SATELLITE COMMUNICATION

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Remote Sensing Satellites

Module-5

4.1 Introduction

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• Remote sensing is a technology used for obtaining information about the characteristics of an object through the analysis of data acquired from it at a distance.
• Satellites play an important role in remote sensing. Some of the important and better known applications of satellites in the area of remote sensing include providing information about the features of the Earth’s surface, such as coverage, mapping, classification of land cover features such as vegetation, soil, water, forests, etc.

• Various topics related to remote sensing satellites will be covered, including their principle of operation, payloads on board these satellites and their use to acquire images, processing and analysis of these images using various digital imaging techniques and finally interpreting these images for studying different features of Earth for varied applications.

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Introduction

Remote Sensing – An Overview

• Remote sensing is defined as the science of identifying, measuring and analysing the characteristics of objects of interest without actually being in contact with them.
• It is done by sensing and recording the energy reflected or emitted by these objects and then processing and interpreting that information.
• Remote sensing makes use of the fact that every object has a unique characteristic reflection and emission spectra that can be utilized to identify that object.
• The gravitational and magnetic fields are also employed for remote sensing applications.

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• One of the potential advantages that this technology offers is that through it various observations can be made, measurements taken and images of phenomena produced that are beyond the limits of normal perception.
• Remote sensing is widely used by biologists, geologists, geographers, agriculturists, foresters and engineers to generate information on objects on Earth’s land surface, oceans and atmosphere.
• Applications include monitoring natural and agricultural resources, assessing crop inventory and yield, locating forest fires and assessing the damage caused, mapping and monitoring of vegetation, air and water quality, etc.

Remote Sensing – An Overview

Classification of Satellite Remote Sensing Systems

• The principles covered in this section apply equally well to ground-based and aerial platforms but here they will be described in conjunction with satellites.
• Remote sensing systems can be classified on the basis of

(a) the source of radiation and

(b) the spectral regions used for data acquisition.

• The satellite remote sensing is the science of acquiring information about the Earth’s surface by sensing and recording the energy reflected or emitted by the Earth’s surface with the help of sensors on board the satellite.
• Based on the source of radiation, they can be classified as:

1. Passive remote sensing systems

2. Active remote sensing systems

1. Passive remote sensing systems

• Passive remote sensing systems either detect the solar radiation reflected by the objects on the surface of the Earth or detect the thermal or microwave radiation emitted by them.

2. Active remote sensing systems

• Active remote sensing systems make use of active artificial sources of radiation generally mounted on the remote sensing platform.
• These sources illuminate the objects on the ground and the energy reflected or scattered by these objects is utilized here.

• Examples of active remote sensing systems include microwave and laser-based systems.
• Depending on the spectral regions used for data acquisition, they can be classified as:

1. Optical remote sensing systems (including visible, near IR and shortwave IR systems)

2. Thermal infrared remote sensing systems

3. Microwave remote sensing systems

Optical Remote Sensing Systems

• Optical remote sensing systems mostly make use of visible (0.3–0.7m), near IR (0.72–1.30 m) and shortwave IR (1.3–3.0 m) wavelength bands to form images of the Earth’s surface.
• The images are formed by detecting the solar radiation reflected by objects on the ground and resemble the photographs taken by a camera.
• Some laser-based optical remote sensing systems are also being employed in which the laser beam is emitted from the active sources mounted on the remote sensing platform.

• The target properties are analysed by studying the reflectance and scattering characteristics of the objects to the laser radiation.
• Optical remote sensing systems employing solar energy come under the category of passive remote sensing systems and the laser-based remote sensing systems belong to the category of active remote sensing systems.
• Passive optical remote sensing systems work only during the day as they rely on sensing reflected sunlight.
• This phenomenon makes them weather dependent because during cloudy days the sunlight is not able to reach Earth.

Optical Remote Sensing Systems

Thermal Infrared Remote Sensing Systems

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• Thermal infrared remote sensing systems employ the mid wave IR (3–5m) and the long wave IR (8–14m) wavelength bands.
• The imagery here is derived from the thermal radiation emitted by the Earth’s surface and objects.
• As different portions of the Earth’s surface are at different temperatures, thermal images therefore provide information on the temperature of the ground and water surfaces and the objects on them.
• As the thermal infrared remote sensing systems detect the thermal radiation emitted from the Earth’s surface, they come under the category of passive remote sensing systems.

• The 10 m band is commonly employed for thermal remote sensing applications as most of the objects on the surface of the Earth have temperatures around 300K and the spectral radiance for a temperature of 300K peaks at a wavelength of 10 m.
• Another commonly used thermal band is 3.8 m for detecting forest fires and other hot objects having temperatures between 500K and 1000 K.

Thermal Infrared Remote Sensing Systems

Microwave Remote Sensing Systems

• Microwave remote sensing systems generally operate in the 1 cm to 1m wavelength band.
• Microwave radiation can penetrate through clouds, haze and dust, making microwave remote sensing a weather independent technique.
• This feature makes microwave remote sensing systems quite attractive as compared to optical and thermal systems, which are weather dependent.
• Microwave remote sensing systems work both during the day as well as at night as they are independent of the solar illumination conditions.

