InSAR On Demand!

Content updated 27 September 2022

 Synthetic Aperture Radar (SAR)  has the advantage of being able to image the surface of the Earth at any time, regardless of cloud cover. The Sentinel-1 satellite constellation collects SAR data globally, with the potential to collect repeat data as often as every six days, making it a very powerful dataset for time series analysis, change detection, and flood extent mapping.

The phase component of the SAR dataset allows for the detection of very small changes in elevation on the earth's surface from one acquisition to the next. This makes SAR an excellent tool for monitoring deformation caused by earthquakes, volcanoes, ground water subsidence, and other geologic and anthropogenic processes.

By differencing phase images collected from the same location in space at two different points in time, users can generate an interferogram that indicates the location and magnitude of surface deformation that has occurred during the time interval between acquisitions. This approach is called interferometric SAR, or InSAR.

To generate interferograms, specialized software and workflows are used to process Sentinel-1 SLC datasets. The learning curve for the processing workflow is steep, and processing InSAR also requires significant computing power. It can take many hours to generate one interferogram on a desktop computer.

The Alaska Satellite Facility (ASF) offers  On-Demand Processing of Sentinel-1  products. This allows you to search for Sentinel-1 scene pairs and submit them for  InSAR processing  directly in the  Vertex Data Search  interface. ASF uses cloud computing to efficiently process the data, and InSAR products are generally available for download about an hour after they are submitted for processing.

On Demand users are given an allotment of 10,000 credits per month to use for processing jobs, and each type of On Demand job costs a different number of credits, as shown in the  Credit Cost Table .

When  selecting processing options  for output products, Vertex displays the number of credits required for each of the different options. Currently the only InSAR option that impacts credit use is the number of looks, which determines the pixel spacing of the output product.

More information about the credit system is available  here . If you have any questions or comments, contact ASF at uso@asf.alaska.edu.

For users working with smaller areas of interest, ASF now also offers  Burst-Based InSAR On Demand  processing. Users can search for and submit individual Sentinel-1 bursts for InSAR processing, rather than using an entire Sentinel-1 SLC scene.

An InSAR job is submitted for On-Demand processing as a pair of Sentinel-1 scenes to be used to generate the interferogram. There are currently two methods of selecting and submitting scene pairs in Vertex: the  Baseline  and  SBAS  search interfaces.

The  Baseline  tool is most appropriate for selecting specific individual pairs to use for interferometry. This is often useful when generating an interferogram for a discrete event, such as an earthquake. Once you find a good acquisition to use as the reference scene, use the Baseline tool to find appropriate secondary scenes.

The  SBAS  tool is designed to compile a series of many InSAR pairs that covers a long period of time. This time series approach is often used when monitoring long-term processes such as groundwater subsidence or magma chamber dynamics.

In both cases, you will start with a  Geographic Search  in Vertex to find a reference scene that you want to use for InSAR. You can then select the  Baseline tool  to find another scene to pair with it to generate a single interferogram, or the  SBAS tool  to compile a time series of InSAR pairs that cover the same footprint as the reference scene.

Scroll through the tutorial below to learn how to find Sentinel-1 scenes in ASF's Vertex Data Search portal

Use the navigation bar at the top of the screen to access the Baseline or SBAS tutorials directly if you're already familiar with the Vertex search interface

Once you've selected a reference scene using the Geographic Search in Vertex, you'll need a secondary scene to match with it to generate an interferogram. The scenes must be acquired fairly close together in space in order for the phase measurements to be coherent enough for interferometry.

The  perpendicular baseline  indicates the distance between the imaging platform locations during the two acquisitions. The phase difference measurements will be better if the scenes are acquired from very similar locations in space. The Baseline tool finds and lists other acquisitions that are within a suitable range of perpendicular baseline values, allowing them to be paired with the reference scene for InSAR processing.

Scroll through the tutorial below to learn how to use the  Baseline tool  to pick InSAR pairs for processing

Time series analysis of InSAR products is often used to monitor landscape or geophysical processes, determine rates of change, gain insight into the characteristics of deformation, and correct for atmospheric impacts or other noise signals. One common method for time series analysis is the Small BAseline Subset (SBAS) approach.

SBAS uses a series of many interferograms generated from SAR images acquired over an extended time period, with each individual InSAR pair having a relatively short time interval between acquisitions. A range of short baselines are used, allowing for pairs to overlap each other, and for each reference scene to be paired with multiple secondary scenes.

The  SBAS Tool  in Vertex assists users in compiling a list of pairs suitable for use in SBAS analysis.

Scroll through the tutorial below to learn how to use the  SBAS tool  to find and submit a time series of InSAR pairs for On Demand processing

Click the arrows or swipe to move through the slideshow, which demonstrates the steps involved in working with Custom Pairs

The InSAR products are downloaded in zip archives, which include GeoTIFFs of the interferograms, coherence and displacement maps, and a number of ancillary rasters, browse images, and accompanying documentation.

Click on the map below to explore some sample On Demand InSAR output files. This InSAR product captures an earthquake event in Ridgecrest, California in July 2019. The map displays the browse image for the wrapped interferogram and the GeoTIFF of the unwrapped interferogram.

If you are new to ASF's On Demand InSAR products, open the README file included in the zip file to find out more information about the files included in the product package. It can be read either as a text file or in a Markdown viewer/editor.

Refer to our  Sentinel-1 InSAR Product Guide  for additional information on the On Demand InSAR processing workflow and options, files included in the product package, and guidance for usage.

To learn about the datasets included in the InSAR product package, and to explore techniques for interacting with and interpreting the data, refer to our  Exploring Sentinel-1 InSAR Products Story Map .