How to use it

To illustrate some of the tool workings and in general suggest ways to use it in your researches, I’m going to guide you through a fast research over the Persian Gulf area, specifically we will check if as consequence of the August 17 and September 14 attacks on Saudi oil infrastructure, additional air defence assets where deployed or activated.

(Very important to underline once more that the technique reveals just interferences caused by powerful equipment operating on frequencies close to those the satellite uses, it cannot detect all military radars, far from it, BUT, coincidence wants that US made Patriot AD system main radar operates on frequencies detectable by the technique, the system is also broadly employed by Persian Gulf states so… )

STEP 1

Once loaded the web app in your web browser you will be presented with an interface very similar to the one in the above picture. One of the most useful features of the app is that of allowing you to configure 2 different maps in a split view so to easily compare the data from both.

STEP 2

The area covered by the 2 SENTINEL-1 satellites data each day is limited, by configuring a time frame the app will compose all satellite passes of that period isolating the strongest interferences and allowing you to study a wider area, but possibly even more importantly it can allow you to precisely pinpoint the emitting source location.

The two satellites composing the SENTINEL-1 constellation orbit in basically and roughly opposite directions and with a slight angular difference between them (very close to polar orbit). The interferences as detected by this technique manifest themselves as wide “lines” perpendicular to the satellite orbit. This lines provide just a very rough (sometimes hundreds of kms) idea of where the source of the emissions could be. BUT, when two satellites, in slightly different orbits pass over the same location, each one registering the interferences as “lines” perpendicular to their orbit, we can then overlay the data from both satellites and have the precise location of the interference source thanks to the intersection of both “lines”.
By expanding the “layers” menu in the top right corner of the app screen, you can isolate each of the satellites ( “Ascending” and “Descending”, due to their orbits) data and see how it contributes to the “Composite” layer which is instead the result of both previous layers overplayed to better isolate the “X” drawn by their intersection.

STEP 3

With both date ranges configured and data loaded for them we can start doing some fast comparison by dragging the split screen handle.

Among other things, the time period configured in this case shows that sometime within few weeks of the Shaybah oil field attack (August 17) a new interference source appeared in south eastern Saudi Arabia. Unfortunately we have only one satellite passing over the area so we can’t leverage the precision brought by the intersection of 2 satellites, but we can still see that the interference line passes over Shaybah and we can infer that a Patriot AD system was plausibly deployed there after the attack. (a Reuters article also confirmed it some weeks after the attack). You could start reducing the timeframe to isolate more precisely the date when the system became active.

You can usually expect at least one satellite passing over the area covered by the Sentinel-1 mission every 3 days, if the interference source isn’t “active” during satellite pass, nothing will be captured, but if it is active during multiple satellite sources the images will show it as more intense and defined.

STEP 4

Conclusion

While very basic, hope this “tutorial” will get you started with the tool and the technique in general, and possibly have you contributing to the advancement of it. There is still much to research about what we can learn from it, if sources can be id’ed thanks to some signal pattern, if something could be used to infer the signal strength etc etc..

Google Earth Engine (GEE) its an amazing tool, but it also has some (minor) limitations, for SENTINEL-1 data for example there is a delay of usually 24 or more hours between data acquisition and actual availability on GEE platform, so pick the right tool according to your needs, http://apps.sentinel-hub.com/eo-browser/ will be among the first to receive the updated data (just few hours after acquisition usually.