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Surface Remote Sensing Basics (in context of Electromagnetic Radiation)

Faiza Azam7-02-07

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Outline

• Definition• Elements of Remote sensing • Sensors and Plaforms• Electromagnetic Radiation Spectrum• Classification of Remote sensing on the basis of

Electromagnetic spectrum.• Some talk about Orbits.• Conclusion

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Definiton: The techniques for acquisition of information about the

features of earth (at different time scales), without any physical contact using devices that use electromagnetic energy, acoustic waves and force fields.

• Classified as Active or Passive.

Main Elements of remote sensing

1. A PLATFORM to hold the instrument.

2. A target OBJECT to be observed.

3. An instrument or SENSOR to observe the target .

4. Key element is INFORMATION (Data).

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1 - Energy Source or Illumination 2 - Radiation and the atmosphere 3 - Interaction with the target 4 - Recording of energy by sensor 5 - Transmission, Reception, and Processing 6 – Interpretation and analysis 7 – Application

REMOTE SENSING ELEMENTShttp://ccrs.nrcan.gc.ca/resource

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SensorsActive and Passive sensing:Active systems:-

• Remote sensing systems that provide their own energy source of illumination.

• Measure amplitude as well as the phase of the reflected signal.• Operated any time and cover wavelentghs not provided by sun.

http://ccrs.nrcan.gc.ca/resource

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SensorsActive and Passive sensing:Passive systems:-

• Remote sensing systems that measure naturally available energy • Measure incoherent energy.• For reflective energies, passive sensors can only be used when sun

is lluminating earth.

http://ccrs.nrcan.gc.ca/resource

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Platforms

Air borne platforms:• Fixed wings aircrafts• Used to collect detailed images (photographic).

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Satellite Platforms

• Remote sensing is conducted from space shuttle or orbiting satellites.

• Provide repetitive coverage of earth‘s surface on continuous basis.

Space Shuttle Satellite

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Energy Spectrum Wavelengths used in Remote Sensing

• Ultraviolet – 0.3 to 0.4 μm

• Visible – 0.4 to 0.7 μm

• Near Infrared – 0.7 to 1.3 μm

• Middle Infrared – 1.3 to 2.8 μm

• Thermal Infrared – 2.4 to14 μm

• Microwave – 1 mm to 1 m

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Electromagnetic Spectrum

Infomation about Sun and stars

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Radio waves

• Wavelength range: < 1 cm – 10, 100 m.

• Transmit radio and t.v. signals.

• Radar imaging: Waves are transmitted from a satellite or airplane antenna. The reflected waves (brightness) is used to form an image of the ground in complete darkness or through clouds because as they can pass through clouds and dry desert sand.

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Microwaves

• Wavelengths range: approx. 1 mm (the thickness of a pencil lead) - 30 cm (one foot).

• Emitted from the Earth, from objects such as cars and planes, and from the atmosphere.

• Two types; Active (scatterometers, alimeters)or Passive(Radiometers)

• Can be detected to give information, such as the temperature of the object that emitted the microwaves.

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Infrared• Wavelengths range: approx. 0.7 to 300 µm

NIR (0.7 - 1.5 µm) & SWIR (1.5 - 3 µm) reflected IR, solar rad.

refleced from earth‘s surface. MWIR (3 - 8 µm) & LWIR (8 - 15 µm) thermal infrared (heat).

• Can be detected to give information, such as reflectance

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Visible

• Wavelengths range: 400 - 700 nm • Coincides with the wavelength of greatest intensity of sunlight

• great utility for the remote sensing of vegetation and for the identification of different objects by their visible colors.

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Ultravoilet• Wavelengths range: 0.3 µm - 3.0 nm.

• Emitted from the sun, most of it is blocked by ozone in the upper atmosphere.

• Used extensively in astronomical observatories. Also concerned with the measurement of ozone.

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Classification of Remote Sensing with respect to Radiation

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Classification

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Orbits

Geostationary orbits Satellites (geostationary), at altitudes of approximately 36,000

kilometres, revolve at speeds which match the rotation of the Earth (so seem stationary).

Usage:

• Collect information continuously over specific areas. (Weather and communications satellites)

• Due to high altitude, some satellites can monitor weather and cloud patterns covering an entire hemisphere of the Earth.

http://ccrs.nrcan.gc.ca/resource

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Orbits

Near Polar Orbits: Platforms follow an orbit (north-south) which, in conjunction with the

Earth's rotation (west-east), Cover most of the Earth's surface. Most such orbits are also sun-synchronous.

Usage:

Being sun synchronous, ensures consistent illumination conditions when acquiring images in a specific season over successive years, or over a particular area over a series of days. So important for monitoring changes

http://ccrs.nrcan.gc.ca/resource

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OrbitsNear Polar Orbits

In near polar orbits, satellite travels northern pole to the southern pole. The paths at poles are called ascending and descending passes, respectively.

For sun-synchronous orbit:

Ascending pass shadowed side of the Earth

Descending pass sunlit side of the Earth

http://ccrs.nrcan.gc.ca/resource

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Conclusion

• What is remote sensing in general.• Main elements of remote sensing• Sensors, platforms• Electromanetic spectrum (used for remote sensing)• Classified remote sensing in terms of

wavelengths.• Orbits

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Future of Remote Sensing