SAOCOM-CS

The AlpTomoSAR experiment was an airborne L-Band tomographic SAR campaign carried out to support tomopgraphic studies related to proposed SAOCOM-CS by tomography for mapping the 3D internal structure of glaciers and ice sheets.

The main objective of the AlpTomoSAR experiment is to assess the added value of L-Band TomoSAR for ice applications by investigating

The level of analogy/complementarity/synergy of TomoSAR w.r.t. Nadir looking systems.

The added value w.r.t. single baseline inversion.

The potential of SAOCOM-CS TomoSAR to retrieve subsurface information over glaciers an ice sheets

The campaign has encompassed coincident in-situ data, GPR, and SAR surveys, gathered in about two weeks from the end of February to the beginning of March 2014.  SAR data were acquired by repeatedly flying over the Mittelbergerferner along an oval-like racetrack configuration, so as to illuminate the scene from two opposite view points.

Pre-processing activities included 2D focusing of raw SAR data and correction of trajectory errors based on InSAR analysis. All data were then back-projected in the 3D space to generate 3D reflectivity maps of the Mittelbergferner glacier, referred through this documents as TomoSAR cubes. A further processing step was implemented to correct for the change of propagation velocity of the signal while traveling in the glacier ice body.

The analysis of TomoSAR cubes shows the complexity of the glacier sub-surface scattering. Most areas are characterized by surface scattering in proximity of the Lidar surface, plus a complex pattern of in-depth volumetric scattering beneath. A significant backscatter signal is observed in the top 10 to 20 m. In other areas, instead, a gap on the order of 10-20 m is observed between surface and in-depth scattering. These results show that modelling scattering from the ice layer in terms of an exponential decay due to uniform wave extinction is definitely insufficient for characterizing glaciers.

The availability of detailed in-situ information allowed to further analyse TomoSAR cubes, and associate the observed features with a physical interpretation.

Corner reflectors, deployed on the snow surface, were observed in TomoSAR cubes to float about 2-3 m above surface scattering. This observation implies that the observed surface in TomoSAR cubes corresponds to the snow/ice interface, whereas the snow volume does not appear to contribute to the signal. This  is in accordance with in-situ measured snow depths. Various subsurface features observed in GPR transects at 600 MHz and 200 MHz clearly showed up in TomoSAR sections as well. In particular: firn bodies, crevasses, and even the bedrock down to 50 m below the ice surface.

Accordingly, the AlpTomoSAR experiment has provided evidence that, for a temperate glacier, L-Band waves penetrate through ice down to tens of meters, and that Tomographic SAR imaging can successfully be employed to derive information in accordance to high frequency GPR surveys.

A discussion of the capabilities of SAOCOM-CS Tomography for imaging glacier subsurface was provided by simulating a stack of SLC SAOCOM-CS pairs according to SAOCOM-CS system parameters and performances, and using AlpTomoSAR 3D data cubes as input to the simulation. Despite a resolution loss, TomoSAR imaging quality for SAOCOM-CS turned out to be sufficient to observe distinct surface and in-depth scatterer, such that ice-internal features are well represented in the SAOCOM-CS TomoSAR cubes.

The AlptomoSAR results point towards new cryospheric applications for the SAOCOM-CS mission, and also illustrate the unexplored possibilities offered by radar missions consisting of two or more satellites flying in formation.

This paper presents an evaluation of above-ground biomass (ABG) retrieval in boreal forests using simulated tomographic synthetic-aperture radar (SAR) data corresponding to the future SAOCOM-CS (L-band 1.275 GHz) mission. A spaceborne sensor provides the advantage of global coverage but also imposes design limitations which reduce performance compared to an airborne SAR. Resolving the vertical distribution of the measured backscatter through tomographic processing serves as a tool to lessen the impact of the reduced performance and counteract other effects, such as shadowing and ground clutter contributions, which adversely affect retrievals from SAR images.

Both the reference biomass data and the tomographic SAR data used for this analysis were collected over the Krycklan river catchment during the BioSAR-2 campaign in 2008. This test area, located in the north of Sweden, has a varied topography with ground slopes up to 21° and contains boreal forest comprised mainly of Norway spruce and Scots pine. During part of the airborne data collection with DLR’s E-SAR system and with future tomography studies in mind, L-band SAR images were obtained from two different headings using six baselines each on 15 October 2008. Two sets of tomographic data have been generated from the original SAR images, with the first utilizing the full E-SAR resolution available. The second set represents the performance expected of SAOCOM-CS, with care taken to include all relevant differences such as the effects of the reduced bandwidth, azimuth resolution and the differing acquisition approach.

