Micro Robotics VM2 controller
The Antarctic continent is covered by an ice sheet up to three miles thick. The ice flows towards the sea under its own weight, finally going afloat to create large ice shelves that extend out over the continental shelf.
At an ice shelf’s seaward edge, tabular icebergs regularly calve off and drift out into the Southern Ocean, where they fragment and melt. The ocean also melts ice from the base of the ice shelves.
If nothing is changing, the amount of ice that arrives in the ice shelf from the inland ice sheet and from the snow that falls on the ice shelf is the same as the amount lost in melting and in iceberg calving.
In that case the ice shelf is thought of as being in “steady state”. The ice shelves are important to the stability of the main ice sheet: they create a plug that reduces the flow of grounded ice from the continent.
Along some parts of the Antarctic coastline changes in the Southern Ocean seem to be increasing the rate of basal melting, leading to thinning of the ice shelves and a reduction in the back-pressure they exert on the inland ice.
The result is an increased flow of ice from the continent into the ocean, which has several consequences, not least of which is an increase in sea level. The effect of the Antarctic and Greenland ice sheets on global sea level change is currently the least well understood, but potentially the largest of all contributions.
Research into the interaction between the ice sheet and the ocean is hampered by the difficulty of observing the ocean near and beneath the ice shelves. Research vessels are often prevented by sea ice from gaining access, and the ice shelves themselves, which range in thickness from 100m to 2km, and have areas up to the size of France, make gaining access to the water beneath challenging and therefore expensive.
There is an easier way. Researchers need to be able to predict changes in ice shelf melt rates that will come from long term changes in climate. But short term changes in climate – seasonal changes, for example – offer a realistic experiment: by measuring the change in melt rates that result from seasonally changing conditions they can begin to understand how to predict the changes that will occur over the longer term.
They need to be able to make numerical simulations of the ice-ocean system that accurately replicate observations of the changing melt rates: once computer models can simulate the ice-ocean interaction in the present climate, they can be used to predict how things are likely to change in the future.
To measure the rate at which ice shelves melt (from as little as a one or two millimetres per day to up to a hundred or so millimetres per day), and, importantly, the way the melt rates change with the seasons, the researchers at the British Antarctic Survey teamed up with radar engineers at University College London to develop an instrument that can be left on the surface of ice shelves, monitoring the change in thickness over a period of a year or more.
The radar antennas point downwards and a reflection is received from the base of the ice shelf.
Crucially, weak reflections are also received from changes in the dielectric properties that result from layering in the snow that made up the ice column. This stack of “internal layers” is used as a datum for the measurement to the ice base, allowing the effects of settling of the antennas, internal thinning of the ice column (as a result of its viscous flow), and compaction of the less dense upper layers of the snow, all to be taken into account.
The radar is a phase-coherent frequency modulated continuous wave (FMCW) system. The phase coherency is the key to instrument’s precision. The frequency range used by the radar is 200-400MHz, giving a range-resolution of about 0.4m.
System at site in the Antarctic
However, by measuring the phase of the echo to a precision of a degree or better, the range-resolution improves to better than a millimetre. For phase-coherency, precise timing needs to be maintained between radar and controller.
For the instrument to be left gathering data for a year or more, the limitations of the power supply need to be taken into consideration. Each measurement lasts only a few minutes, and is typically made once every few hours.
At such a low duty cycle, the power consumption during the period of sleeping between measurements becomes important. The Micro-Robotics VM2 controller that was selected for the project permits a sleep current of less than 0.2mA amounting to a total consumption of less than 2A-hr over a year-long deployment.
Systems deployed to date have used a single 176A-hr, 6V AGM sealed lead-acid battery so that the VM2 makes a negligible contribution to the total battery consumption while sleeping.
By definition, ice shelves are at low altitude, and near the sea, giving wintertime temperatures that are benign compared with the rest of the continent.
Temperatures can still drop to -50 or -60°C for short periods, and to combat these snaps the instruments are buried in the snow to a depth of about a metre, preventing the equipment temperature dropping below -40°C. All components in the radar and controller boards are specified to operate at that temperature.
In addition to the VM2, an Iridium modem is integrated on the controller board, allowing daily reports of instrument health, and enabling the instrument to be remotely re-configured. Mass storage takes the form of dual SD cards, offering a level of redundancy in the case of a card failure. A GPS module is also incorporated.
Although the GPS is primarily used to provide an accurate timestamp, the radar’s position, which changes as the ice shelf flows, is also reported over the Iridium link to make it easier to find the system when it is due to be recovered.
The system has been trialled extensively over the last few Antarctic field seasons, with several records of a year or more in length having been recovered. Presently ten instruments are overwintering, to be recovered in early 2016.
Future plans are to roll the system out to as many Antarctic ice shelves as possible, with the aim of creating a database that will help researchers understand the interactions between the Southern Ocean and the Antarctic Ice Sheet, and ultimately enable them to predict the future contribution of the ice sheet to global sea level change.
Dr Keith W Nicholls is a glaciologist-oceanographer with the British Antarctic Survey
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