Bistatic SAR experiments with the TanDEM-X constellation

TanDEM-X is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model (DEM) of the Earth surface by means of single-pass X-band SAR interferometry. De spite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes have been conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this paper reports about the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X. Even in the absence of essential synchronisation and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

Bistatic inverse synthetic aperture radar (ISAR) is an ISAR system whose transmitter and receiver are placed in different places. The configuration mode makes it have some differences with monostatic ISAR. Combined with the reality of bistatic ISAR, the paper analyzed the two important factors, signal to noise ratio (SNR) and bistatic angle, who affect the imaging quality. The equal SNR curve of bistatic ISAR is Cassini's Oval whose focus are transmitter and receiver, and the equal bistatic angle curve is a series of circular arc whose center is on the vertical bisector of bistaic baseline and pass the bistatic radar. To ensure the imaging quality, if the echo has enough SNR, the imaging segment should choose the area far from the vertical bisector of based line. The research in this study provides significant evidence for the selection of imaging segment.

Bistatic radar imaging can overcome limitations of monostatic radar imaging and obtain abundant target feature information; thus, it is followed with interest. Different from bistatic inverse synthetic aperture radar (Bi-ISAR) imaging, bistatic radar coincidence imaging (Bi-RCI) provides a new tack on the bistatic radar imaging technique. In this paper, a Bi-RCI based on multiple measurement vectors (MMV) for rotating cone-shaped targets is proposed to realize Bi-RCI coherent processing and improve imaging performance. Based on the mixed mode signals, a MMV parametric model is established and measurement number coarse selection is proposed. Finally, a modified sparse Bayesian learning (MSBL) algorithm is introduced to reconstruct the target image. Simulation results demonstrate the validity and the superiority of the proposed method.

TerraSAR-X add-on for Digital Elevation Measurements (TanDEM-X) mission is designed to generate 3-D images of the Earth as the first bistatic synthetic aperture radar (SAR). However, few quantitative studies of TanDEM-X digital elevation model (DEM) quality validation have been conducted specifically in China. This article presents an iterative method to generate high-resolution TanDEM-X DEMs and assesses the vertical accuracy with high-accuracy GPS observations, 1 arc second global DEMs available [Advanced Land Observing Satellite World 3-D-30 m v2.2, Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model (ASTER GDEM) v3, Shuttle Radar Topography Mission (SRTM) v3.1, NASADEM, and X-band SRTM], and TanDEM-X 90 m DEM. The results demonstrate remarkable elevation quality and consistency in coastal areas with root mean square error of 1.7 m and 90% linear error (LE90) of 0.4 m, whereas 3–4 times weaker accuracies in steep mountainous areas. A positive bias of 1–2 m for an overall LE90 measure exists in the dense vegetation and steep-slope mountainous areas. TanDEM-X DEM-based SAR interferometry deformation uncertainty simulation indicates a low or even negligible topographic error contribution of 2–4 mm in mountainous areas and less than 1 mm in coastal areas. It indicates that the TanDEM-X DEM performs better than other global DEMs overall and shows a better elevation consistence with SRTM C-band DEM in the coastal area. As an excellent source of up-to-date information, the TanDEM-X DEM are expected be an advantage for understanding dynamic land use changes and improving identification and delineation of coastal lands, mountainous landslides, and earthquakes disasters.

Spatial Channel Sounding Based on Bistatic Synthetic Aperture Radar Principles

A disadvantage of monostatic SAR is that it cannot image the forward-looking area. In this paper, the possibility of forward-looking imaging with bistatic configuration is verified using DVB-T signals as transmitting signals. The Back Projection (BP) method is used to perform the imaging in each configuration and the point spread functions (PSFs) are presented. The forward-looking imaging ability is analyzed in terms of the resolution for each configuration. From the simulation, we realized forward-looking imaging with bistatic Radar in configuration 3, that is, the squint angle has been reduced to zero. The result show that bistatic forward-looking imaging is feasible and the resolution can achieve the requirement.

