We could lengthen this list considerably, because the future of glaciology is so bright and is so tightly linked to the future of glaciological practice. Ice is central in the study of the Earth, through archival of the best climate records, control on freshwater fluxes affecting climate and ocean circulation and sea level, and modification of landscapes, among others. Snow and sea ice exert extreme feed-backs on the climate system. The study of ice has been small and marginal compared with many other fields, yet has produced the spectacular results we see. With appropriate levels of interest and support, much larger progress is likely in the future.
We thank numerous colleagues, including those in the WAIS and WAISCORES communities, the Tides project and the Matanuskans. Partial funding was provided by the US National Science Foundation through grants 0229609 and 0229629, and by the Gary Comer Foundation.
tions, is ideally suited to satellite-based observations, which provide the only practical means of obtaining synoptic, regional-scale coverage. It should be stressed, however, that, to be of most value, remote-sensing data require thorough validation and calibration with in situ measurements. Rather than making field observations redundant, therefore, remote sensing is crucially dependent on them.
The subject of remote sensing of glaciers and ice sheets could fill a book in its own right and, as a consequence, what is pre sented here is designed to be a primer, rather than a comprehensive review. The aim of this chapter is to introduce the reader to the key relevant concepts of remote sensing and to demonstrate how these concepts have been used to extend our knowledge and understanding of the cryosphere. There are three objectives of this chapter:
1 to present a selective review of what has been achieved, and what advances have taken place, over the past decade;
2 to identify some of the key gaps in satellite/airborne observations;
3 to consider how these gaps may be filled in the near future.
This section offers a brief refresher in some of the key concepts, sensors and missions that are of particular relevance to glaciol-ogy. These concepts are important to understanding the applications discussed but can be safely skipped by any reader with a basic knowledge of satellite remote sensing.
Observations of the Earth from space utilize a relatively small number of wavebands where modulation of the electromagnetic wave by the atmosphere is low. These wavebands are known as atmospheric windows. Three main windows exist in the visible, infra-red and microwave part of the spectrum. For observing the cryosphere, an increasingly useful waveband is the latter, as clouds are transparent in this part of the spectrum and microwave sensors can be used day or night. In addition, since 1991, a continuous record of high-resolution microwave measurements has been available from an instrument known as a synthetic aperture radar (discussed in section 73.3.3).
All of the satellites discussed below are in what are known as low Earth orbits, at altitudes of typically 750-1000 km and are generally in exact repeat cycles. This means that after a certain number of days the pattern of orbit tracks on the ground is repeated. The length of the repeat period varies anywhere from a few days to a month or longer and this clearly affects the temporal resolution with which processes can be observed. The inclination of the orbit (its angle with respect to the Equator) is also an important parameter as it defines the latitudinal limit of the satellite. Typically, for Earth resources missions, this is around 80°, which means that the highest latitude portion of Antarctica and the Arctic ocean may not be covered by a particular sensor. It is also important to remember that the surface of the Earth is, at any time, covered by about 50% cloud and for some areas, such as the marginal sea-ice zone, this value can be substantially higher. In addition, polar regions suffer extended periods of darkness during their winters. Consequently, the revisit time of a satellite sensor operating in the visible or infra-red does not necessarily indicate its temporal sampling rate. Microwave instruments, however, acquire data under almost all conditions (rain can sometimes produce interference). Their temporal resolution is, therefore, directly related to the revisit interval.
Visible sensors receive solar radiation that has been reflected by the surface but also scattered by the atmosphere back to the sensor. This latter component of the signal, known as sky noise or skylight, is an unwanted signal due to scattering by air mole cules and more significantly particulate matter, which can produce 'haze' in an image. In general, however, sky noise does not seriously affect discrimination of surface types over glacier-ized terrain but it can influence, for example, the calculation of the albedo of a surface (Stroeve et al., 1997). Infra-red sensors measure a combination of reflected solar radiation (below a wavelength of about 3 |lm) and thermal radiation emitted by the surface of the Earth and the atmosphere.
There are a number of books on the general principles of remote sensing that cover, in detail, the concepts and the characteristics of the more common and ubiquitous sensors/satellites (Lillesand & Kiefer, 2000; Rees, 2001). These textbooks, however, do not, necessarily, carry details of the sensors relevant to cryospheric applications. Thus, here, we provide only brief details of established sensors and technology with appropriate references where necessary. Greater detail is provided on recently launched and upcoming missions, especially those that have a particular emphasis on polar applications and those instruments that are particularly relevant to mass balance studies of land and sea ice, that are not well represented in the existing literature.
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