Before we set out to fly thousands of kilometres across the sea ice with the EM Bird slung underneath our airship, we want to make sure the instrument is accurate by calibrating it on a plot if ice whose thickness has already been determined. We will choose a calibration zone that is as representative as possible of the different sea-ice morphologies so that EM Bird measurements are ground-truthed on “real-life” terrain. Surface topography will be carried out using a laser scanner and aerial photo-grammetry:
 Laser scanner
Fine-scale high-precision topographic surveys are carried out using a land-based laser scanner. This is a motorised laser that scans the surface via a mirror able to be guided in both horizontal and vertical angles. As the speed of light is known, the time that a laser pulse takes to travel to and from a reflecting feature tells us the exact distance of that point and thus its 3D coordinates as well. This type of instrument has an accuracy of about 5 mm for an object within a radius of 250 metres, and it can measure several thousand points per second. The laser scanner generates a digital model of the surrounding surface directly and in real time, but it can only measure features in its line of sight. Given that the surface of the ice pack is sometimes broken and jagged, the scanner will have to be set up in several different places to ensure that all the “hidden” features are covered.
Aerial photo-grammetry
Topography of larger areas of the ice pack will be surveyed by taking stereoscopic aerial photos (with each one overlapping the previous photo by 60%), using previously calibrated digital cameras. With this method, surface topography can be reconstituted with an accuracy of about 5 cm from a flying height of 100 metres. Aerial topography is much quicker to carry out than using laser scanners and it covers a much wider area while providing archives of the zone surveyed. But it takes much longer to process the data so interpretation would have to be carried out after the expedition is completed. This technique can be implemented from the airship while the EM Bird takes its own measurements, thus providing archives of the area overflown during the 2008 expedition.
Surface topography will be superimposed on a grid of the measurements obtained by the ROV, thus generating a 3D profile of the area of sea ice being surveyed. This profile will then be used to calibrate the EM-Bird.
 |
