2.2 Sensing Devices

Harris (1987) and Richards (1986) review the characteristics of currently available remote sensing imagery from satellite-borne sensing devices. Richards (1986) also details the available aircraft-borne sensors. Curran (1985) describes the use and features of aerial photography and both aerial scanner and satellite scanner imagery. Lillesand and Kiefer (1979) detail various aspects of remotely sensed image acquisition and interpretation, with particular emphasis on aerial photograph data.
Section 3 summarises the platforms most commonly used to carry remote sensing devices and briefly describes some of the sensor systems currently in operation. Aspects of interpreting data from these devices are considered in Section 4.

2.2.1 Multi-spectral scanners

2.2.1.1 Spectral regions:

Multi-spectral scanners (MSS), as the name implies, are a particular class of remote sensing device which sense radiation in multiple wavelength regions of the visible, near infrared, middle infrared and thermal infrared parts of the electromagnetic spectrum. As wavelengths in these regions of the spectrum are strongly affected by atmospheric scattering, the usefulness of these devices for earth surface studies may be limited by atmospheric conditions.

MSS devices digitally record the detected radiation in a number of defined wavelength 'channels' or 'bands'. The principle of this mode of operation is the same as that of using filters on a camera to photograph limited parts of the visible spectrum. For example, when using an appropriate filter to photograph only blue light, a purely red object would appear black since only blue radiation will pass through the filter to expose the film.

For example, the Landsat 1 to 5 satellites (see Section 3) have carried a MSS which senses four regions or 'bands' of the EM spectrum. These are:
green 0.5­p;0.6 µm (band 4)
red 0.6­p;0.7 µm (band 5)
near infrared 0.7­p;0.8 µm (band 6)
near infrared 0.8­p;1.1 µm (band 7)
(Bands 1 to 3 were associated with another instrument carried on the first three satellites).
The spectral sensitivities of these wavelength bands are illustrated in Figure 16 and compared with the sensitivities of colour and colour infrared films in Figure 17.


Figure 16: Spectral sensitivity of Landsat MSS bands. The actual sensitivity within each bandwidth varies with wavelength as shown by the different line types.


Figure 17: Comparison of the spectral sensitivity of Landsat MSS bands with the three emulsion layers used in colour and colour infrared film. [Adapted from Lillesand and Kiefer 1979]

An additional scanning device called Thematic Mapper operates on Landsats 4 and 5. This instrument has the following 7 spectral channels:

blue/green 0.45­p;0.52 µm
green 0.52­p;0.60 µm
red 0.63­p;0.69 µm
near infrared 0.76­p;0.90 µm
near middle infrared 1.55­p;1.75 µm
middle infrared 2.08­p;2.35 µm
thermal infrared 10.40­p;12.50 µm

Details of other multi-spectral scanning devices used for remote sensing are presented and discussed in Section 3, while interpretation of this data is considered in Section 4.

Most sensing systems in the visible and infrared regions of the EM spectrum are passive detectors of reflected solar radiation or emitted thermal radiation. Lidar (Laser Imaging raDAR) is an active remote sensing device which operates in the wavelength range from ultraviolet to near infrared. The laser directs pulsed or continuous radiation through a collimating system while a second optical system collects the returned radiation and focuses it onto a detector. Lidar is only effective in clear atmospheric conditions and such devices are thus aircraft-borne. Applications for lidar currently include mapping (especially in bathymetry), spectroscopy (for air and water pollution studies) and altimetry.