The data used for training are averages over 50-m radius circular plots placed within forest stands to represent homogenous forest and ground slope. Plot-level biomass values, ranging from 5 to 268 t/ha, are extracted from a biomass map based on lidar data calibrated by ground measurements. Stand-wise averages from a subset of forest stands for which detailed in situ measurements are available is used for model validation. Several tomographic parameters are tested using a model incorporating a linear combination of the different polarimetric components. This choice is based on previous results for 2D-SAR data as well as a desire to minimize band-specific model adaptations. Two example parameters, the fraction of the total backscatter intensity originating from a height of 10 m and more above ground and the intensity weighted-average scattering height, result in good biomass retrieval with minimum dependence on incidence angle or local topography. SAOCOM-CS retrievals consistently perform well compared to SAR based results, showing that it is possible to compensate for the decreased performance of an orbiting platform.

Also of interest is a residual dependence on azimuth angle, apparent when comparing parameters fitted to the two opposing flight headings. This points to a possible connection with forest structure or uneven distribution of surface slope at the site and suggest further investigations.

Bistatic Radar for Earth Observation has been recently proposed for interferometric and urban applications, addressing bistatic geometry with relatively small spatial baselines, i.e., quasi monostatic configurations (not very far from backscattering). In this work we explore the scattering behaviour of the earth surface for any bistatic configuration, that is for any observing direction of the passive element, even out of the incidence plane. This study has been accomplished in the frame of an ESA funded project which aims at investigating the possible applications of a companion satellite (SAOCOM-CS) carrying a passive receiver working together with the Argentinian SAOCOM SAR [Gebert et al., 2014]. SAOCOM is an L band constellation of two satellites (SAOCOM-1A and SAOCOM–1B) developed by CONAE and planned to be launched in 2016-2019 timeframe. SAOCOM-CS driving application is the monitoring of boreal forest structure by using a tomographic technique. This work investigates large baseline applications of scattering intensity collected over land surfaces by both SAOCOM and SAOCOM-CS. It is based on electromagnetic model simulations with two main objectives: i) providing a characterization of the target scattering coefficient to support the technical design of a bistatic imaging system, ii) understanding its potential to infer bio-geophysical parameters, namely soil moisture and vegetation biomass, eventually in combination with conventional monostatic data.

The electromagnetic models adopted in this work for bare soil incoherent scattering are the Advanced Integral Equation Model (AIEM) [Wu and Chen, 2004; Brogioni et al., 2014] and the Small Slope Approximation up to the second order (SSA2) [Johnson and Ouellette, 2013]. As far as vegetated targets are concerned we have considered a radiative transfer model with a discrete approach for vegetation elements, to which we refer as the Tor Vergata (TOV) model [Guerriero et al., 2013]. As for the coherent scattering from the rough soil surface, which is important when looking at the specular direction or to account for double bounce effects, we have adopted a revised version of the model from Eom and Fung as described in [Pierdicca et al., 2008].

The performance analysis is based on the computation of the Cramer Rao Lower Bound (CRLB) starting from the sensitivities predicted by the models to the different target parameters. Here it has been considered for the retrieval of soil moisture of a bare soil, considering the effect of unknown roughness variations, and for that of biomass of a vegetated surface, considering the unknown variation of soil moisture. Possible limitations can be related to the noise equivalent sigma naught (NEs⁰), i.e. the lowest measurable s⁰ due to system noise. The retrieval capability can be affected when the signal is below the noise floor, a condition that can be achieved in particular configurations. The results of this study show that the most effective configuration for the passive system is at an azimuth angle close to 90° with respect to the incidence plane, and at VV polarization. This is mainly because at HH polarization the signal is lower and the saturation due to the noise floor is more likely reached.

The availability of a bistatic measurement at VV polarization at about 90° azimuth angle improves also the accuracy of the SMC estimation in vegetated surfaces, with respect to monostatic measurements.

Finally, to illustrate L-band bistatic imaging for different land cover categories, synthetic images have been generated with the addition of the speckle noise. They have been also compared to corresponding monostatic images and a retrieval exercise has been carried out using monostatic or a combination of monostatic and bistatic images to show, although still in a simulated environment, the potential of a multistatic approach. 

