A thorough review of remote sensing could occupy volumes of text, however an introduction to remote sensing concepts is helpful for discussion of remote sensing techniques applied to the study of vegetation. Remote sensing concepts and methods, along with the objects or systems being studied, must be considered when designing and deploying remote sensing instruments, and when using data acquired by various sensors. This chapter presents a brief overview of some general basics.
Remote sensing is a broad term used to describe acquiring information about an object by means of "remote" examination; that is, with no direct contact of the object. Viewing other planets, distant stars and galaxies with telescopes might immediately come to mind, but animal eyes and microscopes also gather information remotely. The discussion included here focuses on ground based, airborne, and spaceborne (satellite) remote sensing instruments that measure and record energy reflected or emitted by the earth's surface. This includes photographic cameras, and electronic imaging and non-imaging sensors.
Instrumental remote sensing began with the telescope (and microscope). A significant advance in remote sensing occurred with the development of photographic film and cameras, which allowed remote sensing information to be recorded. By the midnineteenth century, a working camera had been produced. Shortly after the development of the camera, there are a few documented cases of people taking aerial photographs from balloons.
Especially during the early years of remote sensing development, military applications were responsible for most of the advances in remote sensing. For example, in the American Civil War, balloons were used by the Union forces to obtain strategic information. After the development of airplanes, aerial photography became more practical because of the increased stability, control and reliability associated with the aircraft as a platform for cameras. Significant advances in aerial photography were again made during World Wars I and II. Between these two wars, the civilian use of remote sensing increased in the fields of geology, forestry, agriculture, and cartography.
Before World War II, most of the developments were focused on improved cameras, film, and interpretation techniques. Following the start of World War II, more advanced sensing devices were developed, such as thermal infrared and radar systems. These electronic sensing devices continued to evolve as more sensitive detectors with faster response times were developed, with improved (more detailed) spatial and spectral resolutions (various types of sensor resolution are discussed in Section 2). As sensor design advanced, mechanical systems were developed that mounted digital sensors into scanners. Scanners provided the ability to acquire image scan lines by motion of the sensor or platform. For vegetation monitoring, the development of color-infrared photography improved the ability to classify vegetation and to monitor the health of vegetation.
In the early 1960s, NASA began launching civilian satellites for purposes of Earth observation. Satellites provide stable sensor platforms, with the potential for global coverage. Another advantage of satellites is that they can stay in orbit for several years and transmit acquired data to ground receiving stations located throughout the world. Most of the early satellite remote sensing systems were designed for meteorological monitoring. These satellite sensors had spatial resolutions ranging from one to several kilometers. In addition to satellites, cameras were used aboard manned and unmanned spacecraft.
As sensor systems further developed and improved, more satellite systems were launched that were specifically designed for monitoring vegetation. In the 1970s, NASA began launching the Landsat series of satellites for the purpose of observing Earth's resources. An early Landsat MSS (Multi Spectral Scanner) instrument had a spatial resolution of 80 meters, four spectral bands, and could acquire images of nearly the entire globe every 18 days.
From the 1980s to the present, many other countries have been active in launching civilian Earth resource satellites. Some of these satellites have spatial resolutions finer than 10 meters. Many advancements have been introduced, such as being able to program a satellite and sensor to acquire imagery for specific locations on particular days. A number of civilian satellites carrying radar systems have also been developed during this time period. The earlier radar systems were generally regarded as experimental, although a number of practical applications were derived from them. Radar technology has progressed to the point where it plays a significant role in remote sensing applications.
Ground based and airborne sensors have also undergone much advancement. Ground based instruments are important for laboratory and field measurements that provide reference characterization of the reflectance of a wide variety of earth materials and vegetation. Airborne sensors are used to obtain greater areal coverage than ground-based instruments and platforms, usually at finer spatial resolutions than satellite data. A large variety of airborne sensors and platforms exist today.