Satellites trace sea level change
A major reassessment of 18 years of satellite observations has provided a new, more detailed view of sea-level change around the world. Incorporating the data from a number of spacecraft, the study re-affirms that ocean waters globally are rising by just over 3mm/yr. Measuring ocean surface shape from satellites has a relatively short history. Routine observations began with Europe's ERS-1 spacecraft in 1991, and this has subsequently been followed up by a series of international missions. The benchmark today is arguably Jason/Poseidon - a cooperative venture between the US and Europe (principally France). Now in its third incarnation, the Jason satellite circles the globe making a topographic map of 95% of the Earth's ice-free oceans every 10 days. To do this, it uses a radar altimeter, which constantly bounces microwave pulses off the sea surface.