REFERENCES

Gebert N., Carnicero Dominguez  B., Davidson M.W.J., Diaz Martin M., Silvestrin P., “SAOCOM-CS - A passive companion to SAOCOM for single-pass L-band SAR interferometry”, EUSAR 2014, 10th European Conference on Synthetic Aperture Radar; Proceedings of, Berlin 3-5 June 2014

Wu, T.-D. and Chen K.-S., “A reappraisal of the validity of the IEM model for backscattering from rough surfaces”, IEEE Transactions on Geoscience and Remote Sensing, 42, pp. 743–753, 2004.

Brogioni M., S. Pettinato , G. Macelloni, S. Palosica, P. Pampaloni, N. Pierdicca, and F. Ticconi, “Sensitivity of bistatic scattering to soil moisture and surface roughness of bare soils”, Int. J.  Remote Sens., vol. 31, n. 15, pp., 4227-4255, 2010.

Johnson J. T. and J. D. Ouellette, “Polarization Features in Bistatic Scattering from Rough Surface”, IEEE Trans. Geosci. Rem. Sens., 52,3, 1616-1626, 2014

Ferrazzoli P., L. Guerriero, and D. Solimini, “Simulating bistatisc scatter from surfaces covered with vegetation”, J. Electromagnetic Waves and Appl., Vol. 14, pp. 233-248, 2000.

Pierdicca, N.; Pulvirenti, L.; Ticconi, F.; Brogioni M., "Radar bistatic configurations for soil moisture retrieval: a simulation study", IEEE Trans. Geosci. Rem. Sens., 2008, 46, 3252-3264,2008.

Guerriero L., N. Pierdicca, L. Pulvirenti, P. Ferrazzoli, “Use of Satellite Radar Bistatic Measurements for Crop Monitoring: a Simulation Study”, Remote Sensing, 5(2), pp 864-890, 2013.

The ESA SAOCOM Companion Satellite is a passive-receive only SAR satellite that will be launched and fly in formation with the CONAE L-band SAR SAOCOM-1b in the 2019/2020 time frame. The companion satellite records the SAOCOM radar echoes reflected from Earth’s surface providing new science opportunities and innovative measurements from space such as SAR tomography and bistatic interferometry and radiometry. Based on its unique measurement capabilities, the SAOCOM-CS mission is expected to bring major scientific advances in different applications fields.

A major science objective is the generation of improved forest biomass and vertical structure information over boreal forests using single-pass tomography, providing complete coverage of this important biome. Together with the ESA BIOMASS mission, SAOCOM-CS is expected to help improve our understanding of the terrestrial carbon cycle through the assessment and mapping of terrestrial forest carbon stocks and their geographical distribution.

Additional mission science objectives include the in-orbit demonstration of 3D surface deformation and movement mapping over selected regions of interest. This application makes use of the innovative bistatic measurement capabilities of the mission providing 3D vectors of displacements over time, which are not in general available from current SAR missions. Surface deformation measurements are considered essential to retrieve information on geophysical processes that can be regarded as natural hazards, such as tectonics, volcanism, landslides, and ice motion. The SAOCOM-CS mission also incorporates a suite of in-orbit experiments with a high element of innovation that make use of the unique bistatic (including tomographic and specular) capabilities of the mission and their combination with monostatic measurements. It provides the first opportunity to study the information content of spaceborne bistatic SAR measurements at large scale. Applications addressed include the demonstration of improved soil moisture recovery and measurement of subsurface structures (e.g. in ice and arid zones). These experiments are expected contribute to a new understanding of radar scattering, SAR data processing and the information content of radar measurements as a function of geophysical surface parameters of interest to a wide variety of applications.

The presentation will provide an overview of the SAOCOM-CS mission include mission objectives, implementation and future developments.

The ESA SAOCOM/CS mission will launch a L-band companion SAR satellite to operate cooperatively with CONAE's SAOCOM mission. Companion SAR missions offer a cost-effective solution to boost the performance and capabilities of existing SAR mis- sions. In particular, a whole suite of coherent single-pass ob- servations become feasible, allowing for the interferometric (both single and repeat-pass) and tomographic exploitation of the received data. In the case of SAOCOM/CS, a tomographic phase to image boreal and tropical forests, and a bistatic phase
for repeat-pass interferometric applications (e.g., deformation, 3D motion) are foreseen.
Interferometric and tomographic measurements are however very sensitive to even the slightest degradations in the phase of the received data, and impose stringent requirements on the calibration concept of the mission. 

The presentation will detail an innovative calibration concept based on autonomous (data-based) calibration at different product levels and present an estimation of the expected performance for
ESAs SAOCOM-CS mission.