Wednesday, December 31, 2008

Remote Sensing-Electromagnetic Spectrum: Distribution of Radiant Energies

Electromagnetic radiation (EMR) extends over a wide range of energies and wavelengths (frequencies). A narrow range of EMR extending from 0.4 to 0.7 µm, the interval detected by the human eye, is known as the visible region (also referred to as light but physicists often use that term to include radiation beyond the visible). White light contains a mix of all wavelengths in the visible region. It was Sir Isaac Newton who first in 1666 carried out an experiment that showed visible light to be a continuous sequence of wavelengths that represented the different color the eye can see. He passed white light through a glass prism and got this result:
Use of a prism to disperse visible light into its spectral colors.

The principle supporting this result is that as radiation passes from one medium to another, it is bent according to a number called the index of refraction. This index is dependent on wavelength, so that the angle of bending varies systematically from red (longer wavelength; lower frequency) to blue (shorter wavelength; higher frequency). The process of separating the constituent colors in white light is known as dispersion. These phenomena also apply to radiation of wavelengths outside the visible (e.g., a crystal's atomic lattice serves as a diffraction device that bends x-rays in different directions).

The distribution of the continuum of all radiant energies can be plotted either as a function of wavelength or of frequency in a chart known as the electromagnetic (EM) spectrum. Using spectroscopes and other radiation detection instruments, over the years scientists have arbitrarily divided the EM spectrum into regions or intervals and applied descriptive names to them.The EM spectrum, plotted here in terms of wavelengths, is shown here.

The EM Spectrum, with specific wavelength intervals assigned Type terms.

Beneath is a composite illustration taken from the Landsat Tutorial Workbook (credited there to Lintz and Simonett, Remote Sensing of the Environment, who identify it as a modification of an earlier diagram by Robt. Colwell) that shows in its upper diagram the named spectral regions in terms of wavelength and frequency and in the lower diagram the physical phenomena that give rise to these radiation types and the instruments (sensors) used to detect the radiation. (Although the width of this second diagram scales closely to the width of the spectrum chart above it, the writer experienced difficulty in centering this second diagram on the present page; it needs some leftward offset so that the narrow pair of vertical lines coincides with the visible range in the upper diagram.)

Wavelength and Frequency representations of the Electromagnetic Spectrum.
Mechanisms (Phenomenology) of generation of EM radiation within wavelength intervals; instruments commonly used in detection of radiation within different intervals.

Although it is somewhat redundant, we reproduce here still another plot of the EM Spectrum, with added items that are self-explanatory:

The EM Spectrum, in a diagram produced by Electro Optical Industries, Inc.

Colors in visible light are familiar to most, but the wavelength limits for each major color are probably not known to most readers. Here is a diagram that specifies these limits (the purple on the far left is in the non-visible ultraviolet; the deep red on the far right is the beginning of the infrared). The human eye is said to be able to distinguish thousands of slightly different colors (one estimate placed this at distinguishable 20000 color tints).

The visible spectrum, with specified (somewhat arbitrary) wavelength boundaries for each color shown.

Different names for (wave)length units within intervals (those specified by types) that subdivide the EM spectrum, and based on the metric system, have been adopted by physicists as shown in this table:

Metric units commonly associated with specific Types of EM Radiation.

(Both in this Tutorial and in other texts, just which units are chosen can be somewhat arbitrary, i.e., the authors may elect to use micrometers or nanometers for a spectral location in the visible. Thus, as an example, 5000 Angstroms, 500 nanometers, and 0.5 micrometers all refer to the same specific wavelength; see next paragraph.)

At the very energetic (high frequency and short wavelength) end are gamma rays and x-rays (whose wavelengths are normally measured in angstroms [Å], which in the metric scale are in units of 10-8 cm). Radiation in the ultraviolet extends from about 300 Å to about 4000 Å. It is convenient to measure the mid-regions of the spectrum in one of two units: micrometers (µm), which are multiples of 10-6 m or nanometers (nm), based on 10-9 m. The visible region occupies the range between 0.4 and 0.7 µm, or its equivalents of 4000 to 7000 Å or 400 to 700 NM The infrared region, spanning between 0.7 and 1000 µm (or 1 mm), has four subintervals of special interest: (1) reflected IR (0.7 - 3.0 µm), and (2) its film responsive subset, the photographic IR (0.7 - 0.9 µm); (3) and (4) thermal bands at (3 - 5 µm) and (8 - 14 µm). We measure longer wavelength intervals in units ranging from mm to cm. to meters. The microwave region spreads across 0.1 to 100 cm, which includes all of the interval used by radar systems. These systems generate their own active radiation and direct it towards targets of interest. The lowest frequency-longest wavelength region beyond 100 cm is the realm of radio bands, from VHF (very high frequency) to ELF (extremely low frequency); units applied to this region is often stated as frequencies in units of Hertz (1 Hz = 1 cycle per second; KHz, MHz and GHz are kilo-, mega-, and giga- Hertz respectively). Within any region, a collection of continuous wavelengths can be partioned into discrete intervals called bands.


