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This paper presents a simple mathematical formula for correcting radio-echo-sounding data from an ice sheet or glacier for the effects of varying refractive index. The method requires some knowledge of the variation of refractive index with depth. In rare cases this may be known from direct measurement, but it can be estimated from the density profile. If even this is unknown, we show that the corrections can be estimated to an accuracy of about 3% of the depth to bubble close-off (i.e. to about ±12 m for dry-snow conditions), and that the size of the refraction correction for a glacier or ice sheet with a flat bed is typically 6–10 m under these conditions.
In Chapter 3 we discussed principally the interaction of electromagnetic radiation with the surface and bulk of the material being sensed. However, the radiation also has to make at least one journey through at least part of the Earth's atmosphere, and two such journeys in the case of systems that detect reflected radiation, whether artificial or naturally occurring. Each time radiation passes through the atmosphere it is attenuated to some extent. In addition, as we have already seen in Section 3.1.2 and Figure 3.5, the atmosphere has a refractive index that differs from unity so that radiation travels through it at a speed different from the free-space speed of 299 792 458 m s−1. These phenomena must be considered if the results of a remotely sensed measurement are to be corrected for the effects of atmospheric propagation, or if they are to be used to infer the properties of the atmosphere itself. We have already considered them in general terms in discussing the radiative transfer equation (Section 3.4). In this chapter we shall relate them more directly to the constituents of the atmosphere.
Composition and structure of the gaseous atmosphere
At sea level, the principal constituents of the dry atmosphere are molecules of nitrogen (about 78% by volume), oxygen (21%) and the inert gas argon (1%). There is also a significant but variable (typically 0.1% to 3%) amount of water vapour, often specified by the relative humidity H.
Fully updated and containing significant new material on photography, laser profiling and image processing, the third edition of this popular textbook covers a broad range of remote sensing applications and techniques across the Earth, environmental and planetary sciences. It focuses on physical principles, giving students a deeper understanding of remote sensing systems and their possibilities, while remaining accessible to those with less mathematical training by providing a step-by-step approach to quantitative topics. Boxed examples, additional photos and numerous colour images engage students and show them how the theory relates to the many real-world applications. Chapter summaries, review questions and additional problems allow students to check their understanding of key concepts and practise handling real data for themselves. Supplementary online material includes links to freely available software, animations, computer programs, colour images and other web-based resources of interest.
There are many books that explain the subject of remote sensing to those whose backgrounds are primarily in the environmental sciences. This is an entirely reasonable fact, since they continue to be the main users of remotely sensed data. However, as the subject grows in importance, the need for a significant number of people to understand not only what remote sensing systems do, but how they work, will grow with it. This was already happening in 1990, when the first edition of Physical Principles of Remote Sensing appeared, and since then increasing numbers of physical scientists, engineers and mathematicians have moved into the field of environmental remote sensing. It is mainly for such readers that this book, like its previous editions, has been written. That is to say, the reader for whom I have imagined myself to be writing is educated to a reasonable standard (although not necessarily to first degree level) in physics, with a commensurate mathematical background. I have however found it impossible to be strictly consistent about this, because of the wide range of disciplines within and beyond physics from which the material has been drawn, and I trust that readers will be understanding when they find the treatment either too simple or over their heads.
In Chapter 1 we noted that electromagnetic radiation is fundamental to remote sensing as we have defined it: the information about the sensed object is carried by this radiation. We therefore need to develop an understanding of the essential characteristics of this radiation and of how it interacts with its surroundings. This is a large topic and it is covered in this chapter and the next two. In this chapter we consider electromagnetic radiation in its simplest form, when it is propagating in (travelling through) a vacuum, usually termed ‘free space’. This is practically useful, because for much of its journey towards the sensor the radiation is propagating in a medium that approximates to free space, and it also allows us to develop some of the essential ideas that describe electromagnetic radiation without too much confusing detail.
A particularly important part of this chapter deals with thermal radiation. As we noted in Chapter 1, most passive remote sensing systems detect thermal radiation (in the infrared or microwave regions) or they detect reflected solar radiation. Solar radiation is itself, as explained in this chapter, essentially just another form of thermal radiation, so by developing an understanding of thermal radiation we are able to describe many of the characteristics of the radiation detected by passive systems.
In Chapter 5 we discussed photographic systems, and although these provide a familiar model for many of the concepts to be addressed in this and subsequent chapters, they nevertheless stand somewhat apart from the types of system to be discussed in Chapters 6 to 9. In the case of photographic systems, the radiation is detected through a photochemical process, whereas in the systems we shall now consider it is converted into an electronic signal that can be detected, amplified and subsequently further processed electronically. This clearly has many advantages, not least of which is the comparative simplicity with which the data may be transmitted as a modulated radio signal, recorded digitally and processed in a computer.
