Principles of remote sensing

Remote sensing is, in general, the collecting of information from an object without making direct physical contact with it. The term is usually used in a more restricted sense in which the observation is made from above the object of interest, from a sensor carried on an airborne or spaceborne platform, and the information is carried by electromagnetic radiation, i.e. visible light, infrared or ultraviolet radiation, or radio waves (Rees 2001, 2006). This radiation can occur naturally, in which case the type of remote sensing is said to be passive, or it can be transmitted from the sensor to the object under investigation, in which case the remote sensing is said to be active. Passive remote sensing developed originally from aerial photography, and can be thought of as an extension of the idea of aerial photography to include other parts of the electromagnetic spectrum, other technologies for detecting the radiation and storing the data, and other platforms to carry the sensor. Active remote sensing grew from the military development of radar during the Second World War. One of the most significant factors in the increasing applicability of remote sensing to many investigations in the environmental sciences, amongst other disciplines, has been the use of spaceborne platforms. Although remote sensing instruments were carried into orbit around the Earth in the 1960s, the age of satellite remote sensing effectively began in 1972 with the launch of Landsat 1 satellite. Spaceborne remote sensing provides a number of advantages compared with airborne observations. Information can be obtained from huge areas in a short time, and from locations that could be difficult or dangerous to overfly. As important as these, however, is the scope for continuity of data collection. While an individual satellite mission does not normally have a planned lifetime exceeding three to five years, even this is enormously longer than the period of continuous data collection achievable from an airborne platform. However, missions are often designed to provide continuity of consistent data coverage with previous missions, in some cases for several decades. These are all advantages that make spaceborne remote sensing particularly valuable for the study of glaciers, and the glaciological research community was and remains quick to identify and exploit the possibilities offered by satellite data.