Synthetic Aperture Radar: The hero tool for remote sensing

None can deny that satellite technology has now become an indispensable part of everyday life. From tracking vegetation coverage and change detection to disaster monitoring and post-restoration work after disaster events satellite imagery has indeed proved to be a revolutionary technological advancement with several tricks up its sleeve.

According to an article by US Geological Survey, remote sensing has been defined as the process of detecting and monitoring the physical characteristics of an area by measuring its reflected and emitted radiation at a distance (typically from satellite or aircraft). Special cameras collect remotely sensed images, which help power utilities “sense” things on the surface of the Earth.

Data and images collected from aircraft and satellites provide information that can help with a range of activities. For example – preserving the environment, monitoring climate change, managing the weather and managing agricultural activities, vegetation management for power utilities, better monitoring of incoming disasters and their management, etc. Because of multiple benefits and remote sensing, satellite technology has gained immense popularity in recent decades and rightly so.

The science behind SAR

Synthetic Aperture Radar (SAR) is unarguably the most important tool of remote sensing. It has been widely used for Earth’s remote sensing for over three decades. It is an imaging radar mounted on a moving platform. Unlike traditional optical satellites, SAR equipped satellites operate in a different manner and are superior to traditional satellites. Let’s understand how it works:

To understand SAR, it is pertinent to understand the concept of active microwave sensors. As per a Remote Sensing Tutorial article, active microwave sensors provide their own source of microwave radiation to illuminate the target. Active microwave sensors are categorized into two types: imaging and non-imaging. The most common form of imaging active microwave sensors is RADAR. RADAR is an acronym for RAdio Detection And Ranging, which essentially characterizes the function and operation of a radar sensor. [Image: Comparison of wavelength, frequency and energy for the electromagnetic spectrum. (Credit: NASA)]

Unlike traditional optical satellites, which take reflectivity from the sun, SAR equipped satellites shoot out their own energy source like a radio wave that reflects back from the earth’s surface and is received back by the satellite and recorded. To create a successful SAR image, successive waves are shot out to irradiate a target scene and the echo of each pulse is received and recorded.

According to an article by Alaska Satellite Facility, the instrument measures distances between the sensor and the point on the Earth’s surface where the signal is backscattered. This distance is called a slant range, which can then be projected on the ground representing the ground range. The flight direction is also referred to as a long-track or azimuth direction and the direction perpendicular to the flight path is the across-track or range direction.

The angle between the direction the antenna is pointing and the nadir is the look angle. The angle between the radar beam center and the normal to the local topography is the incidence angle. Both angles are sometimes used synonymously, which is only valid if the InSAR geometry is simplified neglecting earth’s curvature and the local topography. Because the look angle of the sensor significantly affects the behavior of backscatter, it is one of the main parameters determining the viewing geometry and the incidence angle of the backscattered signal. Depending on the characteristics of the illuminated terrain, areas of layover and shadow may occur in the imagery.  [Image: Imaging Radar Geometry (Credit: NASA)]

The wavelength of the sensor is also very important. It is responsible for determining the penetration depth of the transmitted signal into the vegetation layer of the terrain surface. The longer the wavelength, the deeper the penetration can be, particularly in forests. SAR satellites generally operate at designated frequencies with L- band, C – band and X- band being the predominant wavelengths. This SAR process improves the resolution in the “along-track” or “azimuth” direction, which corresponds to the direction of flight.

Thus, a high-resolution, day-and-night, weather-independent image is made. The image produced can be used for various applications like – predicting changes in weather, managing vegetation and disaster, pre and post-restoration work, etc.

Advantages of using Synthetic Aperture Radar

SAR’s most prominent advantage lies in the fact that unlike optical technology, synthetic aperture radar (SAR) can “see” through the darkness, clouds and rain, detecting changes in habitat, levels of water and moisture, effects of natural or human disturbance and changes in the Earth’s surface after events such as earthquakes or sinkhole openings. Typical optical satellites can see what we see. SAR is unique in its imaging capability. It provides very high-resolution images independent of every kind of weather condition.

Persistent surveillance is only possible when there are no interruptions. Radar has a distinct advantage of being able to collect imagery regardless of weather and during both day and night. The ability to revisit the same place repeatedly and get the same image quality regardless of weather, provides an advantage making SAR very well suited for better viewing of assets.

Not just this, according to a paper published by NASA, the wavelengths that remote sensing radars use to observe the earth’s surface are microwaves, typically in the range of a few to tens of centimeters. Because the radar signal loses energy as it travels – at a rate equivalent to the beam width (wavelength/antenna size) – by the time it hits the surface, the beam has spread dramatically. For example, with a signal wavelength of 10 centimeters and an antenna of 10 meters in diameter, the beam width is 1/100 radians (0.6 degrees). From an altitude of 1,000 kilometers, the resulting beam width on the ground becomes a very large 10 km, producing an image resolution which is insufficient for most applications. SAR is the solution to this dilemma as it can greatly improve the resolution.

SAR techniques take advantage of the fact that the radar is moving in orbit to synthesize a virtual 10-km-long antenna from the physical 10-m antenna in the direction of flight. As the radar moves along its path, it sweeps the antenna’s footprint across the ground while continuously transmitting pulses – short signal bursts separated by time – and receiving the echoes of the returned pulses.

Other advantages and applications of SAR are

• used to create very high-resolution 2D and 3D images or reconstruction of objects eg – a landscape
• land cover and land use mapping in areas that are always covered by clouds, such as rainforests
• with a time series of images over the same area, displacements of the surface can be detected for such applications as mining, oil and gas extraction, construction and excavation
• the ability to see land disturbance such as natural or manmade disasters as well as the detection of paths or the movement of troops by picking up subtle changes in vegetation that are revealed in the imagery
• the quick assessment of natural disasters thanks to the ability to rapidly revisit areas as well as the wide swath of observation
• unique properties for forest monitoring, flood monitoring and water quality applications
• independent of the sun
• can make polarimetric observations eg – HH, VV, HV, VH
• has coherency information

SAR is an extremely important earth observation tool that fills in many data gaps that exist with traditional image-based sensors. Since SAR equipped satellites are far superior in quality of their surveillance and can see through clouds, haze and the dark, making it possible to analyze changes on the ground, regardless of the weather conditions or time of day, utilities need to opt for SAR equipped satellite analytics for better disaster management. AiDash is a leading AI-first SaaS company enabling satellite-powered operations & maintenance for utility, energy and other core industries. Our revolutionary Climate Risk Intelligence System (CRIS) (formerly Disaster and Disruption Management System or DDMS) uses high-resolution multispectral and SAR satellite imagery powered with AI to monitor incoming disasters, survey vegetation hazards, help with post-disaster work, wildfire risks and weather-related damage remotely via a web dashboard and mobile app. Intrigued? To learn more about our CIMS model, feel free to drop us a mail at info@aidash.com.