In this chapter, we shall consider electro-optical systems, interpreted fairly broadly to include the visible, near-infrared and thermal infrared regions of the electromagnetic spectrum. The reason for this is a pragmatic one, since many instruments combine a response in the visible and near infrared (VNIR) region with a response in the thermal infrared (TIR) region, and much of the technology is common to both. Within this broad definition we shall distinguish imaging systems, designed to form a two-dimensional representation of the two-dimensional distribution of radiance across the target, and systems used for profiling the properties and contents of the atmosphere. It is clear that an imaging system operating in the VNIR region has much in common with aerial photography, and systems of this type are in very wide use from both airborne and spaceborne platforms. We shall therefore begin our discussion with these systems.
‘Remote sensing’ is, broadly but logically speaking, the collection of information about an object without making physical contact with it. (The term was coined by Evelyn Pruitt of the US Office of Naval Research in the 1950s.) This is a simple definition, but too vague to be really useful (Campbell 2008), so for the purpose of this book we restrict it by confining our attention to the Earth’s surface and atmosphere, viewed from above using electromagnetic radiation. This narrower definition excludes such techniques as seismic, geomagnetic and sonar investigations, as well as (for example) medical and planetary imaging, all of which could otherwise reasonably be described as remote sensing, but it does include a broad and reasonably coherent set of techniques, nowadays often described by the alternative name of Earth observation. These techniques, which now have a huge range of applications in the ‘civilian’ sphere as well as their obvious military uses, make use of information impressed in some way on electromagnetic radiation ranging from ultraviolet to radio frequencies.
One important casualty of our restricted definition of remote sensing is the use of spaceborne methods of measuring the Earth’s gravitational field. Although observations from artificial Earth satellites have been used since the 1970s to measure the Earth’s gravity, our current (at the time of writing, in 2012) ability in this regard is a remarkable indication of the level of space technology. This is the GRACE (Gravity Recovery and Climate Experiment) mission, launched in 2002. Two satellites, each with a mass of around half a tonne, follow the same orbit 500 km above the Earth’s surface. They are approximately 220 km apart, and the distance between them is constantly monitored with an accuracy of 10 µm. This distance changes as the satellites cross regions of different gravitational field strength. The GRACE system is sensitive enough to respond to changes in groundwater in a large river basin. Data from the GRACE mission are described in Chapter 8.
The interaction of electromagnetic radiation with matter is evidently fundamental to remote sensing. The subject is a vast one, embracing many areas of physics, and a fully systematic treatment of it would require at least a book in itself. In this chapter, therefore, we attempt to provide an overview that will be sufficient to gain an understanding of the operation of remote sensing systems. In order to keep the chapter to a manageable length, we reserve a discussion of the interaction of electromagnetic radiation with the Earth’s atmosphere to Chapter 4. Nevertheless, this is still a long chapter, and it is also the most technical in the book. It is not necessary to understand all of the material in this chapter in order to follow the subsequent material but, as usual, a reading of the summaries at the end of each section should give a general understanding of the material.
The chapter first extends some of the concepts of Chapter 2 into a consideration of how electromagnetic radiation propagates in homogeneous dielectric media. The key concepts here are the dielectric constant (also known as the relative electric permittivity) and the refractive index. By allowing these parameters to take complex, rather than purely real, values, the concept of absorption of radiation is included, and by allowing them to vary with frequency (or equivalently with wavelength) we are able to include the idea of dispersion, which will be important in Chapter 8 where we consider ranging systems.
Chapters 5 to 7 considered passive sensors, detecting naturally occurring radiation. In this chapter and the next we shall discuss active sensors, which emit radiation and analyse the signal that is returned by the Earth’s surface or atmosphere. We have already identified three possible classifications of remote sensing systems, distinguishing between passive and active and between imaging and non-imaging, as well as classifying them according to the wavelength of radiation employed. We can also classify active systems according to the use that is made of the returned signal. If we are principally concerned with the time delay between transmission and reception of the signal we shall call the method a ranging technique, whereas if we are also (or mainly) interested in the strength of the returned signal we shall call it a scattering technique. The distinction between the two cannot be made entirely rigorous, but it provides a useful way of thinking about active remote sensing systems. It is clear that ranging systems are simpler both to visualise and, because of their less stringent technical demands, to construct, and we shall therefore consider them first. In Chapter 9 we shall discuss the scattering techniques.
Laser profiling (or laser altimetry) is the simplest application of the LiDAR (Light Detection and Ranging) technique. Conceptually it is extremely straightforward (Baltsavias 1999, Flood 2001). A short pulse of ‘light’ (visible or near-infrared radiation) is emitted towards the Earth’s surface by the instrument, and its ‘echo’ is detected some time later. By measuring the time delay and knowing the speed of propagation of the pulse, the range (distance) from the instrument to the surface can be determined. By transmitting a continuous stream of pulses, a profile of the range can be built up, and if the position of the platform as a function of time is accurately known the surface profile may then be deduced.
