor velocity \(v_{a}\) and the along-track offsets \(\Delta x\) of the \(N\) sub-apertures has to result in equally spaced effective phase centers, thus leading to a uniform sampling of the received signal. This requires the following relation [79]  
\[PRF_{opt}=\frac{2v_{a}}{N\Delta x} \tag{12}\]  
If a non-optimum PRF is chosen, the gathered samples are spaced non-uniformly. This requires a further processing step after down-conversion and quantization of the multi-aperture azimuth signal before conventional monostatic focusing algorithm can be applied. The purpose of the azimuth processing is to combine the N channels, each of bandwidth \(N\cdot PRF\) but sub-sampled with \(PRF\), to obtain a signal effectively sampled with \(N\cdot PRF=PRF_{eff}\). Thus the Nyquist criterion is fulfilled in average after the processing, which yields an output signal that is free of aliasing. Consider again Figure 7 we may view it as a conventional SAR (the central sub-aperture), operating with a PRF one third of that required to adequately sample its beamwidth, together with additional two sub-apertures, on both sides of the central one. This basic idea is that the additional samples obtained by the outer sub-aperture will fill in the gaps in a target's azimuth phase history with a low operating PRF. The ambiguous Doppler spectrum can be recovered unambiguously by applying a system of reconstruction filters.