Retrieval of biophysical parameters of agricultural crops using polarimetric SAR interferometry

An existing two-layer model for forest height estimation is adapted for agricultural crops in order to develop a retrieval algorithm based on polarimetric synthetic aperture radar interferometry. This new inversion scheme is specifically tailored for vertically oriented agricultural crops, with extinction coefficients dependent on the wave polarization. Physical parameters of the vegetation scene are estimated from the location of the measured coherences in the complex plane. The proposed inversion scheme is validated experimentally with indoor wide-band polarimetric measurements on samples of corn and rice fields. Results show that the estimates of the thickness of the vegetation layer and the ground topography are reasonably accurate for a wide range of frequencies and baselines. Moreover, some interesting results are also obtained when using only dual-polarized data, which brings up new applications for present and future spaceborne missions.     

Single-baseline polarimetric SAR interferometry

Examines the application of single-baseline polarimetric SAR interferometry to the remote sensing and measurement of structure over forested terrain. For this, a polarimetric coherent scattering model for vegetation cover suitable for the estimation of forest parameters from interferometric observables is introduced, discussed and validated. Based on this model, an inversion algorithm which allows the estimation of forest parameters such as tree height, average extinction, and underlying topography from single-baseline fully polarimetric interferometric data is addressed. The performance of the inversion algorithm is demonstrated using fully polarimetric single baseline experimental data acquired by DLR's E-SAR system at L-band     

Permanent scatterers in SAR interferometry

Temporal and geometrical decorrelation often prevents SAR interferometry from being an operational tool for surface deformation monitoring and topographic profile reconstruction. Moreover, atmospheric disturbances can strongly compromise the accuracy of the results. The authors present a complete procedure for the identification and exploitation of stable natural reflectors or permanent scatterers (PSs) starting from long temporal series of interferometric SAR images. When, as it often happens, the dimension of the PS is smaller than the resolution cell, the coherence is good even for interferograms with baselines larger than the decorrelation one, and all the available images of the ESA ERS data set can be successfully exploited. On these pixels, submeter DEM accuracy and millimetric terrain motion detection can be achieved, since atmospheric phase screen (APS) contributions can be estimated and removed. Examples are then shown of small motion measurements, DEM refinement, and APS estimation and removal in the case of a sliding area in Ancona, Italy. ERS data have been used     

Exact closed-form geolocation for SAR interferometry

Exact closed-form algorithms for InSAR-derived digital elevation model (DEM) geolocation are described. They are based on the knowledge of orbit parameters and time/Doppler frequency coordinates of each SAR image, as well as of their interferometric phase. The proposed scheme gives a simple framework for deriving the geolocation accuracy     

The wavenumber shift in SAR interferometry

SAR surveys from separate passes show relative shifts of the ground wavenumber spectra that depend on the local slope and the off-nadir angle. The authors discuss the exploitation of this spectral shift for different applications: 1) generation of “low noise” interferograms benefiting phase unwrapping, 2) generation of quick-look interferograms, 3) decorrelation reduction by means of tunable SAR systems (TINSAR), 4) range resolution enhancement, and 5) the combination of SAR data gathered by different platforms (airborne and satellite) for a “long-time coherence” study     

Repeat-pass SAR interferometry over forested terrain

Repeat-pass synthetic aperture radar (SAR) interferometry provides the possibility of producing topographic maps and geocoded as well as radiometrically calibrated radar images. However, the usefulness of such maps and images depends on our understanding of how different types of terrain affect the radar measurements. It is essential that the scene coherence between passes is sufficient. In this paper, the authors derive a general system model including both radar system and scene scattering properties. The model is used to interpret measurements over a forested area where the scene coherence varies between 0.2 and 0.5. The coherence is found to be sensitive to temperature changes around 0°C but surprisingly insensitive to wind speed. The interferometric height discontinuity at the forest to open-field boundary shows good agreement with in situ tree height measurements. For a dense boreal forest, but is observed to decrease for a less dense forest. This suggests the possibility of estimating bole volume from the interferometric tree height and a ground DEM. The decrease of scene coherence over a dense forest with increasing baseline is also used to estimate the effective scattering layer thickness     

Volume decorrelation effects in polarimetric SAR interferometry

Volume decorrelation is an aspect of synthetic aperture radar interferometry that is currently at stake. It stems from the distribution in height of the backscattering cross section in a imaged scene, and it widely affects interferometric coherence images, e.g., in the presence of forests or buildings. The interest in its reduction lies on the fact that "resolving" volume decorrelation and fixing the exact position of the scattering centers in a resolution cell may lead to an estimation of the height of the observed distributed scatterers. Indeed, the determination of the volume scattering contribution is determinant in digital elevation model generation as well as in biomass estimation. However, volume decorrelation itself may be a source of information on the mechanisms that originate it. In this paper, a distinction is made between the behavior of volume decorrelation due to random volumes and stationary distributed targets. The two cases are then studied by means of polarimetry: the response of such targets to different polarizations is considered by applying a series of decompositions of the scattering matrix, and a systematic analysis of the interferometric coherence characteristics is reported.     

