11 Sensors

Dr. Puneeta Pandey

 

1. Aim of the Module

 

The purpose of this chapter is to understand the types of sensors and their applications in remote sensing.

 

2. Introduction

 

Remote sensing is the art and science of obtaining information about an area, object or phenomenon without any physical contact to derive information about that object, area or phenomenon (Lillesand and Keifer, 1994). This requires the use of sensors that may be space-borne (satellites) or air-borne (mounted on balloons, aircraft) that collect data by detecting the energy that is reflected or emitted from Earth surface features. These sensors are not in direct contact with the objects or events being observed. The information needs a physical carrier to travel from the objects/events to the sensors through an intervening medium; which is generally electromagnetic radiation in remote sensing. The end product of remote sensing is an image or a scene that is representative of the area under observation.

 

3. Types of Sensor

 

There are several categories of sensor systems based on source of illumination or the form of data. Based on source of illumination, sensor types include passive and active; while, based on form of data, sensors can be imaging and non-imaging. Some of these sensors are discussed in detail in the following section:

 

3.1 Passive sensors – Passive sensors use sunlight as a source of energy to measure light reflected or emitted from an object or an area. Since sunlight is essential for working of passive sensors, various factors such as time of day and year, latitude and weather conditions, affect the information received from solar illumination. Absence of cloud cover is another important factor since cloud cover can interfere with the transmission of solar radiation. Therefore, data collection using passive sensor should be carried out under clear and dry atmospheric conditions. Passive sensors are the most common sensor type for vegetation related remote sensing since portions of the solar spectrum provide very useful information for monitoring plant and canopy properties. Some examples of passive sensors are:

 

3.1.1 Photographic camera—Photographic camera is a common passive sensor that behaves like a human eye. Both are similar in having a lens and a light-sensitive film/retina and an iris or a shutter for controlling the amount of light that can strike the film/retina. Filters are attached in front of a lens to restrict the wavelength of light permitted to strike the film.

 

There are three basic elements of photographic systems — optics, film, and filters. Optics refers to the lenses responsible for focusing and zooming on an object and the geometry of light retrieval in a camera. The amount of image detail that can be recorded on film is directly related to focal length (the distance between the lens and the film). A photographic film is used to record the image and is coated with a light-sensitive layer known as the emulsion for recording wavelengths between 0.4 to 0.9 micrometers. During the short time that a shutter is open, light strikes the film and leaves a latent image on the emulsion which is then developed and printed. Film speed is another quality of emulsions that refers to the quantity of light needed to expose the emulsion. In many remote sensing applications, filters are used to restrict the light entering the camera. Color filters work by absorbing a range of wavelengths while allowing other wavelengths to pass through. Antihaze filters absorb the shorter ultraviolet and blue wavelengths that are substantially scattered by particulates in the atmosphere. An infrared filter absorbs visible light and only allows infrared light to pass through.

 

3.1.2 Electro-optic radiometers – A radiometer is an instrument designed to measure the intensity of electromagnetic radiation using optical techniques and electronic detectors. Radiometers are similar in design to a camera except that instead of film, they use an electronic detector to record the intensity of electromagnetic energy.

 

Detectors for radiometers are, in general, devised to measure wavelengths from 0.4 to 14 micrometers. Radiometers that measure more than one waveband are called multispectral radiometers. These instruments separate light into discrete wavebands using filters, prisms or other techniques so that multiple waveband or multichannel readings can be taken. Non-imaging radiometers are used for spectral characterization of features, for atmospheric measurements and quantification of solar energy.

 

3.1.3 Passive microwave systems – Passive microwave systems include both non-imaging and imaging systems to detect wavelengths in the microwave region of the spectrum (1mm to 1m). The components of a microwave radiometer are an antenna, receiver, and recording device. Microwave energy emitted from Earth’s surface is collected by an antenna, converted by a receiver into a signal, and recorded. Microwave radiometers measure polarity, wavelength, and intensity of an object to provide useful information about the structure and composition of that object. Most of the applications of passive microwave radiometers have been in measurement of soil moisture and in the fields of atmospheric and oceanographic research.

 

3.1.4 Visible, infrared, and thermal imaging systems – These sensor systems contain various instruments for creating a 2D image of an area in one exposure. Generally a colour composite is generated using three bands in modern multispectral sensors that acquire data for numerous spectral bands at a time.

Figure 1: Multispectral systems (http://www.ok.sc.e.titech.ac.jp/res/MSI/MSI_e.html)

 

There are three basic designs for imaging sensors:

 

a. Frame- It is a two-dimensional array of detectors that acquires an entire image in one exposure.

b. Pushbroom – A pushbroom sensor is a one-dimensional array that obtains an image one line at a time. Each new data line is added as the platform moves forward, building up an image over time.

c. Mechanical scanner- In a mechanical scanner system the sensor acquires only one or several pixels in any given instant, but since the scanner rotates the sensor, an image is produced.

