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Basics of Remote Sensing( Coming Soon)

Sensors

Sensors
A device.
Measure a physical quantity and covert it into a signal which can be read by an observer by an instrument.
For example:
 Mercury-in-glass thermometer-converts the measured temperature into expansion.

Types:
Active
Energy leading to radiation received comes from an external source, e.g., the Sun
Example: Mobile Satellite Service (MSS)
Passive
Energy generated from within the sensor system is beamed outward, and the fraction returned is measured
Example: radar

Types:
Imaging
Measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance.

Non-imaging
The electrons released and captured by the detector.
The electrons are used to excite or ionize a substance like silver (Ag) in film or to drive an image producing device like a TV or computer monitor or a cathode ray tube or oscilloscope or a battery of electronic detectors.

Sensor examples

Sensors for Remote Sensing

If the scene is sensed point by point (equivalent to small areas within the scene) along successive lines over a finite time, this mode of measurement makes up a scanning system.
And if the entire scene is sensed directly with the sensor then its terms as nonscanning system.

Sensor Operation Principle:
The remote sensing apply the EM radiation for most of its source.
That the bulk of the radiation sensed is either reflected or emitted from the target.
The radiation travel through the air and detected by sensors.

Sensor Classes

 

Sensor Classes:
Polarimeter
An instrument used in polarimetry which uses two Nicol prisms, one fixed (the polarizer) and one rotatable (the analyzer), with the sample between them, to measure optical activity and other aspects of polarization.
Mechanism of polarimeter:
The plane of polarization of the sodium light from the polarizer is rotated as it passes through a solution of optically active substances such as sucrose.
The extent of the rotation is determined by rotating the analyzer until no light reaches the observer.
The slits in the analyzer are then at right angles to the final plane of polarization

Polarimeter:

 

What is a Scatterometer?

A scatterometer is a microwave radar sensor used to measure the reflection or scattering effect produced while scanning the surface of the earth from an aircraft or a satellite

Sensor classes:
Scatterometer
Satellite remote sensor.
Active microwave sensors
Determine the wind direction over water.
Send out a signal and measure how much of that signal returns after interacting with the target.
The fraction of energy returned to the satellite (backscatter) is a function of wind speed and wind
direction.

Radiometer
Instrument that quantitatively measures the EM radiation in some interval of the EM spectrum.
Spectrometer
Instrument used to measure properties of light over a specific portion of the electromagnetic spectrum.
Spectro-radiometer
Sensors that collect the dispersed radiation in bands rather than discrete wavelengths.

Active sensor:
Sensor that able to direct energy at an object in the form of electromagnetic radiation (EMR).
Object is scanned and the sensors detect any radiation reflected back from the object.
Types of active remote sensing:
Active Optical Remote Sensing
Active Thermal Remote Sensing
Active Microwave Remote Sensing

Active Optical Remote Sensing
Active optical remote sensing involves using a laser beam upon a remote target to illuminate it, analyzing the reflected or backscattered radiation in order to acquire certain properties about the target.
The velocity, location, temperature and material composition of a distant target can be determined using this method.
Example:
LIDAR( Light Detection and Ranging)

LIDAR( Light Detection and Ranging)
 The instrument works by using a transmitter and a receiver.
 The laser generates pulses which excite the specified target, causing it to absorb radiation at certain wavelengths.
 The target then reflects radiation in the form of photons which are detected by the LIDAR’s sensors and converted
to an electrical signal.

 

Thermal remote sensing deals with information acquired primarily in the thermal infrared range.
The majority of the thermal remote sensing is done using passive sensors

Active Microwave Remote Sensing
Active microwave remote sensing uses sensors that operate in the microwave region of the electromagnetic spectrum.
Example:
RADAR (Radio detection and ranging)

RADAR
The sensor transmits a microwave (radio) signal upon a specified target.
The reflected or backscattered radiation from the target is then detected by the active sensors which measure
the round trip time delay to targets allowing the system to calculate the distance of the target from the sensors.

