Sentinel 1 will be made by the Franco-Italian firm Thales Alenia Space, ESA said.

In a press release, the company estimated the order at 229 million euros (305 million dollars).

Envisat's nominal lifespace of five years expired in March this year, but ESA believes the craft has enough fuel to remain operational until 2010 and possibly beyond.

The replacements for the eight-tonne Envisat will be relatively small and more specialised, enabling ESA to reduce the risk if a payload is lost on launch, said Volker Liebig, the agency's director for Earth observation programmes.

The announcements were made at the Paris Air Show at Le Bourget, north of the French capital.

ESA has earmarked 1.2 billion euros (1.6 billion dollars) for the five Sentinel satellites, as well as between 300 and 350 million euros (399 and 465 million dollars) per year in running costs for them.

Sentinels 1, 2 and 3 are due to be launched in 2011-2012.

Sentinel 1 will be equipped with a radar for monitoring polar ice breakup and iceberg calving. Sentinel 2 will have a camera able to map the world every five to 10 days at a resolution of 10 metres (32.5 feet). Sentinel 3 will bear an altimetric radar and heat sensor to measure differences in the ocean's height and plankton concentrations.

Sentinel 4, deployed in geostationary orbit, and Sentinel 5, in a looping orbit around the poles, will detect changes in Earth's atmospheric chemistry. A firm date for their launch has yet to be decided, and ESA said the payload may be shared with the European meteorological satellite agency Eumetsat.

The total cost of the Envisat project is around two billion euros (4.66 billion dollars), according to ESA's website.