Satellite systems

Remote sensing of ocean colour from space is carried out by a variety of radiometers on spaceborne satellites launched by a number of nations around the globe. Coordination of these studies is carried out by the International Ocean-Colour Coordinating Group (IOCCG, www.ioccg. org), established in 1996. There are two types of orbits for Earth observation satellites, polar-orbiting and geostationary. Polar-orbiting satellites typically operate at an altitude of 700 to 800 km, with a revisit time of two to three days, whereas geostationary satellites operate in time scales of hours, which could in principle provide data on the diurnal variation in phytoplankton abundance and productivity.

Of the several Landsat satellites1219 that have been placed in orbit, the ones most relevant to observing aquatic ecosystems are those that contain the Thematic Mapper scanner. This has broad bands in the blue (450-520 nm), green (520-600 nm), red (630-690 nm) and near-infrared (760-900 nm) plus three infrared bands. The unit element of area viewed is 30 m square. Although primarily designed for prediction of crop production, the Thematic Mapper has received some use for coastal and inland waters. The most recent version, the Landsat 7 Enhanced Thematic Mapper Plus, achieves its 185 km cross-track ground swath by using an oscillating mirror to traverse a 15° cross-track field of view. Its 705 km altitude orbit is polar sun-synchronous, so that it scans the entire Earth surface. The instantaneous field of view is 0.043 mrad (0.00123°). The Advanced Land Imager, on the Earth Observing-1 satellite is a prototype of the next generation of Landsat series satellites, with three additional spectral bands and higher radiometric resolution (16 bit, compared to 8 bit).

Two other spaceborne sensors, not designed for studies on waterbody colour and composition, but which have occasionally been used for this purpose, are the Advanced Very High Resolution Radiometer (AVHRR), and the high resolution radiometer (HRV) in the System Probatoire d'Observation de la Terre (SPOT). The AVHRR, the current version of which is on the NOAA-15 satellite, at an altitude of 807 km, has a band in the red (580-680 nm) and in the near-infrared (725-1000 nm) and four other bands in the near-infrared, three of which are for sea surface temperature measurement. Its resolution at sea level is 1 km, with a 3000 km swath. The HRV on the French SPOT platform (latest version -SPOT 5, launched 2002, 822 km altitude) has a green (500-590 nm), a red (610-680 nm) and a near-infrared (780-890 nm) band, and is distinguished by its high ground-level resolution of 2.5 m (with a 60 km swath), and by being pointable (±26°) both across and along track. It has 8-bit digitization.

In October 1978, the Nimbus-7 satellite was launched, carrying the Coastal Zone Colour Scanner (CZCS), which was designed specifically for remote sensing of the marine environment: its optical system is shown in Fig. 7.3. The CZCS had four bands in the visible region, each 20 nm wide, centred on 443, 520, 550 and 670 nm, one band in the near-infrared (700-800 nm) and another band in the infrared (10.5-12.5 mm) for temperature sensing. To facilitate avoidance of Sun glitter the sensor could be tilted so that it scanned at up to 20° from the nadir, ahead of or behind the spacecraft. It had a mirror at 45° rotating at 8.1 revolutions per second, at which was directed a telescope that gave an instantaneous field of view of 865 mrad (0.05°) corresponding, at the satellite altitude of 955 km, to a unit element of area 825 m square at the surface.588,589 It covered a continuous swath 1636 km wide, which for mapping purposes was broken down into individual sections that might typically be 700 km along the spacecraft's track. The radiometric sensitivity of the CZCS was 60-fold higher than that of the MSS on Landsat and had 8-bit

Nimbus Spacecraft
Fig. 7.3 Optical system of the Coastal Zone Colour Scanner in the Nimbus-7 satellite (by permission, from Hovis, 1978).

digitization. A simple threshold, using the high reflectance of clouds and land in the 700 to 800 nm waveband, was used to distinguish these areas from open water: it is only to these latter picture elements that an atmospheric correction procedure was applied.

The CZCS eventually ceased to function in 1986. During its life it revolutionized our knowledge of the global distribution of ocean colour and, more importantly, of those oceanic constituents, particularly phyto-plankton, which affect ocean colour. The archived data accumulated from this scanner during its years of operation are still being worked on. The implications of this new synoptic view of the oceanic biosphere are so far-reaching for our understanding of the marine ecosystem, and in particular for the global carbon cycle, in which oceanic primary production plays a crucial role, that the urgent necessity to replace the CZCS so that this crucially important information flow could continue, was clearly apparent. A number of other, more advanced, ocean colour scanners were developed and are now in orbit. Some of these we shall now consider.

