Sensors and multispectral scanners

 Sensor -

A sensor is a device that gathers electromagnetic radiations, converts it into a signal and presents it in a form suitable for obtaining information about the objects under investigation. Based upon the form of the data output, the sensors are classified into photographic (analogue) and non–photographic (digital) sensors. A photographic sensor (camera) records the images of the objects at an instance of exposure. On the other hand, a non–photographic sensor obtains the images of the objects in bit-by-bit form. These sensors are known as scanners.

Multispectral Scanners -

In satellite remote sensing, the Multi Spectral Scanners (MSS) are used as sensors. These sensors are designed to obtain images of the objects while sweeping across the field of view. A scanner is usually made up of a reception system consisting of a mirror and detectors. A scanning sensor constructs the scene by recording a series of scan lines. While doing so, the motor device oscillates the scanning mirror through the angular field of view of the sensor, which determines the length of scan lines and is called swath. It is because of such reasons that the mode of collection of images by scanners is referred bit–by–bit. Each scene is composed of cells that determine the spatial resolution of an image. The oscillation of the scanning mirror across the scene directs the received energy to the detectors, where it is converted into electrical signals. These signals are further converted into numerical values called Digital Number (DN Values) for recording on a magnetic tape.

The Multi-Spectral Scanners are divided into the following types:

(i) Whiskbroom Scanners

(ii) Pushbroom Scanners 

 Whiskbroom sensor 

Pushbroom sensor

(i) Whiskbroom Scanners : The whiskbroom scanners are made up of

a rotating mirror and a single detector. The mirror is so oriented that

when it completes a rotation, the detector sweeps across the field of view between 90° and 120° to obtain images in a large number of narrow

spectral bands ranging from visible to middle infrared regions of the

spectrum. The total extent of the oscillating sensor is known as the Total

Field of View (TFOV) of the scanner. While scanning the entire field, the

sensor’s optical head is always placed at a particular dimension called

the Instantaneous Field of View (IFOV). Figure 7.7 depicts the scanning

mechanism of whiskbroom scanners.

(i) Pushbroom Scanners: The pushbroom scanners consist of a number

of detectors which are equivalent to the number obtained by dividing the

swath of the sensor by the size of the spatial resolution (Fig. 7.8). For

example, the swath of High Resolution Visible Radiometer – 1 (HRVR – 1)

of the French remote sensing satellite SPOT is 60 km and the spatial

resolution is 20 metres. If we divide 60 km x 1000 metres/20 metres, we

get a number of 3000 detectors that are deployed in SPOT HRV – 1 sensor.

In pushbroom scanner, all detectors are linearly arrayed and each detector

collects the energy reflected by the ground cell (pixel) dimensions of 20 metres at a nadir’s view.

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