Future Missions

Abstract--- We study the impact of the atmospheric turbulence in those SAR with very long integration time, from minutes to hours, like geosynchronous or geostationary (GEO-SAR).

The Atmospheric Phase Screen cannot be assumed frozen in the synthetic aperture time, like for LEO_SAR,  therefore its estimation and compensation shall be accomplished during azimuth focusing.

Several approach have been published in literature, mostly based on autofocusing or contrasts. Here we exploit a quite different method that derives from the interferometry and has been proposed for airborne and LEO spaceborne SAR.

In order to evaluate performance and compare with proposed approach, we exploit a parametric model for the space-time variogram of the tropospheric delay. Defocusing in terms of Impulse Response Function and decorrelation are evaluated for different frequencies and atmospheric turbulence. 

The interferometric based method, that iterates sub-apertures at different time-frequency resolution is then detailed. Analysis is carried out by considering both thermal and clutter noise. The accuracy of the evaluation of the Atmospheric Phase Screen and the residual IRF is tested both on simulated point targets and distributes SAR scene from Sentinel-1.

is exploited to evaluate imaging and interferometric performances as a function of wavelength, integration time, and turbulence. A critical review of algorithms developed for LEO and GEO SAR shows that the approach based on interferometry is the most suited for the goal.

Performance are evaluated both by APS models and by simulations.

SESAME is an innovative earth exploration mission proposed by a team of scientist dedicated to the observation of land surface topography, topographic change and bio-geophysical parameters for the scientific understanding and modelling of dynamic processes of the geosphere and biosphere. SESAME´s system concept is to build a single-pass cross-track SAR interferometer using two receive-only C-band radar satellites flying in close formation relative to each other, and at an along-track distance of roughly 200 km with respect to Sentinel-1C or D, which will be used as a transmitter of opportunity. It will provide, for the first time, systematic bistatic SAR acquisitions as well as a geometric diversity that allows a feature not yet provided by any other SAR system that is the retrieval of the North-South deformation component with high accuracy by means of DInSAR.

Specifically, the main SESAME products objectives are (1) precise, high-resolution digital elevation models (DEMs) over land surfaces and ice, (2) maps of topographic change obtained by DEM differencing, and (3) maps of 3D surface velocity.

In the frame of SESAME mission performance, both single points and global analysis have been carried out. A mission timeline and simulator have been developed that, for every point onto a regular grid of latitude and longitude coordinates, provide the observation geometry and frequency, together with the effective baselines between the two companion satellites at the time of the acquisition. Jointly, the SAR system performance (NESZ, AASR, RASR, SNR, …) are exploited for the performance computation.

SESAME single-pass across-track interferometer is characterized by the spread of height of ambiguities                          necessary to satisfy the trade-off between small  to be sensitive to small height variations and larger to deal with height ambiguities.  Beside the geometry of the system, the final height accuracy is strongly affected by phase degradation due to noise-like decorrelation sources, number of independent looks, which depends on the required final product resolution, instrument errors and geometrical uncertainties, and final DEM-level calibration.

Concerning the decorrelation sources, a key factor is the penetration into a finite volume. Over Greenland, Antarctica and glaciers, for increasing penetration depth, the coherence loss due to the ice volume scattering increases; on the contrary, a higher height of ambiguity determines higher coherence [1].

Similarly, in densely forest areas, a relation between coherence loss due to the penetration into the forest volume and , tree height, forest transmissivity and radar backscatter from the ground surface and the vegetation layer holds. Increasing the tree height and reducing the height of ambiguity, the decorrelation increases; as well, the lower the backscattering the lower the degree of coherence [2].

The knowledge of the total coherence allows the derivation of the interferometric phase errors and correspondingly of the height errors [3]. The height accuracy computed into ice shows that for a single acquisition, the target requirement of 2 m accuracy is satisfied for a minimum resolution of 100m x 100m while with a DEM posting of 200 m the height error is always lower than 1 m. For densely forest areas and final product resolution of 50 x 50 m² the expected error (for different trees heights) always satisfies the target accuracy of 3 meters. The goal accuracy of 1.5 meter is reached with DEM posting of 100 m.

Additionally, an improvement of DEM accuracy can be achieved by combining overlapping data segments from successive satellite passes: the redundant interferometric signals can be used to partially compensate the performance decay at the swath border and to improve, thereby, the overall height accuracy [3].

The final performances, where also systematic errors (additional 5 deg phase error)  are shown in terms of height accuracy (68% confidence interval) in Figure 1

On the other side, the different acquisition geometries offered by Sentinel-1 and the two SESAME companion satellites, in ascending and descending, provide a stack of interferometrically compatible images from which 3D displacement velocity can be retrieved.