Passive Sensor:
Passive sensors detect electromagnetic radiation emitted from an object.
Record incoming radiation that has been scattered, absorbed and transmitted from the Earth in transit from its original source, the Sun.

Gamma-ray spectrometer
Passive sensor that detects gamma rays.
The sources for the radiation is are generally upper-soil layers as well as rock layers.
Caused by radioactive decay.
 Used to explore mineral deposits

Aerial cameras
Used in aerial photography.
Aircraft serve as a platform as well as many low-earth orbiting satellites deploy many aerial cameras.
Used for topographic mapping.

Thermal infrared video cameras
Equipped to detect radiation in the near-infrared range.
Sometimes combined with active sensors, such as radar, to provide additional information.
Aircraft as well as satellites can serve as platforms.

Multispectral scanner
Records information in the visible and infrared spectrum.
Scans the Earth’s surface for various wavelength bands.
Satellites act as platforms for such passive sensors.
Used for geological purposes.

Imaging Spectrometer
Similar to the multispectral scanner.
Scans very narrow wavelength bands of the spectrum.
Satellites are used as platforms.
Used for determining the mineral composition of the Earth’s surface and concentrations of suspended matter in surface water.

As a satellite revolves around the Earth, the sensor “sees” a certain portion of
the Earth’s surface. The area imaged on the surface, is referred to as the
swath.

 

Orbit will be elliptical or near circular
• Time taken by a satellite to complete one revolution in its orbit around the earth is called the Orbital period.
• Angle of inclination of orbital plane with respect to equator is measured clockwise (typically 99o for RS Satellite).

Classification of satellites :
1.Based on the purpose
Communication satellite
Earth resources satellite
Spy satellite
Weather satellite
2. Based on the Orbit around which they revolve
Geostationary orbit satellites
Inclined orbit satellites
Polar orbiting satellites

Geostationary Satellites remain over the same point above the Earth, traveling at the same speed as the rotation of the Earth. (35,786 km )

 

Polar Orbiting Satellites circle/orbit round the Earth in a polar direction, and withn each orbit pass over the equator about 30 degrees west of the previous orbit. This is because the Earth continues to rotate below the satellite.

Sensor Performance Parameters

  • SPATIAL – THE PHYSICAL DIMENSION ON EARTH IS RECORDED :  SPATIAL RESOLUTION
  • SPECTRAL – RELATING OF WAVELENGTH CHARACTERISTICS OF    EMR MEASURED NUMBER OF BANDS, BANDWIDTH:SPECTRAL RESOLUTION
  • RADIOMETRIC – ACCURACY AND MINIMUM CHANGE POSSIBLE IN   RADIANCE MEASUREMENT :  RADIOMETRIC RESOLUTION
  • TEMPORAL – FREQUENCY OF OBSERVATION :  TEMPORAL RESOLUTION.

Spatial Resolution when discrete detectors are used :

IFOV (ANGULAR)  =  a/f

IGFOV (LINEAR)   =  (a/f)H

FOV defines the swath coverage.

 

Radiometric Resolution

-A MEASURE OF THE INSTRUMENT’S CAPABILITY TO DIFFERENTIATE SMALLEST CHANGE IN REFLECTANCE/ EMITTANCE

-NOISE EQUIVALENT DIFFERENTIAL RADIANCE    NELL

-NOISE EQUIVALENT DIFFERENTIAL REFLECTANCE   NELP

-NOISE EQUIVALENT DIFFERENTIAL TEMPERATURE   NELT

DEPENDS ON

1.SATURATION RADIANCE

2.NUMBER OF BITS FOR DIGITISATION

S/N

Temporal Resolution

1.Minimum time required for the satellite orbit to trace the same track on ground is called Temporal Resolution and is normally expressed in days.

2.For a nadir looking sensor on an orbiting satellite, (~1000 km height), the revisit at equator approximately is as given below:

  1. Revisit period can be improved by swath steering.
  1. In case of IRS-1C/1D PAN camera, the swath of 70 km can be steered anywhere in the range of ±26° from nadir thereby improving the revisit capability to 5 days.

Opto mechanical Scanner

Pushbroom Scanner

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