Sentinel-1 SAR data beta release
From the moment we launched Sentinel Hub we were receiving questions about support for Sentinel-1 data. However, Synthetic-Aperture Radar (SAR) data are much more complex to process and to use, which is why we had to delay this problem. Putting aside the new formats, the major issue with Sentinel-1 is that products are not orthorectified. This makes them quite cumbersome to use over OGC web-services, which are all tightly related to location. A few months ago European Space Agency asked us to look into this subject in order to complete Sentinels’ coverage in EO Browser. We were a bit reluctant, but so far ESA’s guidance proved to be most beneficial for Sentinel Hub, so we gave it a go. Today we are happy to announce that the Sentinel Hub is richer for Sentinel-1 data as well. Although in beta operation at the moment — some tweaks still need to be solved — you can already explore it through EO Browser and our web services.
This is our first entry in Synthetic-Aperture Radar world, and there was a lot to be learned. Reading the Sentinel-1 files, understanding SAR, polarizations and orthorectification was quite a tough nut to crack. And we are still not done learning.
Sentinel-1 imagery is not available with terrain correction by default. This means that such imagery cannot be precisely positioned on the Earth. Positioning errors can be hundreds of meters in magnitude. To correct this, orthorectification is required. In essence, orthorectification requires a Digital Elevation Model (DEM) along with the precise satellite orbit and the source data. Combined, this gives enough information to position the pixels correctly on the ground, resulting in precise, usable imagery. The tricky part is how to do this fast enough for every single pixel.
Another challenge was data fusion. We need to combine Sentinel-1 data, which reside on EO Cloud in Poland, with MapZen’s DEM, available through Amazon Web Services (AWS) in US.
At the moment, EO Cloud is prioritizing Europe to download backlog of data, but complete Ground Range Detected (GRD) dataset will be available once everything is downloaded. Since there is more than 1PB of data to be downloaded, it takes some time. For now, only the GRD data is available, but we are already looking into possibilities to include Single Look Complex (SLC) data as well.
Interesting facts about Sentinel-1
The Sentinel-1 imagery is provided by two polar-orbiting satellites, operating day and night performing C-band synthetic aperture radar imaging, enabling them to acquire imagery regardless of the weather. Sentinel-1A was launched on 3 April 2014 and Sentinel-1B on 25 April 2016.
Main applications are for monitoring sea ice, oil spills, marine winds, waves & currents, land-use change, land deformation among others, and to respond to emergencies such as floods and earthquakes.
The identical satellites orbit Earth 180° apart and at an altitude of almost 700 km, offering a global revisit time of six days. At the equator, the repeat frequency is just three days and less than one day over the Arctic. Europe, Canada and main shipping routes are covered in less than three days.
Sentinel-1’s radar can operate in four modes:
- Interferometric wide-swath mode at 250 km and 5×20 m resolution
- Wave-mode images of 20×20 km and 5×5 m resolution (at 100 km intervals)
- Strip map mode at 80 km swath and 5×5 m resolution
- Extra wide-swath mode of 400 km and 20×40 m resolution
Explore the data yourself
The Sentinel-1 data is available on EO Browser and also through our Sentinel Hub services.
Originally published at sentinel-hub.com.
