Remote Sensing Goes Cold

Average thickness of Arctic sea ice in spring as measured by CryoSat between 2010 and 2015. Image courtesy of ESA/CPOM

Remote sensing over the Polar Regions has poked its head above the ice recently.

On the 8th February The Cryosphere, a journal of the European Geosciences Union, published a paper by Smith et al titled ’Connected sub glacial lake drainage beneath Thwaites Glacier, West Antarctica’. It described how researchers used data from ESA’s CryoSat-2 satellite to look at lakes beneath a glacier.

This work is interesting from a remote sensing viewpoint as it is a repurposing of Cryosat-2’s mission. It’s main purpose is to measure the thickness of the ice sheets and marine ice cover using its Synthetic Aperture Radar (SAR)/Interferometric Radar Altimeter, known as SIRAL, and it can detect millimetre changes in the elevation of both ice-sheets and sea-ice.

The team were able to use this data to determine that the ice of the glacier had subsided by several metres as water had drained away from four lakes underneath. Whilst the whole process took place between June 2012 and January 2014, the majority of the drainage happened in a six month period. During this time it’s estimated that peak drainage was around 240 cubic metre per second, which is four times faster than the outflow of the River Thames into the North Sea.

We’ve previously highlighted that repurposing data – using data for more purposes than originally intended – is going to be one of the key future innovation trends for Earth Observation.

Last week, ESA also described how Sentinel-1 and Sentinel-2 data have been used over the last five months to monitor a crack in the ice near to the Halley VI research base of the British Antarctic Survey (BAS). The crack, known as Halloween Crack, is located on the Brunt ice Shelf in the Wedell Sea sector of Antarctica and was identified last October. The crack grew around 600 m per day during November and December, although it has since slowed to only one third of that daily growth.

Since last November Sentinel-2 has been acquiring optical images at each overflight, and this has been combined with SAR data from the two Sentinel-1 satellites. This SAR data will be critical during the Antarctic winter when there are only a few hours of daylight and a couple of weeks around mid-June when the sun does not rise.

This work hit the headlines as BAS decided to evacuate their base for the winter, due to the potential threat. The Halley VI base, which was only 17km from the crack, is the first Antarctic research station to be specifically designed to allow relocation to cope with this sort of movement in the ice shelf. It was already planned to move the base 23 km further inland, and this was successfully completed on the 2nd February. Further movement will depend on how the Halloween Crack develops over the winter.

Finally, the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) project was announced this week at the annual meeting of the American Association for the Advancement of Science. Professor Markus Rex outlined the project, which will sail a research vessel into the Arctic sea ice and let it get stuck so it can drift across the North Pole. The vessel will be filled with a variety of remote sensing in-situ instruments, and will aim to collect data on how the climate is changing in this part of the world through measuring the atmosphere-ice-ocean system.

These projects show that the Polar Regions have a lot of interest, and variety, for remote sensing.

Gliding Across The Ice

ESA’s Earth Explorer CryoSat. Image courtesy of ESA/AOES Medialab.

ESA’s Earth Explorer CryoSat. Image courtesy of ESA/AOES Medialab.

There’s been a flurry of reports in the last couple of weeks, reporting melting ice and retreating glaciers in Greenland and the Himalayas respectively.

A paper by McMillan et al (2016), titled ‘A high-resolution record of Greenland mass balance’ and published in Geophysical Research Letters earlier this month, highlighted that Greenland’s melting ice has contributed twice as much to sea level rise than in the previous twenty years. The research used CryoSat-2 radar altimetry between 1 January 2011 and 31 December 2014 to measure elevation changes in the Greenland ice.

The main instrument on ESA’s CryoSat-2 satellite is a Synthetic Aperture Radar (SAR)/Interferometric Radar Altimeter known as SIRAL, although also carries a second version of this instrument as a back-up. The SIRAL instrument has been enhanced to detect millimetre changes in the elevation of both ice-sheets and sea-ice. It sends out bursts of radar pulses, with an interval of 50 μs between them, covering a 250 m wide strip of the Earth and measures the time of the return signal to determine the height of the satellite above the Earth. It requires a very accurate measurement of its position to calculate this, and so it also carries a Doppler Orbit and Radio Positioning Integration by Satellite (DORIS) instrument to determine its orbit.

The research team discovered that the Greenland Ice Sheet lost an average of 269 ± 51 Gt/yr of snow and ice during the investigative period, which compared well with other independent measurements from sensors such as the Gravity Recovery and Climate Experiment (GRACE) satellite and results from climate models. This snow and ice loss corresponds to a 0.75 mm contribution to global sea-level rise each year.

It was reported this week that research undertaken by the Indian Space Research Organisation, Wadia Institute of Himalayan Geology and other institutions have revealed that the majority of the glaciers in India are retreating; albeit at different rates. Using remote sensing data up to 2006, the study looked at 82 glaciers in the Bhagirathi and Alaknanda river basins and found that there had been an overall loss of 4.6% of the glaciers within the region. The Dokriani glacier in Bhagirathi is retreating between 15 and 20 metres per year since 1995, whereas the Chorabari glacier in the Alaknanda basin is retreating 9-11 metres per year.

It’s interesting to read the retreating glacial picture alongside the research published by Schwanghart et al (2016), titled ‘Uncertainty in the Himalayan energy–water nexus: estimating regional exposure to glacial lake outburst floods’, in Environmental Research Letters. Here the research team completed the first region wide risk assessment of floods from glacial lakes, even though this only covered around a quarter dams in the Himalaya’s. The study mapped 257 dams against more than 2,300 glacial lakes within the region and found that over 20% of the dams are likely to be overwhelmed with flood water as rock systems that surround glacier-fed lakes fail. Due to the hydro-electric power needs of the region, more dams have been built in recent years, putting them closer to glacier-fed lakes.

The potential danger of this issue is demonstrated by the collapse of Zhangzangbo, a glacier-fed lake in southern Tibet, in 1981 where 20 million cubic meters of floodwater damaged hydroelectric dams and roads causing damage of approximately $4 million.

These three reports also show the potential danger melting ice and glaciers pose both locally and globally. Remote sensing data, particularly from satellites such as CryoSat-2, can help us monitor and understand whether this danger is increasing.

Satellite Data Continuity: Hero or Achilles Heel?

Average thickness of Arctic sea ice in spring as measured by CryoSat between 2010 and 2015. Image courtesy of ESA/CPOM

Average thickness of Arctic sea ice in spring as measured by CryoSat between 2010 and 2015. Image courtesy of ESA/CPOM

One of satellite remote sensing’s greatest strengths is the archive of historical data available, allowing researchers to analyse how areas change over years or even decades – 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.

It’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?