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Geostationary Illuminator with LEO receivers
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The performance and capabilities of a bistatic spaceborne synthetic aperture radar (SAR) are analyzed, where the transmitter and receiver are in different orbits. Such a configuration may be optimized for a broad range of applications like frequent monitoring, wide swath imaging, single-pass cross-track interferometry, along-track interferometry, r...
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Context 1
... transmitter and receiver satellites may be in low earth orbit (LEO), medium earth orbit (MEO), or geosynchronous orbit (GEO) and the whole satellite constellation can be optimized for different applications like frequent monitoring, wide swath imaging, single-pass cross-track interferometry or resolution enhancement. Figure 1 shows an example for a spaceborne constellation which combines a geostationary transmitter with multiple passive receivers in low earth orbit (see also [1] [2]). This specific combination has the potential to provide a cost- effective solution to the frequent monitoring problem, since only low-cost receivers with small antennas have to be duplicated to shorten the revisit times. ...
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... constellation of Figure 1 is especially attractive for applications demanding frequent monitoring of selected regions on the earth's surface. Short revisit times are usually only achievable by increasing the number of satellites. ...
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... this section, the performance of the bistatic configuration shown in Figure 1 will be investigated. Since the following analysis takes full account of the bistatic imaging geometry, it is also applicable to other bistatic SAR constellations, like a global earth observation system with continuous monitoring based on multiple MEO satellites. ...
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Citations
... Although the Newton iterative method is a general method to solve the non-linear InSAR equations with high precision, the method is too time-consuming to apply to the novel InSAR system because the InGEO transmitter offers the ability to observe the earth continuously with wide swath, and the time overhead of the Newton iterative method is not tolerated. Therefore, it is necessary to propose an effective algorithm to improve the efficiency for this novel system characterised with high spatial resolution, wide swath, high flexibility and a short revisit cycle [11][12][13]. ...
Spaceborne interferometric synthetic aperture radar (InSAR) technology is an effective method to obtain digital elevation model (DEM) data. The bistatic InSAR configuration of the inclined‐geosynchronous (InGEO) transmitter with low earth orbit (LEO) receivers (InGEO‐LEO) is a novel InSAR system to acquire terrain information. This novel system is characterised with high resolution, wide swath and timeliness, but the Newton iterative method is time‐consuming to solve the bistatic InSAR equations for fast DEM generation. For the conventional LEO bistatic InSAR system, the closed‐form solution is an effective method to improve the efficiency of solving InSAR equations, which is invalid in the InGEO‐LEO InSAR system because of the significant geometry difference caused by the orbits of InGEO transmitter and LEO receivers. To address this issue, we analyse the bistatic InGEO‐LEO geometry in detail and exploit the bistatic InSAR equations to propose an approximate closed‐form solution (ACS) for the novel system. Compared with the general Newton iterative method, the ACS significantly improves the efficiency of geolocation for the bistatic InGEO‐LEO InSAR system with high precision. Simulation experiments are carried out to verify the effectiveness and superiority of ACS.
... Compared with the monostatic case, the separation characteristic of GTLR-BiSAR has benefits, such as lower military vulnerability, saved costs, anti-interference, more abundant scattering information, and real-time monitoring, etc. [11,12]. These advantages make the GTLR-BiSAR system an appropriate spaceborne system for Earth observation missions with high-resolution, wideswath, high flexibility, and shorter revisit cycle characteristics [13,14]. However, the timevarying relative positions of the GEO transmitter and LEO receivers bring challenges to their methods were both proposed for the stripmap working mode, the azimuth Doppler aliasing problem in SS and TOPS modes will make their methods invalid. ...
... , and rewrite Equation (14) in ascending power of f τ , successively: ...
... Remote Sens. 2022,14, 2006 ...
