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README.md

@@ -14,7 +14,7 @@ From a photograph containing topographic features, can the location and view ang
 
 Create an open source package for chrono-locating a audio/video source by matching electrical network frequency variations (from low frequency hums in source audio) with recorded variations in a grid frequency database (i.e. https://osf.io/m43tg/). This is theoretically possible (many IEEE articles about it) and reportedly in use by state-level justice entities, but is it practical for the OSINT hobbyist? To our knowledge, no one has publicly developed a public, non-academic proof-of-concept of this.
 
-### Complex SAR imagery
+### Interferometric SAR imagery
 
 Synthetic aperture radar imagery (as captured by, e.g., Sentinel-1) has certain advantages over optical imaging for open source investigation. It is unaffected by cloud cover and can even be used to detect radar installations, as has been previously documented. An additional advantage, little explored in the open source investigation community, is that these images are complex, that is, they contain both magnitude and phase information. While difficult to work with for several reasons (not accessible in Google Earth Engine, requires "unwrapping", etc.), it has some useful applications for computing topography or topographic changes. It is often used in academic research to visualize crust deformations following an earthquake or long-term land subsidence in agricultural areas. Could this phase data also be used for open source investigations, for example to see deforestation or bombing impacts?
 

interferometric-sar-imagery/experiments.md

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+## Experiments
+
+### Logan Williams, June 10 2021
+#### Blast/impact crater mapping
+
+Can InSAR images be useful for mapping of deformations caused by airstrikes? I tried following the [University of Alaska's guide](https://asf.alaska.edu/how-to/data-recipes/create-an-interferogram-using-esas-sentinel-1-toolbox/) to generating an InSAR image using two Sentinel-1 passes from May 8 2021 and May 20 2021 in the Gaza Strip.
+
+Results were not encouraging. Displacement was only non-zero in some areas, and it did not appear to represent real damage on the ground. Additionally, the non-zero displacement was mostly measured in areas with low phase coherence, indicating that the data is pretty suspect to begin with.
+
+This failed for possibly several reasons.
+
+- Coherence was generally low in the Gaza Strip area. This was especially true in many of the agricultural areas, as somewhat expected. This could have caused the phase unwrapping to fail entirely.
+- Any significant impact sites would likely significantly change the radar reflection, guaranteeing incoherence.
+
+That said, areas with destruction reported by UNITAR did _not_ appear to show low coherence. They actually seem to correlate with higher coherence, as is typical of urban areas. I don't currently have an explanation for this. The quick overlay below shows areas (in red) that have reported damage, and is brighted in areas with high coherence.
+
+![](images/2021-06-10/gaza-sar.jpg)
+
+- The changes in elevation expected as a result of an airstrike could be quite large, possibly above the "altitude of ambiguity," where phase unwrapping becomes more difficult if not impossible.
+
+Other observations
+
+- A radio installation near the Egypt/Gaza border is very visible in the imagery.
+
+Overall: maybe there is some way to make this work, but it seems difficult and like it would have few advantages over conventional visible/infrared imagery. 

interferometric-sar-imagery/references.md

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+## References
+
+* A Wikipedia overview of the concept: https://en.wikipedia.org/wiki/Interferometric_synthetic-aperture_radar
+
+### Academic applications
+
+* Subsidence monitoring in a groundwater basin with heavy withdrawals https://www.spatialsource.com.au/gis-data/predicting-disaster-strikes-insar-australia
+* Visible surface deformation caused by underground nuclear testing https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2003GL018179
+* Tracking infrastructure degradation in Syria https://ieeexplore.ieee.org/abstract/document/8128442