A new data processor coming online means enhanced quality data for the CryoSat user community. ESA’s ice mission remains in excellent health, outliving its planned lifespan to offer years of observations ahead.
This month sees the release of improved ground data processing software for CryoSat. The result, comments Mission Manager Tommaso Parrinello, should be a “leap in quality” in results tracking shifts in polar ice thickness over time.
Launched on 8 April 2010, CryoSat-2 is Europe’s first dedicated ice satellite, equipped with an innovative radar altimeter design for greatly improved coverage of sea ice and land ice margins – the very zones where the influence of climate change is thought to be greatest.
Up until now these areas of the cryosphere have been more or less invisible to standard altimeters optimised for sea surface height measurements.
"Standard radar altimeters have a ground footprint of several kilometres," remarks Tommaso. “This means over sea ice the returning signal is made up of a mixture of ice and water. But CryoSat’s Synthetic Aperture Radar Interferometry Altimeter (SIRAL) achieves an along-track resolution of around 250m, a factor of six times better. This allows us to distinguish sea ice from the water leads running between them.
“So we measure the elevation of the ice but also the water it is drifting on – the difference between the two being known as the ‘freeboard’. Only around 10% of that ice is above the water the rest is beneath it, just like ice in a drink. And we use the freeboard measurements to derive the ice thickness and then ice volume in turn.”
Over land ice margins, CryoSat switches on a second antenna to go into interferometry mode, measuring the phase distance between signals reaching the two antennas to derive their differing travel times and therefore the relief features in the across-track direction.
Finally CryoSat can also shift into ’Low Rate Mode’ (LRM) over open ocean to work like a standard radar altimeter – its high 92° inclination and atypical non-Sun synchronous orbit means it offers useful ocean coverage, and discussion is ongoing about producing an oceanic data product in future.
Getting the best quality data to users
As CryoSat Mission Manager, based in ESA’s ESRIN Earth Observation centre in Frascati, Italy, Tommaso holds overall responsibility for the mission, from the satellite in space to the ground segment and the interfaces that get the data to its users.
“It’s a real end-to-end responsibility,” says Tommaso. “It also includes interacting with the scientific community, our main users. It’s a two-way relationship, in effect: we listen to what they say and try to implement it, trying to give them the best quality data possible.”
As an Earth Explorer mission applying innovative technology to target a previously unobserved parameter of the Earth system, CryoSat-2 went through a lengthy commissioning process post-launch.
“We finished commissioning the satellite at the end of 2010, during which time we were really concentrating on the spacecraft health and calibrating the instrument,” Tommaso explains. “Since then we’ve been giving data out to users. You can receive Level 1b data, which is basically the initial engineering measurements – really most of interest to researchers wanting to become familiar with how the instrument works – and the more refined Level 2 geophysical measurements.
“We’re not keeping anything back from users. As far as we’re concerned sharing helps us solve problems and improve things. So from January to July 2011 users helped identify several improvements and bugs, allowing us to make changes to the data processors as we understood their workings better. Now we’re launching the new processor that incorporates all those improvements, and the result should be a leap in quality.
“Next we need to perform more measurements, especially over those parts of the sea never before measured by altimetry, to get enough data to average out a good value for the freeboard. Once that’s done then users will be able to easily produce their own Level 3 and Level 4 maps – the Arctic maps produced up until now were produced by a group of scientists using our first couple of months’ worth of data combined with their own measurements and calculations.
“We have to be sure the data is good and cannot be misinterpreted, because any message coming out of CryoSat results is a message about climate change, with all its attendant political factors.”
Extended mission offers synergy with other satellite sensors
The mission was originally planned to run up until 2013 but ESA’s Earth Observation Directorate plans to request funding from ESA’s Member States to extend mission operations, potentially as far as 2017. “The mission remains in great health,” comments Tommaso.
With this long potential lifespan in mind a dedicated study – ‘CryoSat Plus’ – is considering how mission data could be used in synergy with other satellite sensors.
CryoSat’s fellow Earth Explorers are high on the list, Tommaso explains: “GOCE is producing the most accurate geoid – the surface of an ideal global ocean in the absence of tides and currents, shaped only by gravity – which obviously has value for calculating the freeboard.
“And the SMOS radiometer, while designed to gather data on soil moisture and ocean salinity, has also turned out to be sensitive to sea ice thickness. SMOS has a much bigger footprint than CryoSat, but it could still be useful to cross-check observations.”
The loss of the original CryoSat satellite on launch in 2006 prevented combined observations with its NASA equivalent ICESat, which relied on laser altimetry to chart ice thickness.
ICESat ceased operations in 2009 and ICESat-2 is not scheduled for launch until 2015. But last April CryoSat-2 did perform joint testing with IceBridge, NASA’s largest airborne programme over Earth’s polar ice, bridging the data gap between the two ICESat missions.
CryoSat’s technological legacy
Provided all goes well CryoSat-2 should still be operational when ICESat-2 is launched. By the time it does conclude, the mission will have left behind it a lasting technological as well as scientific legacy.
Next year sees the launch of Sentinel-3, ESA’s first satellite for operational oceanography whose Synthetic Aperture Radar (SRAL) altimeter is derived from CryoSat-2’s SIRAL, able to reproduce CryoSat-2’s high-resolution SAR mode for sea ice (although not its interferometry mode for land ice margins).
And Jason-CS, the next in the US-European Jason series of ocean radar altimetry missions due to launch in 2017, will employ a similar SRAL instrument, while the satellite platform itself will also be based on CryoSat-2.