Remote Sensing Technology, Remote Sensing Satellites and its Applications

Introduction

Remote Sensing is a technique used to gather information about an object without actually coming into contact with it. The characteristics of an object can be determined by using reflected or emitted electro-magnetic radiation from the object. Actually each object has a unique and different characteristic of reflection or emission. Thus, remote sensing is a technology to identify and understand the object through the reflection or emission. Remote sensing systems are commonly used to survey, map and monitor the resources and environment of earth. They can also be used to explore other planets.

The technical term ‘remote sensing’ was first used in the United States in the 1960s. Since Landsat-1, the first earth observation satellite was launched in 1972, remote sensing has become widely used.

The electro-magnetic radiation which is reflected or emitted from an object is the usual source of remote sensing data. However, any media such as gravity or magnetic field can be utilized in remote sensing. A device to detect the electromagnetic radiation reflected or emitted from an object is called a remote sensor or sensor. Cameras or scanners are examples of remote sensors.

Types of Remote Sensing Instruments

Passive Instruments

They detect natural energy that is reflected or emitted from the observed scene. Passive instruments sense only radiation emitted by the object being viewed or reflected by the object from a source other than the instrument. Reflected sunlight is the most common external source of radiation sensed by passive instruments. Scientists use a variety of passive remote sensors.

Various Passive Remote Sensing Instruments

  • Radiometer: An instrument that quantitatively measures the intensity of electromagnetic radiation in some band of wavelengths in the spectrum. Usually a radiometer is further identified by the portion of the spectrum it covers; for example, visible, infrared, or microwave.
  • Imaging Radiometer: It includes a scanning capability to provide a two-dimensional array of pixels from which an image can be produced is called an imaging radiometer. Scanning can be performed mechanically or electronically by using an array of detectors.
  • Spectrometer: A device designed to detect, measure and analyze the spectral content of the incident electromagnetic radiation is called a spectrometer. Conventional, imaging spectrometers use gratings or prisms to disperse the radiation for spectral discrimination.
  • Spectroradiometer: A radiometer that can measure the intensity of radiation in multiple wavelength bands (i.e. multispectral). Often the bands are of a high spectral resolution- designed for the remote sensing of specific parameters such as sea surface temperature, cloud characteristics, ocean colour, vegetation, trace chemical species in the atmosphere, etc.

Active Remote Sensing Instruments

They provide their own energy (electromagnetic radiation) to illuminate the object or scene they observe. They send a pulse of energy from the sensor to the object and then receive the radiation that is reflected from that object. Scientists use several types of active remote sensors:

  • RADAR (Radio Detection and Ranging): A radar uses a transmitter operating at either radio or microwave frequencies to emit electromagnetic radiation and a directional antenna or receiver to measure the time of arrival of reflected radiation from distant objects. Distance to the object can be determined since electromagnetic radiation propagates at the speed of light.
  • Scatterometer: A scatterometer is a high frequency microwave radar designed specifically to measure reflected radiation. Over ocean surfaces, measurements of reflected radiation in the microwave spectral region can be used to derive maps of surface wind speed and direction.
  • LIDAR (Light Detection and Ranging): A Lidar uses a laser (light amplification by stimulated emission of radiation) to transmit a light pulse and a receiver with sensitive detectors to measure the reflected light. Distance to the object is determined by recording the time between the transmitted and reflected pulses and using the speed of light to calculate the distance travelled. Lidars can determine atmospheric profiles of aerosols, clouds, and other constituents of the atmosphere.
  • Laser Altimeter: A laser altimeter uses a lidar to measure the height of the instrument platform above the surface. By independently knowing the height of the platform with respect to the mean Earth’s surface, the topography of the underlying surface can be determined.

Types of Remote Sensing

Satellite Remote Sensing

The remote sensing satellites are equipped with sensors looking down to the earth. They are “the eyes in the sky” constantly observing the earth as they go round in orbits. In satellite remote sensing of the earth, the sensors are looking through a layer of atmosphere separating the sensors from the Earth’s surface being observed. So, the analysis of the effects of atmosphere on the electromagnetic radiation travelling from the earth to the sensor through the atmosphere provides vital inputs. The atmospheric constituents cause wavelength dependent absorption and scattering of radiation. These effects lead to the deterioration of quality of images.

An important consequence of atmospheric absorption is that certain wavelength bands in the electromagnetic spectrum are strongly absorbed and effectively blocked by the atmosphere. The wavelength regions in the electromagnetic spectrum usable for remote sensing are determined by their ability to penetrate atmosphere. These regions are known as the atmospheric transmission windows.

Remote sensing systems are often designed to operate within one or more of the atmospheric windows. These windows exist in the microwave region, some wavelength bands in the infrared, the entire visible region and part of the near ultraviolet regions. Although the atmosphere is practically transparent to x-rays and gamma rays, these radiations are not normally used in remote sensing of the earth.

Optical and Infrared Remote Sensing

In optical remote sensing, optical sensors detect solar radiation reflected or scattered from the earth, resembling photographs taken by a camera high up in space. The wavelength region usually extends from the visible and near-infrared to the short-wave infrared.

There are also infrared sensors measuring the thermal infrared radiation emitted from the earth, from which the land or the sea surface temperature can be derived.

Microwave Remote Sensing

There are some remote sensing satellites which carry passive or active microwave sensors. The active sensors emit pulses of microwave radiation to illuminate the areas to be imaged. The images of the earth surface are formed by measuring the microwave energy scattered by the ground or sea back to the sensors. These satellites carry their own flashlight emitting microwaves to illuminate their targets. So, the images can be acquired day and night.

