Introduction to Remote Sensing

In this page we present some concepts related to Remote Sensing and to images generated by orbital sensors, for one of the main functions of SPRING lies on the treatment of these images via the functions of enhancement and classification.

The topics presented herein are:

See also:
About Digital Images.
Reading an Image with IMPIMA.
Performing an Image Registration.
RADAR Image Processing.
RS Image Processing.


Origins and Evolution of RS

The origins of remote sensing dates back to the experiments of Newton (1822), who discovered that a ray of light (white light) when traversing a prism would unfold or disperse into a multicolored beam - a spectrum of colors.

Since then the scientists have been widening the scope of their studies on so interesting a subject. They verified that white light was but a synthesis of different types of light, a kind of a vibration composed of many other different vibrations. Later they discovered that each color of the decomposed spectrum corresponded to a different temperature, and that the red light when hitting on the surface of a body would heat it more that violet light.

Besides the visible red, there are some radiations that are invisible to the eye, called infrared waves, rays or radiation. Soon, an experiment by Titter revealed another type of radiation: ultra-violet. Always taking their experiments further, scientists managed to prove that the light wave was in fact just one of the many different types of electromagnetic wave.

Some authors trace the origins of remote sensing down to the development of photographic sensors, when aerial photographs were taken from balloons.

It becomes evident that remote sensing is the fruit of a multidisciplinary effort that has involved, and still does, many different advancements in physics, chemistry, biosciences, and geosciences, computing, mechanics, etc..

The evolution of remote sensing is connected to some of the following main events:

  • 1822 - Development of the theory of light;
               Newton - decomposition of white light;
               Use of a primitive camera.
  • 1939 - Development of optical equipment;
               Research of new photosensitive substances.
  • 1859 - Use of photographic cameras onboard balloons.
  • 1903 - Use of aerial photographs for cartographic purposes.
  • 1909 - Aerial photographs taken from planes.
  • 1930 - Systematic coverage of the territory for the survey of natural resources.
  • 1940 - Development of radiometers sensitive to the infrared radiation;
    • - Use of infrared film in World War II for the detection of camouflage.
  • 1944 - First experiments with the use of multispectral cameras.
  • 1954 - Development of microwave radiometers.
    • - Initial tests for the development of side looking radars.
  • 1961 - Development of optical and digital processors.
    • - First side looking radars.
  • 1962 - Development of manned and unmanned spacecraft;
    • - Launch of meteorological satellites;
    • - First orbital photography from MA-6-Mercury.
  • 1972 - Orbital photography taken from the Gemini Program;
    • - Other spatial programs are born including natural resources satellites: SEASAT, SPOT, ERS, LANDSAT.
  • 1983 - Launch of Landsat 4, SIR-A, SIR-B, MOMS;
  • 1991 - Launch of ERS-1.

DEFINITION

A definition of remote sensing could be: "It's the use of sensors for the acquisition of information about objects or phenomena without a direct contact between them".

Sensors : equipment capable of collecting energy from the object, converting it into a signal capable of being registered and presented in a way that is adequate for the extraction of information.
Energy : in the majority of times refers to the electromagnetic energy or radiation.

A more specific concept could be: "It's the set of activities related to the acquisition and the analysis of data from remote sensors", where:

Remote
Sensors: photographic or opto-electronic systems capable of detecting and registering, as images or not, the flux of radiant energy reflected or emitted by distant objects.

A flux of electromagnetic radiation while propagating through space can interact with surfaces and objects, being reflected, absorbed, and even reemitted by them. The variations that these interactions produce in the flux are strongly dependent on the physicochemical properties of the elements on the surface. Ahead we will discuss with more detail the interactions between radiation and matter.

Everything in nature is in constant vibration, emitting or modifying electromagnetic waves (energy) and presenting "perturbations" in the magnetic and gravimetric fields of the earth. All the instruments that detect and transform this energy could be classified as sensors: radio, television, photographic camera, etc..

During the data acquisition phase by the sensors we can distinguish the following basic elements: radiant energy, radiation source, object (target), trajectory and sensor (optical imaging system and detector). The following figure presents such elements and exemplifies the many ways that electromagnetic radiation can take before hitting the sensor system.

A photographic camera with flash could be given as an example of sensor system: "when the camera system is activated the flash is triggered and emits radiation. The radiation travels to the target and is reflected by it towards the camera's optical system. The reflected radiation if focused onto the film plane, that is a photochemical radiation detector. An image of the radiation pattern is recorded on the film and is later chemically developed."



Trajectories of radiation

Every time that work is done, some type of energy is transferred from one body to another, or from one place in space to another. Of every possible forms of energy, one is of special interest to remote sensing, being  the only one that does not need some material means in order to propagate,  that is the radiant or electromagnetic energy. The most familiar example of radiant energy and the one of utmost importance is the solar energy, that propagates through empty space from the Sun to the Earth.