Monday, December 29, 2008

Opinion Poll: Terror & Geospatial


Remote Sensing-Electromagnetic Spectrum:Transmittance, Absorptance, and Reflectance

Any beam of photons from some source passing through medium 1 (usually air) that impinges upon an object or target (medium 2) will experience one or more reactions that are summarized in this diagram:

Some objects are capable of transmitting the light through their bulk without significant diminution (note how the beam bends twice at the medium 1/medium 2 interface but emerges at the same angle as entry). Other materials cause the light energy to be absorbed (and in part emitted as longer wavelength radiation). Or, the light can be reflected at the same angle as it formed on approach. Commonly the nature of the object's surface (owing to microscopic roughness) causes it to be scattered in all directions.

The primary source of energy that illuminates natural targets is the Sun. Solar irradiation (also called insolation) arrives at Earth at wavelengths which are determined by the photospheric temperature of the sun (peaking near 5600 °C). The main wavelength interval is between 200 and 3400 nm (0.2 and 3.4 µm), with the maximum power input close to 480 nm (0.48 µm), which is in the visible green region. As solar rays arrive at the Earth, the atmosphere absorbs or backscatters a fraction of them and transmits the remainder.

Upon striking the land and ocean surface (and objects thereon), and atmospheric targets, such as air, moisture, and clouds, the incoming radiation (irradiance) partitions into three modes of energy-interaction response: 

(1) Transmittance (τ) - some fraction (up to 100%) of the radiation penetrates into certain surface materials such as water and if the material is transparent and thin in one dimension, normally passes through, generally with some diminution. 

(2) Absorptance (α) - some radiation is absorbed through electron or molecular reactions within the medium ; a portion of this energy is then re-emitted, usually at longer wavelengths, and some of it remains and heats the target; 

(3) Reflectance (ρ) - some radiation (commonly 100%) reflects (moves away from the target) at specific angles and/or scatters away from the target at various angles, depending on the surface roughness and the angle of incidence of the rays. 

Because they involve ratios (to irradiance), these three parameters are dimensionless numbers (between 0 and 1), but are commonly expressed as percentages. Following the Law of Conservation of Energy: τ + α + ρ = 1. 

A fourth situation, when the emitted radiation results from internal atomic/molecular excitation, usually related to the heat state of a body, is a thermal process. The theory underlying thermal remote sensing is treated in Section 9.

When a remote sensing instrument has a line-of-sight with an object that is reflecting solar energy, then the instrument collects that reflected energy and records the observation. Most remote sensing systems are designed to collect reflected radiation


Friday, December 19, 2008

Haryana, India to use remote sensing for monitoring drought

Chandigarh, India: Haryana government is planning to use remote sensing satellites for monitoring drought situation in the state, an official spokesman said here.

He said a Memorandum of Understanding (MoU) had been signed between Hisar based Haryana Space Application Centre (HARSAC), an autonomous body of Science and Technology, Haryana and National Remote Sensing Centre (NRSC) Hyderabad to monitor the drought situation at block level.

Advance Wide Image Field Sensor data from resource satellite of India would be used for the purpose, he added. After analysing and interpreting the satellite data acquired at frequent intervals, a monthly bulletin, indicating the situation in each block, would be prepared.

The bulletin would be circulated to all departments so that required contingency measures to save the crops in the susceptible blocks could be taken, he said. He said the project, a milestone in drought management, would be taken up jointly by HARSAC and NRSC for three years after which the technology would be transferred to HARSAC. 