The use of image stacks allows one to separate the APS from the deformation. The selected model to obtain the performance for each individual LOS is based on the work on the Hybrid Cramér-Rao Lower Bound (HCRLB) in XX and XX accounting for both target decorrelation (temporal decorrelation, thermal noise, ambiguity noise), which can be fought by increasing the number of interferometric looks, and atmospheric errors, which ask for an increased number of acquisitions. The 3D motion vector can be retrieved using a weighted least-squares (WLS) approach.

A special aspect to consider in SESAME bistatic acquisitions is the fact that turbulent part of the troposphere is correlated even for large along-track distances. This implies that the atmospheric signal is correlated for both acquired signals (both LOS), and therefore the difference of the two LOS, which is mostly oriented in the North-South direction, has almost no atmosphere, hence resulting in a better performance in the inversion of the 3D motion for the NS component.

The performance (see Figure 2), computed assuming the mission duration of 5 years, are obtained combining different LoS, given the mission acquisition plan: indeed, the actual performance may vary as a function of the geographical location (backscatter, coherence properties, atmospheric conditions, etc.) and systematic phase errors caused by imperfect knowledge of the sensor position. The temporal frequency and distribution of acquisitions plays an important role in mitigating phase effects. The available stack of images is those acquired by Sentinel1 monostatically, in ascending and descending and those acquired bistatically by SESAME.

The performances derived for SESAME mission reveal highly potentiality of such multistatic mission that at the same time provides single-pass cross-track interferometric capabilities as well as different acquisition geometry. The final paper will describe more in detail the employed model and several cases of study showing the performance obtained for different physical parameters characterizing the land surfaces as well as products requirements.

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Tandem-L is a proposal for a highly innovative L-band SAR satellite mission for the global observation of dynamic processes on the Earth’s surface with hitherto unparalleled quality and resolution [1], [2], [3] , [4], [5], [6]. Thanks to the novel imaging techniques and the vast recording capacity with up to 8 terabytes/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere. By this, the new L-band SAR mission will make an essential contribution for a better understanding of the Earth system and its dynamics.

The Tandem-L mission concept is based on the use of two SAR satellites operating in L-band (23.6 cm wavelength) with variable formation flight configurations and is distinguished by its high degree of innovation with respect to the methodology and technology. Examples are the polarimetric SAR interferometry (PolinSAR) for measuring forest height, multi-pass coherence tomography for determining the vertical structure of vegetation and ice, the utilization of the latest digital beamforming techniques in combination with a large deployable reflector for increasing the swath width and imaging resolution, as well as the formation flying of two cooperative radar satellites with adjustable spacing for single-pass interferometry . The mission proposal Tandem-L has been elaborated during a phase-A study that lasted until December 2015 and is now being further developed in the scope of a phase B1 study [7], [8]. The systematic acquisition concept is based on two imaging modes: 1) 3-D structure mode with a bistatic radar operation and 2) Deformation imaging mode with differential SAR interferometry (DinSAR), both allowing the following mission objectives to be achieved:

-        global measurement and monitoring of 3-D forest structure and biomass for a better understanding of ecosystem dynamics and the carbon cycle,

-        systematic recording of small and large scale deformations of the Earth’s surface with millimeter accuracy for earthquake, volcano and landslides research as well as risk analysis and mitigation,

-        quantification of glacier movements, 3-D ice structure and melting processes in the polar regions for improved predictions of future sea level rise,

-        fine scale measurements of soil moisture and its variations close to the surface for a better understanding of the water cycle and its dynamics,

-        systematic observation of coastal zones and sea ice for environmental monitoring and ship routing,

-        monitoring of agricultural fields for crop yield forecasts, as well as

-        generation of highly accurate global digital terrain and surface models which form the basis for a wide range of further remote sensing applications.

The current goal of Tandem-L is to interferometrically image large parts of the Earth’s landmass up to twice per week. Based on the User Requirements Document [7], a set of 26 preliminary geophysical products have been defined during Phase A and summarized within the Mission Requirements Document [8]. Above and beyond the primary mission goals, the unique data set acquired by Tandem-L has therefore immense potential for developing new scientific and commercial applications and services. According to the current planning, and subject to timely financial approval, the Tandem-L satellites could be launched at the end of 2022.

This paper will present the mission and data acquisition concept [9] along with the techniques adopted for the derivation of geo-/bio-physical parameters. Further, the assessment of the accuracy of the image products derived by means of DinSAR, PolinSAR and tomography will be presented along with image examples obtained from airborne, TanDEM-X and Sentinel-1 data.

References

[1]     A. Moreira, G. Krieger, I. Hajnsek, K. Papathanassiou, M. Younis, P. Lopez-Dekker, S. Huber, M. Villano, M. Pardini, M. Eineder, F. De Zan, A. Parizzi, “Tandem-L: a highly innovative bistatic SAR mission for global observation of dynamic processes on the Earth’s surface,” IEEE Geosci. Remote Sens. Mag., vol. 3, pp. 8-23, 2015.

[2]     G. Krieger, I. Hajnsek, K. Papathanassiou, M. Eineder et al. “The Tandem-L mission proposal: monitoring Earth’s dynamics with high resolution SAR interferometry,” Proc. IEEE Radar Conf., May 2009.