• Another advantage that a microwave remote sensing system offers is that it provides unique information on sea wind and wave direction that cannot be provided by visible and infrared remote sensing systems.
• The need for sophisticated data analysis and poorer resolution due to the use of longer wavelength bands are the disadvantages of microwave remote sensing systems.

Microwave Remote Sensing Systems

Remote Sensing Satellite Orbits

• Remote sensing satellites have sun-synchronous subrecurrent orbits at altitudes of 700–900 km, allowing them to observe the same area periodically with a periodicity of two to three weeks.
• They cover a particular area on the surface of the Earth at the same local time, thus observing it under the same illumination conditions.
• This is an important factor for monitoring changes in the images taken at different dates or for combining the images together, as they need not be corrected for different illumination conditions.

• Generally the atmosphere is clear in the mornings, hence to take clear pictures while achieving sufficient solar illumination conditions, remote sensing satellites make observations of a particular place during morning (around 10 a.m. local time) when sufficient sunlight is available.
• As an example, the SPOT satellite has a sun-synchronous orbit with an altitude of 820 km and an inclination of 98.7◦.
• The satellite crosses the equator at 10:30 a.m. local solar time.

Remote Sensing Satellite Orbits

• Satellite orbits are matched to the capabilities and objectives of the sensor(s) they carry.
• Remote sensing satellites generally provide information either at the regional level or at the local area level.
• Regional level remote sensing satellite systems have a resolution of 10m to 100m and are used for cartography and terrestrial resources surveying applications, whereas local area level remote sensing satellite systems offer higher resolution and are used for precision agricultural applications like monitoring the type, health, moisture status and maturity of crops, etc., for coastal management applications like monitoring photo planktons, pollution level determination, bathymetry changes, etc.

Remote Sensing Satellite Orbits

• As the satellite revolves around the Earth, the sensors onboard it see a certain portion of the Earth’s surface.
• The area imaged on the surface is referred to as the swath. The swath width for space-borne sensors generally varies between tens of kilometers to hundreds of kilometres.
• The satellite’s orbit and the rotation of Earth work together to allow the satellite to have complete coverage of the Earth’s surface.

Remote Sensing Satellite Orbits

Remote Sensing Satellite Payloads

10.4.1 Classification of Sensors

• The main payloads on board a remote sensing satellite system are sensors that measure the electromagnetic radiation emanating or reflected from a geometrically defined field on the surface of the Earth. Sensor systems on board a remote sensing satellite can be broadly classified as:

1. Passive sensors

2. Active sensors

• A passive system generally consists of an array of sensors or detectors that record the amount of electromagnetic radiation reflected and/or emitted from the Earth’s surface.
• An active system, on the other hand, emits electromagnetic radiation and measures the intensity of the return signal.
• Both passive and active sensors can be further classified as:

1. Scanning sensors

2. Non-scanning sensors

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Remote Sensing Satellite Payloads

Various types of sensors on board remote sensing satellites

• Scanning sensors have a narrow field of view and they scan a small area at any particular time.
• These sensors sweep over the terrain to build up and produce a two-dimensional image of the surface.
• Hence they take measurements in the instantaneous field-of-view (IFOV) as they move across the scan lines.
• The succession of scan lines is obtained due to the motion of the satellite along its orbit.
• It may be mentioned here that the surfaces are scanned sequentially due to the combination of the satellite movement as well as that of the scanner itself (Figure 10.6).
• The scanning sensors can be classified as image plane scanning sensors and object plane scanning sensors depending upon where the rays are converged by the lens in the optical

system.

Various types of sensors on board remote sensing satellites

• A non-scanning sensor views the entire field in one go. While the sensor’s overall field-of-view corresponds to the continuous movement of the instantaneous field-of-view in the case of scanning sensors, for non-scanning sensors the overall field-of-view coincides with its instantaneous field-of-view.
• Figure 10.7 shows the conceptual diagram of a non-scanning

satellite remote sensing system.

Various types of sensors on board remote sensing satellites

Non-scanning satellite remote sensing system

Types of Images

• The data acquired by satellite sensors is processed, digitized and then transmitted to ground stations to construct an image of the Earth’s surface.
• Depending upon the kind of processing used, the satellite images can be classified into two types, namely primary and secondary images.
• Secondary images can be further subcategorized into various types.

Primary Images

• The raw images taken from the satellite are referred to as primary images. These raw images are seldom utilized directly for remote sensing applications but are corrected, processed and restored in order to remove geometric distortion, blurring and degradation by other factors and to extract useful information from them.

10.7.2 Secondary Images

• The primary images are processed so as to enhance their features for better and precise interpretation.
• These processed images are referred to as secondary images.
• Secondary images are further classified as monogenic images and polygenic images, depending upon whether one or more primary images have been used to produce the secondary image.

Image Classification

• The processed satellite images are classified using various techniques in order to categorize the pixels in the digital image into one of the several land cover classes.
• The categorized data is then used to produce thematic maps of the land cover present in the image.
• Multispectral data are used to perform the classification and the spectral pattern present within the data for each pixel is used as the numerical basis for categorization.
• The objective of image classification is to identify and portray, as a unique grey level (or colour), the features occurring in an image in terms of the object or type of land cover they actually represent on the ground.

