Introduction to Remote Sensing

Objectives

• Define remote sensing and identify the components of a remote sensing system
• Provide several examples of remote sensing technologies
• Describe the advantages of using remote sensing technologies to study the Earth and its environment.

Introduction

Imagine you have been asked to investigate how temperature changes in a large body of water such as the Chesapeake Bay affect shellfish populations. Or imagine that you have been asked to perform an analysis of how land use in your region has changed over the past decade. Such projects are very real and very important on a regional scale, and the results of very similar environmental research are often used as the basis for policy decisions by local and state governments.

As you consider such a task, numerous questions come to mind. What kinds of measurements would you need to make? How often would you need these measurements? How much area should your research cover? What tools are available for such a research project? What are the costs involved? In many cases the answers to these questions identify a need for measurements and observations on temporal (time) and spatial scales that are impossible for a single person (or even a well organized group of researchers) to meet. Additionally, the manpower and the funding is often not available to carry out such research using traditional methods of field research.

These problems are increasingly faced by researchers by turning to remote sensing as a cost effective tool for performing environmental research on local and regional scales. Remote sensing is not a new concept and has been used extensively in global environmental research over the past several decades. However, recent advances in remote sensing technologies, lower cost, and greater availability of remotely-sensed data has made it a much more attractive solution for local and regional governments, schools, and universities interested in performing environmental research that may have real impact on their communities.

What is Remote Sensing?

Remote sensing can be defined as the study of something without making actual contact with the object of study. More precisely, it can be defined as:

"The acquisition and measurement of data/information on some property(ies) of a phenomenon, object, or material by a recording device not in physical, intimate contact with the feature(s) under surveillance"

(http://rst.gsfc.nasa.gov/Intro/Part2_1.html)

Whatever working definition you use to describe remote sensing, the key concept is that remote sensing involves making observations remotely, or without physical contact with the object under investigation. The remote nature of these technologies allow us to make observations, take measurements, and produce images of phenomena that are beyond the limits of our own senses and capabilities.

Remote sensing tools can be used to study things on all scales, ranging from the smallest particles within the atom to the universe as a whole. While remote sensing tools are often associated with researchers and scientists who conduct scientific inquiry, they are also a familiar part of our everyday lives and often turn up in places that may surprise the unsuspecting observer.

Components of a Remote Sensing System

While the definition of remote sensing describes a very wide array of technologies and types of research, all remote sensing technologies are based on certain common concepts, and all remote sensing systems consist of the same basic components. These four basic components of a remote sensing system include a target, an energy source, a transmission path, and a sensor.

The target is the object or material that is being studied. The components in the system work together to measure and record information about the target without actually coming into physical contact with it. There must also be an energy source which illuminates or provides electromagnetic energy to the target. The energy interacts with the target, depending on the properties of the target and the radiation, and will act as a medium for transmitting information from the target to the sensor. The sensor is a a remote device that will collect and record the electromagnetic radiation. Sensors can be used to measure energy that is given off (or emitted) by the target, reflected off of the target, or transmitted through the target.

Once the energy has been recorded, the resulting set of data must be transmitted to a receiving station where the data are processed into a usable format, which is most often as an image. The image is then interpreted in order to extract information about the target. This interpretation can be done visually or electronically with the aid of computers and image processing software.

Infrared Weather Satellite Image over continental U.S.

Weather satellite imaging of the Earth is a familiar example of a remote sensing system. The target in such a system is the Earth's surface, which gives off energy in the form of infrared radiation (or heat energy). This energy travels through the atmosphere and space and reaches the sensor, which is mounted on a satellite platform. Varying levels of this energy are recorded, transmitted to ground stations on the Earth, and converted into images that depict differences in temperature across the planet's surface. In a similar manner, other weather satellite sensors measure the visible light energy from the sun as it is reflected off the Earth's surfaces, transmitted through space to the satellite sensor, and recorded and sent to Earth for processing.

Another familiar form of remote sensing that occurs on a relatively smaller scale is medical imaging technologies such as Magnetic Resonance Imaging (MRI), sonograms, and X-Ray imaging. These technologies all use forms of energy to produce images of the inside of the human body. In each, various forms of energy are produced by a machine and directed at the target object. The sensors measure how this energy is absorbed, reflected, or transmitted through the target object, and the results are compiled into an image. These technologies provide the obvious benefit in that they allow observation and measurement of the internal systems in the human body without potentially dangerous invasive surgery.

Remote sensing is not limited to investigations within our own planet. Most forms of astronomy are examples of remote sensing, since the targets under investigation are such vast distances from Earth that physical contact is impossible. Astronomers therefore must collect and analyze the energy given off by these objects in space by using telescopes and other sensing devices. This information is recorded and used to draw conclusions about space and our universe.

Environmental Remote Sensing

Environmental remote sensing is the study of the Earth and the interactions between its natural systems through the use of remote sensing technologies. The benefits of using remote sensing techniques to study our planet are many. For example, remote sensing tools allow us to investigate larger portions of the Earth than previously possible. In fact, much of the environmental research over the past two decades has focused on investigating the entire planet as a single system. The Earth, to human observers, appears quite large. If we were to limit our investigations to what we, as humans, can directly observe with our senses, we could never begin to understand the complex interactions between the Earth's land masses, oceans, and atmosphere. Detailed measurements of the planet from sensors mounted on satellites have provided scientists with the information needed to begin to understand, describe, and model the various components of the Earth's ecosystem. This research is now being used to determine the effects of pollution on the global climate and to develop policy that will attempt to control such pollution in the future.

[TOMS Ozone image depicting ozone hole over Antarctica]

Remote sensing of our environment also allows research to occur on time scales, both in duration and frequency, that were previously impossible. Research can now continue over weeks, months, years, and decades, giving us a better understanding of the cycles that occur naturally in our environment. Measurements of key environmental indicators can be automated and made remotely at any time interval necessary to provide better insight into natural phenomena. For example, satellite data depicting the degree of vegetative cover in a particular region can be produced on a weekly basis over the course of several years, providing researchers with valuable insight into how weather, climate change, and human interaction may affect our agricultural production. In another case, weather satellites can image a single thunderstorm every minute to detect important changes in the cloud structure that occur on a very short time scale and may indicate the development of severe weather. In both cases, the measurements obtained remotely with the assistance of satellite sensors would be impossible to duplicate using traditional methods of field research.

[Image produced by Laboratory for Atmospheres,
NASA Goddard Space Flight Center.]

Another benefit of remote sensing is that it often allows the investigation of portions of the Earth that are difficult or dangerous to reach. Satellite and aircraft sensors can help us investigate the Earth's polar regions, upper atmosphere, forest fire and volcanic activity, remote oceans, and desert landscapes without having to send people into these dangerous locations.

References

NASA Earth Observatory: Remote Sensing

http://earthobservatory.nasa.gov/Library/RemoteSensing/

© CGIS at Towson University