Multispectral scanning is a technique that gathers information at many wavelengths using a scanning apparatus.The most popular scanning devices for remote sensing are multispectral scanners (MSS).
Several spectral bands, such as visible, near-infrared, and shortwave infrared, are used in multispectral imaging (MSI) to take pictures.The foundation of MSI is the notion that every object has a distinct spectral fingerprint that represents the light wavelengths that are transmitted, reflected, or absorbed by it.
There are numerous uses for multispectral imaging, such as:
Military: Initially created for military target reconnaissance and identification
Earth mapping: A technique for charting the planet’s features, including flora, coastlines, and landforms
Contrary to hyperspectral imaging, which may capture tens to hundreds of spectral bands, multispectral imaging offersimaging typically captures a smaller number of bands, often ranging from 3 to 10
Many electronic (as opposed to photographic) remote sensors acquire data using scanning systems, which employ a sensor with a narrow field of view (i.e. IFOV) that sweeps over theterrain to build up and produce a two-dimensional image of the surface. Scanning systemscan be used on both aircraft and satellite platforms and have essentially the same operatingprinciples. A scanning system used to collect data over a variety of different wavelengthranges is called a multispectral scanner (MSS), and is the most commonly used scanningsystem. There are two main modes or methods of scanning employed to acquire multispectralimage data – across-track scanning, and along-track scanning.
Across-track scanners scan the Earth in a series of lines where each line is scanned from one side of the sensor to the other, using a rotating mirror.
Across-track scanners also called a whiskbroom scan the Earth in a series of lines. The lines are oriented perpendicular to the direction of motion of the sensor platform (i.e. across the swath). Each line is scanned from one side of the sensor to the other, using a rotating mirror (A). As the platform moves forward over the Earth, successive scans build up a two-dimensional image of the Earth´s surface. The incoming reflected or emitted radiation is separated into several spectral components that are detected independently. The UV, visible, near-infrared, and thermal radiation are dispersed into their constituent wavelengths. A bank of internal detectors (B), each sensitive to a specific range of wavelengths, detects and measures the energy for each spectral band and then, as an electrical signal, they are converted to digital data and recorded for subsequent computer processing.
The IFOV (C) of the sensor and the altitude of the platform determine the ground resolution cell viewed (D), and thus the spatial resolution. The angular field of view (E) is the sweep ofthe mirror, measured in degrees, used to record a scan line, and determines the width of theimaged swath (F).Airborne scanners typically sweep large angles (between 90º and 120º),while satellites, because of their higher altitude need only to sweep fairly small angles (10-20º) to cover a broad region. Because the distance from the sensor to the target increasestowards the edges of the swath, the ground resolution cells also become larger and introducegeometric distortions to the images. Also, the length of time the IFOV “sees” a groundresolution cell as the rotating mirror scans (called the dwell time), is generally quite short andinfluences the design of the spatial, spectral, and radiometric resolution of the sensor.
Along-track scanners also use the forward motion of the platform to record successive scan lines and build up a two-dimensional image, perpendicular to the flight direction. However, instead of a scanning mirror, they use a linear array of detectors (A) located at the focal plane of the image (B) formed by lens systems (C), which are “pushed” along in the flight track direction (i.e. along track). These systems are also referred to as pushbroom scanners, as the motion of the detector array is analogous to the bristles of a broom being pushed along a floor. Each individual detector measures the energy for a single ground resolution cell (D) and thus the size and IFOV of the detectors determines the spatial resolution of the system. A separate linear array is required to measure each spectral band or channel. For each scan line, the energy detected by each detector of each linear array is sampled electronically and digitally recorded.
Along-track scanners with linear arrays have several advantages over across-track mirror scanners. The array of detectors combined with the pushbroom motion allows each detector to “see” and measure the energy from each ground resolution cell for a longer period of time (dwell time). This allows more energy to be detected and improves the radiometric resolution.
The increased dwell time also facilitates smaller IFOVs and narrower bandwidths for each detector. Thus, finer spatial and spectral resolution can be achieved without impacting radiometric resolution. Because detectors are usually solid-state microelectronic devices, they are generally smaller, lighter, require less power, and are more reliable and last longer because they have no moving parts. On the other hand, cross-calibrating thousands of detectors to achieve uniform sensitivity across the array is necessary and complicated. Regardless of whether the scanning system used is either of these two types, it has several advantages over photographic systems.
The spectral range of photographic systems is restricted to the visible and near-infrared regions while MSS systems can extend this range into the thermal infrared. They are also capable of much higher spectral resolution than photographic systems. Multi-band or multispectral photographic systems use separate lens systems to acquire each spectral band. This may cause problems in ensuring that the different bands are comparable both spatially and radiometrically and with registration of the multiple images. MSS systems acquire all spectral bands simultaneously through the same optical system to alleviate these problems. Photographic systems record the energy detected by means of a photochemical process which is difficult to measure and to make consistent. Because MSS data are recorded electronically, it is easier to determine the specific amount of energy measured, and they can record over a greater range of values in a digital format.
Photographic systems require a continuous supply of film and processing on the ground after the photos have been taken. The digital recording in MSS systems facilitates transmission of data to receiving stations on the ground and immediate processing of data in a computer environment.