 

3.2 Active Sensors

 

Active sensors supply their own illumination energy and then measure the energy that has returned back after interacting with the surface. Examples include Radio Detection and Ranging (RADAR) and Light Detection and Ranging (LIDAR) systems. Since active remote sensing systems do not require solar illumination of surfaces or perfect weather conditions to collect useful data; consequently, they can be deployed at night or in conditions of haze, clouds, or light rain conditions,

 

3.2.1 RADAR – Radio Detection and Ranging Systems use microwave wavelengths ranging from 1 millimeter to 1 meter. Microwave pulses are transmitted at a target or surface, and the timing and intensity of the return signal is recorded. Factors determining the strength of a radar return signal are geometric and electrical properties of the surface or object that reflects the signal. The transmission characteristics of radar depend on the wavelength and polarization of the energy pulse. Common wavelength bands used in pulse transmission are K-band (11-16.7 mm), X-band (24-37.5 mm), and L-band (150-300 mm). Besides wavelength, polarization of the transmitted energy is an important aspect. Pulses can be transmitted or received in either an H (horizontal) or V (vertical) plane of polarization. Radar can be very useful in areas with nearly constant cloud cover and finds application in diverse fields such as geology, snow and ice studies, oceanography, agriculture, and vegetation studies.

 

3.2.2 LIDAR– Light Detecting and Ranging) systems use laser light in the wavelength of 0.3 to 1.5 micrometers as an illumination source. A short pulse of light is emitted from a laser and a detector receives the light energy (photons) after it has been reflected, or absorbed and remitted, by an object or surface. Thus, the travel time of round trip of a laser pulse is calculated, which gives an idea about the distance between the sensor and the target.

Figure 2: LIDAR (Vermont Center for Geographic Information – Vermont.gov)

 

 

Lidar is referred to as rangefinders or as laser altimeters if deployed on an aircraft or spacecraft; and used to measure elevation, slope, and roughness of land, ice, or water surfaces. Lidar systems that measure the received intensity of the backscattered light are used for atmospheric monitoring applications such as characterization of various gases, aerosols and particulate matter. Besides, Lidar systems can also make fluorescence measurements that help in identifying and quantifying the amount of plankton and pollutants in the marine environment. Leaf fluorescence can also help to identify plant species.

 

4. Imaging and non-imaging sensors

 

Remote sensing data are the recorded representation of radiation reflected or emitted from an area or object. When measuring the reflected or emitted energy, either imaging or non-imaging sensors can be used.

 

4.1 Imaging sensors: Data from imaging sensors can be processed to produce an image of an area, within which smaller parts of the sensor’s whole view are resolved visually. Image data provide an opportunity to understand spatial information by looking at spatial relationships, object shapes, and to estimate physical sizes based on the data’s spatial resolution and sampling.

 

4.2 Non-imaging sensors: Non-imaging sensors usually are hand-held devices that register only a single response value, called as ‘point data’, and therefore, no image can be made from the data. Non-image data give information for a small area or surface cover type, and can be used to characterize the reflectance of various materials occurring in a larger scene.

 

6. Conclusions

 

At the end of this module, you would have been able to gain an understanding of the types of sensor such as active and passive sensors as well as imaging and non-imaging sensors used in remote sensing. While active and passive sensors are distinguished on the basis of source of illumination with active sensors using their own source of energy and passive relying on sunlight; imaging and non-imaging sensors are distinguished on the basis of the form of data. These sensors have varied applications in environment such as geology, vegetation mapping, planktonic diversity, atmospheric properties as well as ice or water surfaces.

 

 

Bibliography / Further Reading

 

  • American Society of Photogrammetry (1975) “Manual of Remote Sensing”, Falls Church, Va.
  • Avery, T.E., and G.L. Berlin, Fundamentals of Remote Sensing and Airphoto Interpretation, Macmillan, New York, 1992.
  • Campbell, G.S., and J.M. Norman, An Introduction to Environmental Biophysics, 2nd ed., Springer, New York, 1997.
  • Campbell, J.B., Introduction to Remote Sensing, 3rd ed., Guilford Press, New York, 2002. Colwell, R.N. (Ed.) 1983. Manual of Remote Sensing. Second Edition. Vol I: Theory,
  • Curran, P.J. 1985. Principles of Remote Sensing. Longman Group Limited, London.
  • Dozier, J., and T.H. Painter, “Multispectral and Hyperspectral Remote Sensing of Alpine Snow Properties,” Annual Review of Earth and Planetary Sciences, vol. 32, 2004, pp. 465-494.
  • Elachi, C. 1987. Introduction to the Physics and Techniques of Remote Sensing. Wiley Series in Remote Sensing, New York.
  • Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, Wiley, New York, 1987.
  • Gibson P.J (2000) “Introductory Remote Sensing- Principles and Concepts” Routledge, London.
  • http://www.ccpo.odu.edu/SEES/veget/class/Chap_5/5_3.htm
  • Joseph, G. 1996. Imaging Sensors. Remote Sensing Reviews, 13: 257-342.
  • Lillesand, T. M., Kiefer, R. W., Chipman, J. W. (2004). “Remote sensing and image interpretation”, Wiley India (P). Ltd., New Delhi.
  • Sabins, F.F. 1997. Remote Sensing and Principles and Image Interpretation. WH Freeman, New York.