TYPES OF EARTH OBSERVATION IMAGERY — Active Imaging
In the last few articles, we gave an introduction and a brief history of earth observation. When it comes to earth observation and remote sensing, there are two types of imaging techniques namely Active imagery and Passive imagery. In this article, let us briefly discuss about Active imagery.
Active imaging involves using an onboard transmitter to send out electromagnetic pulses from the satellite towards the surface of the earth. The reflected pulses are captured by a receptive sensor and the surface is visualized based on temporal resolution. Active imaging is independent of solar illumination on the surface.
There are a variety of ways Active imaging can be done. The most common active sensor in EO satellites is Synthetic Aperture Radar(SAR). SAR transmits electromagnetic pulses to the Earth’s surface where they get deflected by surface elements. SAR is also capable of detecting and recording return pulses with help of an antenna. The intensity of return pulse and time elapsed between plays an important role in generating SAR imagery.
The SAR interferometry technique can be used to accurately measure surface topography, ground deformation and subsidence, and glacier movements. It uses the phase of two or more complex radar images that get acquired from different positions. The phase of each SAR image pixel contains a range of information that is accurate to a small fraction of the radar wavelength. So, the determination of path length differences is accurately done.
In the across-track interferometry, the radar images get obtained from mutually displaced flight tracks with accurate precision in the measurement of surface topography.
The Digital Elevation Model (DEM) which could be programmatically created with the SAR base data helps in land classification and change detection, by subtracting the topographic information from the interferogram. This is done through SAR interferometric measurement with the changes of the range distance between the two acquisitions.
Light Detection and Ranging (Lidar) sensor uses the same principle as SAR but works in the IR, visible, or UV wavelengths. It helps in the precise measurement of topographic features, profiling clouds, measuring winds, etc.
The Atmospheric Lidar (ATLID) is another technique that provides the vertical profiles of aerosols and thin clouds. A laser light is beamed up and the scattered light is captured to analyse various air-borne particles. Though it is majorly ground-based, few satellite missions have done it using satellites.
Radar altimeters are another type of active sensor to measure the surface topographic profile along the satellite track. It helps to find reliable measurements of satellite height above the ocean. It works by measuring the time interval between transmission and reception of electromagnetic pulses thereby analyzing the earth’s surface underneath. Information from radar measurements helps in studying the lateral extent of sea eyes, the altitude of the iceberg above the sea level. It also provides information on sea surface winds and wave heights.
GNSS- reflectometry (GNSS-R) is comparatively a new category of satellite navigation for remote sensing. GNSS is an abbreviation for Global Navigation Satellite System. Different countries have different systems in place like GPS (USA), GLONASS (Russia), IRNSS (India), Galileo (European Union), Beidou (China), etc. These satellites are parked in the medium earth orbit or MEO at an altitude of about 20,200Km above the surface of the earth. They consistently transmit navigation signals towards the surface of the earth which are picked up by receivers (like GPS receivers) embedded in various objects to provide location accuracy.
The navigation signals that are reflected back from the earth’s surface are captured by a satellite in low earth orbit to measure various aspects of the surface. The receiver that is used for doing measurements can be either placed in land or in a satellite. It is possible to predict sea wind retrieval, the salinity of seawater, ice layer density, and the measurements of land humidity using the GNSS-R method.
Radar Scatterometry helps in scanning the earth’s surface from an aircraft or satellite by transmitting microwave energy pulses towards the earth’s surface. The reflected energy captured by a receiver antenna is compared with the transmitted signal to filter out the noise and the information about the respective surface is obtained. It is used to measure the speed and direction of the wind near the sea surface.
It also measures the backscatter from small waves in the sea. Radar Scatterometry data are the sources of information for Numerical Weather Prediction (NWP), Oceanography, and climate studies. It also provides information about the sea ice cover. Sea ice reflects more of the radar energy emitted by the sensor than the surrounding area. The image appears to be much brighter.
Figure: Composite radar scatterometer image of Antarctica, 19 July 2003, from the QuikSCAT satellite (Source: David Long, Brigham Young University Centre for Remote Sensing)
Source: (https://nsidc.org/cryosphere/seaice/study/active_remote_sensing.html)
That’s a brief about active imaging techniques. In the next article, we will discuss about passive imagery and various techniques.