There are two main classification methods, namely:

1. Supervised Classification

2. Unsupervised Classification

• With supervised classification, the land cover types of interest (referred to as training sites or information classes) in the image are identified.
• The image processing software system is then used to develop a statistical characterization of the reflectance for each information class.
• Once a statistical characterization has been achieved for each information class, the image is then classified by examining the reflectance for each pixel and making a decision about which of the signatures it resembles the most.

Image Classification

Image Interpretation

• Extraction of useful information from the images is referred to as image interpretation.
• Interpretation of optical and thermal images is more or less similar.
• Interpretation of microwave images is quite different.

Interpreting Optical and Thermal Remote Sensing Images

These images mainly provide four types of information:

1. Radiometric information

2. Spectral information

3. Textural information

4. Geometric and contextual information

• Radiometric information corresponds to the brightness, intensity and tone of the images.
• Panchromatic optical images are generally interpreted to provide radiometric information.
• Multispectral or colour composite images are the main sources of spectral information.
• The interpretation of these images requires understanding of the spectral reflectance signatures of the objects of interest.
• Different bands of multispectral images may be combined to accentuate a particular object of interest.
• Textural information, provided by high resolution imagery, is an important aid in visual image interpretation.

1. Radiometric information

• The texture of the image may be used to classify various kinds of vegetation covers or forest covers.
• Although all of them appear to be green in colour, yet they will have different textures.
• Geometric and contextual information is provided by very high resolution images and makes the interpretation of the image quite straightforward.
• Extraction of this information, however, requires prior information about the area (like the shape, size, pattern, etc.) in the image.

1. Radiometric information

Interpreting Microwave Remote Sensing Images

• Interpretation of microwave images is quite different from that of optical and thermal images.
• Images from active microwave remote sensing systems images suffer from a lot of noise, referred to as speckle noise, and may require special filtering before they can be used for interpretation and analysis.
• Single microwave images are usually displayed as grey scale images where the intensity of each pixel represents the proportion of the microwave radiation backscattered from that area on the ground in the case of active microwave systems and the microwave radiation emitted from that area in the case of passive microwave systems.
• The pixel intensity values are often converted to a physical quantity called the backscattering coefficient, measured in decibel (dB) units, with values ranging from +5 dB for very bright objects to −40 dB for very dark surfaces.

• The higher the value of the backscattering coefficient, the rougher is the surface being imaged.
• Flat surfaces such as paved roads, runways or calm water normally appear as dark areas in a radar image since most of the incident radar pulses are specularly reflected away.
• Trees and other vegetations are usually moderately rough on the wavelength scale.
• Hence, they appear as moderately bright features in the image.
• Ships on the sea, high rise buildings and regular metallic objects such as cargo containers, built-up areas and many man-made features, etc., appear as very bright objects in the image.

Interpreting Microwave Remote Sensing Images

• Multitemporal microwave images are used for detecting land cover changes over the period of image acquisition.
• The areas where no change in land cover occurs will appear in grey while areas with land cover changes will appear as colourful patches in the image.

• The brightness of areas covered by bare soil may vary from very dark to very bright depending on its roughness and moisture content.
• Typically, rough soil appears bright in the image.
• For similar soil roughness, the surface with a higher moisture content will appear brighter.

Interpreting Microwave Remote Sensing Images

GIS in Remote Sensing

• The geographic information system (GIS) is a computer-based information system used to digitally represent and analyse the geographic features present on the Earth’s surface.
• The GIS is used to integrate the remote sensing data with the geographic data, as it will help to give a better understanding and interpretation of remote sensing images.
• It also assists in the automated interpretation, detecting the changes occurring in an area and in map revision processes.

• For example, it is not enough to detect land cover change in an area, as the final goal is to analyse the cause of the change or to evaluate the impact of the change.
• Hence, the remote sensing data should be overlaid on maps such as those of transportation facilities and land use zoning in order to extract this information.
• In addition, the classification of remote sensing imagery will become more accurate if the auxiliary data contained in the maps are combined with the image data.

GIS in Remote Sensing

The GIS performs the following three main functions:

1. To store and manage geographic information comprehensively and effectively.

2. To display geographic information depending on the purpose of use.

3. To execute query, analysis and evaluation of geographic information effectively.

Applications of Remote Sensing Satellites

• Data from remote sensing satellites is used to provide timely and detailed information about the Earth’s surface, especially in relation to the management of renewable and non-renewable resources.
• Some of the major application areas for which satellite remote sensing is of great use is assessment and monitoring of vegetation types and their status, soil surveys, mineral exploration, map making and revision, production of thematic maps, planning and monitoring of water resources, urban planning, agricultural property management planning, crop yield assessment, natural disaster assessment, etc.

