Introduction

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 examples towers, balloons, model aircraft and kites, but we will 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.

Aircraft

Aircraft of various types provide exceptionally convenient and operationally flexible platforms for remote sensing, carrying payloads ranging from a few tens of kilograms to many tonnes. With a suitable choice of vehicle, a range of altitudes can be covered from a few tens of metres, where atmospheric propagation effects are negligible, to many thousands of metres, above most of the Earth's atmosphere. The choice of flying altitude will obviously have an impact on the scale, spatial coverage and spatial resolution of the data collected.

Introduction

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 that the information has not yet reached its final destination. Firstly, it is still at the sensor and not with the data user. Secondly, 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 here than outline its general features. There are many books on the subject to which the interested reader may be referred, for example Mather (1987), Richards (1993) and Schowengerdt (1997).

Transmission and storage of data

It is clear that the data must be brought from the sensor to the place where they are to be analysed.

Introduction

The Global Positioning System (GPS) is a satellite-based positioning system operated by the United States Department of Defense. It has been operational since 1993, and although it is primarily intended for military use, it is, subject to one or two provisos, also available for world-wide civilian use. It is now widely used in remote sensing – in field work, to determine the position of training areas for image classification (see section 11.3.4.2), and also by some remote sensing satellites, for precise determination of the satellite's own position. This appendix therefore presents a brief introduction to GPS and its capabilities. A much fuller discussion is given by, for example, Leick (1995).

The fundamental idea behind GPS is not a new one. It is a radio-positioning system in which timing signals are transmitted at known times from a number of radio beacons at known locations. By measuring the times at which these signals are received, the distances to the various beacons can be calculated, and hence the position of the receiver can be deduced. What sets GPS apart from its predecessors is that the beacons are carried on satellites, providing genuinely global coverage.

Space segment

The ‘space segment’ of the GPS system consists of 24 satellites in circular orbits around the Earth. The semi-major axis (radius) of these orbits is about 26 600 km and the inclination is 55°, giving them a nodal period (see equation (10.13)) of 43 082 s, or exactly half a sidereal day.

Introduction

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 in 1858, 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 chapters 2 to 5 of Avery and Berlin (1992). 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. 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.

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 those people 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 for such readers that this book, like its first edition, 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.

This book attempts to follow a logical progression, more or less following the flow of information from the remotely sensed object to the user of the data. The first four chapters lay the general foundations.

Introduction

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. 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 careful attention to the factors that determine it. The treatment that follows in this chapter is similar to that of Robinson (1994), and is expanded upon by Ulaby et al. (1981, 1982, 1986). 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.

Antenna theory

Angular response and spatial resolution

As we have remarked before, electromagnetic radiation is detected through its influence on electrons, which are excited to higher energy states by the incident photons. The energy of a microwave photon is typically only a few microelectron-volts, which is too small to excite an electron across an atomic or molecular band-gap.

Two planets collected most of the rocky material that was left in the inner nebula after the gas and volatile elements were swept away by early solar activity. They finished up with nearly an equal share. Here I investigate why these twins that have almost the same mass and density have turned into such different worlds.

Venus

Venus, rising in the morning or setting in the evening skies, is the most brilliant object in the sky, after the Sun and the Moon, and has been admired since antiquity. Because it is an apparent twin of the Earth, it has always been of interest as the only similar planet in the solar system. When Venus was found to have an atmosphere, it did not take much imagination to make it a hotter version of the Earth. In popular literature, it was clothed with thick tropical forests and swamps, that were populated with various monsters. Dinosaur-like creatures were favorites of science-fiction writers [1].

Early observers thought that the planet was either spinning rapidly, similar to the 24-hour period of the Earth, or perhaps on a monthly period. But Venus was discovered from radar observations to be unique among the planets. Although the atmosphere of Venus rotates in about four days, the planet itself has a retrograde rotation that takes 243 days [2] (Table 12.1). Venus orbits around the Sun in 225 days and so the day on Venus is longer than the year.

