Ground controllers say they're maneuvering a trio of European satellites into precise orbits to start delivering the best-ever survey of Earth's magnetic field.

Since the European Space Agency's Swarm constellation was launched last November, engineers have been busy preparing the three satellites for providing data to further our understanding of the complex and constantly changing field.

Essential to life, the magnetic field protects us from cosmic radiation and charged particles that bombard Earth in solar winds.

To optimize sampling in space and time the satellites are being placed in different orbits, ESA scientists said.

Two are being lowered to an altitude of about 287 miles where they will orbit almost side by side, about 90 miles apart as they pass over the equator.

The third satellite is being placed in a higher orbit of 315 miles and will orbit closer to the Earth poles.

"We are taking the satellites to their new heights through careful thrust and aim to achieve the constellation for science operations around mid-April," mission System Engineer Ralf Bock said in a release from ESA headquarters in Paris.