Effects of Orbit and Pointing Geometry of a Spaceborne Formation for Monostatic-Bistatic Radargrammetry on Terrain Elevation Measurement Accuracy (original) (raw)
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Geoscience and Remote Sensing, IEEE …, 2009
This paper aims to investigate the performance of stereoradargrammetric methods applied to spaceborne monostatic-bistatic synthetic aperture radar (SAR) data for digital elevation model (DEM) generation. Stereoradargrammetric techniques for robust DEM generation were successfully experienced on monostatic repeat-pass SIR-A, SIR-B, SIR-C/X-SAR, ERS1/2, JERS-1, and Radarsat data. However, novel configurations achievable by modern spacecraft flying in formation will allow for the attainment of very large baselines between the antennas in a single-pass bistatic geometry so that the height determination accuracy can benefit from both stereo effect and simultaneous acquisition. Five models for relief reconstruction by monostatic-bistatic SAR stereoradargrammetry are presented, and an error budget is assessed for each of them. Results of the sensitivity analysis exhibit metric accuracy, and therefore, the technique could be applied for height reconstruction as a methodology complementary to SAR interferometry.
TanDEM-X: 10 Years of Formation Flying Bistatic SAR Interferometry
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
On June 21, 2010, the TanDEM-X mission was launched and opened a new era in spaceborne radar remote sensing. The first formation flying radar system was built by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second, TerraSAR-X-like satellite TanDEM-X. The resulting large single-pass SAR interferometer features flexible baseline selection, enabling the acquisition of highly accurate cross-track interferograms not impacted by temporal decorrelation and atmospheric disturbances. The primary objective of the mission was the generation of a global Digital Elevation Model (DEM) with unprecedented accuracy (12-m horizontal resolution and 2-m relative height accuracy). The main mission phase for DEM data acquisition has been finished in 2014; the processing of the global TanDEM-X DEM was concluded in September 2016. The final DEM product is well within specifications and features an extremely low percentage of void areas. It is of fundamental importance for a wide range of commercial and scientific applications. But the scientific exploitation of TanDEM-X is not limited to the DEM. TanDEM-X has unique capabilities, including along-track interferometry, and new bistatic and multistatic SAR techniques, that support numerous secondary mission objectives. Indeed, some of these experiments were directly performed during the DEM acquisition phase, when suitable satellite formation geometries were available. Moreover, regular acquisitions over selected super test sites enabled multitemporal analyses. A dedicated science phase after the DEM acquisitions included up to 4 km cross-track baselines, operation in the so-called Dual-Receive Antenna mode, as well as a period in pursuit monostatic flight formation. Comparisons of the TanDEM-X DEM with that of SRTM, or among multitemporal TanDEM-X data, revealed dramatic, ongoing, changes in Earth's topography, especially over ice and forests. In the last 3.5 years the mission has further acquired data for a global change layer showing the height changes relative to the first global DEM dataset. The so-called "Change DEM" is planned for release in 2021. Despite being well beyond their design lifetime, both satellites are still fully functional and have enough consumables for several additional years. Therefore, bistatic operations continue with a focus on changes in the cryosphere, biosphere, and densely populated urban areas.
TanDEM-X: A Satellite Formation for High-Resolution SAR Interferometry
IEEE Transactions on Geoscience and Remote Sensing, 2007
TanDEM-X is a mission proposal for an innovative spaceborne radar interferometer which has been evaluated in a phase A study by a joint DLR and EADS/Astrium team. The mission concept is based on two TerraSAR-X radar satellites flying in close formation to achieve the desired interferometric baselines in a highly reconfigurable configuration. The main goal of the TanDEM-X mission is the generation of a world-wide, consistent, timely, and high-precision digital elevation model according to the HRTI/DTED-3 standard as the basis for a wide range of scientific research, as well as for operational, commercial DEM production. Secondary mission goals of TanDEM-X are moving target indication with a distributed four aperture displaced phase centre system, the measurement of ocean currents and the detection of ice drift by along-track interferometry, high resolution SAR imaging based on a baseline induced shift of the Doppler and range spectra (super-resolution), the derivation of vegetation parameters with polarimetric SAR interferometry, large baseline bistatic SAR imaging for improved scene classification, as well as regional very high resolution DEM generation based on spotlight and large baseline interferometry. The feasibility of these modes will be investigated and predictions about the achievable performance will be made.
Earth Observation with SAR Satellite Formations: New Techniques and Innovative Products
This paper discusses the use of small satellites for future radar remote sensing applications. After a short introduction, we give first an overview of the TanDEM-X mission to be launched in autumn 2009. Here, special emphasis is put on the demonstration of innovative synthetic aperture radar (SAR) imaging techniques. Then, novel SAR configurations are introduced which make synergistic use of multiple small satellites flying in close formation. Performance examples demonstrate their unique capabilities for advanced Earth observation applications. Among these opportunities are the generation of digital elevation models with decimetre accuracy, the monitoring of ocean currents, and the measurement of small vertical displacements from snow accumulation, vegetation growth, and thawing permafrost soils. Challenges associated with the use of small satellites are pointed out and solutions to overcome them are presented.
