One of satellite remote sensingâ€™s greatest strengths is the archive of historical data available, allowing researchers to analyse how areas change over years or even decades â€“ for example, Landsat data has a forty year archive. It is one of the unique aspects of satellite data, which is very difficult to replicate by other measurement methods.
However, this unique selling point is also proving an Achilles Heel to industry as well, as highlighted last week, when a group of 179 researchers issued a plea to the European Commission (EC) and the European Space Agency (ESA) to provide a replacement for the aging Cryosat-2 satellite.
Cryosat-2 was launched in 2010, after the original Cryosat was lost during a launch failure in 2005, and is dedicated to the measurement of polar ice. It has a non sun-synchronous low earth orbit of just over 700 km with a 369 day ground track cycle, although it does image the same areas on Earth every 30 days. It was originally designed as three and half year mission, but is still going after six years. Although, technically it has enough fuel to last at least another five years, the risk of component failure is such that researchers are concerned that it could cease to function at any time
The main instrument onboard is a Synthetic Aperture Interferometric Radar Altimeter (SIRAL) operating in the Ku Band. It has two antennas that form an interferometer, and operates by sending out bursts of pulses at intervals of only 50 microseconds with the returning echoes correlated as a single measurement; whereas conventional altimeters send out single pulses and wait for the echo to return before sending out another pulse. This allows it to measure the difference in height between floating ice and seawater to an accuracy of 1.3cm, which is critical to measurement of edges of ice sheets.
SIRAL has been very successful and has offered a number of valuable datasets including the first complete assessment of Arctic sea-ice thickness, and measurements of the ice sheets covering Antarctica and Greenland. However, these datasets are simply snapshots in time. Scientists want to continue these measurements in the coming years to improve our understanding of how sea-ice and ice sheets are changing.
Itâ€™s unlikely ESA will provide a follow on satellite, as their aim is to develop new technology and not data continuity missions. This was part of the reason why the EU Copernicus programme of Sentinel satellites was established, whose aim is to provide reliable and up to date information on how our planet and climate is changing. The recently launched Sentinel-3 satellite can undertake some of the measurements of Cryosat-2, it is not a replacement.
Whether the appeal for a Cryosat-3 will be heard is unclear, but what is clear is thought needs to be given to data continuity with every mission. Once useful data is made available, there will be a desire for a dataset to be continued and developed.
This returns us to the title of the blog. Is data continuity the hero or Achilles Heel for the satellite remote sensing community?