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Chapter: Civil : Remote Sensing Techniques and GIS : Platforms and Sensors

Important Questions and Answers - Platforms and Sensors

Civil - Remote Sensing Techniques and GIS - Platforms and Sensors



1.What is passive  sensors?                             

  Passive  sensors  can  only  be  used  to  detect  energy  when    the   naturally occurring  energy    is  available.  For  all  reflected  energy,  this  can  only take  place during the  time  when  the  sun  is  illuminating  the  Earth.  There  is no  reflected energy  available from  the  sun  at  night.  Energy  that  is naturally emitted  (such  as thermal  infrared) can  be detected day  or night,  as long  as the amount of energy  is large enough to be recorded.                 


2.What is Active sensors?                             

  On the other  hand,  provide their  own  energy  source  for illumination. The sensor  emits  radiation which  is directed toward  the  target  to  be  investigated.  The radiation reflected from that target  is detected and  measured by the sensor.


3.Write the advantages of active sensors?               

Advantages  for  active  sensors  include  the  ability  to  obtain  measurements anytime, regardless of the time  of day  or season.  Active  sensors  can be used  for examining  wavelengths  that  are  not    sufficiently provided  by  the  sun,  such  as microwaves, or to  better  control  the  way  a target  is illuminated. However, active systems   require  the  generation  of  a  fairly  large  amount  of  energy   to adequately  illuminate  targets.  Some  examples   of  active  sensors  are  a  laser fluorosensor and  a synthetic aperture radar  (SAR).             


4. What are the types of Platforms?                     

The vehicle or carrier for remote sensor is borne is called the Platform.' The typical platforms are satellite and aircraft, but they can also include radio controlled airplanes, balloons, pigeons, and kites for low altitude remote sensing, as well as ladder and cherry pickers for ground investigation.


5.Differentiate  Geostationary  orbit  and  Polar  sun  synchronous orbit.


Geostationary orbit

High altitude (36,000km)

Remains in same position above the Earth

Used by meteorological and communications satellites

Sees Earth disk (between third and quarter of Earths surface) High temporal frequency (c.30 mins typical)

Polar sun synchronous orbit Low altitude (200-

1000km) Goes close to poles

Higher spatial resolution than geostationary Lower temporal resolution than geostationary


6. What is Resolution?

In general resolution is defined as the ability of an entire remote-sensing system, including lens antennae, display, exposure, processing, and other factors, to render a sharply defined image.It is the resolving power of the sensor to detect the smallest meaningful elemental area in different spectral bands in s defined gray level at a regular interval.


7.What are the elements of resolution?

The four elements of resolutions are Spatial, Spectral, Radiometric and Temporal.


8. Write short notes about Spatial resolution.

It is the minimum elemental area the sensor can detect or measure. The resolution element is called pixel (picture element).

Example: IRS LISS 1-72.5m; LISS II-36.25m Land sat MSS-80m; Land sat TM-30m



9. Write short notes about Spectral resolution.

It refers to the sensing and recording power of the sensor in different bands of EMR. The sensors can observe an object separately in different bands or colors.

Examples: IRS-4 bands; Land sat MSS-4 bands; Land sat MSS TM-7 bands SPOT-4 bands

It is the ability if the sensor to distinguish the finer variation of the reflected radiation from different objects.

It is the smallest amount of energy that can be detected by sensor and differentiate the same in a defined scale.

It is recorded in digital number (DN) for different bands of the satellite.

The radiometric value of the pixel is the average of the values coming from every part of the pixel.

Example: IRS-128 gray level; Land sat MSS-64; Land sat TM-256; SPOT-256(it is to be noted that '0'is also a value in the gray scale).


11.  Write short notes on Temporal resolution.

It is the time interval between two successive surveys of a particular place of the earth by the sensor or satellite.

Examples: IRS-22days; Land sat 16/18days; SPOT-16days.


12.  Write   the   types   of   Microwave Sensors?

Active  microwave  sensors  are  generally  divided  into  two  distinct  categories:

imaging and non-imaging. The most common form of imaging active microwave sensors is RADAR.


13.What is RADAR?

RADAR is an acronym for RAdio Detection And Ranging, which essentially characterizes the function and operation of a radar sensor. The sensor transmits a microwave (radio) signal towards the target and detects the backscattered portion of the signal.


14. What are the types of DATA products?

The data for all the sensors of IRS -1C/1D are supplied on digital media like

a) Computer copatible tapes (CCTs)

b) Cartridge tapes

c) Floppies

d)  CD-ROM products


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