Coregistration

Coregistering InSAR Products

The sub-pixel coregistration of SAR images is a strict requirement and critical component of any interferometric processing chain. It is an essential step for the accurate determination of phase difference, and applications such as DEM map generation, interferometric deformation analysis, etc.

The interferometric modules of the toolbox will accurately co-register one or more slave images with respect to a master image. The co-registration procedure is completely automatic. Apart from defining the processing parameters, no additional input nor intervention from the user is required. For example the distribution of correlation (optimization) windows are done in automatic manner for both master and slave image. Also, the refinement of the coregistration offsets is done in a fully automatic way, including downloading and interpolation of the a-priori digitial-elevation-model.

Brief Implementation Details

The implementation of the coregistration procedure is based on the cross-correlation technique. Since this technique for an optimal alignment tend to be slow for very large search windows, the procedure is usually separated in two main steps: coarse and fine coregistration. In the coarse coregistration, the offsets are approximated either by using the satellite orbits and timing as a reference, and/or by defining an approximate common points in master/slave images and performing correlation matching with large windows. The subsequent fine coregistration applies automation correlation technique to obtain sub-pixel alignment accuracy. After the coregistration offsets are computed, the estimation of the coregistration polynomial (CPM) and interferometric resampling of slave images to the master geometry is performed.

Overview of Coregistration Processing Chain

The interferometric coregistration is performed by create stack, coarse fine coregistration and resampling.

Input Images and Data Support

Input SAR images may be fully ("full frame") or only partially overlapping ("subset"), they have to be from acquisitions taken at different times using compatible, in the interferometric sense, sensors, and input images must belong to the same type (i.e., them must be complex).

While in principle the implementation of the InSAR coregistration is flexible enough to allow processing of real (detected) products, for now only complex (single-look-complex) data is supported.

Create Stack

The Create Stack operator collocates the master and slave images based into a single reference (master) geometry. Basically the slave image data is subset into geometry of the master image. With performing this operation the master and slave images share the same geo-positioning information, and have the similar dimensions. For overlap and geometry calculation either orbital data, or annotated tie-point-grids (i.e., ground-control-points) can be used. In other words the coarse coregistration is performed using orbital information or annotated GCPs. The method based on orbits is recommended for all platforms, since especially in case of old sensors (ERS1/2) annotated GCPs prove not to be reliable through-out the whole mission lifetime.

More details on this operator are given in the operator help - Create Stack.

Cross Correlation

The Cross Correlation operator creates an alignment between master and slave images by matching automatically distributed correlation optimization windows to their corresponding slave windows. There are two steps: coarse and fine registration. The offsets between master and slave are computed by maximizing the cross-correlation between master and slave images on a series of imagettes defined across the images. First on coarse level, with large windows and lower oversampling factors, later on fine level, with smaller windows and higher oversampling factors.

For details and specifics on the operators input parameters, readers are referred to operators help Cross Correlation.

Warp

With the master-slave offsets computed, a coregistration polynomial (CPM) is estimated by the Warp operator, which resamples pixels in the slave image into pixels in the master image.

This resampling is performend in two-steps: (1) reconstruction of the continuous signal from its sampled version by convolution with an interpolation kernel, and (2) sampling of the constructed signal at the new sampling locations.

For details of the Warp operator, readers are referred to Warp operator.

Coregistration with SNAP

Processing steps that are listed below should give satisfactory results for most of the interferometric combinations.

1. From the Radar menu, in sub-menu Coregistration, select Coregistration.
2. Add a master product to the ProductSet Reader.
3. Add any slave products to the ProductSet reader or drag and drop products from a Project.
4. In the CreateStack tab, select no resampling for InSAR products. A resampling method may be selected for detected product which are of differnt passes.
   
5. In the Cross Correlation enter the number of correlation optimization windows to create.
6. In the Warp tab select the thresholds and coregistration polynomial order.
7. In the Write tab enter in the output product name and location.
8. Press the Run button to begin processing.