The basics including the electromagnetic spectrum and active and passive sensing
1 Remote sensing and the electromagnetic spectrum
The electromagnetic spectrum provides the basis for remote sensing. While the human eye is limited to the visible part of the spectrum, remote sensing is able to use other parts of the electromagnetic spectrum, as shown below. The benefit of this is that different regions (spectral wavelengths) of the electromagnetic spectrum are sensitive to different properties of the atmosphere and Earth’s surface and can provide information tailored to particular environmental applications.
The human uses remote sensing at visible wavelengths (from red to violet) to detect the colour of an object. The spectral wavelengths used for remote sensing can be associated with microwave, infrared, visible and ultraviolet part of the electromagnetic spectrum, covering a much broader range of the electromagnetic spectrum than the human eye. At the same time, many remote sensing instruments combine information across multiple spectral wavelengths, for example commonly including both visible and near infrared (multispectral sensing), but in some cases involving over a hundred discrete spectral wavelengths (hyperspectral sensing).
2. Active vs passive remote sensing
As well as covering different parts of the electromagnetic spectrum, remote sensing can be separated into active and passive categories that reflect differences in the source of radiation used in their measurements. Passive sensors measure the energy from the earth’s surface. This energy is either directly emitted from the earth’s surface or is reflected from the Sun. Active sensors create the initial energy source that is measured. In simple terms, consider a camera as a passive sensor but a camera using a flash as an active sensor.
Tables 1 and 2 summarise many of the key differences between active and passive sensors operating in different parts of the electromagnetic spectrum, all of which have particular strengths and weaknesses in relation to remote sensing and environmental applications. An important part of planning the use of remote sensing is to ensure that the appropriate sensors are matched to both the environmental context (project location, etc) and application (e.g. information needs).