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Intruments and measurements
The ocean is a vast body of water covering almost three quarters of the Earth’s surface. To find out what the ocean is doing it needs to be observed in detail. There are many different observational instruments in use, in the sea and in space, to scan and measure the oceans. Plans for the implementation of an integrated, comprehensive, global observing system have been realised and every day new devices are added to help to improve the observations.
There are two main sources of observed data from instruments:
a) Satellites (measurements from space)
b) In-situ (measurements obtained through direct contact with the ocean)
Impression of satellite swath - credits to the Canada Centre for Remote Sensing Polar orbiting satellites (in comparison to geostationary satellites) can scan the whole globe in a rather short period of time and provide comprehensive data sets. They fly at around 850 km altitude and need about 96-100 min for one orbit. During one orbit satellites can only look at a stripe of the earth’s surface (swath), so it will take several orbits to scan the whole of the earth’s surface. Depending on the satellite this time can vary and usually takes 10 to 35 days. Taking into account data assembly, data interpolations and information about previous observations, the latest picture of the ocean state is available for example after 3 days for SSH (Sea Surface Height) altimeter data, and after 6-12 hours for SST (Sea Surface Temperature) data.
In-situ measurements are done at and under the ocean surface and give a less comprehensive picture of the global ocean, because the instruments can only measure at one spot at a time. But in combination with the satellite observations they are an important component of the observing system to provide complete observational data sets. One major advantage of in-situ measuring instruments is their ability to measure data inside the ocean, meaning under the ocean surface. Satellites cannot do this.
Slocum ocean glider - credits to the Office of Naval Research Important in-situ measuring systems include moored buoys, free drifting buoys, XBT, CTD, CPR, Ocean Gliders and Tide gauges. One in-situ measuring network in particular has contributed enormously to the success. The GODAE/CLIVAR pilot project Argo, which started in 2000, has almost completed deployment of the planned 3,000 free-drifting profiling floats. This allows, for the first time, continuous monitoring of the temperature, salinity, and velocity of the upper 2000m of the ocean, with all data being relayed and made publicly available within hours after collection.
The data that are collected by in-situ and satellite instruments provide oceanographers with many different quantities. They are used to describe the ocean state.
- Temperature
- Salinity
- Currents
- Sea surface topography
- Surface/internal waves
- Colour/biology
- Turbulence
- Nutrients
It is extremely important to sustain the global ocean observing system. Successful forecasts and information about the ocean state very much depend on a reliable observing system. Without it marine safety, off-shore industry, fisheries, coastal management and naval operations would be challenged, also research activities would suffer. Using in-situ and satellite observations combined helps to calibrate the measurements and complement the datasets.
(Last Updated: 16-10-2007)