Synthetic-Aperture Radar (SAR), Earth Observation, and Mapping

• Synthetic-aperture radar (SAR) has been increasingly used for Earth observations.

• Advancements in computing and satellite systems enable more use of this technology.

• We can expect SAR to be a key part of future developments in satellite observations.

The use of synthetic-aperture radar (SAR) has been somewhat limited, at least in spaceborne imagery and use in remote sensing. In large part, this is because of the complexities of processing and providing the data that are easy to understand, outside of a limited group of computational and geospatial specialists. However, now SAR is becoming more widely available and increasingly part of Earth observation satellites and data provided, including those sponsored by private companies such as ICEYE.^[1] 

In a MapScaping podcast, ICEYE’s Eric Jensen discusses the increasing benefits provided by SAR.

What is Synthetic-Aperture Radar?

Technology that makes SAR possible has been in development since World War II, as effectively the data represent developments in radar that have occurred since that time. The data consist of microwave data sent by an active sensor, that is the data are emitted from an energy source within an observation platform such as a satellite, and the energy then is reflected back from the surface of objects such as Earth. The radar microwave data that area reflected are then read.

The microwaves are at a scale of usually centimetres wavelength, making them much larger than light waves. The radar instrumentation and antenna determine the spatial resolution of the data. Synthetic aperture is used to effectively create a sensor that works like a large antenna, which allows that radar collection to utilize a much smaller data receiver, enabling more fine-scale resolution to be observed.

The instrumentation on SAR allows different pulses to not only be actively sent but also data can be recorded from different angels. Data can be then compiled to create composite or multiple imagery of features. Different wavelengths are measured, making SAR effective for range finding using different frequency bands, helping to measure surface roughness and different scales of surface properties.

Bands, such as U band, enable vegetation and soil property measurements. Additionally, there is a wide variety of band types, including KA, K, KU, C, S, and others that are used.

Higher frequency bands are less able to penetrate objects such as clouds. Larger wavelengths allow relatively easier penetration of clouds; radar data also allow you to operate at night. The qualities of penetrating cloud cover and for the satellites useful for night conditions have made SAR vital for Earth observation given few other sensors have such qualities.^[2] 

Data from different SAR bands allow interferometry, that is measuring and superimposing bands, to be used to measure discrepancies in objects, such as elevation changes or displacement on surfaces. The key data that SAR measures is distance, with the time in which signals bounce back to the sensors allow distance to be calculated.

Typically, digital elevation models (DEMs) are created from the output of SAR. However, different frequencies can be used to differentiate the land surface, tree cover, or other objects on the earth’s surface. ^[3]

Rapid Earth Observation Data Acquisition

Recently, satellite sensors are now orbiting not only frequently but can apply multiple sensor measurements, which means we can collect rapid data acquisition over given areas. Satellites such as the Sentinel-2 can better detect which SAR frequencies are emitted when there are multiple frequencies used.

This allows many benefits, including monitoring vegetation or landform change that happens at fast temporal scales, such as within hours or minutes. This can be useful to monitor events such as forest fires, deforestation, or even oil spills. In the case of oil spills, the densities of oil could be measured by SAR frequencies to differentiate it from water.

Because of SAR’s key benefits in operating in night-time and cloudy environments, SAR has become instrumental for rapid landscape change monitoring required for environmental monitoring. Operating licenses governed by the International Telecommunication Union (ITU) and Federal Communication Commission (FCC) are also better regulating and coordinating how radar data are used, that is when and where data are recorded, particularly as radar can interfere with vital radar communications such as at airports.

Geospatial 2.0 and SAR

Advancements in processing powers and rapid, automated data provisions means that we can have near real-time monitoring of important events on Earth. Increasingly, time differences between one image and another is measured by minutes and hours, rather than days.

Geospatial specialists such as Jensen see SAR as part of what people have called Geospatial 2.0, where spatial data are provided as a service rather than as unprocessed or lightly processed data, with big spatial data and big spatial data analytics at its core.

Analysts and scientists are increasingly using services that input what is desired, such as information about forest fires or given objects, and that data are provided directly to answer questions more rapidly. This saves time from having to download and process data.

With deep learning and segmentation techniques, you can also obtain specific information on objects of interest rather than get large views or areas of little interest. Satellites are also becoming smaller and have shorter orbital lives.

Satellites, in the near future, may also be easier to modify, becoming more like platforms for applications similar to phones today. This will allow new applications and services provided to satellites, enabling use we have yet to conjure even but which we can deploy almost as quickly as we can develop them. 

The rise of SAR is, therefore, in the background of rapidly changing ways in which we see how satellites operate. Additionally, computational power and automated analyses have enabled SAR data to require little processing and results can be provided more rapidly. With SAR’s advantages in night and cloud regions, this means SAR will increasingly be a critical component of our geospatial answers in Earth observation. 

References

^[1]    For more on ICEYE, see:  https://www.iceye.com/.

^[2]    More information on SAR and wavelength properties can be found here: https://earthdata.nasa.gov/learn/backgrounders/what-is-sar.

^[3]    Technologies and background to SAR can be found here:  Wang, W.-Q. (2013). Multi-antenna synthetic aperture radar (affiliate link). Boca Raton: CRC Press, Taylor & Francis Group.

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