Civil - Remote Sensing Techniques and GIS - Platforms and Sensors
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.
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
SPOT MSS HRV-120m; SPOT MSS HRV II-10m
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 0is 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
d) CD-ROM products