This paper deals with the imaging problem for sliding spotlight (SS) and terrain observation by progressive scan (TOPS) modes in bistatic configuration of the geostationary (GEO) transmitter with a low earth orbit satellite (LEO) receiver, named GTLR-BiSAR system. A unified imaging algorithm is proposed to process the GTLR-BiSAR data acquired in SS or TOPS modes. Our main contributions include four aspects. Firstly, the imaging geometry of this novel configuration is described in detail. Furthermore, the GTLR-BiSAR signal expressions were deduced in both time and frequency domains. These signal expressions provide great support for the design of processing the algorithm theoretically. Secondly, we present a unified deramping-based technique according to the special geometry of GTLR-BiSAR to overcome the azimuth spectrum aliasing phenomenon, which typically affects SS and TOPS data. Thirdly, the spatial variance of GTLR-BiSAR data were thoroughly analyzed based on the range-Doppler (RD) geolocation functions. On the basis of a former analysis, we put forward the azimuth variance correction strategy and modified the conventional chirp scaling function to solve the range variance problem. Finally, we completed the derivation of the two-dimensional spectrum after the range chirp scaling. On the basis of spectrum expressions, we compensated for the quadratic and residue phase, and the azimuth compression was completed by SPECAN operation. In addition, we provide a flow diagram to visually exhibit the processing procedures. At the end of this paper, the simulation and real data experiment results are presented to validate the effectiveness of the proposed algorithm.
... A rectangular window is used in both the range and azimuth processing, instead of an antenna pattern, i.e. no weighting is used, which enables more quantitative evaluation of imaging results. The oversampling ratio is 1.2 in range; thus, the theoretical range resolution is 0.75 m, which is based on the definition of resolution in [39,40]. According to the PRF and velocity of the senor, we can roughly calculate the azimuth oversampling rate is 3.63 and the theoretical azimuth resolution is 1 m. Figure 11 provides the results from the main steps of our proposed algorithm. ...
In this paper, a modified azimuth nonlinear chirp scaling (NLCS) algorithm is derived for high-squint bistatic synthetic aperture radar (BiSAR) imaging to solve its inherent difficult issues, including the large range cell migration (RCM), azimuth-dependent Doppler parameters, and the sensibility of the higher order terms. First, using the Lagrange inversion theorem, an accurate spectrum suitable for processing airborne high-squint BiSAR data is introduced. Different from the spectrum that is based on the method of series reversion (MSR), it is allowed to derive the bistatic stationary phase point while retaining the double square root (DSR) of the slant range history. Based the spectrum, a linear RCM correction is used to remove the most of the linear RCM components and mitigate the range-azimuth coupling, and, then, bulk secondary range compression is implemented to compensate the residual RCM and cross-coupling terms. Following this, a modified azimuth NLCS operation is applied to eliminate the azimuth-dependence of Doppler parameters and equalize the azimuth frequency modulation for azimuth compression. The experimental results, with better focusing performance, prove the high accuracy and effectiveness of the proposed algorithm.
... In addition to the aforementioned multistatic (bistatic or monostatic) baseline, the TDX mission concept demonstrated the along-track interferometry (ATI) with four inter-or intra-satellite phase centers. The ATI technique can be used to resolve the velocity of ocean currents, ice drift, and other on-ground objects (e.g., transportation traffic) [64][65][66]. ...
Space-based radar sensors have transformed Earth observation since their first use by Seasat in 1978. Radar instruments are less affected by daylight or weather conditions than optical counterparts, suitable for continually monitoring the global biosphere. The current trends in synthetic aperture radar (SAR) platform design are distinct from traditional approaches in that miniaturized satellites carrying SAR are launched in multiples to form a SAR constellation. A systems engineering perspective is presented in this paper to track the transitioning of space-based SAR platforms from large satellites to small satellites. Technological advances therein are analyzed in terms of subsystem components, standalone satellites, and satellite constellations. The availability of commercial satellite constellations, ground stations, and launch services together enable real-time SAR observations with unprecedented details, which will help reveal the global biomass and their changes owing to anthropogenic drivers. The possible roles of small satellites in global biospheric monitoring and the subsequent research areas are also discussed.
