greater value to scientists eager to learn more about the ever-changing seas is the US- French Topex-Poseidon, the most advanced oceanography satellite yet built. Unlike ERS-1, Topex-Poseidon's orbit is inclined 66 degrees to the equator, so it is not able to scrutinise the polar ice packs. However, the satellite's radar can survey 90 per cent of the world's ice-free oceans. From an orbit 1,336km (830 miles) above the planet, its onboard instruments can measure variations in ocean topography to within 3cm (1.2in).
The system works by sending short pulses of microwave energy which bounce back from the ocean below and return to the satellite. The round-trip travel time of the reflected pulses gives the distance between the spacecraft and the surface. It also offers information on wave height and wind speed.
Small adjustments are necessary to allow for atmospheric conditions. However, the most significant errors in the altimetry are caused by not knowing the precise height of the satellite's orbit. This is a major problem with a low-flying satellite such as ERS-1, which is influenced by atmospheric drag and tossed around by minor variations in the pull of gravity. In an effort to overcome this problem, Topex-Poseidon flies higher than ERS-1. By using three independent and complementary systems, the satellite's orbital position has been pinpointed to within 3cm. Twelve laser-ranging stations around the globe give intermittent information by bouncing laser beams off reflectors on the satellite. There is also a French-built receiver system which determines the satellite's velocity by measuring subtle changes in the frequencies of signals transmitted from the ground. The third system is an experimental receiver which uses signals from the network of Global Positioning System satellites to fix its location.
Once the orbit has been precisely determined, scientists can translate the mass of distance measurements into a global map of sea level relative to the centre of the Earth.
The view that water always finds its own level does not work on a large scale. Ships have to go up and down slopes, just the same as cars or trains. The ocean topography varies considerably according to local differences in the gravitational pull of the Earth. This usually means that the mountains and valleys of the sea floor are reflected on the surface. There is a difference in height of nearly 160m (525ft) between the highest summits and lowest troughs on the ocean surface.
These highs and lows are also influenced to a lesser extent by winds, temperature and water density. This is particularly noticeable on the western side of oceans, where water tends to pile up against continents. Between the highs and lows on the ocean surface, the global range of altitudes due to these other factors is quite small, only about 2m, but this 'dynamic topography' is significant.
The highs and lows of the sea surface act like their equivalent pressure systems in the atmosphere. Massive circulations known as gyres have been mapped in great detail by Topex-Poseidon. Around their perimeters, most noticeably on the western margins, ocean currents transport millions of cubic metres of water each second around the globe. Topex-Poseidon observations made over many months clearly show large- scale variability as huge eddies spin off from these currents.
'What we really want to know is what the currents are doing,' said Dr Philip Woodworth, Topex-Poseidon principal investigator at Proudman Oceanographic Laboratory on Merseyside. 'We measure sea level because we can convert it into currents. The greater the slope of the sea, the faster the current.'
Ocean variability takes many forms. Topex-Poseidon has been able to watch how the sea level falls in winter as the ocean cools, and how it rises in summer as the ocean expands. A temperature change of 1C in the upper ocean layer alters sea level by 1cm. Maps of wave height also show how strong winter winds generate giant waves more than 6m high in the mid-latitudes, compared with much gentler winds and waves in the summer.
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