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 characteristics. 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 routinely.

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 information. 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 valuable time-series data.

Satellite sensors are used to map natural infrared and microwave 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 chlorophyll, integrating the effects of temperature, nutrients, and the physical structures and processes of the ocean surface layer. Suspended 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 accuracies adequate for determining many features of ocean circulation. This is the only practicable method for global-scale continuous observation of circulation. Synthetic aperture radar on satellites can measure sea state, internal waves, and ice conditions with very high spatial resolution.

Although satellite data yield a nearly continuous view of the ocean, it is important to note that complementary in situ obser-

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