436 REMOTE SENSING/Passive Sensors raphy A digital camera is built on a full 2D (two dimensional) CCD panel The only difference between a digital camera and a film camera is that it records an image through a 2D CCD panel to a memory chip instead of using a film The new generation of passive sensors will be largely based on digital camera technology that takes an image by an instantaneous frame rather than scanning line by line The consequence is that the constraints on platform flight parameters can be relaxed, image resolution (spatial and spectral) can be improved, and image geometric correction processing can be further streamlined Broadband Reflective Multispectral Imagery Broadband reflective multispectral sensors and thermal sensors are addressed separately being based on different physics In practice, however, these two groups of passive sensors are often mounted in the same instrument as different bands This is true for many sensor systems, such as TM, Enhanced Thematic Mapper Plus (ETMỵ), and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) (Table 1), used for geological applications Broadband reflective multispectral image data are the most widely used for geological studies and regarded as an effective operational tool for mapping tectonic structures and lithology, for mineral exploration, for logistic planning, and for field survey navigation Extending the scope of applications from Earth observation to planetary study, this group of sensors are often the major tools for data collection from other planets A typical example is the exploration of Mars The American Landsat satellite family TM, ETMỵ, and French SPOT (Syste`me Pour 1Observation de la Terre) satellite family high resolution visible (HRV) are the most commonly used Earth observation systems, providing broadband multispectral and panchromatic imagery data of global coverage As shown in Table 1, these types of sensor systems operate in: the visible spectral range with bands equivalent to three primary colours; blue (380– 440 nm), green (440–600 nm), and red (600–750 nm); the near infrared (NIR) range (750–1100 nm), and the short wave infrared (SWIR) range (1550–2400 nm) The number of bands and the spectral width in VNIR (visible near infrared) and SWIR spectral ranges depend on the atmospheric windows and sensor design For instance, the spectral width of SWIR bands needs to be much wider than visible bands if the same spatial resolution is to be achieved, as is the case for TM bands and 7, because the solar radiation in the SWIR spectral region is significantly weaker than that in the visible spectral range In general, the ‘broad’ band means that the spectral range is significantly wider than a few nanometres, except in the case of hyperspectral sensor systems described below Broadband reflective multispectral sensor systems are a successful compromise between spatial resolution and spectral resolution With relatively broad spectral bands, such a sensor system offers reasonable spatial resolution with high SNR (Signal Noise Ratio) and meanwhile, operating in a wide spectral range from VNIR to SWIR, such a system can provide images of multispectral bands, enabling identification of major ground objects and discrimination of various land cover types With the dramatic improvement of sensor technology, from mechanical scanners to push-broom scanners, and to CCD digital cameras, the spatial resolution of broadband multispectral imagery is improving all the time For Sun synchronous near polar orbit satellites, the spatial resolution of this type of sensors has been improved from 80 m (Landsat MSS) in the 1970s to a few metres, on current systems, as shown by the examples in Table The VNIR spectral range is used by nearly all broadband reflective multispectral sensor systems This spectral range is within the solar radiation peak and thus allows the generation of high resolution and high SNR images It also covers diagnostic features of major ground objects such as the few examples below: Vegetation: minor reflection peak in green, absorption in red, and then significant reflection peak in NIR, often called the ‘red edge’ Water: strong diffusion and penetration in blue and green, and nearly complete absorption in NIR Iron oxide (red soils, gossans, etc.): absorption in blue and high reflectance in red Many satellite sensor systems did not use the blue band, in order to avoid strong Rayleigh scattering effects occurring in the atmosphere that can make an image ‘hazy’ A popular configuration is to offer three broad spectral bands in green, red, and NIR, such as the case of SPOT and the most recent commercial high spatial resolution space-borne sensors (Tables and 2) In a computer graphic system, we can display the three band images as a colour composite with NIR displayed in red, red in green, and green in blue Such a colour composite image is called a standard false colour composite This image is the most effective for mapping healthy vegetation The SWIR spectral range is regarded as the most effective for lithological mapping and mineral exploration, because most rock types have high reflectance in 1.55–1.75 mm and clay minerals (often products of