Land Cover Classification

• Land cover mapping and classification corresponds to identifying the physical condition of the Earth’s surface and then dividing the surface area into various classes, like forest, grassland, snow, water bodies, etc., depending upon its physical condition.
• Land cover classification helps in the identification of the location of natural resources

(a) Digital satellite image showing the land cover map of Onslow Bay in North

Carolina taken by Landsat’s thematic mapper (TM) in February 1996 and (b) the land cover classification map derived from the satellite image in Figure 10.18(a)

Land Cover Change Detection

• Land cover change refers to the seasonal or permanent changes in the land cover types.
• Seasonal changes may be due to agricultural changes or the changes in forest cover and the permanent changes may be due to land use changes like deforestation or new built towns, etc.
• Detection of permanent land cover changes is necessary for updating land cover maps and for management of natural resources.
• Satellites detect these permanent land cover changes by comparing an old image and an updated image, with both these images taken during the same season to eliminate the effects of seasonal change.

Water Quality Monitoring and Management

• Satellite imagery helps in locating, monitoring and managing water resources over large areas.
• Water resources are mapped in the optical and the microwave bands.
• Water pollution can be determined by observing the colour of water bodies in the images obtained from the satellite.
• Clearwater is bluish in colour, water with vegetation appears to be greenish-yellow while turbid water appears to be reddish-brown.
• Structural geographical interpretation of the imagery also aids in determining the underground resources.
• The changing state of many of the world’s water bodies is monitored accurately over long periods of time using satellite imagery.

• Scientists measured the water quality by observing the ratio of blue to red light in the satellite data.
• Water quality was found to be high when the amount of blue light reflected off the lakes was high and that of red light was low.
• Lakes loaded with algae and sediments, on the other hand, reflect less blue light and more red light.
• Using images like this, scientists created a comprehensive water quality map for the water bodies in the region.

Water Quality Monitoring and Management

Flood Monitoring

• Satellite images provide a cost effective and potentially rapid means to monitor and map the devastating effects of floods.
• Figures show the false colour composite images of the Pareechu River in Tibet behind a natural dam forming an artificial lake, taken by the advanced space-borne thermal emission and reflection radiometer (ASTER) on NASA’s Terra satellite on 1 September 2004 and 15 July 2004 respectively.
• From the two images it is evident that the water levels were visibly larger on 1 September 2004 than they were on 15 July 2004.
• The lake posed a threat to communities downstream in northern India, which would have been flooded if the dam had burst.

Flood monitoring using remote sensing satellites (Courtesy: NASA). These images are grey scale versions of original colour images

Urban Monitoring and Development

• Satellite images are an important tool for monitoring as well as planning urban development activities.
• Time difference images can be used to monitor changes due to various forms of natural disasters, military conflict or urban city development.
• These images have a resolution of 1m and were taken by the IKONOS satellite.
• Remote sensing data along with the GIS is used for preparing precise digital basemaps of the area, for formulating proposals and for acting as a monitoring tool during the development phase.
• They are also used for updating these basemaps from time to time.

Images of Manhattan (a) before and (b) after the 11 September 2001 attacks, taken by the IKONOSsatellite (Satellite imagery courtesy of GeoEye). These images are grey scale versions of original colour images.

Measurement of Sea Surface Temperature

• The surface temperature of the sea is an important index for ocean observation, as it provides significant information regarding the behaviour of water, including ocean currents, formation of fisheries, inflow and diffusion of water from rivers and factories.
• Satellites provide very accurate information on the sea surface temperatures.
• Temperature measurement by remote sensing satellites is based on the principle that all objects emit electromagnetic radiation of different wavelengths corresponding to their temperature and emissivity.
• The sea surface temperature measurements are done in the thermal infrared bands.

Sea surface temperature map derived from the thermal IR image taken by the GMS-5 satellite (Reproduced by permission of © Japan Meteorological Agency). The image is the grey scale version of the original colour image.

Deforestation

• Remote sensing satellites help in detecting, identifying and quantifying the forest cover areas.
• This data is used by scientists to observe and assess the decline in forest cover over a period of several years.
• The images in Figure 10.24 show a portion of the state of Rondˆonia, Brazil, in which tropical deforestation has occurred.
• Figures 10.24 (a) and (b) are the images taken by the multispectral scanners of the Landsat-2 and -5 satellites in the years 1975 and 1986 respectively.
• Figure 10.24 (c) shows the image taken by the thematic mapper of the Landsat-4 satellite in the year 1992. It is evident from the images that the forest cover has reduced drastically.

Images taken by the multispectral scanners of (a) Landsat-2 satellite in the year 1975

and (b) Landsat-5 satellite in the year 1986. (c) Image taken by the thematic mapper of the Landsat-4 satellite in the year 1992 (Data available from US Geological Survey). These images are grey scale versions of original colour images.

Global Monitoring

• Remote sensing satellites can be used for global monitoring of various factors like vegetation, ozone layer distribution, gravitational fields, glacial ice movement and so on.
• Figure 10.25 shows the vegetation distribution map of the world formed by processing and calibrating 400 images from the NOAA remote sensing satellite’s AVHRR sensor.
• This image provides an unbiased means to analyse and monitor the effects of droughts and long term changes from possible regional and global climate changes.

• Figure 10.26 shows the global ozone distribution taken by the global ozone monitoring experiment (GOME) sensor on the ERS-2 satellite.
• Measurement of ozone distribution can be put to various applications, like informing people of the fact that depletion of the ozone layer poses serious health risks and taking measures to prevent depletion of the ozone layer.
• The figure shows that the ozone levels are decreasing with time.
• Satellites also help us to measure the variation in the gravitational field precisely, which in turn helps to give a better understanding of the geological structure of the sea floor.
• Gravitational measurements are made using active microwave sensors.

