This variable, also known as the dynamic height or dynamic topography gives very useful information on currents. It is provided by altimetry satellites. The Mercator systems use altimetry data from 3 satellites: Envisat (European Space Agency), GFO (US Navy), Jason-1 (Nasa/Cnes).

Other satellites measure the temperature at the surface of the sea. These are mainly infrared sensing satellites from NOAA (the American Meteorology department), but also from the European Space Agency (ESA).

These floats are also called 'drifting profilers'. They are released at sea from oceanography ships. They sink down to 2,000 metres and are moved at that depth by ocean currents. After 10 days, they rise again to the surface and send the temperature and salinity measurements taken during the 10 days to non-geostationary Argos satellites. The satellite then relays this information to ground stations. The ground stations process the measurements and relay them in turn to oceanography centres which have requested them. The diagram opposite shows a measurement cycle for an Argo float.

These are also called 'fixed-point profilers'. As with the Argo floats, they record the temperature and salinity and send their measurements to non-geostationary satellites. They are anchored to the bottom of the sea and do not move.

These are floating buoys released from oceanography ships. They are equipped with a satellite GPS system. From the successive positions transmitted, the speed and direction of the surface current are deduced. The buoys also measure the temperature of the sea surface and sometimes a few atmospheric parameters such as air pressure and temperature.

These are also called CTDs, for Conductivity, Temperature, Depth. As the name indicates the CTDs measure the temperature and salinity vertically along a water column. They are also used to take sea water samples for chemical analysis (by filling small cylinders around the CTD).

The glider is a miniature, remote controlled submarine, 2 metres long, with wings spanning 1.2 m). They regularly rise to the surface and communicate with their operators by satellite in order to send in real time the data acquired during dives and on the other hand, to evaluate their drift due to currents, in order to correct for it, for the next dive. Their mission may possibly be changed while they are floating on the surface. The advantage is that they are remote-controlled whereas floating profilers drift freely according to the currents.

The XBT (eXpendable BathyThermograph) is a probe launched from a ship or an aircraft, which measures the temperature of the ocean as the instrument sinks towards the bottom. The speed of descent is known. The depth is thus deduced from the time taken. The depth of the measurement depends on the length of the wire (between 300m and 2000m). The wire is linked to a small buoy containing a radio transmitter which communicates the data to the ship which continues its route while recording it. The data is then relayed via the worldwide transmission system towards oceanography data centres which check it, validate it and distribute it to operational centres.