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Advanced concepts for high-resolution wide-swath (HRWS) imaging. (a) ScanSAR with multiple azimuth channels. (b) Single-channel SAR with multiple elevation beams. (c) Digital beamforming with reflector antenna. (d) Staggered-SAR.
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Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independent of weather conditions and sunlight illumination and is therefore very attractive for the systematic observation of dynamic processes on the Earth’s surface. However, conventional SAR systems are limited, in that a wide swath can only be achieved at the expense...
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... order to keep the antenna length down, several new instrument architectures and modes have been proposed [12][13]. One example is the combination of displaced phase centers in azimuth with ScanSAR or TOPS mode (see Figure 1 (a)). As in classical ScanSAR, azimuth bursts are used to map several swaths. ...
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... enables an increase of the coverage area without the necessity to either lengthen the antenna or to employ burst modes. Figure 1 (b) provides an illustration, where three narrow receive beams follow the echoes from three simultaneously mapped image swaths that are illuminated by a broad transmit beam. A sufficiently high antenna is needed to separate the echoes from the different swaths by digital beamforming on receive, while a wide beam can either be accomplished by a separate small transmit antenna or a combined transmit-receive antenna together with tapering, spectral diversity on transmission or sequences of subpulses. ...
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... sufficiently high antenna is needed to separate the echoes from the different swaths by digital beamforming on receive, while a wide beam can either be accomplished by a separate small transmit antenna or a combined transmit-receive antenna together with tapering, spectral diversity on transmission or sequences of subpulses. An interesting alternative to a planar antenna is a reflector, fed by a multichannel array, as illustrated in Figure 1 (c). A parabolic reflector focuses an arriving plane wave on one or a small subset of feed elements. ...
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... drawback of the multi-beam mode is the presence of blind ranges across the swath, as the radar cannot receive while it is transmitting. The Staggered SAR concept (Figure 1 (d)) overcomes this drawback by continuously varying the PRI in a cyclic manner, so allowing the imaging of a wide continuous swath without the need for a long antenna with multiple apertures [14][15]. ...
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... as several urban areas, characterized by high backscatter, are present in the vicinity of a large lake, characterized by low backscatter, azimuth sidelobes and ambiguities of strong targets can be compared with the impulse response expected from simulations. Figure 10 shows this comparison for the most prominent scatterer of the scene. As apparent from the azimuth cuts, measurements on data show very good agreement with predictions from simulations. ...
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Citations
... The design of a PRI sequence with slow and fast change is analyzed in Ref. [8]. The principle of slow changing sequences of PRIs is to minimize the PRI span such that the blind ranges are almost uniformly spread over the slant ranges of the illuminated area. ...
... As for fast PRI variation, the principle to design sequences of PRIs is to avoid missing two consecutive samples in the raw azimuth signal for all slant ranges of interest. In order to eliminate the effects caused by nonuniform sampling in azimuth direction, like spectrum distortion and false targets, many reconstruction methods have been proposed, such as back projection (BP) algorithm [6,7], multichannel reconstruction, linear interpolation, best linear unbiased (BLU) interpolation [8], etc. Although the BP algorithm is available [9,10], it suffers from significant computational complexity due to the point-by-point correlation in the time domain. ...
... where k * means that the k * th pulse is transmitting while the first pulse is received from the slant range R min , ∆ is the difference between two consecutive PRIs and ∆ min is the minimum of the difference. Furthermore, k * is the key factor to avoid consecutive pulses losing [8], and it is defined as: ...
This paper focuses on an improved imaging algorithm for spotlight synthetic aperture radar (SAR) with continuous Pulse Repetition Interval (PRI) variation in extremely high-resolution. Conventional SAR systems are limited in that a wide swath cannot be achieved with a high azimuth resolution in the meantime. This limitation can be overcome by Pulse Repetition Frequency (PRF) variation in a SAR system. However, there are problems such as the ambiguities of point targets or extended targets caused by nonuniform sampling. A reconstructive method, Nonuniform Discrete Fourier Transform (NUDFT) has been presented in the current literature, but it is rather computationally expensive. In this paper, a modified sinc interpolation based on NUDFT is proposed, which is used to reconstruct the uniformly sampled echo in time domain. Since the interpolation kernel length is relatively short, it is more computationally efficient. Then, the two-step processing approach combined with the modified sinc interpolation is further presented, which has much better accuracy than that combined with the conventional sinc interpolation. Both the simulated data and the extracted GF-3 data experiment demonstrate the validity and accuracy of the proposed approach.
... The idea of optimizing the sequence of PRIs by imposing that two consecutive samples in azimuth (although in the range-compressed data) are never missed has been suggested in [61]. In case linear PRI trends are considered, this criterion leads to shorter sequences with a much faster PRI change, compared to the previously described criterion [66], [67], [70]. ...
Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independently of weather conditions and sunlight illumination and is therefore very attractive for the systematic observation of dynamic processes on the Earth’s surface. However, conventional SAR systems are limited in that a wide swath can only be imaged at the expense of a degraded azimuth resolution. This limitation can be overcome by using systems with multiple receive subapertures displaced in along-track, but a very long antenna is required to map a wide swath. If a relatively short antenna with a single aperture in along-track is available, it is still possible to map a wide area: Multiple subswaths can be, in fact, simultaneously imaged using digital beamforming in elevation, but “blind ranges” are present between adjacent swaths, as the radar cannot receive while it is transmitting. Staggered SAR overcomes the problem of blind ranges by continuously varying the pulse repetition interval (PRI). A proper selection of the PRIs, together with an average oversampling in azimuth, allows an accurate interpolation of the non-uniformly sampled raw data on a uniform grid, so that resampled data can be then focused with a conventional SAR processor. This concept therefore allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple subapertures. As an additional benefit, the energy of range and azimuth ambiguities is spread over large areas: Ambiguities therefore appear in the image as a noise-like disturbance rather than localized artifacts. In this thesis, the impact on performance of the selected sequence of PRIs, the adopted interpolation method and the processing strategy are addressed. Design examples for single-, dual-, and fully-polarimetric staggered SAR systems are presented, based on both planar and reflector antennas, in L band and C band. The impact of staggered SAR operation on image quality is furthermore assessed with experiments using real data. As a first step, highly oversampled F SAR airborne data have been used to generate equivalent staggered SAR data sets and evaluate the performance for different oversampling rates. Moreover, the German satellite TerraSAR X has been commanded to acquire data over the Lake Constance in staggered SAR mode. Measurements on data show very good agreement with predictions from simulations. Furthermore, a data volume reduction strategy, based on on-board Doppler filtering and decimation, has been developed to cope with the increased azimuth oversampling of staggered SAR. Finally, a patented extension of the staggered SAR concept is provided, where the phase centers are continuously varied as well, so that it is possible to transmit pulses according to a sequence of different PRIs and acquire uniformly sampled data without the need of any interpolation. The staggered SAR concept is currently being considered as the baseline acquisition mode for Tandem L, a proposal for a polarimetric and interferometric spaceborne SAR mission to monitor dynamic processes on the Earth’s surface with unprecedented accuracy and resolution.