Global Monitoring

Vegetation distribution map of the world (Reproduced by permission of © NOAA/NPA). The image is the grey scale version of the original colour image.

Global ozone distribution (Reproduced by permission of © DLR/NPA). The image is the grey scale version of the original colour image

Predicting Disasters

• Remote sensing satellites give an early warning of the various natural disasters like earthquakes, volcanic eruptions, hurricanes, storms, etc., thus enabling the evasive measures to be taken in time and preventing loss of life and property.
• Geomatics, a conglomerate of measuring, mapping, geodesy, satellite positioning, photogrammetry, computer systems and computer graphics, remote sensing, geographic information systems (GIS) and environmental visualization is a modern technology, that plays a vital role in mitigation of natural disasters.

Weather Satellites

• Use of satellites for weather forecasting and prediction of related phenomena has become indispensable.
• Information from weather satellites are used for short term weather forecasts as well as for reliable prediction of the movements of tropical cyclones, allowing re-routing of ships and a preventive action in zones through which hurricanes pass.
• Meteorological information is also of considerable importance for conducting of military operations such as reconnaissance missions.

• Due to the inherent advantages of monitoring from space, coupled with developments in the sensor technology, satellites have brought about a revolution in the field of weather forecasting.
• The end result is that there is a reliable forecast of weather and other related activities on a routine basis.

Weather Satellites

Weather Forecasting – An Overview

• Weather forecasting, as people call it, is both a science as well as an art. It is about predicting the weather, which can be both long term as well as short term.
• The short term predictions are based on current observations whereas long term predictions are made after understanding the weather patterns, on the basis of observations made over a period of several years.

• Weather watching began as early as the 17th century, when scientists used barometers to measure pressure.
• Weather forecasting as a science matured in the early 1900s when meteorological kites carrying instruments to measure the temperature, pressure and the relative humidity were flown.
• After that came the era of meteorological aircraft and balloons carrying instruments for weather forecasting.

Weather Forecasting – An Overview

Weather Forecasting Satellite Fundamentals

• Weather forecasting satellites are referred to as the third eye of meteorologists, as the images provided by these satellites are one of the most useful sources of data for them.
• Satellites measure the conditions of the atmosphere using onboard instruments.
• The data is then transmitted to the collecting centres where it is processed and analysed for varied applications.
• Weather satellites offer some potential advantages over the conventional methods as they can cover the whole world, whereas the conventional weather networks cover only about 20% of the globe.

• Satellites are essential in predicting the weather of any place irrespective of its location.
• They are indispensable in forecasting the weather of inaccessible regions of the world, like oceans, where other forms of conventional data are sparse.
• A forecasters can predict an impending weather phenomenon using satellites 24 to 48 hours in advance.
• These forecasts are accurate in more than 90 cases out of 100.

Weather Forecasting Satellite Fundamentals

Images from Weather Forecasting Satellites

• Weather forecasting satellites take images mainly in the visible, the IR and the microwave bands.
• Each of these bands provides information about different features of the atmosphere, clouds and weather patterns.
• The information revealed by the images in these bands, when combined together, helps in better understanding of the weather phenomena.
• The images in the visible band are formed by measuring the solar radiation reflected by Earth and the clouds.

• The IR and the microwave radiation emitted by the clouds and Earth are used for taking IR and microwave images respectively.
• Some images are formed by measuring the scattering properties of the clouds and Earth when microwave or laser radiation is incident on them.
• This is referred to as active probing of the atmosphere.

Images from Weather Forecasting Satellites

Visible Images

• Satellites measure the reflected or scattered sunlight in the wavelength region of 0.28 to 3.0 m.
• The most commonly used band here is the visible band (0.4 to 0.9 m).
• Visible images represent the amount of sunlight being reflected back into space by clouds or the Earth’s surface in the visible band.
• These images are mainly used in the identification of clouds. Mostly, weather satellites detect the amount of radiation without breaking it down to individual colours.
• So these images are effectively black and white.
• The intensity of the image depends on the reflectivity of the underlying surface or clouds

IR Images

• Another common type of satellite imagery depicts the radiation emitted by the clouds and the Earth’s surface in the IR band (10 to 12 m).
• IR images provide information on the temperature of the underlying Earth’s surface or cloud cover.
• This information is used in providing temperature forecasts, in locating areas of frost and freezes and in determining the distribution of sea surface temperatures offshore.

• Since the temperature normally decreases with height, IR radiation with the lowest intensity is emitted from clouds farthest from the Earth’s surface.
• The Earth’s surface emits IR radiation with the highest intensity.
• In IR images clouds appear dark as compared to Earth.
• High lying clouds are darker than low lying clouds.
• High clouds indicate a strong convective storm activity and hence IR images can be used to predict storms.

