Marc Bara

Interferometric SAR signal analysis in the presence of squint

This paper develops an analysis of the SAR impulse response function from the interferometric point of view, with the intention of studying its phase behavior in the presence of high squint angle values. It will be pointed out that in this case, a phase ramp is present in the range direction, which, in combination with a certain degree of misregistration between the two images induces an offset in the generated interferometric phase. This behavior, if not compensated, imposes strong limits on the performance of the interferometric techniques in a squinted case, especially for airborne SAR systems. The article proposes two new techniques, which are appropriate to correct the phase bias coming from this source. The first one is based on a modification of the azimuth compression filter, which cancels the phase ramp of the range impulse response function for one specific squint value. In case the SAR processing is performed with variable squint over range, the authors propose a second method oriented to estimating the expected misregistration and thus, the phase bias by means of an iterative approach. Simulated data as well as real corner reflector responses are used to show that the correct topography can be recovered precisely even in the presence of phase bias coming from the squinted geometry.

Calibration requirements for airborne SAR interferometry

One of the main limitations on the use of SAR interferometry to automatically generate Digital Elevation Models (DEM) is related with both the accurate calibration of the system parameters and its stability from flight to flight. The unstable movements of an airborne SAR platform can be corrected during the processing step, as long as those movements are recorded in an accurate manner. Errors and time drifts in the system parameters or on the plane position and attitude measurements lead to geolocation errors in the final DEM. A calibration method based on the so-called “sensitivity equations”, which relate the target location error with the error on the estimation of the system parameters, is described. The equations have been derived by differentiating the interferometric geolocation equations in closed form for the most general case in a squinted geometry. The interferometer parameters considered are the baseline length and elevation, common time delay, interferometric constant phase, plane position and attitude angle offsets, etc. The equations can be used to both calibrate a single DEM from different ground control points (GCP) spread along the swath and the frill interferometer from well-known located comer reflectors (CR).

Interferometric calibration for DEM enhancing and system characterization in single pass SAR interferometry

In this paper the comparison between different methods to calibrate a digital elevation map (DEM) generated from airborne differential SAR interferometry will be presented. The traditional methods applied phase ramp corrections along the swath derived from the errors in some ground control points (GCP) to improve the DEM. A more advanced technique is proposed based on the sensitivity equations, which are derived by differentiating the basic location equations with respect to the different parameters of the error model. The behavior of the different techniques have been tested with both synthetic and real interferometric SAR data.

Sensitivity equations and calibration requirements on airborne interferometry

The usage of an airborne interferometric SAR to mass map production is strongly dependent on the capability of performing the data processing in a near automatic mode. One of the main limitations is related to both the accurate calibration of the system parameters, its stability from flight to flight and the correct in-flight recording of aircraft position and attitude. The unstable movements of an airborne SAR platform, if recorded in an accurate manner, can be corrected during the processing step. Measurement errors and time drifts lead to location errors in the final DEM. The system requirements in both accuracy and stability to fulfil a quality-mapping requirement can be resolved from the sensitivity equations. The same equations can be also used to calibrate the system parameters from the location errors on the imaging of well-known targets, usually corner reflectors.

Geocoding techniques for interferometric and polarimetric airborne SAR data

This paper presents a detailed study of geocoding algorithms for interferometric SAR data from airborne sensors. They allow to relate a phase value in slant-range coordinates to a position in a cartesian reference system and, hence, a height value on a topographic map. Two basic approaches, which follow forward or backward strategies, are proposed in this paper. Another aspect is related to the kind of the acquisition mode: single- or repeat-pass. In general, a wide variety of techniques and combinations (single or repeat pass, direct or indirect transformations, one or two steps, zero-Doppler or squinted geometry) exist. This paper reviews the state-of-the-art of these combinations, as well as their evaluation, comparison and discussion by means of real data from the DLR’s E-SAR system. The discussion includes the problem of polarimetric data geocoding. In that case, the map-projected height information derived from a X-band single-pass system is used to geocode non-interferometric channels (for example polarimetric L-band data).

Generation of precise wide-area geocoded elevation models with ERS SAR data

The authors present an improved technique for the generation of digital elevation models (DEM), capable of dealing with full scene images (100/spl times/100 km) coming from an interferogram obtained with ERS satellite data. Starting from an interferometric processor aimed to the geocoding of smaller areas, now the authors expound the new improvements based on the use of ground control points (GCP) in order to calibrate some imprecisions which appear in the case of very wide swaths. A generated DEM of the test zone of Tarragona (Spain) and its error assessment are presented.

Design criteria and comparison between conventional and subaperture SAR processing in airborne systems

This paper compares two different approaches for designing airborne SAR systems. The first one is the most common where conventional processing is employed, and therefore wide antenna beams are to be used in order to avoid ambiguities in the final image due to attitude variations. A second approach is proposed to lower the requirements such system imposes based on subaperture processing. The idea is to follow the azimuth variations of the Doppler centroid, without increasing the hardware requirements of the system. As it is shown in this paper, this processing procedure must be complemented with precise radiometric corrections, because the platform may experience small attitude variations, which could increase/decrease the target observation time, inducing a significant azimuth modulation in the final image. This leads to the definition of a new criterion concerning maximum attitude deviations for an airborne platform.

Development and validation of a multisensor system for ocean pollution detection

The increase in maritime traffic has caused an increment on the risk of accidents that can damage the environment. In recent times, the efficiency on oil spill detection and monitoring with spaceborne systems has been demonstrated. It is however important to remark that satellite data must be complemented with other monitoring platforms and sensors, for better temporal coverage and for an improvement on detection and analysis of the spill. RAPSODI is an European project for the development of a new anti-pollution remote sensing system that results from the integration of airborne sensors: SAR, SLAR, IR, UV and a microwave radiometer and spaceborne data. One of the main operational goals of this project was a real size experiment in completely controlled situations and environment. In this campaign, heavy fuel oil was released and treated. This experiment has also allowed monitoring the efficiency of the dispersing products used by the oil spill response community. In this context, this paper describes the development of the airborne SAR mode optimized for oil spill detection, and the planning and first results of the experimentation campaign.

Precise geometry simulation of interferometric SAR signal for air- and spaceborne sensors

A complete interferometric synthetic aperture radar (SAR) simulator has been developed. It achieves precise results by using geodetic models to represent the Earths shape, and considering SAR effects such as range migration, Doppler shifts, or perturbations of movement in the airborne case, which are included via the inverse Extended Chirp Scaling Algorithm (ECSA).

WLMS-RG combined unwrapping method

In this paper we evaluate a combined phase unwrapping method that improves the results in difficult areas (containing noise and discontinuities) taking advantage of the Region Growing (RG) and Weighted Least Mean Square (WLMS) algorithms. The performance of the combined method is based on the following steps. Starting from the wrapped phase, a binary mask is calculated using a first RG solution, in order to generate a mask which is able to consider low-quality pixels. Afterwards, the WLMS solution is calculated using the wrapped phase and the RG mask. To obtain this WLMS solution it is not necessary to compute many iterations, since the objective of this step is just to ease the subsequent RG process. Therefore, the calculation time is smaller than with a WLMS solution separately, because it needs a larger number of iterations to converge. As commented, at this stage the RG algorithm is applied in order to unwrap the error of the WLMS solution, which is expected to have less noise than the input interferogram. Therefore, the RG is able to handle its values in a relatively easy manner. Finally, the first WLMS solution and the unwrapped error are added to obtain the desired unwrapped phase.