Chapter 1
Remote sensing and the cryosphere

Marco Tedesco

The City College of New York, City University of New York, New York, USA

Summary

This introductory first chapter provides a general overview on remote sensing and an introduction to the cryosphere, exposing the reader to general concepts. The chapter is mainly oriented toward those readers with minimal or no experience on either of the two subjects. Remote sensing can be defined as to that ensemble of techniques, tools, data and sensors that allow us to study the Earth and its processes from airborne, spaceborne and in situ sensors without being in physical contact with the object under examination.

In the first part of this chapter, a brief history of remote sensing is introduced, describing early tools and applications (such as the pioneering work from air balloons and cameras attached to pigeons), followed by a basic introduction to the electromagnetic spectrum and electromagnetic radiation. The reader is then presented with a description of remote sensing systems, divided into the categories of passive (aerial photography, electro-optical sensors, thermal systems, microwave radiometers and gravimetric systems) and active systems (LiDAR, radar). Concepts such as spatial, temporal, spectral and radiometric resolutions are also introduced. In the second part of the chapter, the several elements of the cryosphere are introduced, together with a description of their basic physical properties and a general overview of their spatial distribution and the impact on other fields (such as biology, ecology, etc.).

1.1 Introduction

This chapter contains a general overview on both remote sensing and the cryosphere and briefly introduces the reader to their general concepts. Both topics are vast, and it is not possible to cover them in their entirety here. Nevertheless, it is helpful to provide an introductory overview of the two fields, with the references in this chapter (and throughout the book) suggesting reading material for those interested in more details.

1.2 Remote sensing

Remote sensing is the collection of information about an object or phenomenon without physical contact with the object. For practical applications, throughout this book we will refer to remote sensing as that ensemble of techniques, tools, data and sensors that allow us to study the Earth and its processes from airborne, spaceborne and in situ sensors without being in physical contact with the object under examination.

Remote sensing of the Earth began with the development of flight. The first photographs of Paris were taken from air balloons as early as 1858 by Gaspard-Félix Tournachon, a French photographer known as Nadar (http://www.papainternational.org/history.asp). In the 1880s, Arthur Batut attached cameras to kites to collect pictures over Labruguière, France. The apparatus also included an altimeter so that the scale of the images could be estimated. At the beginning of 1900, the Bavarian Pigeon Corps had cameras attached to pigeons, taking pictures every 30 seconds (http://www.sarracenia.com/astronomy/remotesensing/primer0120.html; Jensen, 2006).

Systematic aerial photography began with World War I and was improved during World War II. At the end of the Wars, the development of artificial satellites allowed remote sensing to begin performing measurements on a large scale, leading to the modern remote sensing era. More information on the history of remote sensing can be found, for example, in Jensen (2006).

1.2.1 The electromagnetic spectrum and blackbody radiation

Remote sensing of the Earth is based on the interaction between electromagnetic waves and matter, with the exception of those approaches based on gravimetry. The interaction between materials and electromagnetic waves depends on both the characteristics of the electromagnetic radiation (e.g., frequency) and on the chemical and physical properties of the targets. In many cases, the source of the electromagnetic radiation is the sun, which can be approximated as a black body (an idealized body that absorbs all incident electromagnetic radiation, regardless of frequency) at a temperature of about 5800 K. Though a large number of remote sensing applications deal with the visible portion (400-700 nm) of the electromagnetic spectrum (Figure 1.1), visible light occupies only a fraction of it. Indeed, a considerable portion of the incoming solar radiation is in form of ultraviolet and infrared radiation, and only a small portion is in form of microwave radiation.

Figure 1.1 Spectral regions used for thermal and passive microwave sensing (Adapted from Lillesand et al., 2007).

Before reaching a spaceborne or airborne sensor, the electromagnetic radiation propagates through the atmosphere, hence interacting with the different atmospheric components. For example, as the sunlight enters the atmosphere, it interacts with gas molecules, suspended particles and aerosols. Because of the preferential scattering and absorption of particular wavelengths and elements, the radiation reaching the Earth is a combination of direct filtered solar radiation and diffused light scattered from the sky.

As the filtered and diffused solar radiation reaches the Earth, it interacts with surface targets (e.g., soil, snow, vegetation, ocean, etc.). Each of these materials interacts with the electromagnetic radiation through absorption, transmission and scattering, depending on its physical properties (e.g., leaves reflects back most of the radiation in the green regions, water reflects blue radiation more than red and green, etc.). Before reaching the sensors and being recorded by the instruments onboard, the upwelling radiation passes again through the atmosphere.

It follows that the atmospheric components (such as water vapor, carbon dioxide and ozone, Figure 1.2) drive the design of the sensors used to study the Earth from space. On the other hand, in the case of thermal infrared radiation (e.g., 800-1400 nm), the sensors will detect the radiation emitted by the surface as a result of the solar heating. In the case of passive microwave remote sensing, the instruments will record the naturally emitted radiation by the objects, because the incoming solar radiation in the microwave region is negligible.

Figure 1.2 Solar spectral irradiance incident on the top of the atmosphere and transmitted through the atmosphere to the Earth's surface. Major absorption bands in the atmosphere are also shown (NASA).

Features characterizing the data collected by remote sensing platforms are spatial, temporal, spectral and radiometric resolutions. Spatial resolution is a measure of the spatial detail that can be distinguished in an image, being, in turn, a function of the sensor design and its altitude above the surface. Temporal resolution is the frequency of data acquisition (e.g., how many acquisitions are collected within a day) or the temporal interval separating successive data acquisitions. Spectral resolution is the ability of the system to distinguish different parts of the range of measured wavelengths (e.g., the number of measured bands and how narrow each band is).

For recording purposes, the energy received by an individual detector in a sensor must be "quantized" (e.g., divided into a number of discrete levels that are recorded as integer values). Radiometric resolution quantifies the number of levels: the more levels that are recorded, the greater is the radiometric resolution. Many current satellite systems quantize data into 256 levels (8 bits of data in a binary encoding system, ), but other systems can have higher radiometric resolution (e.g., 12 bits, levels).

Remote sensing instruments can be categorized into active and passive. Passive sensors measure the radiation that is naturally emitted or reflected by the target. For example, sensors operating in the visible range, measuring the solar radiation reflected by an object on Earth, are passive sensors. Other passive sensors are microwave radiometers, measuring the microwave radiation naturally emitted by the targets (in this case, as mentioned, the solar contribution is negligible). In this book, sensors collecting gravimetric data are also classified as passive. Active sensors emit energy and measure the amount of energy that is reflected or backscattered by the target. Examples of active sensors are radar (Radio Detection and Ranging) or LiDAR (Light Detection and Ranging).

1.2.2 Passive systems

1.2.2.1 Aerial photography

The first collection of aerial photography was performed by the French photographer and balloonist Gaspard-Félix Tournachon, known as Nadar, in 1858 over Paris (though it was destroyed, and the first surviving collection of aerial photography consists of a view of Boston taken from 630 m by James Wallace Black and Samuel Archer King in 1860). Aerial photography was crucial during World War I, supporting many strategic decisions for battlefronts, for example. After the wars, aerial photography became more and more accessible and was used by commercial companies and government agencies for many purposes. More information on the history of aerial photography can be found at http://www.papainternational.org/history.asp (History of Aerial Photography, Professional Aerial Photographers Association, accessed December 28, 2012). Aerial photography is nowadays used for cartography, land use planning and management, archeology and environmental...