IR Images

Water Vapour Images

• The visible and the IR images discussed so far are passed unobstructed through the Earth’s atmosphere.
• These images tell little about the atmosphere as for these wavelengths the atmosphere is transparent.
• Satellite images are also constructed using IR wavelengths that are absorbed by one or more gases in the atmosphere, like water vapour, carbon dioxide, etc.
• Radiation around the wavelength band of 6.5 m is absorbed as well as emitted by water vapour.

• The water vapour channel on weather forecasting satellites works around this wavelength.
• It detects water vapour in the air, primarily from a height of 10 000 feet to 40 000 feet up from the Earth’s surface.
• The level of brightness of the image taken in this band indicates the amount of moisture present in the atmosphere.
• The radiation emitted from the bottom of the water vapour layer is absorbed by the water vapour present above it.

Water Vapour Images

Microwave Images

• Weather satellites also utilize the microwave band, mostly within the wavelength region from 0.1 to 10 cm.
• They use both passive as well as active techniques for making measurements in the microwave band.
• Passive techniques measure the amount of microwave radiation emitted from the Earth’s surface and clouds.
• Active microwave probing involves the use of sensors that emit microwave radiation towards the Earth and then record the scattered or the reflected radiation from the clouds, the atmosphere and the Earth’s surface.

Weather Forecasting Satellite Orbits

• Weather forecasting satellites are placed into either of the two types of orbits, namely the polar sun-synchronous low Earth orbit and the geostationary orbit.
• Polar sun-synchronous weather forecasting satellites revolve around the Earth in near polar lowEarth orbits, visiting a particular place at a fixed time so as to observe that place under similar sunlight conditions.
• These orbits are similar to those discussed earlier in the case of remote sensing satellites.

• Polar weather forecasting satellites, due to their low altitudes, have better spatial resolution as compared to the geostationary satellites.
• Hence they help in a detailed observation of the weather features like the cloud formation, wind direction, etc.
• However, these satellites have a poorer temporal resolution, visiting a particular location only one to four times a day.
• Hence, only a few weather satellite systems have satellites in these orbits.

Weather Forecasting Satellite Orbits

Weather Forecasting Satellite Payloads

• Weather forecasting satellites carry instruments that scan Earth to form images.
• These instruments usually have a small telescope or an antenna, a scanning mechanism, a detector assembly that detects the incoming radiation and a signal processing unit that converts the output of the detectors into the required digital format.
• The processed output is then transmitted to receiving stations on the ground.
• The most commonly used instrument on a weather forecasting satellite is the radiometer.

Weather Forecasting Satellite Applications

• Satellites play a major role in weather forecasting. All the daily weather forecast bulletins which we hear every day are broadcasted on the basis of data sent by weather forecasting satellites.
• Satellites have helped in predicting the paths of tropical cyclones far more reliably than any other weather forecasting tool.
• They also help in predicting the frost, rainfall, drought and fog and so on that is of immense help to farmers.
• Various combinations of satellite images are used to identify clouds and determine their approximate height and thickness.

• The cloud and water vapour patterns are used to identify cyclones, frontal systems, outflow boundaries, upper level troughs and jet streams.
• As a matter of fact, not even a single tropical cyclone has gone unnoticed since the use of satellites for weather forecasting.
• Moreover, these satellites provide early frost warnings, which can save millions of dollars a day for citrus growers.
• They also play an important role in forest management and fire control

Weather Forecasting Satellite Applications

Measurement of Cloud Parameters

• Satellite imagery enables meteorologists to observe clouds at all levels of the atmosphere, both over land and the oceans. Generally, both visible and IR images are used together for the identification of clouds.
• Visible images give information on thickness, texture, shape and pattern of the clouds.
• Information on cloud height is extracted using IR images.
• False colour IR images are used for a detailed analysis of clouds.
• Information from visible and IR images can be combined to identify the types of clouds and the weather patterns associated with them.
• This helps in the prediction of rainfall, thunderstorms and hurricanes. Moreover, information on the movement of clouds is a valuable input in predicting the wind speed and direction.

Rainfall

• Imagery from space is also used to estimate rainfall during thunderstorms and hurricanes.
• This information forms the basis of flood warnings issued by meteorologists.
• Satellite images of the clouds are processed and analysed to predict the location and amount of rainfall.
• It is possible to determine the cloud thickness and height using visible and IR images respectively. Both these images are combined to predict the amount of rainfall, as it depends both on the thickness and height of clouds.
• Thick and high clouds result in more rain.

• The clouds in their early stage of development produce more rain. Therefore, regular observations from GEO satellites, which can track their development, are used for rainfall prediction.
• Measurements in the microwave band help in determining the intensity of rain as scattering depends on the number of droplets in a unit volume and their size distribution.
• As an example, during Hurricane Diana, using images from a GOES satellite, it was calculated that there would be nearly 20 inches of rainfall over North Carolina in a two-day period. The actual recorded rainfall was 18 inches.

Rainfall

Wind Speed and Direction

• Determination of wind speed and direction is essential to provide an accurate picture of the current state of the atmosphere.
• Wind information can be determined by tracking cloud displacements in successive IR and visible images taken from geostationary weather forecasting satellites. However, these measurements can only be taken when the cloud cover is present.
• To overcome this, successive water vapour channel images are used to track the movement of wind fields. However, both of these methods are not accurate.
• A more accurate method is to make simultaneous measurements of both the temperature profile as well as the position of the cloud tops.
• The VISSR atmospheric sounder (VAS) instrument on the GOES satellite is used to perform such measurements.