... A rectangular window is used in both the range and azimuth processing, instead of an antenna pattern, i.e., no weighting is used, which enables more quantitative evaluation of imaging results. The oversampling ratio is 1.2 in range; thus, the theoretical ground range resolution is 2.66 m, which is based on the definitions of resolution in [29] and [30]. According to the PRF and velocity of the satellites, we can roughly calculate the azimuth oversampling rate is 1.33, so the theoretical azimuth resolution is 2.45 m. ...
LuTan-1 [(LT-1), i.e., TwinSAR-L] mission is an innovative spaceborne bistatic synthetic-aperture radar (BiSAR) mission focusing mainly on differential interferometry, which will be launched in 2020. This article introduces some important aspects of the LT-1 mission for the first time, including the formation configuration, imaging mode, application scenarios, and the efficient baselines between the master satellite and the slave satellite. To realize the high accurate topography and deformation measurements, a wide swath BiSAR focusing algorithm with phase reserving ability should be developed. This article proposes a modified bistatic range-Doppler algorithm based on 2-D principle of stationary phase spectrum, which reduces the phase error introduced by the root term expansion and has an excellent focus performance and a good phase reserving ability. Finally, the spaceborne bistatic simulation experiments using orbital parameters and imaging mode of the LT-1 mission, including point targets and scene targets, are utilized to illustrate the validity and accuracy of the proposed algorithm.
... with the development of Geosynchronous Earth orbit synthetic aperture radar (GEO-SAR) and multi-channel SAR technology, the Geosynchronous-Low Earth orbit (GEO-LEO) bi-static multi-channel high-resolution and wide-swath synthetic aperture radar (HRWS SAR) system is proposed [1]- [5], where the GEO satellite is utilized to be the transmitted platform and the LEO satellite is employed to be the received platform. As the transmitted platform of SAR system, the GEO-SAR is with a capability of widecover for the surface of earth. ...
For the geosynchronous-low earth orbit (GEO-LEO) bi-static multi-channel high-resolution and wide-swath synthetic aperture radar (HRWS SAR) system, a novel azimuth Doppler signal reconstruction algorithm is developed based on an equivalent baseline approximating in the azimuth Doppler chirp Fourier transform domain. The main difficulty for the GEO-LEO bistatic multi-channel HRWS SAR system is that the baseline is variant with the scene in the conventional Doppler Fourier transform domain during azimuth signal reconstruction. The developed approach first employs the linear range cell migration correction and chirp Fourier transform to obtain the coarse-focused ambiguity SAR image. More importantly, the baseline component can be approximated a uniform baseline length for the whole image scene and the ambiguous Doppler spectrum can be reconstructed using the available Doppler signal reconstruction approach. Finally, the experimental results for a simulation GEO-LEO bi-static multi-channel HRWS SAR system are employed to validate the theoretical investigations and the proposed approach.
... This is essentially the opposite configuration of PASSAT (where now we have a single transmitter on the ground and multiple receivers in space), where a number of key issues directly applicable to this concept (such as image formation or multistatic operation) have already been considered. In other work, the German Aerospace Centre has considered a constellation of receive-only LEO satellites, with a satellite in geostationary orbit acting as the transmitter [21]. ...
Over the last decades spaceborne synthetic aperture radar (SAR) has proved its capability as a remote sensing technique of great power and value. A number of precise SAR instruments (e.g., [1]–[4]) have been developed by space and defense agencies around the world and are being used today for a wide range of applications in wide-area land (e.g., [5]–[8]) and sea monitoring (such as [9]–[11]).
... After that, some extended NLCS and Range Migration Algorithm (RMA) and modified BP algorithm were proposed and gained good results192021222324. Furthermore, some bistatic system concepts were advanced [25]. However, those synchronization errors could not be eliminated in the absence of accurate orbit data and some important system parameters information, including squint angle, orbit height, satellite velocity, off-nadir angle, and accurate sampling time, while all above imaging algorithms were based on the synchronized echo and thus could not be directly implemented to nonsynchronous echo. ...
