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62 OCEANOGRAPHY IN THE NEXT DECADE 



that travel to great depths and transmit their data via acoustic 

 signals or return to the surface and broadcast to satellites. Sensor 

 packages could transmit data for any ocean observation that can 

 be measured in situ. Disposable sensor packages can transmit 

 data via a thin wire that detaches when the sensor reaches its 

 maximum depth, but free-falling sensors could travel much deeper 

 and measure more accurately than present devices. 



• VOS could tow instruments to measure various ocean char- 

 acteristics. When ship routes and sampling locations coincide, 

 passing ships could retrieve data from moored instruments (e.g., 

 moored current meters and inverted echo sounders) by acoustic 

 signals. At the least, commercial ships equipped with acoustic 

 Doppler current profilers could measure upper ocean currents and 

 heat fluxes routmely. 



Remote Sensing by Satellite Satellites observe vast areas of the 

 ocean surface daily, obtaining data at a far faster rate than surface 

 vessels and instruments (e.g., Stewart, 1985). Satellites also aid 

 physical oceanography by transmitting data gathered by in situ 

 ocean sensors combined with highly accurate positioning infor- 

 mation. The availability of surface and subsurface instruments 

 has greatly increased the volume of data beyond that possible 

 from ship-based observations, and the instruments provide valu- 

 able time-series data. 



Satellite sensors are used to map natural infrared and micro- 

 wave radiation emitted from the sea surface. Infrared radiation 

 provides images of sea surface temperature patterns. Radiation in 

 the microwave frequencies is used to map sea ice distribution in 

 areas of the globe that are otherwise poorly accessible. Phytoplankton 

 blooms can be monitored from space with sensors that respond to 

 the visible and near-infrared radiation reflected by plankton chlo- 

 rophyll, integrating the effects of temperature, nutrients, and the 

 physical structures and processes of the ocean surface layer. Sus- 

 pended sediment is also measured by visible sensors. Surface 

 winds are measured by satellite scatterometers. The sea surface 

 slope can be measured from space by radar altimeters with accu- 

 racies adequate for determining many features of ocean circula- 

 tion. This is the only practicable method for global-scale con- 

 tinuous observation of circulation. Synthetic aperture radar on 

 satellites can measure sea state, internal waves, and ice condi- 

 tions with very high spatial resolution. 



Although satellite data yield a nearly continuous view of the 

 ocean, it is important to note that complementary m situ obser- 