Ground-level Temperature Measurements

Satellite data cannot produce detailed information about the temperature profile of the lowest

few hundred metres of the atmosphere, but it can provide some physically important observations.

Infrared radiometers can make widespread observations of maximum and minimum

temperatures. High resolution IR satellite imagery is used to produce heat maps of Earth.

However, where standard ground-based measurements are available, satellite measurements

are generally not used. They are used at those places where ground-based measurements are not feasible.

However, in some conditions satellite measurements of the ground-level temperature

are more accurate than the ground-based measurements. For example, when it is exceptionally

cold and the radiative contribution of the atmosphere is minimal, satellite observations can

provide considerably more information than ground-based measurements.

Air Pollution and Haze

• Air pollution and haze are recognizable in visible imagery by their grey appearance.
• Satellite images have shown that the pollution level is lowin the morning and increases as the day passes by.
• Satellite data is also used to infer the effect of air pollution on weather.
• Using satellite data, it has been found that haze bands may act as boundaries along which thunderstorm activities can develop.
• Satellite measurements have indicated that the increase in air pollution leads to an increase in the amount of rainfall.

Fog

• Fog is detected using visible satellite imagery. Fog appears as a flat textured object with sharp edges in these images.
• The level of brightness of the image is a measure of the thickness of the fog.
• Satellite images also provide information on the clearance of fog during the day.

Oceanography

• Weather forecasting satellites are a useful tool for oceanography applications.
• Satellite images are used to map locations of different ocean currents and to measure ocean surface temperatures accurately.
• Polar orbiting satellites compute around 20 000 to 40 000 global ocean temperature measurements daily.
• This information on the ocean surface temperature is utilized by meteorologists to observe ocean circulation, to locate major ocean currents and to monitor its effect on climate and weather changes.

• The observation of these temperatures before and after the occurrence of hurricanes helps to show the way in which these hurricanes pick up energy from oceans.
• This helps to predict their behaviour and to improve forecasts of their motion.
• Satellite observations have shown that hurricanes result in cooling of the ocean surface.
• The stronger the hurricane, the more cooling effect it has on the temperature of the ocean surface.

Oceanography

Severe Storm Support

• One of the most important applications of weather forecasting satellites is in the prediction of hurricanes, tropical storms, cyclones and so on.
• Satellites are crucial to detecting and tracking intense storms through their various stages of development.
• This allows meteorologists to issue advanced warnings before the storms actually hit.
• These advanced warnings have saved lives of millions of people.

Fisheries

• Commercial fishery operations have also benefited from data supplied by weather satellites.
• Information on ocean currents and sea temperatures help in finding the location of tuna or salmon fishes.
• It also assists in tracking the movement of fish eggs and larvae. Satellite data can be used to study hypoxia, a condition of severe lack of oxygen at deep sea levels that can completely block the growth and development of sea life.

Snow and Ice Studies

• Weather satellites are used to observe snow cover on land surfaces and to monitor ice on lakes, rivers and other water bodies.
• These data help meteorologists to estimate the climate of the place and to plan irrigation and flood control methodologies.
• Snow cover estimates are especially helpful in mountain regions where a large part of the water supply comes from melting of snow. It is also used to issue winter storm warnings.
• Satellite ice monitoring provides useful information to the shipping industry.
• Information on the progression of freezing seasonal temperatures allows farmers to take timely measures to protect their crops.

Navigation Satellites

• Navigation is the art of determining the position of a platform or an object at any specified time.
• Satellite-based navigation systems represent a breakthrough in this field that has revolutionized the very concept and application potential of navigation.
• These systems have grown from a relatively humble beginning as a support technology to that of a critical player used in a vast array of economic, scientific, civilian and military applications.
• Two main satellite-based navigation systems in operation today are the Global Positioning System (GPS) of the USA and the Global Navigation Satellite System (GLONASS) of Russia

Development of Satellite Navigation Systems

• Various navigation methods have been used over the ages including marking of trails using stones and twigs, making maps, making use of celestial bodies (sun, moon and stars), monumental landmarks and using instruments like the magnetic compass, sextant, etc.
• These traditional methods were superceded by ground-based radio navigation techniques in the early 20th century.
• These ground-based systems were widely used during World War II.
• These systems, however, could only provide accurate positioning services in small coverage areas.

• Accuracy reduced with an increase in the coverage area. Satellite-based navigation systems were developed to provide accurate as well as global navigation services simultaneously.
• These systems emerged on the scene in the early 1960s.
• They provided an accurate universal reference System that extends everywhere over land as well as sea and in near space regardless of weather conditions.
• These systems were originally developed for military operations, but their use for civilian applications soon became commonplace

Development of Satellite Navigation Systems

Doppler Effect based Satellite Navigation Systems

• The first satellite navigation system was the Transit system developed by the US Navy and John Hopkins University of the USA back in the early 1960s.
• The first satellite in the system, Transit I, was launched on 13 April 1960.
• It was also the first satellite to be launched for navigation applications. The system was available for military use in the year 1964 and to civilians three years later in 1967.
• The system employed six satellites (three active satellites and three in-orbit spares) in circular polar LEOs at altitudes of approximately 1000 km.