Analysis of satellite configurations for spaceborne SAR interferometry
2002
Spaceborne bi-and multistatic SAR is an attractive approach for alongtrack and across-track interferometry. This may be obtained by a set of low cost, passive receiver equipped micro satellites which simultaneously record the signals transmitted from an already existing spaceborne satellite. In this paper we introduce several multistatic SAR configurations, their fuel consumption, and present the results of an interferometric perfomance analysis for the satellites ALOS, ENVISAT, and TerraSAR-X. This analysis includes errors due to SNR, range and azimuth ambiguities, geometric decorrelation for flat surfaces. We finally conclude with an estimation of the required position accuracy for the different proposed formations. 1-Introduction Interferometric SAR is an attractive and prosperous approach to extract geophysical parameters from the earth's surface. To achieve high quality along-track and across-track interferograms on a global scale, several spaceborne bi-and multi-static SAR missions have been suggested. By combining already operating transmit satellites with new spaceborne configurations one can benefit and keep the costs very low. In this paper we introduce the concepts of the Cartwheel, Pendulum, and Carpe, discuss the interferometric baselines and Doppler-Centroids. In the following section we estimate the interferometric performance for PALSAR onboard ALOS, ASAR onboard Envisat, and TerraSAR-X. This includes the computation of the Noise Equivalent Sigma Zero (NESZ). Then, we derive the Signal to Noise ratio (SNR) including ambiguities in azimuth and range. From this, we derive the interferometric phase error. With the results, we are able to get the relative height accuracy for a given resolution cell and baseline for a flat earth model. We conclude with an estimation of the required positioning accuracy. 2-Spaceborne Concepts An important design goal for multi-static SAR formations is achieving constant baselines between the receiving spacecrafts. For along-track interferometry this is easily obtained by chosing circular orbits with equal period T 0 and a time lag ∆t between the satellites. However, this solution is not possible for across-track interferometry as the vertical and the horizontal across-track separation do not remain constant in their orbits. The relative motion of free moving satellites in close formations may be approximated by Hill's equations [5], also known as Clohessy-Wiltshire equations [2], which describe the relative motion in a rotating reference frame [12]. This transformation allows for a linearization of the differential equations characterizing the satellite dynamics. For undisturbed, Keplerian motion
IEEE Transactions on Geoscience and Remote Sensing, 2000
We report about the first X-band spaceborneairborne bistatic synthetic aperture radar (SAR) experiment, conducted early November 2007, using the German satellite TerraSAR-X as transmitter and the German Aerospace Center's (DLR) new airborne radar system F-SAR as receiver. The importance of the experiment resides in both its pioneering character and its potential to serve as a test bed for the validation of nonstationary bistatic acquisitions, novel calibration and synchronization algorithms, and advanced imaging techniques. Due to the independent operation of the transmitter and receiver, an accurate synchronization procedure was needed during processing to make high-resolution imaging feasible. Precise phase-preserving bistatic focusing can only be achieved if time and phase synchronization exist. The synchronization approach, based on the evaluation of the range histories of several reference targets, was verified through a separate analysis of the range and Doppler contributions. After successful synchronization, nonstationary focusing was performed using a bistatic backprojection algorithm. During the campaign, stand-alone TerraSAR-X monostatic as well as interoperated TerraSAR-X/F-SAR bistatic data sets were recorded. As expected, the bistatic image shows a space-variant behavior in spatial resolution and in signal-to-noise ratio. Due to the selected configuration, the bistatic image outperforms its monostatic counterpart in almost the complete imaged scene. A detailed comparison between monostatic and bistatic images is given, illustrating the complementarity of both measurements in terms of backscatter and Doppler information. The results are of fundamental importance for the development of future nonsynchronized bistatic SAR systems.
Geometrical considerations of spaceborne SAR surveys
2014
In this paper, the conventional approximation of stationary locally range-dependent straight orbits for LEO spaceborne SAR surveys is analysed. This approximation breaks down in demanding scenarios, e.g., high-resolution, squinted, or bistatic SAR, which are likely to play an important role in future spaceborne SAR missions. The dependencies of the image formation parameters with respect to topography and squint are analysed. Further, an exemplary analysis of the expected variations along the orbit, i.e., azimuth variance, is presented. To complete the analysis, the impact of the intrapulse motion of the satellite in squinted geometries is considered. The understanding of the residual geometric components including the parameters having an impact on them is a rst step to the development of ef cient SAR image formation schemes for future challenging spaceborne SAR missions.