Multitemporal repeat pass SAR interferometry of boreal forests

Multitemporal interferometric European Remote Sensing 1 and 2 satellite tandem pairs from a forest test site in Finland are examined in order to determine the stem volume retrieval accuracy. A form of multitemporal filtering is introduced to investigate what forest stands show a multitemporal consistency in coherence. It is found that a large stand size is a major factor to obtain accurate retrievals. The effect of heterogeneity of forest stands is also discussed. Based on the stands showing highest multitemporal consistency different models for scattering and coherence are compared. The interferometric water cloud model is chosen for stem volume retrieval. The variation of the model parameters with meteorological parameters is investigated and the results illustrate that the best imaging conditions are obtained for subzero temperatures and windy conditions. It is shown that for the 20 stands showing highest multitemporal consistency the stem volume can be retrieved with a relative error of 21%, deteriorating when the number of testing stands is increased, e.g., for 80 stands the error is 48%. For 37 large forest stands representing 48% of the investigated area the relative stem volume error is 26%. With experience from another site in Sweden we may conclude that the error level for a multitemporal interferometric synthetic aperture radar evaluation of stem volume for large forest stands (>2 ha) in a well managed and homogeneous boreal forest may be expected to be in the 15% to 25% range, deteriorating for small and heterogeneous stands and for images acquired under nonwinter conditions.     

Topographic SAR interferometry formulation for high-precision DEM generation

In repeat-pass synthetic aperture radar (SAR) interferometry, the approximations, allowing the phase-to-height conversion, prevent high-resolution mapped relief. In this paper, we present a more general and exact formulation giving a new relationship between the interferogram phase and the target height. It is based on the interferometric SAR geometry and on a better expansion of the path length difference between the sensor and the target. This formulation emphasizes the impact of baseline uncertainties on digital elevation model (DEM) accuracy. A quantitative assessment of the required baseline accuracy is computed. The impact of orbital parameters, used in the new formulation, on interferogram generation is studied. Thus, a new simulator algorithm is developed and tested on a set of reference and simulated DEM examples. Examples of interferogram simulation and DEM generation validate this new approach in the case of a mountainous area in Mustang (Nepal).     

Correction of residual motion errors in airborne SAR interferometry

The main limitation for a wider applicability of airborne repeat pass interferometric SAR data is the presence of small uncompensated motion errors. The effect of residual motion compensation errors is addressed and a new technique to minimise their influence on the interferometric phase is proposed     

Differential tomography: a new framework for SAR interferometry

A new interferometric mode crossing the differential synthetic aperture radar (SAR) interferometry and multibaseline SAR tomography concepts, that can be termed differential SAR tomography, is proposed. Its potentials, coming from the joint elevation-velocity resolution capability of multiple scatterers, are discussed. Processing is cast in a bidimensional baseline-time spectral analysis framework, with sparse sampling. The use of a modern data-dependent bidimensional spectral estimator is proposed for joint baseline-time processing. Simulated results are reported for different baseline-time acquisition patterns and two motion conditions of layover scatterers, showing that this new challenging interferometric technique is promising.     

ENVISAT ASAR data reduction: impact on SAR interferometry

An onboard synthetic aperture radar (SAR) signal-data reduction algorithm called Flexible Block Adaptive Quantization (FBAQ) was developed in 1994 for the Advanced SAR (ASAR) on the ENVISAT satellite. This paper presents work done in a follow-on study that examined the impact of the data-reduction algorithm on the accuracy of two digital elevation models (DEM) produced by using the technique of repeat-pass SAR interferometry. All three allowable data compression ratios were investigated to determine the maximum compression ratio appropriate for SAR interferometry. Based on the scenes studied it was concluded that a reduction from 8 to 4-bits/sample was the maximum data reduction ratio appropriate for precision SAR interferometry, while 8 to 3-bits/sample and 8 to 2-bits/sample encoding were only appropriate for less-demanding wide-swath applications     

Synthesis of microstrip reflectarrays as planar scatterers for SAR interferometry

The use of printed reflectarrays as planar scatterers for SAR interferometry applications is proposed. A phase-only synthesis is applied to obtain a scattered field in a prescribed direction. A reflectarray working at ERS1/2 frequency (5.3 GHz) is simulated to validate the method. Measurements are performed on a 10 GHz size-reduced prototype.     