• The last Transit satellite was launched in the year 1988.
• The main limitations of the system were that it provided only two-dimensional services and was available to users for only brief time periods due to low satellite altitudes.
• The high speed receivers were not able to use the system. The system was terminated in the year 1996.
• The Transit system was followed by the Nova navigation system, which was an improved system having better accuracy.

Doppler Effect based Satellite Navigation Systems

Trilateration-based Satellite Navigation Systems

• Doppler-based navigation systems have given way to systems based on the principle of ‘trileration’, as they offer global coverage and have better accuracy as compared to the Doppler-based systems.
• In this case, the user receiver’s position is determined by calculating its distance from three (or four) satellites whose orbital and the timing parameters are known.
• The receiver is at the intersection of the invisible spheres, with the radius of each sphere equal to the distance between a particular satellite and the receiver, with the centre being the position of that satellite

Global Positioning System (GPS)

• The GPS comprises of three segments, namely the space segment, control segment and user segment.
• All the three segments work in an integrated manner to ensure proper functioning of the system.

12.2.1 Space Segment

• The space segment comprises of a 28 satellite constellation out of which 24 satellites are active satellites and the remaining four satellites are used as in-orbit spares.
• The satellites are placed in six orbital planes, with four satellites in each plane.
• The satellites orbit in circular medium Earth orbits (MEO) at an altitude of 20 200 km, inclined at 55◦ to the equator (Figure 12.10).
• The orbital period of each satellite is around 12 hours (11 hours, 58 mins).
• The MEO orbit was chosen as a compromise between the LEO and GEO orbits.

• If the satellites are placed in LEO orbits, then a large number of satellites would be needed to obtain adequate coverage.
• Placing them in GEO orbits would reduce the required number of satellites, but will not provide good polar coverage.
• The present constellation makes it possible for four to ten satellites to be visible to all receivers anywhere in the world and hence ensure worldwide coverage.

12.2.1 Space Segment

Space segment of GPS

12.2.2 Control Segment

• The control segment of theGPSsystem comprises aworldwide network of five monitor stations, four ground antenna stations and a master control station.
• The monitor stations are located at Hawaii and Kwajalein in the Pacific Ocean, Diego Garcia in the Indian Ocean, Ascension Island in the Pacific Ocean and Colorado Springs, Colorado.
• There is a master control station (MCS) at Schriever Air Force Base in Colorado that controls the overall GPS network.
• The ground antenna stations are located at Diego Garcia in the Indian Ocean, Kwajalein in the Pacific Ocean, Ascension Island in the Pacific Ocean and at Cape Canaveral, USA.

Control segment of GPS

12.2.3 User Segment

• The user segment includes all military and civil GPS receivers intended to provide position, velocity and time information.
• These receivers are either hand-held receivers or installed on aircraft, ships, tanks, submarines, cars and trucks.
• The basic function of these receivers is to detect, decode and process the GPS satellite signals. Some of the receivers have maps of the area stored in their memory.
• This makes the whole GPS system more user-friendly as it helps the receiver to navigate its way out.
• Most receivers trace the path of the user as they move.

• Certain advanced receivers also tell the user the distance they have travelled, their speed and time of travel.
• They also tell the estimated time of arrival at the current speed when fed with destination coordinates.
• There is no limit to the number of users using the system simultaneously.
• Today many companies make GPS receivers, including Garmin, Trimble, Eagle, Lorance and Magellan.

12.2.3 User Segment

Applications of Satellite Navigation Systems

• Satellite navigation represents one of the dual use space technologies that has found extensive applications both in military and civilian fields.
• These systems have been in use over the past two and a half decades and have replaced the conventional navigation methods in most cases.
• Some of the main military application areas include weapon guidance, navigation, tracking, etc.
• Civilian applications include construction and surveying, seismic surveying, airborne mapping, vehicle navigation, automotive, marine, military and aviation surveying.
• They are also used in endeavours like aerial refuelling, rendezvous operations, geodetic surveying and various search and rescue operations.

12.8.1 Military Applications

• Satellite navigation systems have proved to be a valuable aid for military forces.
• Military forces around the world use these systems for diverse applications including navigation, targeting, rescue, disaster relief, guidance and facility management, both during wartime as well as peacetime.
• GPS and GLONASS receivers are used by soldiers and also have been incorporated on aircraft, ground vehicles, ships and spacecraft

12.8.2 Civilian Applications

• Initially developed for military applications, satellite navigation systems soon became commonplace for civilian applications as well.
• In fact, civil applications outnumber military uses in terms of range of applications, number of users and total market value.
• Satellite navigation systems are finding newer and newer commercial applications due to decreasing cost, size and introduction of new features.
• Civilian applications include marine and aviation navigation, precision timekeeping, surveying, fleet management, mapping, construction & surveying, aircraft approach assistance, geographic information system (GIS), vehicle tracking, natural resource and wildlife management, disaster management and precision agriculture etc.

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