 

Introduction to Remote Sensing


Electromagnetic Spectrum

The electromagnetic radiation (waves) is defined by many physical characteristics (intensity, wavelength, frequency, energy, polarization, etc..). However, independently of these characteristics, every electromagnetic wave is essentially identical, presenting an independency on relation to the existence or non-existence of a propagation medium (an important property of this energy transfer process). This independency is easy to understand in the following figure, the electric field and the magnetic field are perpendicular to each other and both oscillate perpendicular to the direction of the wave propagation, thus the electric field generates the magnetic field while the magnetic field generates the electrical field.

Where: E = Electric Field

M = Magnetic Field



The speed of the electromagnetic wave propagation in the vacuum is the speed of light (3 x 108 m/s). The number of waves that pass through a point in space in a certain time interval defines the frequency (f) of the radiation. The frequency of the wave is directly proportional to its velocity of propagation.  The higher the velocity of propagation, the higher number of waves that will pass through a certain point in a certain time (t) and the higher it will be its frequency. The velocity of propagation (v) will be constant in a certain medium of propagation.

The electromagnetic wave can also be characterized by its wavelength (lambda) that can be expressed by the equation:

The range of wavelengths or frequencies where we can find electromagnetic waves is unlimited With the present technology we can generate or detect electromagnetic waves in a wide range of frequencies, extending from 3 Hz to 300.000.000 GHz, or wavelengths in the range between 108 meters to 0.01 A (angstrons) or 10-12 m.

The spectrum is subdivided in ranges, representing regions that possess characteristics that are specific to the physical processes, generators of energy in each range, or  to the physical mechanisms that detect this energy. Depending on the range of the spectrum, we work with energy (electron-volt), wavelength (micrometer), or frequency (Hertz). For example: in the range of gamma and cosmic rays we use the energy; in the range of UV (ultraviolet) or IR (infrared) we use the wavelength, while in the microwave and radio region we use the frequency. The main ranges of the spectrum are described below and represented in the following figure:

Electromagnetic Spectrum.


    Radio waves
    : low frequencies and long wavelengths. The electromagnetic waves in this range are used for communications over long distances, for, besides being little attenuated by the atmosphere, they are reflected by the ionosphere, allowing for their propagation over range distances. 

    Microwaves: situated in the range between 1 mm to 30 cm, or 3 x 1011 Hz (300 GHz) to 1010 Hz (10 GHz). In this range of wavelengths we can create beams of electromagnetic radiation that are very concentrated, used in radars. Little atmospheric attenuation, or blockage by clouds, makes radar a very good observation means in any weather condition.

    Infrared: of great importance for Remote Sensing. Encompasses radiation with wavelengths ranging from 0,75 µm to 1,00 mm. The IR radiation is easily absorbed y many substances (a warming effect).

    Visible: is defined as the radiation capable of producing the sensation of vision of the normal human eye. Small variation in the wavelength (380 to 750 nm). Of importance to Remote Sensing, since images collected in this range, will generally present a great correlation with the visual experience of the interpreter.

    Ultraviolet: wide range of the spectrum (10 to 400 nm). Photographic film is more sensitive to ultraviolet radiation, than visible light. Used for the detection of minerals by luminescence and marine pollution. Strong atmospheric attenuation in this range presents a great obstacle against its use.

    X
    Rays: range of 1 A to 10 nm (1 A = 10-10 m). Are mainly generated by the stoppage or disacceleration of high energy electrons. Since they are constituted by high energy photons, the X-Rays are extremely penetrating, being thus a powerful tool in the research of the structure of the matter.

    GAMMA Rays: are the most penetrating rays from the emissions of radioactive substances. There is in principle no upper limit to the frequency of gamma rays, despite we have an upper frequency range called cosmic rays.

* The most used range in Remote Sensing lies between 0,3 µm and 15,0 µm (known as the optical spectrum), for in this range the optical components of reflection and refraction, such as lenses, mirrors, prims, etc.., are used to collect and reorient the radiation.

Electromagnetic radiation sources

The sources of Electromagnetic radiation (EMR) can be divided in natural (Sun, Earth, Radioactivity) and artificial (Radar, Laser, etc..).

The Sun is the most important natural source, for its energy, when interacting with the many substances on the surface of the Earth, originates a series of phenomena (reflection, absorption, transmission, luminescence, warming, etc..) that are investigated by Remote Sensing.

Any electromagnetic energy source is characterized by its spectrum of emission, that can be continuous or distributed among discrete ranges. The Sun, for example, emits radiation continuously distributed in the range that spreads from X-Rays down to the microwave region, though concentrated in the 0,35 µm - 2,5 µm interval.

Any substance with a temperature above absolute zero (0o K or -273o C) emit electromagnetic radiation, as a result of its atomic and molecular oscillations. Such radiation can strike the surface of another substance where it could be reflected, absorbed or transmitted. In the case of absorption, the energy is usually reemitted, usually in a different wavelength.

