Introduction

This chapter reviews the retrieval of organic and inorganic, dissolved and suspended material in the water column. While the last chapter discussed the properties of clear water and defined the irradiance reflectance, water-leaving radiance and the radiance received at the satellite, this chapter shows how the presence of dissolved and suspended material alters the wavelength-dependence of the water-leaving radiances from their clear-water values. As the following shows, satellite observations in the visible and near infrared allow retrieval of the oceanic chlorophyll a (Chl-a), the principal photosynthetic pigment associated with oceanic plant life.

Most oceanic plants are microscopic single- or multi-celled free-floating plants called algae or phytoplankton, from the Greek phyton, meaning plant, and planktos, meaning wandering (Jeffrey and Mantoura, 1997). Phytoplankton use photosynthesis to fix inorganic carbon into organic forms of carbon such as carbohydrates. They reproduce asexually, are globally distributed, consist of tens of thousands of species and make up about 25% of the total planetary vegetation (Jeffrey and Mantoura, 1997). Jeffrey and Vesk (1997) and Lalli and Parsons (1993) provide an introduction to the kinds and variety of phytoplankton species.

Introduction

Ocean winds drive the ocean currents, transfer gases, heat, moisture, energy and momentum between the atmosphere and ocean, and, through these processes, influence regional and global climate. In driving the large-scale ocean circulation and small-scale mixing, winds contribute to ocean ecosystems. Hurricanes, typhoons and winter mid-latitude storms present a threat to shipping and coastal regions (Atlas et al., 2011).

Surface winds are the largest source of momentum for the generation of surface waves and basin-scale ocean currents. The oceanic distribution of vector winds determines the height distribution and propagation direction of ocean swell, and allows prediction of the effect of this swell on ships, offshore structures and seacoasts. Because, in regions such as the Southern Hemisphere, surface observations of ocean winds by island weather stations, moored meteorological buoys and ships are sparse, without scatterometers, large areas of ocean would lack wind observations. Scatterometer data are particularly important for improvement of the forecast accuracy in such regions (Chelton et al., 2006). For numerical weather forecasts, Bi et al. (2011) show that the inclusion of these ocean winds leads to improvements in days 4 through 7 of the forecasts.

Introduction

The radar altimeter transmits short pulses of energy vertically downward toward the ocean surface, then receives the reflected signal. The return yields information on the global distribution and variability of sea surface height, ocean swell amplitude and scalar wind speed. Specifically, the time difference between the transmitted and received signals measures the distance or range between the satellite and the sea surface, the shape of the return yields the significant wave height (SWH) and the magnitude yields the scalar wind speed. If the satellite orbit is precisely determined and the range is corrected for a variety of ionospheric, atmospheric and ocean surface factors, these observations measure, to an accuracy of 2–3 cm, the changes in sea surface height (SSH) associated with tides, geostrophic currents and other oceanic phenomena.

This chapter describes how the altimeter works, discusses its sources of uncertainty and describes some of its oceanographic applications. Wunsch and Stammer (1998) and the collection of papers edited by Fu and Cazenave (2001) contain more detailed and extended discussions of the TOPEX altimeter results, and Chelton et al. (2001b) describe the physics of the TOPEX altimeter and its associated error budget. Morrow and Fu (2010) organized and provided the introduction to a special issue of Marine Geodesy on the JASON-2 mission; see JASON-2 (2010) for a listing of the papers. Fu (2010) describes the current state of the altimeter measurements and their application to the ocean circulation. At scales of 150–200 km, multiple altimeters provide near global coverage of the temporal and spatial scales of ocean variability, the meridional transports of heat and the distribution and properties of ocean eddies.

Introduction

A radar is an active microwave device that transmits short directional pulses of energy, then operates as a sensitive receiver to measure the returned energy or radar echo. The term radar is an acronym for radio detection and ranging. The oceanographic value of radar is due to its response to different surface conditions. When the pulse interacts with a surface that is strongly reflective, the return is strong or bright; when the surface is non-reflective, the return is weak or dark. The properties of the reflected and scattered pulse are called backscatter. Because of the large variety of oceanic surface phenomena that modulate the backscatter, radars can retrieve wind speed and direction, ocean swell properties and the presence of heavy rain. They can also make precise measurements of distance and observe phenomena such as internal waves, sea ice, oil and biological slicks and human-made structures such as ships and oil platforms.