The immediate successor to the CZCS was the SeaWiFS (Sea-viewing Wide-field-of-view Sensor), a joint Earth Observation Satellite (EOSAT)/ NASA project. It was launched on 1 August 1997, on the relatively small Pegasus rocket, which was first carried up to the low stratosphere on board a Lockheed L-1011 aircraft and released at an altitude of 12 km. The Pegasus rocket then engaged, and carried the spacecraft up to a low Earth orbit at 278 km. The SeaStar spacecraft was then released from the Pegasus, and using its own hydrazine propulsion system, raised the satellite to its final 705 km circular orbit.

The scanner is of the whiskbroom type with a rotating telescope, and measurement is carried out in eight wavebands isolated with dichroic beam splitters and interference filters. There are six 20-nm spectral bands in the visible region, centred on 412, 443, 490, 510, 555 and 670 nm, and two bands for atmospheric correction in the near-infrared at 745 to 785 nm (blocked between 760 and 770 nm to minimize interference from the atmospheric oxygen absorption band) and 845 to 885 nm. As the name indicates, the SeaWiFS has a very wide field of view, ±58.3° to either side of the track, which, from its orbit height of 705 km, gives a swath width of 2800 km. To avoid Sun glitter, the instrument can be directed at the nadir, or at 20°, along track or behind. The sea level resolution is 1.13 km, and the radiance values are l0-bit digitized. Figure 7.4 shows the instrument design, and in Fig. 7.5 it is shown deployed on the SeaStar spacecraft on which it is in fact (unlike CZCS on Nimbus-7) the only instrument.

The European Space Agency developed the Medium Resolution Imaging Spectrometer (MERIS).370 This is a programmable, mediumresolution imaging spectrometer operating in the visible and near-infrared range. It is one of the instruments on the ENVISAT satellite, which was launched by an ARIANE 5 rocket on 1 March 2002 into a near-circular, near-polar orbit at 800 km. The optical system, of which there are five identical units within the instrument each with its own diffraction grating, covers the spectral range 390 to 1040 nm, but measurements are confined to 15 spectral bands - which can be selected on command from the ground within this range. For ocean remote sensing, there are eight bands in the visible, centred on 412.5, 442.5, 490, 510, 560, 620, 665 and 681.25 nm, each 10 nm wide except for the 681.25 nm (chlorophyll fluorescence peak) band which is 7.5 nm wide. Of the seven bands in the near-infrared (700-1040 nm) three are used to calculate the atmospheric contribution to the radiance measured in the visible bands above the atmosphere (see §7.3), and the remainder for measurement of water vapour, clouds, vegetation and oxygen absorption. Scanning is of the pushbroom type, and the two-dimensional photodetector arrays are silicon matrix CCDs (charge-coupled devices); the radiance values are 12-bit digitized.

bench

Direction of scan (west to east)

Fig. 7.4 Design of the SeaWiFS instrument for remote sensing of the ocean from space. By permission, Orbital Sciences Corporation, USA.

The instrument field of view is 68.5° centred about the nadir. This wide field of view is shared between the five identical optical modules, each of them receiving 14° of the total field. The spatial resolution at sea level is 300 m when the instrument is operated in full resolution mode (e.g. in coastal waters) and 1200 m in reduced resolution mode (e.g. over the ocean). From the orbit height of —800 km the swath width is 1150 km, and complete coverage of the Earth's surface is carried out every three days.

The Terra and Aqua satellites, developed by NASA, both carry the Moderate Resolution Imaging Spectrometer (MODIS), which can be used for ocean colour sensing. Both spacecraft are in near-polar, circular orbits at an altitude of 705 km. Terra (launched in December 1999) passes from north to south across the equator in the morning, while Aqua

Fig. 4.11 Coccolithophorid blooms in the Atlantic Ocean, west of Ireland and of Cornwall, and also in the Celtic Sea, Monday 18 May 1998. Image collected at NERC Satellite Receiving Station, Dundee, Scotland. (Courtesy of the SeaWiFS Project, NASA Goddard Space Flight Center.)
Kingdoms Kalamar Atlas

Fig. 7.10 Distribution of phytoplankton in the ocean around Tasmania (Southern Ocean, south of the Australian continent), 27 November 1981, derived from CZCS data. Phytoplankton chlorophyll a concentration is colour coded from red (high) to blue (low). Courtesy of the NASA GSFC Earth Sciences Data and Information Services Center (GES DISC).

Fig. 7.10 Distribution of phytoplankton in the ocean around Tasmania (Southern Ocean, south of the Australian continent), 27 November 1981, derived from CZCS data. Phytoplankton chlorophyll a concentration is colour coded from red (high) to blue (low). Courtesy of the NASA GSFC Earth Sciences Data and Information Services Center (GES DISC).

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