Microwaves have an additional advantage as they can also penetrate clouds. Images can be acquired even when there are clouds covering the earth surface. A microwave imaging system which can produce high resolution image of the earth is the synthetic aperture radar (SAR). The intensity in a SAR image depends on the amount of microwave reflected by the target and received by the SAR antenna.

History of the Indian Remote Sensing Satellite System

IRS-1A, the first of the series of indigenous state-of-art operating remote sensing satellites, was successfully launched into a polar sun-synchronous orbit on March 17, 1988 from the Soviet Cosmodrome at Baikonur. The successful launch of IRS-1A was one of the remarkable achievements of the ISRO. It depicted the maturity of satellite to address the various requirements for managing natural resources of the nation. The operational availability of data products to the user organisations further strengthened the operationalisation of remote sensing applications and management in the country.

IRS-1A was followed by the launch of IRS-1B, an identical satellite, in 1991. These two satellites in the IRS series have been the workhorses for generating natural resources information in a variety of application areas, such as agriculture, forestry, geology and hydrology etc.

From then onwards, series of IRS spacecrafts were launched with enhanced capabilities in payloads and satellite platforms. The whole gamut of the activities from the evolution of IRS missions by identifying the user requirements to utilisation of data from these missions by user agencies is monitored by National Natural Resources Management System (NNRMS).  The NNRMS is the nodal agency for natural resources management and infrastructure development using remote sensing data in the country.

There have been various thematic IRS missions premised on applications like natural resources monitoring, ocean and atmospheric studies and cartographic applications. This resulted in the realisation of theme based satellite series:

  • Land/water resources applications (RESOURCESAT series and RISAT series);
  • Ocean/atmospheric studies (OCEANSAT series, INSAT-VHRR, INSAT-3D, Megha-Tropiques and SARAL); and
  • Large scale mapping applications (CARTOSAT series).

IRS-1A development was a major milestone in the IRS programme. The journey of Indian remote sensing programme has been full of remarkable achievements. India has become a role-model for the rest to follow in the case of development of remote-sensing applications.  India has made remarkable strides in building and launching the state-of-the-art Indian remote sensing satellite as well as in operational utilisation of the data in various applications.

Today, the array of Indian Earth Observation (EO) Satellites with imaging capabilities in visible, infrared, thermal and microwave regions of the electromagnetic spectrum have helped the country in realising major operational applications.

Applications of Remote Sensing Satellites

  • With the help of active and passive sensors of remote sensing satellites land-use pattern can be mapped. It also helps in determining soil moisture and can estimate crop production.
  • It helps in observing the flow of ocean currents and circulation.
  • It can also assist in studying glacier melts and effects on sea levels.
  • It helps in mapping the degradation and loss of wetland ecosystems.
  • It can also help in proper operationalisation of precision farming.
  • It can help in tracking of displaced refugees to help deliver aid and services using satellite imagery. The United Nations High Commissioner of Refugees (UNHCR) can get a lot of help by getting data on refugees. The remote sensing satellites can help in mapping the influx of refugees.
  • It can help in monitoring active volcanoes using thermal remote sensing.
  • It can make an inventory of potential landslides.
  • It helps in mapping ocean floors for further scientific research.
  • Remote sensing satellites are quite helpful in comparing climatic factors from past to present. Example: NASA is mapping different climate factors on a monthly basis to see how much these variables change. Climate variables like carbon monoxide, chlorophyll and aerosol are being mapped as a function of time with the help of remote sensing satellites. Thus, remote sensing satellites are going to make imperishable contribution in further climate change research. The data can be utilized for framing suitable adaptation and mitigation strategies for dealing with climate change.
  • Studying geology of earth’s surface: Some of the remote sensing applications in geology include bedrock mapping, lithological mapping and structural mapping. The  multispectral reflectance has provided valuable information on rock composition while radar has also been useful in studying surface roughness.
  • It helps in measuring albedo for Earth’s radiation budget. (Albedo measures the percent of reflected sunlight).
  • Improving efficiency and safety of air traffic control: Air traffic control directs aircrafts from the ground to prevent collision and improve the flow of traffic. Unfortunately, there are excessive costs involved and huge emissions are also ejected due to the prevalence of so many inefficient routes. The next generation of air traffic moves from ground-based radar to a satellite-based GPS system. The new air traffic control system aims at improving routes, reducing traffic delays and saving money. It also intends to assist planes land faster and help navigate through weather with the use of satellites.
  • Remote sensing satellites can save time and manpower by effectively and quickly locating missing aircraft in case of plane accidents.
  • The remote sensing satellites can be used for military surveillance. They can track down undeclared nuclear installations run some countries by 3-D mapping of uranium enrichment site.
  • They can help in effectively measuring sea-level rise.
  • They can help in making inventory and evaluation of glaciers, glacial lakes, and glacial lake outburst floods (GLOFs) etc.

Conclusion

The science of acquiring information about the earth using instruments which are remote to the earth’s surface, usually from aircraft or satellites is called remote sensing. The instruments may be visible light, infrared or radar to obtain data. Remote sensing offers the ability to observe and collect data for large areas relatively quickly. It is an important source of data for Geographic Information System (GIS).

Remote sensing from space has been used for science, national security and commercial purposes. Remote sensing data are used for applications such as mapping land use, managing forested land, estimating crop production, monitoring grazing conditions, assessing water quality, and protecting wildlife. Remote sensing also helps scientists understand how human activity affects the environment. For example: sensors monitor the health of forests threatened by pollution and can map the destruction of tropical rainforests.

Scientists use infrared images to determine the condition of vegetation, survey temperature changes in bodies of water, locate damage in underground pipelines, and map certain geographic features above and below the ground. The images from satellites are used in the search for mineral and petroleum deposits. In this way, we see that remote sensing technology and remote sensing satellites have multiple applications which help mankind.

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