Remote sensing of the earth from space

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  • This article was downloaded by: [Universitaets und Landesbibliothek]On: 29 November 2013, At: 04:22Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

    Contemporary PhysicsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tcph20

    Remote sensing of the earth from spaceF. W. Taylor aa Atmospheric,Oceanic and Planetary Physics , Clarendon Laboratory, OxfordUniversity , EnglandPublished online: 20 Aug 2006.

    To cite this article: F. W. Taylor (1996) Remote sensing of the earth from space, Contemporary Physics, 37:5, 391-405,DOI: 10.1080/00107519608217544

    To link to this article: http://dx.doi.org/10.1080/00107519608217544

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  • Contemporary Physics. 1996, volume 37, number 5, pages 391-405

    Remote sensing of the Earth from space

    F. W. Taylor

    Remote sensing is the popular name for the measurement ofphysical quantities at a distance, usually by quantitative spectroscopic methods. Its application to the study of the Earth's atmosphere und surface from satellites is a rapidly growingfield, with many applications, and new or improved techniques are constantly being developed. In this article we look at some of the basic principles and technical challenges involved, and at the beneJts which are accruing in a number of research areas in earth science, with particular emphasis on the use of remote sensing to understand global change.

    1. Introduction The Earth, suspended in space, emits a varying stream of electromagnetic radiation in all directions at wavelengths ranging from the ultraviolet to the microwave. On the dark side of the planet, the photons which make up this energy flux have their origins in emission from the Earth's surface and atmosphere, and their intensity and wavelength distributions are complicated functions of the composition and physical state of the emitter. On the daylit side of the globe, there are additional photons of solar origin which have been reflected from the surface or from clouds, and scattered from aerosols or, at the shorter wavelengths, from the molecules of gas which make up the atmosphere. Overall about one third of the flux from the Sun, on average, is returned to space. The rest is absorbed, its energy reappearing later as long-wave infrared and microwave emission. Like the emitted flux, the backscattered and reflected photons also carry information about the planet as a physical system, and about the human environment below.

    The experimental techniques by which the rich informa- tion content of the outgoing photon flux is harvested and analysed has become known as remote sensing. The information is coded into the intensity as a function of wavelength, and so can be acquired by measuring pre-selected, and carefully calibrated, parts of the electromagnetic spectrum of the object of interest. This approach should not be confused with what we might call surveillance from space, using high-resolution photography and television, usually limited to the visible part of the spectrum. Such observations can be important scientifically as well as economically and strategically, and are often included under the remote sensing banner. However, remote sensing

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    Author '.F address: Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Oxford University, England

    techniques based on the use of imaging are usually of limited intrinsic interest to physicists and we will consider them only marginally in this article. On the other hand, where spectroscopy or radiometry is interpreted in terms of vertical projiles of quantities such as atmospheric temperature or ozone abundance, for example, the techni- que (sometimes called remote sounding in this case) involves advanced optical methods and sophisticated mathematical approaches to information retrieval, and we will be discussing these in some detail.

    The main purpose of this paper is to describe the measurement techniques and the interpretation methods of remote sensing, and to show how these can be applied to various important aspects of the behaviour of the Earth viewed as a physical system. Space techniques are the most interesting, because the combination of systemic, long-term global coverage and clever measurement and retrieval methods is what has made remote sensing such a valuable new tool in so many areas. Obviously, many of the methods can also be employed from ground- or aircraft-based platforms, and frequently they are. However, it is their use from satellites (and not only those orbiting the Earth) that has generated an information explosion which is revolutionizing our understanding of the behaviour of the planet and its surroundings.

    The most basic observations we can make are those of the energy budget of the whole Earth, from an investigation of the balance between the ingoing and outgoing fluxes of radiative energy which drive this formidable thermody- namic engine. Going beyond that, infrared spectroscopy and radiometry are well-known approaches to measuring the temperature and composition of atmospheric gases and solid surfaces, and are now routinely used from space platforms to observe the atmosphere, oceans and land surface. The stratospheric ozone layer, pollution in the

    0010-7514/96 $12.00 0 1996 Taylor & Francis Ltd

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  • 392 F. W. Taylor

    Incoming Solar Radiation

    0 5 10 15 20 25 WAVELENGTH/microns

    Ozone Water Vapour Carbon Dioxide

    Figure 1. Schematic mean spectra for the Sun and the Earth, approximated by black bodies at their effective temperatures of about 6000 K and 260 K respectively. The positions of the main absorption bands of the three most important minor constituents in the Earths atmosphere are indicated below.

    lower layers of the atmosphere, and the greenhouse gases which drive global warming, are all conveniently studied in this way. Mineralogy, vegetation (including agricultural products and some of the species in the sea), the icy cryosphere, and volcanism and its products are also candidates. Radar and lidar can be used to investigate land and ice topography, sea-state, and to infer wind fields.

    Operationally, remote sensing is used to observe the weather, snow cover, crop development and so on. In some cases, present remote sensing instruments and methods are too crude to replace older ways of obtaining data completely. For example, satellite measurements of atmo- spheric temperature and humidity near the surface are not good enough to supersede the traditional balloon-borne radiosonde for providing the input to weather-forecasting models, except of course where the latter is not available (which is, however, a good deal of the time over most of the globe). Current meteorological satellite sensors lack vertical resolution and can be confused by clouds. However, their use has already improved weather forecasting and further progress remains rapid, with advances being made which bode well for the future.