A principal task in writing in the first edition of this book was to examine the series of events that led to the formation of the solar system. The conclusion, so greatly illuminated by the previous three decades of planetary exploration by spacecraft, was that random events had predominated in the construction of the great variety of planets and satellites. Thus it was unlikely that duplicates might be found elsewhere. This was in contrast to earlier views that the solar system was as orderly as a clock and that, given sufficient computer power, one might simulate the construction of such a clockwork system from first principles according to the laws of physics and chemistry.

In the 1992 edition, I commented that “the … common occurrence of disks around young … stars strengthens the case for the existence of other planetary systems. If so, would they resemble our own? Would we see something like the Galilean satellite system of a few equal-sized planets, systems with one giant planet, or a single brown dwarf companion?” After contemplating the satellite systems around our giant planets, I concluded that “no simple sequence of reproducible events has occurred in our solar system. Other planetary systems … will be different in detail to our own. What their satellites might look like is only for bold spirits to predict” (p. 251).

The pre-Copernican view

Pre-Copernican theories in which the Earth was the center of the universe have long lost the attention of scientists. This is not only because such theories have been superseded since the Copernican Revolution, but also because in such hypotheses the origin of the Earth, Sun and planets is inextricably bound up with the origin of the universe. The Earth could hardly be younger than the rest of the universe if it occupied the central position. We are now aware that the solar system is less than one-third of the age of the observable universe. This makes it no longer necessary, as was the case with the authors of the Book of Genesis, to seek a common origin for Earth, Moon, Sun and stars. Most of this progress has been made by the discovery of new facts, not by theories. Galileo's observations, like those of Darwin, have done more to give us a correct view of the world than most of the theorising about it over the centuries.

The Greeks

The Babylonian and Greek astronomers observed the strange motion of the planets against the fixed positions of the stars [1]. In this manner, they became aware that there were two classes of heavenly objects in addition to the Sun and the Moon. It is curious that although the ancient astronomers devoted much study to the movements of the planets, they did not spend much time considering the origin of the solar system.

“After the Sun, the Moon of all the heavenly bodies is that which interests us the most; its phases afford us a measure of time so remarkable that it has been primitively in use among all people” [1]. The Moon and Mercury represent special cases even by the standards of the solar system. Mercury is unique due to its high density, with an iron/silicate ratio about twice that of the other inner planets (Section 11.1). In contrast, the Moon is of interest because of its low density and low metal/silicate ratio [2]. Explanations for the peculiar nature of both bodies have a long history and much effort has been expended in attempts to fit one or both into overall schemes of planetary formation, but without conspicuous success.

The Moon has played a central role in the recent development of theories of the origin and evolution of the solar system. This is not without irony, as it has proven one of the most difficult objects to explain. It is in plain sight, accessible even to naked-eye observation, as Harold Urey (1893–1981), who persuaded NASA to go to the Moon, was accustomed to remind us. The Moon was often thought to be a kind of Rosetta Stone, so that the general belief in pre-Apollo times was that we could discover much about the origin of the solar system by going to the Moon [3].

In this chapter I discuss ideas on the construction of planets. These notions are naturally heavily biased toward observations within our solar system, although I include some comments based on the discovery of extrasolar planets. Although limited, these are already shedding light on some of the problems and importantly on the distinction between planets and brown dwarfs. The conclusions from these new bodies appear to be broadly consistent with present ideas. The extrasolar planets that are found in close orbits probably originated in a similar manner to our own gas giants, although subject to later migration, something that may have also happened to our own giants.

The problem of building planets is fundamental to the entire question of the origin of the solar system. Historically, this latter question has frequently been considered to have been solved, but the wide variety of explanations and solutions that have been offered, from the creation myths of primitive societies, to the more recent, but numerous, scientific attempts, have generally collapsed when faced with new information about the system [1].

There are two principal difficulties. The first dilemma is that until very recently planetary scientists, like historians, had only one example, the present scene, together with whatever relics have survived from previous epochs, to tell the tale of former events [2].