In this chapter we complete our survey of the principal types of remote sensing instrument by discussing those active systems that make direct use of the backscattered power. Optical (LiDAR) systems are used for sounding clouds, aerosols and other atmospheric constituents, for characterising surface albedo, and for measuring wind speeds. These are discussed briefly in Section 9.1. However, the bulk of this chapter is concerned with microwave (radar) systems. (‘Radar’ is an acronym, originally standing for ‘radio detection and ranging’. The functions that can be performed by microwave scattering systems now extend far beyond detection and ranging, but the term continues to be in very general use.)
In Section 9.2 the ground-work established in Chapter 3 is extended to a derivation of the radar equation, which shows how the power detected by a radar system is related to the usual measure of backscattering ability, the differential backscattering cross-section σ0. The remainder of the chapter discusses the main types of system that employ this relationship. The first and simplest is the microwave scatterometer (Section 9.3), which measures σ0, usually only for a single region of the surface but often for a range of incidence angles. As described here, this is not an imaging system, although the distinction between microwave scatterometers and imaging radars is not a precise one.
In Chapters 5 and 6 we considered passive remote sensing systems in which the diffraction resolution limit λ/D, while important, was not usually a critical parameter of the operation. In this chapter we consider our last major class of passive remote sensing system, the passive microwave radiometer. This is a device that measures thermally generated radiation in the microwave (usually 5–100 GHz) region. (Frequencies much below 1 GHz are unsuitable because of the large signal contributed by the Galaxy, as well as the difficulty of achieving adequate spatial resolution.) As we discussed in Section 2.6, the long ‘tail’ to the Planck distribution at relatively low frequencies means that measurable amounts of radiation are emitted even in this range of frequencies.
Because microwave wavelengths are so much greater than those of visible or even of thermal infrared radiation, the resolution limit plays a much more important role, and we shall need to give more attention to the factors that determine it. More detailed technical treatments of antenna theory are given by Ulaby, Moore and Fung (1981) and by Sharkov (2003), amongst others. Much of the technology and nomenclature of passive microwave radiometry was originally developed in the field of radio astronomy, and further details can also be found in works on that subject.
Aerial photography, as we remarked in Chapter 1, represents the earliest modern form of remote sensing system. Despite the fact that many newer remote sensing techniques have emerged since the first aerial photograph was taken over 100 years ago, aerial photography still finds many important applications, and there are many books that discuss it in more detail than will be possible in this chapter. The interested reader is referred, for example, to Berlin and Avery (2003) and a detailed treatment of photogrammetry and stereogrammetry is give by Kraus (2007). Aerial photography is familiar and well understood, and is a good point from which to begin our discussion of types of imaging system. In particular, it provides a convenient opportunity to introduce some of the imaging concepts that will be useful in discussing some less familiar systems in later chapters.
Photography responds to the visible and near infrared parts of the electromagnetic spectrum. It is, in the context of remote sensing, a passive technique, in that it detects existing radiation (reflected sun- and skylight), and an imaging technique in that it forms a two-dimensional representation of the radiance of the target area. In this chapter we shall consider the construction, function and performance of photographic film, especially its use in obtaining quantitative information about the geometry of objects. Although film-based aerial photography is still dominant, digital photography is beginning (at the time of writing) to rival it, so the chapter includes a brief comparison of the two methods. The chapter then discusses the effects of atmospheric propagation, and concludes by describing the characteristics of some real instruments and giving a brief account of the applications of the technique.
In this chapter we consider aircraft and satellites as platforms for remote sensing. There are other, less commonly used, means of holding a sensor aloft, for example towers, balloons, model aircraft and kites, but we do not discuss these. The reason for this, apart from their comparative infrequency of use, is that most remote sensing systems make direct or indirect use of the relative motion of the sensor and the target, and this is more easily controllable or predictable in the case of aircraft and spacecraft. Figure 10.1 shows schematically the range of platforms, and their corresponding altitudes above the Earth’s surface.
The spatial and temporal scales of the phenomenon to be studied will influence the observing strategy to be employed, and this in turn will affect the choice of operational parameters in the case of an airborne observation or of the orbital parameters in the case of a spaceborne observation. After a brief introduction to the use of aircraft as platforms for remote sensing, this chapter focusses on the use of artificial satellites.
The general direction of this book has been to follow approximately the flow of information, from the thermal or other mechanism for the generation of electromagnetic radiation, to its interaction with the surface to be sensed, thence to its interaction with the atmosphere, and finally to its detection by the sensor. It is clear that the information has not yet reached its final destination. First, it is still at the sensor and not with the data user. Second, the ‘raw’ data will in general require a significant amount of processing before they can be applied to the task for which they were acquired.
In this chapter we shall discuss the more important aspects of the processes to which the raw data are subjected. For the most part, it will be assumed that the data have been obtained from an imaging sensor so that the spatial form of the data is significant. The principal processes are transmission and storage of the data, preprocessing, enhancement and classification. The last three processes are generally regarded as aspects of image processing, a major field of study in its own right, and we shall not be able to do much more than outline its general features. There are many books on the subject to which the interested reader may be referred, for example Campbell (2008), Schowengerdt (2007), Mather and Koch (2010), Burger and Burge (2005).