About every six days, ghostly images of most areas on Earth emerge from the European Space
Agency’s Sentinel-1 program.
The technology capturing these images, Synthetic aperture radar—also known as SAR—is booming.
In the coming years, it may reveal tiny changes on every patch of earth daily.
Optical imagery satellites, like the U.S. Geological Survey’s Landsat, use reflected
light to passively take photos from high in the sky. Sentinel-1 takes its snapshots actively,
by using radars.
Unlike optical technology which produces the best images on sunny days, SAR penetrates
clouds and works at night.
An antenna onboard the satellite emits microwave pulses to the surface of the earth and records
the echoes that bounce back. The more signals it gathers, the higher the resolution of the image.
The “synthetic aperture” of SAR satellites describes an important advance.
Instead of building an extremely large antenna to capture returning waves, these satellites combine
signals from a relatively small antenna moving through space to sharpen resolution.
By comparing measurements from different positions, SAR data can reveal topography.
Images are often colorized to highlight important features, such as flooded regions.
What started as a tool for military surveillance is now essential to earth scientists examining
changes in landscapes over time.
Comparing SAR images from the same vantage point at
different times can reveal surface movements—from meters down to millimeters.
The technique is known as interferometric SAR, or InSAR.
One of the first applications emerged in 1989, when researchers observed the subtle swelling
of fields in California’s Imperial Valley after crops were irrigated.
In 1993, a groundbreaking study used InSAR to reveal how the ground shifted after a powerful
earthquake rippled through Landers, California.
Colors on the map show meters of movement,
concentrated along the faults.
The technique also revealed changes in the land miles away
from the earthquake’s epicenter.
Two years later, glaciologists used InSAR to measure the flow of ice streams in Greenland,
which accelerated as the planet warmed.
More recently, geophysicists were able to detect signs of deformation at several volcanoes
in the East African Rift Zone—some of which were thought to be inactive.
The Corbetti volcano, near the city of Awassa, Ethiopia, has been inflating by almost 7 cm a year.
The launch of twin Sentinel-1 satellites, in 2014 and 2016, provided researchers with
regular, freely available data to observe long-term changes.
Norway used Sentinel-1 data to create a deformation map
of the entire country, in order to identify rockslide hazards.
Inadvertently, they discovered that parts of the train station in Oslo were sinking.
Private companies are also investing in SAR technology.
Iceye, a Finnish space start-up,
plans to launch eight or more satellites in 2021—adding its growing fleet already in orbit.
Ultimately, they hope to capture snapshots of most places on Earth several times per
day and produce images with 50 centimeter or higher resolution.
Researchers envision close to real-time SAR observations to change the way we live.
It could provide crucial information on natural disasters, such as floods.
Quickly identifying the most inundated areas can guide response efforts.
The applications are just beginning to be conceived.