Remote sensing
A device used for above purpose is called remote sensor.
Remote sensors are mounted on the aircraft or satellite platforms.
The sensors received the reflected or emitted radiations from a particular place on the earth's surface and measure the energy.
These measurements being made at a large number of points form the image of the earth's surface.
The sensed images are interpreted in terms of geographical variations.
Remote sensing takes place an important role in collection of dates from dangerous and unreachable areas.
During the data collection areas and objects are not disturbed.
Types of remote sensing
The system of remote sensing is classified into two types. They are
1) Active remote sensing
2) passive remote sensing
1) Active remote sensing
Active remote sensing is the process of sensing the distant object in which the sensors direct radiations towards the object and then they detect the amount of energy reflected by the object.
Example: Rader, Rader altimeter and synthetic aperture radar.
2) Passive remote sensing
Passive remote sensing is the process of sensing the distant object in which the sensors detect only the amount of energy naturally reflected by the object.
Example: Aerial photography, Indian remote sensing and microwave radiometer.
Components of remote sensing
The process of remote sensing system is divided into three stages namely.
1) Data acquisition
2) Data analysis
3) Reference data
1) Data acquisition
In this process, five distinct elements are involved. They are
1) Energy sources
2) Propagation of energy through the atmosphere.
3) Energy interaction on the earth surface.
4) Spaceborne sensors record the reflected energy.
5) Conversion of sensors data in the form of pictures or digital information.
The electromagnetic radiation emitted by the energy sources are propagated through the atmosphere and incident on the surface of the earth.
Then the radiation are reflected back towards the spaceborne remote sensors.
The sensors generate the receiving data into picture or digital forms.
2) Data analysis
The data analysis is divided into two separate process. they are
1) Visual interpretation technique
2) Digital analysis
In visual interpretation techniques, various viewing instruments are used to examining and analysis the pictorial data.
In digital analysis the computers are used to analysis the digital data. All dates are viewed in the information products by users.
3) Reference data
The reference data is a total set collection of information already sensed about the objects, area and any other incidents.
The reference data is used in the following purposes.
1) To analyses and interpret remotely sensed data.
2) To calibrate a sensor.
3) To verify the information extracted from remote sensing data.
The collection of reference data may be depends upon time critical or time stable measurements.
The time critical measurements are those conditions, water pollutents ate climatic conditions.
The time stable measurements are those materials do not with time and other geological measurements like buildings, land areas.
Radar- principle and working
The term RADAR is an abbreviation of ' Radio detection and ranging '.
Radar is an active sensor used to detect the range, altitude, direction and speed of the target.
Principle
Radar works on the principle of "radio echoes". The radar antenna transmits pulses of radio waves towards target.
When these waves incident on the target, a part get reflected and back to the radar antenna.
By using the time delay between transmission and reception, the position of the target can be easily detected.
Working
The radar system consist of two system,
1) Transmitting system and
2) Receiving system
The transmitting system consist of a transmitter and a pulser. The receiving system consists of a receiver and an indicator.
Both the systems are linked with transmitter receiver switch. It connects the antenna to the transmitter during reception.
so this switch is called ' duplexer '. A timer circuit is also linked the two system.
The transmitter generate high power short pulses and formed periodically by using the pulsar.
These pulses reach the antenna and transmitted into space. If the transmitted pulses hit any target, they get reflected back to the same antenna.
These echo signals are sent to the receiver through T.R switch. These signals are weak but amplified by the receiver and sent to the indicator.
The indicator is a cathode ray tube (CRT) it indicates both original transmitted pulses and receiving pulses along a straight line.
Now the timer generate the
pulses, supplied to the both transmitting and receiving system.
pulses, supplied to the both transmitting and receiving system.
So the indicator shows both the transmitting pulse and receiving pulse simultaneously with a gap.
The gap is used to measure the range of the target.
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