Bistatic inverse synthetic aperture radar (ISAR) can help image the high-speed target for its advantage in tracking and more observation angle to provide more information than monostatic ISAR. However, the complex image geometry makes it difficult to achieve a clear image of the target with geometry distortion correction and calibration. Furthermore, high-speed motion will make the image blurred or defocussed. To address these problems, a bistatic ISAR (B-ISAR) imaging method for high-speed motion target with geometric distortion correction and calibration is proposed. According to the motion decomposition idea, we established the B-ISAR echo model of the high-speed motion target. Then, based on the range Doppler algorithm, we deduce the analytic formula of the geometric distortion factor and calibration factor, and transform the imaging problem into a parameter estimation problem. With the sparsity of the scattering points, the required parameters are solved using the expectation maximization algorithm based on the maximum a posteriori probability criterion. With the estimated parameters, a clear B-ISAR image of a high-speed motion target with geometric correction and calibration is obtained. The simulations show that the proposed method has better resolution and simultaneously attains geometric distortion correction and calibration of the image.

Passive bistatic radar (PBR) is a well-known topic but research is mainly focused on detection and estimation, and relatively little work has been done on PBR imaging. This paper studies the feasibility of using existing signals for imaging airborne aircraft. The use of the FM band for that purpose is motivated by the geographic prevalence of radio broadcasting and the large size of an FM cell. For the construction of radar images, tomography principles are applied. It centered around a Fourier transform of the radar cross-section (RCS) of airplanes acquired at different frequencies, incident angles and scattered angles in order to form a radar image. However, a sufficiently large part of the Fourier space must be filled to form an interpretable image, and this depends on the available frequencies, bistatic angles and aspect angles. Those parameters are constrained by the bistatic configuration considered and the trajectory of the observed airplanes. The relationships between these parameters and PBR imaging performance are analyzed and discussed in this paper. The main contribution is the successful construction of interpretable PBR images given a realistic configuration and limited trajectories for the airplanes. We discuss the use of these images for applications such as automatic target recognition.

Started in June 2010, the TanDEM-X satellite joined the TerraSAR-X satellite in space to perform the conjoint interferometric TanDEM-X mission to acquire a truly global Digital Elevation Model (DEM) of unprecedented accuracy [1]. Since the very first interferometric acquisitions, the Integrated TanDEM-X Processor (ITP) delivered operationally “Raw”-DEMs and complex products of mono- and bistatic data. The RawDEMs are scenes of about 50km × 30km, generated for a dedicated DEM Mosaicking and Calibration Processor (MCP) which produces the final DEM. The so-called Coregistered Single-look Slant-range Complex (CoSSC) products are provided for each of these scenes in different flavors for production internal purposes and system performance monitoring as well as for scientific use. The capabilities of the ITP go far beyond the primary mission objective of DEM generation alone: it also provides the operationally available end-user products from different experimental modes as e.g. pursuit monostatic, dual polarization bistatic data, alternating bistatic in single and dual polarization and different bistatic and alternating bistatic spotlight modes. This paper focuses on the accuracy of the generated products, the ITPs contribution to the achieved accuracy of the data and the direct effect of it on the use and interpretation of RawDEM heights for temporal change detection. Also the basic characteristics of the operational experimental products are introduced‥

A novel synthetic aperture radar (SAR) constellation called Constellation of Geostationary and Low Earth Orbit SAR (ConGaLSAR), including one geostationary illuminator and several low Earth orbit CubeSat transponders, was previously proposed with outstanding revisiting. The MirrorSAR technology used in transponder significantly simplifies the satellite; however, it brings complexity to the imaging geometry, resulting in failures of the traditional bistatic imaging method. The introduction of probe signals achieves accurate measurement of the additional transponding distance. In this article, a three-step imaging method is proposed to realize accurate imaging processing aiming at the transponding mechanism and wide swath observation: 1) linear time variation for range cell migration grid correction; 2) second range compression for the additional squint angle introduced in the previous step; and 3) range cell migration correction in the range-Doppler domain for nonuniform cell grid caused by bistatic SAR (BiSAR). Digital simulations and real BiSAR data experiments are conducted to verify the effectiveness and accuracy of the algorithm. Moreover, the influence of the elevation is analyzed, confirming its excellent tolerance of elevation errors under the condition of a 3-m resolution and a 40-km swath width, which conforms to the system design of ConGaLSAR.