Analysis of topographic decorrelation in SAR interferometry usingratio coherence imagery

Topographic decorrelation due to the local surface slope has been an obstacle to interferometric synthetic aperture radar (InSAR) applications. A modified spatial decorrelation function is derived as a function of the baseline and topography. This function explains the origin of the total topographic decorrelation phenomenon on the slopes directly facing radar illumination and layover, which may mislead InSAR coherence image interpretation. The authors define critical terrain slope (or critical incidence angle) as the angle for which two SAR signals completely decorrelate regardless of surface stability. It is found that the width of the critical terrain slope increases with the increase of the component of the baseline perpendicular to the radar look direction. A new analytical method, the ratio coherence imagery, is then introduced to highlight total topographic decorrelation against the temporal decorrelation features. The applications of this methodology are demonstrated in selected locations in the Sahara Desert, Algeria, and Almerı´a, Spain, using ERS-1 and ERS-2 SAR data     

Retrieval of forest stem volume using VHF SAR

The ability to retrieve forest stem volume using CARABAS (coherent all radio band sensing) SAR images (28-60 MHz) has been investigated. The test site is a deciduous mixed forest on the island of Oland in southern Sweden. The images have been radiometrically calibrated using an array of horizontal dipoles. The images exhibit a clear discrimination between the forest and open fields. The results show that the dynamic range of the backscattering coefficient among the forest stands is higher than what has been found with conventional SAR using microwave frequencies. The backscatter increases with increasing radar frequency. This work shows an advantage compared to higher frequencies for stem volume estimation in dense forests     

Current measurements by SAR along-track interferometry from a Space Shuttle

We present one of the first studies on ocean current retrievals from interferometric synthetic aperture radar (InSAR) data acquired during the Shuttle Radar Topography Mission (SRTM) in February 2000. The InSAR system of SRTM was designed for high-resolution topographic mapping of the Earth's land surfaces, using two SAR antennas on a Space Shuttle with a cross-track separation of 60 m. An additional along-track antenna separation of 7 m resulted in an effective time lag of about 0.5 ms between the two images, which could theoretically be exploited for target velocity retrievals. However, the feasibility of ocean current measurements with SRTM has been questionable, since the time lag was much shorter than the theoretical optimum (about 3 ms at X-band) and the signal-to-noise ratio over water was quite low. Nevertheless, some X-band InSAR images of coastal areas exhibit clear signatures of tidal flow patterns. As an example, we discuss an image of the Dutch Wadden Sea. We convert the InSAR data into a line-of-sight current field, which is then compared with results of the numerical circulation model KUSTWAD. For tidal phases close to the conditions at the time of the SRTM overpass; we obtain correlation coefficients of up to 0.6 and rms differences on the order of 0.2 m/s. Furthermore we find that SRTM resolves current variations down to spatial scales on the order of 1 km. This is consistent with predictions of a numerical InSAR imaging model. Remaining differences between SRTM- and KUSTWAD-derived currents can be attributed mainly to residual motion errors in the SRTM data as well as to a limited representation of the conditions at the time of the SRTM overpass in the available KUSTWAD results.     

Permanent scatterers analysis for atmospheric correction in ground-based SAR interferometry

Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, complementary or alternative to spaceborne SAR interferometry, for terrain monitoring, and for detecting structural changes in buildings. It has been noted that, in spite of the very short range, compared with the satellite configuration, in GB-SAR measurement the disturbances due to atmospheric effects cannot be neglected either. The analysis of the interferometric phases of very coherent points, called permanent scatterers (PSs), allows the evaluation of the atmospheric disturbance and the possibility of removing it. In this paper, the PS analysis is carried out both on a test site facility and on a real campaign (Citrin Valley, Italy) that provided data with a temporal baseline of about ten months.     

Ground-based SAR interferometry for terrain mapping: theory and sensitivity analysis

This short communication studies the problem of terrain mapping by means of the ground-based synthetic aperture radar (GB-SAR) interferometry technique. A phase-to-height relationship tailored for the GB-SAR interferometric configuration is introduced and verified by a simulated analysis. A sensitivity study is carried out aiming to optimize the use of a GB-SAR system for the terrain mapping and to derive the precision of this technique.