Earth’s Ionosphere drops to a new low

click here to read 

Monday, December 15, 2008

CRRI’s 'Hawk Eye' on Indian roads

At first sight, one can mistake this vehicle to be an ultra tech robotic remote-controlled vehicle, but it is the country's first 'Hawk Eye' that is surveying almost 50,000 kilometres of country's highways and roads to gauge their quality and see if they are fit for freight movement and travel. The database will be used to create GIS maps of highways in the country. The project is being undertaken by Central Road Research Institute (CRRI) and GIS survey and global positioning system (GPS) will help better navigation across the country. 

Mounted on a jeep, the Hawk Eye's instrumentation system includes a laser profiler along with pavement view cameras to measure road surface, collect and process digital images of pavements and other roadside elements. The vehicle has been used in countries like China, Malaysia, Singapore, South Korea and Thailand. Hawk Eye requires high-speed paved roads for operation and is sensitive to rough weather conditions like dust storms and heavy rains. It can gather data while travelling at a speed of 30 to 100 km per hour. 

An advantage Hawk Eye enjoys over other instruments with CRRI is its ability to look at the surface of the road and measure cracking areas. Any area with a crack of above three centimetres is detected by the vehicle's sensors and a preventive measure is suggested by the computers. This helps improve life of highways, which is normally around 20 years in the case of bitumen roads.

Source :

Friday, December 12, 2008

Fishy project to save Indian lake

 Thousands of fish have been released into a picturesque lake in India's northern Nainital in an attempt to restore the areas lost grandeur.

Nainital is a hill resort 2,000 meters above the sea level in Kumaon hills. It's known as the "Switzerland of India" because of its picturesque lakes.

play video


Tuesday, December 9, 2008

ISRO to deploy smaller launchers for LEO satellites

The Indian Space Research Organisation (ISRO) is building a smaller launcher designed to put remote-sensing satellites, weighing less than 500kg, into low earth orbits (LEO). Such launchers will cost 40 per cent less than existing rockets. 

An orbit 400-500km above the earth is designated as a low-earth orbit. Satellites launched into LEOs circle the earth at shorter intervals than their heavier geo-synchronous cousins and so can return to cover a specific point on the planet at shorter intervals. 

The newly designed three stage launcher will cater to the country's military as well as international customers.

"This (launcher) is for strategic reasons. There is also demand from international customers," an ISRO official is quoted as saying. He also said the new launcher would take around six months to build. According to the official, ISRO was developing a variant of its Polar Satellite Launch Vehicle (PSLV). 

The PSLV, Isro's workhorse, can launch satellites of 1.3 tonnes into polar orbits. Since 2007 stripped-down versions of the rocket have carried lighter Israeli and Italian satellites into LEOs. "We had to first send it to polar orbit, burn the rocket for long, before we placed TecSAR in the low (earth) orbit," said another official at the space agency. "That (detour) consumed 60% of the energy of the rocket."

TecSAR is a 300kg Israeli spy satellite which ISRO placed in LEO in January this year. Earlier it had launched a 352kg Italian astronomical satellite Agile in April 2007.

It currently costs Rs100 crore to launch a satellite on a PSLV rocket.

The Indian Air Force's first dedicated satellite intended to gather navigational information will be launched in July next year. The satellite, according to IAF chief ACM Fali H Major, would serve as the force's eye in the skies. 

Source :

Sunday, December 7, 2008

China delays finishing mammoth water project

China has postponed completing a huge water transfer project to quench its national capital's thirst, citing stubborn pollution worries for pushing the target date back four years to 2014, official media said on Saturday.

The South-North Water Diversion scheme will channel water from the Yangtze River and its tributaries to ease shortages across northern China, where population growth and frantic industrialization have drained dams and underground reserves.

The main "central route" stretching 1,267 kms (787 miles) from the Danjiangkou Dam in central Hubei province to Beijing was due to be finished in 2010.

But not now.

Hubei officials said on Friday that pollution and ecological strains in the rivers feeding the dam will make that impossible, Hubei's Changjiang Times said, in a report reprinted by the official Xinhua news agency ( ).

Thursday, December 4, 2008

A global view of HIV infection

llustration: GfK GeoMarketing”

To mark World AIDS Day on December 1, GfK GeoMarketing is provided a map that illustrates the global distribution of HIV cases (data source: WHO/GfK GeoMarketing; map: GfK GeoMarketing).

According to the latest statistics from the World Health Organization (WHO), the highest rates of infection occur in southern Africa and the Russian Federation.