In practice these four processes: emission, absorption, reflection, and transmission occur simultaneously and their relative intensity characterize the substance under investigation. Depending on its physical and chemical characteristics, those four processes occur with different intensities in different regions of the spectrum. Such spectral behavior of the many substances is called spectral signature and is used by Remote Sensing to distinguish the many substances from one another.

Atmospheric propagation effects of EMR

When collecting data from a remote sensor, be it onboard a satellite or airplane, the collected signal, most of the time, is the radiation from the Sun, that interacts with the atmosphere before striking the target and returning to the sensor after interacting with the atmosphere again. Even if the measured signal is the radiation emitted by the target, it interacts with the atmosphere before reaching the sensor.

There are regions of the electromagnetic spectrum to which the atmosphere is opaque, that is, it does not allow that radiation to pass through. Those regions define the "atmospheric absorption bands". The regions of the spectrum where the atmosphere is transparent to the electromagnetic radiation from the Sun are known as "atmospheric windows".

We should thus always consider the following factors associated to the atmosphere, since they interfere with Remote Sensing: absorption, air masses effects, spreading due to gaseous molecules or particles in suspension, refraction, turbulences, radiation emission by the atmospheric constituents, etc..

This way we conclude that the attenuation of radiation is given by:


Absorption

the energy of an electromagnetic radiation beam is transformed into other forms of energy. It is a selective attenuation observed in various constituents, like water vapor, ozone, carbon monoxide, etc.. In many cases it can be ignored, because it is too small.

Spreading

the energy of a collimated beam of electromagnetic radiation is removed by a change in the direction. When interacting with the atmosphere, by the process of spreading, will generate a diffuse field of light, that will propagate in all directions.

    There are two types of spreading:

    • (a) - Molecular or Rayleigh spreading: essentially produced by the molecules of gases of the atmosphere. It is characterized by the fact that its intensity is inversely proportional to the fourth power of the wavelength (). That explains the blue color of the sky, where the wavelength in this range is shorter.

    • (b) - MIE spreading: it occurs when the size of the spreading particles is of the order of the radiation wavelength.

    • (c) - Non selective spreading: it occurs when the diameter of the particles are much greater than the wavelength. The radiation of different wavelengths will be spread with the same intensity. The white appearance of the clouds is explained by this process.
    • * The attenuation of the radiation can explain the reddish color of dusk, that is, due to the greater length of atmosphere that the radiation has to cross, and where the shorter wavelengths (blue) of the light are captured, allowing through the red component of the solar light.
    • ** Due to the attenuation factor it is important that a planning be done before the data acquisition and the interpretation processes.

       


Sensor Systems

As we've seen every material and natural phenomena absorb, transmit, reflect and emit selectively the electromagnetic radiation. With the present development it is possible to measure with reasonable precision from the distance, the spectral properties of those materials and phenomena.

Any sensor system presents the following components needed to capture the electromagnetic radiation (see the figure below).


System Components.

    where: collector = receives the energy through a lens, mirror, antennas, etc..
    detector = captures the energy of a certain range of the spectrum;
    processor = the recorded signal is submitted to a processing (development, amplification, etc..) through which the product is obtained;
    product = contains the information needed by the user.


Sensor Types

The sensors can be classified as a function of the energy source or as a function of the product type it produces.

As a function of the energy source:


    A-) PASSIVE :  does not have an internal source of radiation. It measures the solar radiation reflected or the radiation emitted by the targets, for example: photographic systems.

    B-) ACTIVE : have their own source of electromagnetic radiation, working in strict ranges of the spectrum, for example: radars.

As a function of the product type:

    A-) Non-imagers: do not provide an image of the sensored surface, for example: radiometers (output in numbers or graphics) and spectroradiometers (spectral signature). They are essential for the acquisition of detailed information about the spectral behavior of the objects on the surface of the earth.

    B-) Imagers: give as a result an image of the observed surface. Provide information about the spatial variation of the spectral response of the observed surface.

      B.1 - framing systems: acquire the whole image of the scene at one instant (ex: RBV)

      B.2 - scanning systems: for example: TM - MSS - SPOT.

      B.3 - photographic system

The non-photographic imagers (scanning systems) came to fill the gap left by the then most used optical sensor device: the photographic camera. This, besides presenting easier operation and lower costs presents a limitation in the capture of the spectral response, due to the films that cover only the range between the near ultraviolet and the thermal infrared. This type of sensor is also limited to the overpasses hours and due to atmospheric phenomena do not allow the frequent observation of the ground from high altitudes.

Since the data from these non-photographic sensors are colleted in the form of electrical signal, they can be easily transmitted to distant stations, where an electronic process will make its discriminatory analysis.


The table below presents a comparative analysis of the photographic sensors and scanning imagers.



Photographic Sensors
Scanning Imagers
Geometric resolution
high *
medium
Spectral resolution
medium
high *
Repetitively low
high *
Synoptic vision
low
high *
Database
analogical
digital *
* greater advantage over the other

Introduction to RS