Applications of Synthetic Aperture Radar Satellites to Environmental Monitoring
November 9, 2020
By Ricardo Rendon Cepeda
Since the launch of the first joint NASA-USGS Landsat Mission in 1972, Earth observation satellites have been powerful tools for scientists and policymakers to understand planetary changes. In particular, Earth observation data plays a central role in environmental monitoring by enabling users to compare images of Earth over time and examine large-scale phenomena such as melting sea ice, deforestation, droughts, and floods.
As the saying goes, a picture is worth a thousand words; in the case of satellite imagery, considerably more so. However, sometimes these pictures are worth much less or nothing at all, since traditional imagery faces capture obstacles such as cloud cover and nightfall. Essentially, traditional Earth observation satellites need an unobstructed and illuminated view of the Earth in order to capture meaningful images. These limitations have created a need for new types of spaceborne technologies like Synthetic Aperture Radar (SAR) satellites.
At its core, satellite imagery is made possible by remote sensing instruments that measure energy across the electromagnetic spectrum. In traditional Earth observation satellites, optical instruments sense energy in the visible, infrared, thermal, and microwave portions of the spectrum in order to produce photographs. These optical instruments are known as passive sensors because they measure energy emitted from another source; i.e. the natural energy from the sun being reflected off the surface of the Earth. SAR instruments, on the other hand, are active sensors. This means they can emit their own energy towards the Earth and measure how that energy is scattered after coming into contact with the Earth’s surface. These instruments use radar technology to sense energy in the microwave and radio portions of the spectrum. Compared with traditional optical imaging, SAR imaging provides more details about the surface of the Earth because of the way in which SAR signals interact with particular surfaces (e.g. buildings, trees, mountains, lakes, etc.). Optical imaging is similar to taking a picture of the Earth, whereas SAR imaging is more similar to measuring the topography of the Earth.
Since SAR instruments don’t depend on the Sun’s energy to collect surface data, SAR satellites can operate just as well during the day or night. Additionally, SAR signals can penetrate through clouds to “see” the covered surface underneath, allowing satellites to have a full view of the Earth’s surface regardless of atmospheric or lighting conditions. SAR can also “see” through other types of cover such as smoke, vegetation, snow, or sand, depending on the satellite’s designated operating band (which indicates the sensor’s associated frequency and wavelength). SAR bands are helpful in categorizing the penetration strength and thus the potential applications of a satellite, such as Germany’s TanDEM-X (low-penetration X-band), Canada’s RCM (moderate-penetration C-band), and Japan’s ALOS-2 (high-penetration L-band).
A common feature among all SAR satellites is their more detailed collection of data about the Earth’s surface, which makes SAR imagery a highly valuable tool for scientists and policymakers to better understand our changing environment.
Given the increased threats of environmental phenomena to national and global security, SAR can provide additional information to assess and respond to climate change, ecosystem loss, natural disasters, and more. Below are just a few examples of how SAR is being used for such purposes:
- Agriculture. Differences in surface roughness are indicative of field ploughing, soil tillage, and crop harvesting.
- Floods. Differences in surface reflection can help distinguish heavy flooding, light flooding, urban areas, and permanent bodies of water.
- Land subsidence. Differences in measurements over time can reveal displacements of land, such as sinking ground caused by the extraction of underground natural resources.
- Snow cover. Differences in surface reflection can help forecast snowmelt by distinguishing wet snow, dry snow, and snow-free areas.
- Wildfires. Penetration through thick smoke can provide more accurate and timely information about the extent of a forest fire and can help quantify vegetation loss.
- Wetlands. Penetration through wetland areas can reveal flooded vegetation where land is covered by shallow water.
Despite the many advantages of SAR, it’s still an underused technology compared to traditional optical imagery.
To alleviate this issue, the Department of Commerce and the National Oceanic and Atmospheric Administration (NOAA) recently promulgated a new rule entitled Licensing of Private Remote Sensing Space Systems (hereafter, “remote sensing rule”). This rule substantially revised the regulations for licensing the operation of private remote sensing space systems under the Land Remote Sensing Policy Act of 1992, with the aim of making U.S. private actors competitive with their foreign counterparts. An excerpt of the new rule is shown below, which notably uses SAR as a specific example:
“Take, for example, the U.S. SAR industry. Commerce license conditions prevent such licensees from imaging at finer than 0.5 meters impulse response (IPR), while some foreign competitors sell data at .24 meters IPR. Even a regulatory approach that allows U.S. licensees to sell data at .24 meters IPR would only let U.S. industry meet, not exceed, their foreign competition. This creates a market opportunity for foreign entities to sell data at finer than .24 meters IPR. The U.S. Government has no control over such foreign SAR systems and must adapt to protect its operations, making such a regulatory approach ultimately ineffective and counterproductive.”
The remote sensing rule, and this particular excerpt, is more than a step in the right direction for the U.S. SAR industry. Already, foreign competitors such as Finland’s ICEYE are operating constellations with .25-meter resolution imaging, and have proven themselves effective in applied environmental monitoring for forestry, agriculture, land subsidence, and more. The new rule will allow US private actors, such as Umbra, to operate under bare minimum regulation as Tier 1 space systems up to the point that they meet similar foreign offerings. Policymakers and businesses alike must keep a close watch on foreign developments to ensure that the U.S. private industry is not left behind.
Though this new rule is specifically aimed at creating a more competitive space for U.S. private actors, SAR is presently being led by government space agencies, with major missions on the horizon such as the joint NASA-ISRO NISAR Mission and the ESA’s Biomass Mission. The Biomass Mission is particularly exciting because it will carry the first-ever spaceborne P-band instrument. This instrument can more accurately quantify forest biomass and advance our knowledge of the global carbon cycle. Nonetheless, the remote sensing rule will certainly allow the commercial SAR sector to flourish at just the right time, with important milestones underway such as Capella Space’s recent launch marking the U.S.’s first commercial SAR provider in orbit.
If the growth of commercial SAR imaging is anything like the growth of commercial optical imaging, then the future is bright. With increasing demand to better understand our environment and changing planet, space is wide open for more aerospace companies to emerge and quite literally make waves.
Ricardo Rendon Cepeda is a research intern with the Technology Policy Program at the Center for Strategic and International Studies in Washington, DC.
The Technology Policy Blog is produced by the Technology Policy Program at the Center for Strategic and International Studies (CSIS), a private, tax-exempt institution focusing on international public policy issues. Its research is nonpartisan and nonproprietary. CSIS does not take specific policy positions. Accordingly, all views, positions, and conclusions expressed in this publication should be understood to be solely those of the author(s).