Remote sensing

Introduction

There is an increasing demand for accurate, timely information on environmental and natural resources, including spatial relationships and temporal changes and trends, local to global. In the broadest sense, remote sensing is the measurement or acquisition of information of an object or phenomenon, by a recording device that is not in physical or intimate contact with the object. In practice, remote sensing is the utilization at a distance (as from aircraft, spacecraft, satellite, or ship) of any device for gathering information about the environment. Advantages of this technology are:

• Observation of a large geographical area
• Long-term and fast collection of data
• Lower collecting costs
• "Inaccessible" regions become accessible (e.g. Antarctica)
• Object is not being destroyed

Disadvantages are:

• Lower spatial resolution (depending on the type of sensor)
• Need for the installation of complex systems (which have a long testing phase)
• Captured data need to be calibrated via in-situ data
• Noise caused by another source than the desired one
• Atmospheric effects degrade the quality of the images and need to be corrected

General principles of remote sensing

Remote sensors are devices that measure and record specific types of energy. In remote sensing this energy is electromagnetic radiation which is reflected or emitted by all natural and synthetic objects on Earth. The electromagnetic spectrum is the continuous range of electromagnetic radiation. The spectrum can be divided in the following regions: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves. Remote sensing involves measurement of energy in many parts in the EM spectrum and takes place in spectral bands.A spectral band is defined as a discrete interval of the EM spectrum. Satellite sensors for example have been designed to measure responses within particular spectral bands to enable the discrimination of the major Earth surface materials. Scientist choose a particular spectral band for data collection depending on what they wish to examine. The data captured and recorded by the sensors must be analyzed by interpretive and measurement techniques in order to provide useful information about their subjects. The technique varies from simple traditional methods of visual interpretation to complicated methods using computer processing. The output is usually an image.

Types of sensors

Remote sensing systems can be divided into 2 categories: active or passive sensors. In active sensors (e.g. radar) energy is transmitted from the sensor so they provide their own energy source for illumination. The sensor emits radiation which is directed toward the object or surface to be investigated. The radiation reflected from that target is detected and measured by the sensor. Passive systems (e.g. photographic camera) measure energy that is naturally available or depend on an external energy source (for example the sunlight).

Remote sensing applications

• Agriculture
• Environmental monitoring and risks
• Geology
• Oceanography
• Climatology
• Ecology

See also

• Optical remote sensing