TanDEM-X: A radar interferometer with two formation-flying satellites

Digital Elevation Model (DEM) generated by bistatic Synthetic Aperture Radar Interferometry (InSAR) usually has systematic plane and elevation errors. However, the DEM adjustment method based on the function model can only correct systematic elevation error, affecting the plane accuracy of DEM mosaic products. Given this issue, this paper proposes a three-dimensional InSAR-DEM block adjustment method based on general models with a separate adjustment strategy, that is, first correct vertical error using the adjustment method based on the function model and then correct horizontal error using the method based on Rational Function Model (RFM) after compensating for elevation errors. In order to extract the Tie Points (TPs) and Ground Control Points (GCPs) used in adjustment, we propose a Normalized Correlation Coefficient matching method based on Complex Slope Maps (NCC-CSM). We conduct experiments using 29 TanDEM-X Co-registered Single look Slant range Complex (CoSSC) data. The accuracies of adjustment results are verified using ICESat-2 ATL08 data and selected GCPs, respectively. The adjustment experiments show that the elevation accuracy of InSAR-DEM is improved from 1.70 m to 1.35 m, the absolute and relative plane accuracies are improved by 5.58 m and 11.53 m, respectively, and the horizontal and vertical geometric discontinuity in the overlapping area disappear. The matching experiments of TPs and GCPs show that the proposed NCC-CSM method is superior to the traditional NCC method based on DEM (NCC-DEM), especially in flat areas.

Conventional Synthetic Aperture Radar (SAR) Coherent Change Detection (CCD) has been found to be of great utility in detecting changes that occur on the ground. The CCD procedure involves performing repeat pass radar collections to form a coherence product, where ground disturbances can induce detectable incoherence. However there is usually a difference in the radar collection geometry which can lead to incoherent energy noise entering the CCD. When sensing flat terrain in a far-field regime, the incoherence due to collection geometry difference can be removed through a conventional global Fourier image support trimming process. However, it has been found that when the terrain is either in a near-field regime or contains non-flat topography, the optimal trimming process is substantially more involved, so much so that a new per-pixel SAR bistatic back-projection imaging algorithm has been developed. The new algorithm removes incoherent energy from the bistatic SAR CCD collection pair on a per-pixel basis according to the local radar geometry and topography, leaving a higher coherence CCD product. In order to validate the approach, change detection measurements were conducted with GB-SAR, a ground-based indoor radar measurement facility.

TerraSAR-X, to be launched at the end of May, 2007, carries the tracking, occultation and ranging (TOR) Category A payload instrument package. The TOR consists of a high-precision dual-frequency GPS receiver, called integrated GPS occultation receiver (IGOR), for precise orbit determination and atmospheric sounding and a laser retro-reflector (LRR) serving as target for the global satellite laser ranging (SLR) ground station network. The TOR is supplied by the GeoForschungsZentrum Potsdam (GFZ) Germany, and the Center for Space Research (CSR), Austin, Texas. The objective of the German/US collaboration is twofold: provision of atmospheric profiles for use in numerical weather predictions and climate studies from the occultation data and precision SAR data processing based on precise orbits and atmospheric products. GFZ also supplies the TOR instrumentation for the follow-on mission TanDEM-X. For the scientific objectives of the TanDEM- X mission, i.e. bi-static SAR together with TerraSAR-X, the dual- frequency GPS receiver is of vital importance for the millimeter level determination of the baseline between the two spacecrafts. In the following, the TOR instrumentation is characterized by the features of and accuracies achievable with the GPS receiver and the LRR. The data flow is shown, as GFZ operates a high latitude data receiving station for fast data access, and runs a data system for preprocessing, archiving and retrieval. With this data system, higher level products are generated for TerraSAR- X, including near real-time orbits by GFZ and post-processed precise science orbits by GFZ and CSR. In the case of TanDEM- X, GFZ operationally determines the inter-satellite baseline on the millimeter level. CSR produces calibration orbits to support the GFZ operational activity. Orbit results in terms of latency and accuracy from current missions are given and achievable results for TerraSAR-X and TanDEM-X are discussed.

A nonlinear chirp scaling algorithm (NCSA) is proposed for bistatic SAR data processing with high squint angles in tandem configuration. Besides the dependence of the azimuth frequency, the dependence of range is considered for the Doppler chirp rate of tandem bistatic SAR. The proposed algorithm can compensate the range dependence of both the range cell migration (RCM) and the second range compression (SRC) terms well by appropriately setting the coefficients of the phase term, which is obtained after the nonlinear chirp scaling process in the two-dimensional wavenumber domain. Only fast Fourier transforms (FFTs) and phase multiplies are required, and fast imaging is implemented in the frequency domain. Satisfying focusing qualities verify the effectiveness of the proposed algorithm by simulations.