Successful Seasons for BAS Radar Controllers
The British Antarctic Survey (BAS) has completed several successful seasons with a set of VM2-based radar controllers that make detailed and accurate measurements of ice sheet thickness. Keith Nicholls of BAS explains the background: Researchers at the British Antarctic Survey (BAS) and University College London (UCL) have developed a radar instrument capable of monitoring the rate of melting of Antarctica’s floating ice shelves. The device is capable of measuring to better than 1 mm the changes in the thickness of a 2-km thick ice shelf that result from melting at its base.
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 100 m to 2 km, 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.
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.
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 0.2 ma, amounting to a total consumption of less than 2 A-hr over a year-long deployment. Systems deployed to date have used a single 176 A-hr, 6-V AGM sealed lead-acid battery so that the VM2 makes a negligible contribution to the total battery consumption while sleeping.
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.
Deployment in the AntarcticThe 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.
Keith Nicholls, British Antarctic Survey
Housing showing Micro-Robotics controller board in the lid
Radar antennas being buried 1m below surface level. There are two such trenches at right angles, each containing 8 antennas to enable synthetic antennas to be assembled.
The control board housing in place
BAS have now adopted the VM2 controller for several other projects.