Two specialized radars discussed in this and following chapters are the scatterometers and imaging radars. By transmitting a pulse, receiving the return, then correcting the return for atmospheric interference and instrument noise, a scatterometer makes quantitative measurements of the backscatter from small surface areas. With these corrections, any radar that measures backscatter can serve as a scatterometer (Ulaby et al., 1981, pp. 9–10). In oceanography, the backscatter return from a scatterometer is used to retrieve vector wind speed. In contrast, the backscatter from an imaging radar consists of the return from a large surface area, which is then binned according to either time delay or Doppler shift. This subdivides the field-of-view (FOV) into many small areas, which, when combined with the radar motion, produces an image of the area covered by the FOV, where the image consists of the relative changes in backscatter.

Introduction

Passive microwave radiometers provide a powerful, nearly all-weather technique for retrieval of a wide variety of ocean, sea ice and atmospheric geophysical variables. In contrast to the lenses and mirrors used in the VIR, antennas are used with passive microwave radiometers to receive the Earth-emitted radiances, and with radars to transmit and receive energy pulses. The present chapter describes the radiometer antennas and their terminology, discusses how radiometers work and summarizes the properties of the past, present and near-future operational passive microwave imagers. The next chapter describes the atmospheric transmissivity in the microwave, discusses the dependence of the surface emissivity on a variety of geophysical variables, gives examples of the retrieval algorithms and concludes with examples.

Microwaves occupy that part of the electromagnetic spectrum between 1 and 500 GHz in frequency, or between 0.3 m and 1 mm in wavelength. The microwave band is bounded at low frequencies by the television broadcast bands, the presence of which makes Earth observations difficult, and at high frequencies by the far-infrared wavelengths where alternative methods exist for detection. The frequency dependence of the atmospheric transmissivity and interference from other users restricts the frequencies used in retrieval of oceanic variables to specific windows in the range 1–90 GHz. The importance of microwave observations is that the ocean surface emissivities and atmospheric transmissivities depend both on frequency and on geophysical variables such as atmospheric water vapor and liquid water, rain rate, sea surface temperature and salinity, wind speed, and sea ice type and extent. As Chapter 9 discusses in detail, to retrieve these variables, the instruments must observe the ocean at several different frequencies.

Introduction

This chapter describes those properties of electromagnetic radiation (EMR) relevant to remote sensing. Specifically, Section 3.2 gives a brief description of the nature of electromagnetic radiation, its propagation in different media and its polarization. Section 3.3 describes several different ways of describing radiation fluxes. Section 3.4 discusses blackbody radiation, Planck's equation and the concepts of emission and absorption. Section 3.5 discusses the basic optics applicable to an instrument operating in the visible and infrared, then describes the operation and spatial resolution of an ideal instrument. The section concludes with a discussion of terms such as bandwidth and signal-to-noise ratio that are applicable to real instruments.

Descriptions of electromagnetic radiation

As many textbooks describe, EMR has a dual nature, in that it behaves both as discrete quanta of radiation and as electromagnetic waves (Jackson, 1975; Born and Wolf, 1999). In the quantum description, radiation propagates as photons, which are massless, discrete bundles of energy released by atomic or molecular changes of state.

Introduction

This chapter reviews three sets of missions that do not easily fit into the previous chapters. The first set includes three gravity missions, the Challenging Minisatellite Payload mission (CHAMP) launched in July 2000, the US/German Gravity Recovery and Climate Experiment (GRACE) mission launched in March 2002 and the European Gravity field and steady-state Ocean Circulation Explorer (GOCE) launched in March 2009. The second set includes three altimeter missions used for studies of sea and glacier ice. These are the two NASA laser altimeter missions, the Ice, Cloud, and land Elevation Satellite-1 (ICESat-1) that operated from 2003 to 2009, the ICESat-2 satellite planned for launch in 2017, and the dual-beam radar altimeter on the ESA CryoSat-2 satellite. The third set includes the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA/Argentinian Aquarius/SAC-D missions used to measure sea surface salinity (SSS), respectively launched in November 2009 and in June 2011.

Gravity missions

There are two reasons for the importance of a detailed knowledge of the Earth's gravity field. First, in the steady state, altimetric retrieval of sea surface height depends on the shape of the ocean geoid and its accompanying gravity field. Second, understanding the time variability of the Earth's gravity field contributes to our knowledge of the global water cycle. This variability includes the loss or gain of mass in the polar ice caps, changes in the mass distribution associated with the oceanic general circulation, redistribution of mass caused by shifts in bottom currents and the effects of runoff, precipitation and evaporation. Before gravity satellites, many different kinds of measurements contributed to the modeling of the geoid. These included combinations of land- and ship-based gravity measurements and data from the altimeter satellites. The launch of gravity satellites presented the first opportunity to derive the geoid from a single set of measurements (GRACE, 2013).