ERS-2, the space agency's veteran spacecraft, and Envisat, the largest environmental satellite ever built, both carry Synthetic Aperture Radar instruments that provide high resolution images of the Earth's surface.

"By combining two or more SAR images of the same site, slight alterations that may have occurred between acquisitions can be detected," ESA said. "This technique, known as SAR interferometry, … has proved to be very useful for applications such as glacier monitoring, surface deformation detection and terrain mapping."

ESA engineers configured the first SAR tandem mission, which took place from September 2007 to February 2008, and the current one that began Nov. 23, to ensure the satellites both acquire data over the same area just 28 minutes apart. Such a short separation, the ESA said, allows for detection of changes that occur quickly, such as the movement of some glaciers.

The current tandem mission is to run until Jan. 27. Both ERS-2, launched in 1995, and Envisat, launched in 2002, have exceeded the time they were intended to remain in orbit.