Lecture 6

Scanning Systems

Across Track Multispectral Scanner (MSS)

          This form of imaging is used in Landsat series of satellite. The scanning system (Fig.5.6) employs a single detector per band of the multispectral signal. It has an electrical motor, to the axel of which is attached a solid metal cylinder whose free end is cut at 45 degrees to the axis of its rotation and highly polished to act as a scanning mirror. The field of view (FOV) is restricted by an aperture so that the mirror will receive signals in almost nadir view from 2000 ground resolution cells that makes one scan line. The signal received by the rotating scanning mirror from a ground resolution cell (corresponding to GIFOV) is a white one and contains spectral information in different bands. This white beam is reflected by the mirror in the flight direction (parallel to the ground) and is allowed to pass through a monochromator/spectroscope which splits the composite beam into its color components. The detectors with their designed apertures are so placed that they now receive the spectral information from the ground resolution cells in the Specified bandwidths of various color components of the white signal

The dwell time of the scanner is compared by the formula 

Dwell Time = (Scan rate per line)/(Number of ground resolution cells per line).

The Spatial Resolution of the scanner

          =ground resolution of the scanner

          =GIFOV x Attitude of the scanner

Along track multispectral

Scanner/ Push Broom Scanner

          In this type of scanner, the scan direction is along the track (direction of night) and hence the name along track scanner (Fig.5.7). It is also called push broom scanner because the detectors are analogous to the bristles of a push broom sweeping a path on the floor.

          Development of charge-coupled device (CCD) has contributed to the successful design of the along track scanner. In this the sensor elements consist of an array of silicon photodiodes arranged in a line. There are as many silicon photodiodes as there are ground resolution cells (corresponding to IFOV) accommodated within the restricted FOV of the sensor optics. Each silicon photodiode, in turn, is coupled to a tiny charge storage cell in an array of integrated circuit MOS (metal oxide semiconductor) device forming a char e coupled device (CCD) (Fig.5 3). When light from a ground resolution cell strikes a photodiode in the array, it generates a small current proportional to the intensity of light falling on it and the current charges the storage cell placed behind the diode. The charged cells form part of an electronic shift register which can be activated to read out the charge stored in the cells in a sequential fashion. The output signals are correlated with the shift pulses, and digitized to reconstitute the image.

The spatial resolution of the sensor = ground resolution cell

                                                          =GIFOV x altitude of the scanner

The dwell time for the long track scanner is given by 

Dwell time = (ground resolution cell dimension)/ (velocity of sensor platform) 

Side Viewing /Side Looking Scanner

          The across track and along track scanners described above are used in passive remote sensing in visible, infrared and microwave regions of the electromagnetic spectrum. These scanners always receive signals in the nadir view. However if the user demands (on payment basis) to observe a dynamic scene frequently, then there is provision in SPOT and IRS satellites to steer the cameras to look off-nadir at the required scene, some days before to some days after the normal nadir viewing date (fig 5.9).

          But scanners of active remote sensing, like the scanners used 'for radar remote sensing, in their normal mode of scanning look to the sides and not to the nadir for technical reasons which will be described later. Therefore such scanners are called side looking airborne radar (SLAR). The most sought after sophisticatedly designed synthetic aperture radar (SAR) belongs to the side looking scanner system.