[INTERNAL] InSAR and DinSAR processing chains using ENVI SARscape
MariM
InSAR (Interferometry) and DinSAR (Differential Interferometry) processing chains allow to respectively build DEM and displacement maps, based on the phase information contained in SAR images. This process is usually more accurate than standard photogrammetric approach.
Requirements to complete InSAR or DInSAR processing chain :
- The processing steps require 2 SAR images acquired above the same area, by the same sensor.
- The data should be SLC data (Single Look Complex) because they should contain phase information.
- The data should be acquired be acquired from 2 different locations for DEM generation. It is better if data are acquired with a small temporal baseline for DEM generation.
- The data should be acquired at 2 different dates for Displacement map generation. It is better is data are acquired from the same -or near- locations for displacement map generation.
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The steps below illustrate the kind of processing chain you can follow when processing interferometric SAR data.
Step 1 - Baseline estimation.
This step is optional but recommended to estimate the baseline between the 2 input images. If this baseline is larger than the critical baseline no DEM or displacement map could be generated.
Step 2 - Interferogram generation.
This steps consists in computing the phase difference between the 2 input images.
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Technical note: how to relate phase difference to elevation or displacement
Let's assume that the 2 input images have the same amplitude information because they are acquired above the same area and so on the backscattered signal from one single target is the same on the 2 images
The images are acquired :
- from 2 different locations and at the same time for DEM generation
or
- at 2 different dates and from a similar location for displacement map generation
The phase of the signal recorded by the sensor for each interferometric image changes because the distance between each target and the sensor is different between the 2 acquisition. This difference is due to the topography in the first case and to the displacements in the second case. Mathematical equations relate phase variations to topographic variations or displacements.
The phase difference between the 2 input images is computed in the complex domain by multiplying one image by its conjugate.
An interferogram is composed of fringes: one fringe corresponds to a 2Π phase variation related to topographic variation or displacement.
A coregistration step is automatically computed during this step
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Step 3 - Interferogram flattening.
The phase difference can be divided in different components.:
For DEM generation:
- a flat component which corresponds to flat terrain phase variations
and
- a topographic component which corresponds to the terrain elevation above the flat area
For Displacement maps:
* a flat component which corresponds to flat terrain phase variations
and
* a topographic component which corresponds to flat terrain phase variations
and
* a displacement component due to the movement between the 2 acquisitions
The flattening step consists in removing the flat component of the phase (and eventually the known topographic component when a DEM is input during the flattening step), so that only the component of interest (topography or displacement will be analyzed afterwards.
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Technical note:
The way the flattening steps runs in ENVI SARscape depends on the input data that are provided :
* No DEM but an average elevation of the ellipsoid: the flattening step will only remove the flat phase component base don the average elevation of the ellipsoid. This configuration can be used for DEM generation but is not recommended: there will be too much remaining fringes in the output flattened interferogram to compute an accurate phase unwrapping afterwards.
* a DEM: the flattening step will remove the flat phase component and the phase topographic component corresponding to this initial DEM - and called the "known topography"-.
This is the recommended configuration for DEM generation. The output flattened interferogram will then only include the residual topographic component which will be easier to unwrap afterwards. Both known and residual topography will be combined at the last step to build an accurate DEM
* an Accurate DEM: the flattening step will remove the flat phase component and the entire topographic phase component. This is the recommended configuration for displacement map generation: the residual phase will then correspond to the displacement phase component.
During the flattening step, a synthetic interferogram is computed based on the input data (DEM or ellipsoid elevation). It is subtracted to the original interferogram to output the residual phase interferogram.
In SARscape ENVI the flattening step can be computed at the same time as the interferogram generation by providing a DEM in input. This is the recommended configuration because it will improve the coregistration step too.
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Step 4 - Interferogram Filtering and coherence map generation.
The filtering step consists in cleaning the flattened interferogram before computing the coherence map. The coherence map measure the correlation between the 2 images based on the phase information. If the coherence is too low (close to 0) no DEM generation or map displacement can be computed. For example if the ground aspect is too different between the 2 images acquired at 2 different dates ,the decorrelation effect can be too important and the coherence will be very low.
Step 5 - phase unwrapping.
This step consists in converting the discontinuous phase difference (fringes) of the flattened interferogram into a continuous signal.
The phase difference values (i.e. the interferogram) are always included between -Π and +Π. Consequently a continuous variation of topography is recorded as a discontinuous signal which corresponds to several fringes. The phase unwrapping rebuilds a continuous phase variation from the discontinuous recorded signal. The output image does not show fringes anymore.
Step 6 - Refinement & Re-flattening.
This step consists in refining orbital parameters and removing eventual phase ramp components using GCP. You will find more details about this step in the following Help Article.
Step 7 - DEM or displacement map generation.
It is this is the final step of the interferometric chain. It consists in generating the DEM or the displacement map. Phase values are converted to elevations or displacements based on mathematical equations. The geocoding of the DEM and displacement map is computed at the same time.