This paper proposes a novel image formation algorithm for the bistatic synthetic aperture radar (BiSAR) with the configuration of a noncooperative transmitter and a stationary receiver in which the traditional imaging algorithm failed because the necessary imaging parameters cannot be estimated from the limited information from the noncooperative data provider. In the new algorithm, the essential parameters for imaging, such as squint angle, Doppler centroid, and Doppler chirp-rate, will be estimated by full exploration of the recorded direct signal (direct signal is the echo from satellite to stationary receiver directly) from the transmitter. The Doppler chirp-rate is retrieved by modeling the peak phase of direct signal as a quadratic polynomial. The Doppler centroid frequency and the squint angle can be derived from the image contrast optimization. Then the range focusing, the range cell migration correction (RCMC), and the azimuth focusing are implemented by secondary range compression (SRC) and the range cell migration, respectively. At last, the proposed algorithm is validated by imaging of the BiSAR experiment configured with china YAOGAN 10 SAR as the transmitter and the receiver platform located on a building at a height of 109 m in Jiangsu province. The experiment image with geometric correction shows good accordance with local Google images.
... A further point involves bistatic and multistatic SAR systems. In this case, individual transmit and receive antennas are carried by separate satellites, which can form multiple baselines for various interferometric applications [52]. Furthermore, this configuration offers an opportunity of using continuous signals like FMCW. ...
In this paper, we introduce a novel multiple–input multiple–output (MIMO) synthetic aperture radar (SAR) concept for multimodal operation. The proposed system employs waveforms based on the orthogonal frequency division multiplexing (OFDM) technique and digital beamforming (DBF) on receive. Thereby, it becomes feasible to maximize spatial degrees of freedom, which are necessary for the multimodal operation. The proposed MIMO SAR system produces multiple high-resolution wide-swath SAR imageries that are used for coherent postprocessing. Through this paper, we aim to open up a new perspective of using MIMO concept for a wide-swath SAR imaging with high resolution in interferometric and polarimetric modes, based on OFDM and DBF techniques. Therefore, this paper encompasses broad theoretical backgrounds and general system design issues as well as specific signal processing techniques and aspects.
... The expression in [19] is a valid approximation for the NESZ at the center of the image, where the joint antenna pattern is substituted by a constant gain factor G Tx · G Rx over the whole aperture. The NESZ of the bistatic system is approximated as ...
... However, and particularly in a bistatic scenario, its meaning is more clearly identified once projected on a flat scene. The onground range resolution is computed using the formula given in [19] δr ...
... 1) Conventional Approach: Assuming there is no significant amplitude modulation in the target response and the processing is done mainly on the lower order portion of the bistatic range history, the formula presented in [19] approximates well the along-track resolution ...
We report about the first X-band spaceborne-airborne bistatic SAR experiment,
using the German satellite TerraSAR-X as transmitter and DLR's new airborne
radar system F-SAR as receiver. The data were acquired early November 2007.
The importance of the experiment is not only due to its pioneering character, but
also due to its potential to serve as a test-bed for the validation of
non-stationary bistatic acquisition procedures, novel calibration and
synchronisation algorithms, and advanced imaging techniques, as required
for future non-synchronised bistatic SAR missions.
High-resolution bistatic imaging requires in general a perfect synchronisation of the
radar systems, which in the present case is achieved by evaluating the range
histories of several reference targets. After synchronisation, non-stationary
focussing is performed using a bistatic backprojection approach.
Benefiting from the flexibility of both acquisition systems, stand-alone TerraSAR-X
monostatic as well as interoperated TerraSAR-X/F-SAR bistatic data sets have been
recorded; the simultaneous availability of monostatic and bistatic images will help
to understand their similarities and differences. As expected, the bistatic
image shows a space-variant behaviour in spatial resolution and
signal-to-noise ratio; both outperform their monostatic
counterparts in almost the complete imaged scene.
A detailed comparison between the monostatic and bistatic images is
given, illustrating the complementarity of both measurements in terms of
available backscattered energy and Doppler information.