    2. Radiation measurements and energy budgets

    2.1. Introduction Figure 1 shows the spectrum of the Sun, approximated as a 6000K black body, together with that of the Earth, approximated as a 250 K black body. The integrated energy under the two curves is equal, except for the very small

    contribution from internal sources of heat within the Earth. Obviously, the details of these two spectra are interesting and important: the solar spectrum deviates significantly from a black body at many wavelengths which drive important processes on the Earth, for example, in the ultraviolet bands responsible for ozone production. Although the Earth, on average, radiates about the same total energy to space as a 250 K black body, the terrestrial spectrum is rich in molecular vibration-rotation lines which contain most of the information about the atmosphere, and continuum fluctuations which represent weak and/or complex atmospheric bands plus surface and cloud emissivity variations. The most pronounced of these spectral features are those caused by absorption and emission in the bands of the principal atmospheric minor constituents, especially water vapour and carbon dioxide, as indicated at the bottom of the figure.

    2.2. Solur irrudiunce The integrated energy from the Sun fluctuates slightly with time, with an amplitude of the order of 0.1%, but the intensity at some wavelengths varies much more than this, as a function of conditions on the Sun [l]. All of these variations can be of considerable interest since the response of the Earth is difficult to predict and, if it can be observed, is quite informative about the Earth-Sun relationship. For example, it has long been discussed whether weather conditions at the surface are influenced by short-term solar variability or not, and the debate continues. It is certainly the case that the abundances of certain important trace constituents, such as nitric oxide, vary dramatically in the upper atmosphere with solar activity (cf. section 5.2).

    The best series of measurements of the total solar input are from the active cavity radiometer irradiance monitor (ACRIM) series of instruments, which have flown on a variety of spacecraft since the Solar Maximum Mission in 1980, and are planned to continue to the year 2000 and beyond. The basic principle is to maintain the temperature of a black body, in the form of a cavity, at a constant value by electrical heating while a shutter over the entrance aperture facing the Sun is opened and closed. The thermodynamic state of the cavity remains constant and a measurement of the current required to compensate for the removal of solar heating gives a straightforward estimate of the latter. The more recent versions of this type of instrument [2] include many detailed refinements and achieve very high accuracies.

    2.3. The solar spectrum The latest measurements of the solar spectrum from space have been made by instruments known as SUSIM (Solar Ultraviolet Spectral Irradiance Monitor [3]) and SOLSTICE (Solar Stellar Irradiance Comparison Experiment [4]) flying

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  • Remote sensing of the Earth from space 393

    on the Upper Atmosphere Research Satellite (UARS) in the last few years since 1991. The spectral region of main interest is the ultraviolet, since the variability is greatest here (of the order of 10% at some wavelengths) and these are the more energetic photons which drive photochemical reactions in the atmosphere. SUSIM and SOLSTICE are grating spectro- meters which, between them, cover the range from 119 to 420nm. Since the absolute intensity as a function of wavelength is required, calibration is a major issue; SUSIM uses a series of standard lamps, while SOLSTlCE has an innovative approach by which the input aperture is variable, allowing the observation of a number of standard UV stars.

    The results from instruments of this type [5] show, as might be expected, large variations, especially in the Lyman- a band, with the same frequency as the 27-day solar rotation and with sunspot activity, although other, smaller, periodic and episodic variations are also seen. The 27-day cycle produces a corresponding signal in the atmospheric tempera- ture at very high levels, and sunspot eruptions on the Sun produce particle fluxes which have been observed to strongly modulate the thermospheric nitric oxide abun- dance, over localized regions and for a short time, as already noted. It is not yet clear whether there is any measurable effect of solar fluctuations on the lower atmosphere.

    2.4. Figure 2 shows schematically the principal ways in which the energy from the Sun is redistributed within the climate system, and finally re-emerges as long-wave infrared radiation. The individual components are variable of course, on a wide range of distance and time scales. For example, the difference between incoming and outgoing radiative energy varies markedly as a function of latitude because of the transport of heat, generally in a poleward direction, by the atmosphere and the oceans, and this transport has a seasonal dependence which is not the same every year. Climate models, like those now being extensively

    The energy balance of the Earth

    m Emitted to space Waqs and Eddies

    Reflected from 1 Absorbed 1 cloudsand uVvlM Adv&im of latent and sensible heat

    DY atmosphere I

    Emitted by atmosphere

    Convection

    I I I 1 by atmosphere 1 / Reflected from surface Absorbed by surface

    EARTHS SURFACE Emitted by surface

    Figure 2. Some important atmospheric processes (radiative in plain script, chemical in capitals, and dynamical in italics) involved in converting the sunfall into outgoing thermal radiation.

    employed to attempt medium-to-long term forecasts and to assess the likely progress of global warming, need to have the correct fluxes of reflected solar and emitted thermal radiation at the upper boundary as a necessary, if not sufficient, condition for realistic predictions. The extent and microphysical nature of cloud cover is one of the most important and unpredictable variables affecting these [6].

    The current state-of-the-art in making the relevant observations is represented by the multi-satellite Earth Radiation Budget Experiment (ERBE). The measurements are simple in principle but actually very difficult in practice, mainly because the complete range of wavelengths has to be sampled at every outgoing zenith angle all over the Earth. The ERBE scanners [A have three broad spectral channels, covering the wavelength ranges from 0.2 to 5...

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