[3]     G. Krieger, I. Hajnsek, K. Papathanassiou, M. Younis, A. Moreira, “Single-pass synthetic aperture radar (SAR) missions,” Proc. IEEE, vol. 98, no. 5, pp. 816-843, 2010.

[4]     S. Huber, M. Younis, A. Patyuchenko, G. Krieger, and A. Moreira, “Spaceborne reflector SAR systems with digital beamforming,” IEEE Trans. Aerospace Electron. Syst., vol. 48, pp. 3473-3493, 2012.

[5]     M. Villano, G. Krieger, and A. Moreira, “Staggered SAR: high resolution wide-swath imaging by continuous PRI variation,” IEEE Trans. Geosci. Remote Sensing, vol. 52, no. 7, pp. 4462–4479, 2014.

[6]     S. Huber, M. Villano, M. Younis, G. Krieger, A. Moreira, B. Grafmüller, R. Wolters, “Tandem-L: design concepts for a next generation spaceborne SAR system,” EUSAR 2016.

[7]     Tandem-L User Requirements Document, V. 1.0, May 2016.

[8]     Tandem-L Mission Requirements Document, V. 1.0, April 11, 2016.

[9]     M. Bachmann, D. Borla Tridon, F. De Zan, G. Krieger, M. Zink, “Tandem-L observation concept – an acquisition scenario for the global scientific mapping machine,” EUSAR 2016.

This paper presents an overview of SESAME (SEntinel-1 SAR Companion Multistatic Explorer), a mission concept that would extend the capabilities of Sentinel-1 by adding a pair of close formation-ying receive-only spacecraft in order to enable single-pass interferometric observations.

The potential of bistatic companion (or add-on) missions to greatly extend the capabilities of regular monostatic missions has been recognized and explored by a number of authors. A major milestone, and a direct precursor of SESAME's mission proposal, was the interferometric Cartwheel concept [3], in which a set of of three formation- ying satellites would have own with ENVISAT. The interferometric Cartwheel was one of the main inspirations of the TanDEM-X mission [1, 2], has been the rst mission to use a pair of formation spacecraft to generate single-pass interferometric data, and also the rst multistatic mission. While the main goal of the TanDEM-X mission was to generate a high resolution accurate global Digital Elevation Model (DEM), it has also served to demonstrate the great value of time-series of single-pass interferometric data.

The SESAME mission is dedicated to the observation of land surface topography, topographic change and bio- geophysical parameters in order to advance the scientic understanding and modelling of dynamic processes of the geosphere and biosphere. The observations focus at processes that are associated with distinct temporal changes of shape and elevation of land surfaces and ice bodies, as well as forest height and biomass. Available topographic databases with (near) global coverage are lacking the capability to capture and quantify key features as required for studying dynamic processes that are shaping and transforming the land surfaces, ice bodies and vegetation cover. The SESAME mission will be able to ll this critical gap by providing repeat acquisitions of precise, spatially-detailed elevation data over land surfaces including ice covered areas and forests.

The primary SESAME objectives respond directly to specic challenges of ESA's Earth Observation Science Strategy for Cryosphere, Solid Earth and Land Surface, exploiting the Single-Pass Interferometric SAR (SP-InSAR) capability of the mission.

SESAME's system concept is to build a single-pass cross-track SAR interferometer using two receive-only C- band radar satellites ying in close formation relative to each other, and at an along-track distance of roughly 200 km with respect to Sentinel-1[4] C or D, which will be used as a transmitter of opportunity. Aside from single- pass interferometric acquisitions, the geometric diversity resulting from the proposed conguration also allows the retrieval of the North-South deformation component by means of DInSAR.

Formation ying provides the opportunity to dynamically recongure the measuring apparatus according to specic observational requirements. This will be exploited by organizing the mission in phases, repeated from year to year, in which the formation conguration will be adjusted to the needs of particular applications or to provide optimum geometries over specic latitudes.

The space segment consists of two 200 kg class spacecraft carrying a receive-only radar payload. The radar design is simple, yet highly innovative by the envisioned use of two small antennas spaced approximately 4.5 m in ight direction. This architecture solves one of the major challenges related to companion SAR systems by providing adequate ambiguity rejection despite the small total area of the antennas used. SESAME will use a synchronization link for mutual synchronization. System-level synchronization with Sentinel-1 is not required.

The baseline operating concept is to acquire only one of the three sub-swathes of Sentinel-1's IWS mode. Access to the different sub-swathes will be provided in successive passes through attitude steering of the spacecraft. This implies that SESAME will require several Sentinel-1 repeat cycles to provide global coverage. Besides allowing the use of xed beams, this also relaxes the data volumes acquired, making them manageable by the proposed small platform concept.

The final paper will provide a general overview of the mission, including the observation concept and timeline, and a discussion of the main trade-offs.