The BBC's Esa and Nasa line up satellites to measure Antarctic sea-ice says:
Authorisation was given on Tuesday for Europe's Cryosat-2 spacecraft to raise its orbit by just under one kilometre.
This will hugely increase the number of coincident observations it can make with the Americans' Icesat-2 mission.
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Nasa's Icesat-2, which orbits the globe at about 500km in altitude, uses a laser to measure the distance to the Earth's surface - and hence the height of objects. This light beam reflects directly off the top of the snow.
Esa's Cryosat-2, on the other hand, at around 720km in altitude, uses radar as its height tool, and this penetrates much more deeply into the snow cover before bouncing back.
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On Tuesday, managers at Esa gave final approval for the joint campaign known as Cryo2Ice.
Cryosat will fire its thrusters on 16 July to climb a few hundred metres higher into the sky. The manoeuvre, which will take a couple of weeks to complete, will not compromise the longevity of the mission as the spacecraft has ample fuel on board.
Esa's Cryosat's mission manager, Dr Tommaso Parrinello, told BBC News: "Icesat is quite a bit below us so we can't go down to meet them, but by going up we find this incredible resonant orbit in which for every 19 orbits for us and 20 orbits for them - we will meet at the poles within a certain time lag. Basically, every 1.5 days, we meet over the poles within a few hours of each other and that means we can observe the same ice almost simultaneously.
Question: Why will it take "a couple of weeks to complete" CRYOSAT 2's orbit raising of "a few hundred meters"? Is it using electric propulsion, or is this a low delta-v way to attain proper phasing, or something else?
