This section contains the answers to frequently asked questions about GlobICE products.
If you have questions that are not answered in this section and in the Product User Guide, please contact Alan Muir by email: asm(at)mssl.ucl.ac.uk
Q1: How does GlobICE track sea ice motion?
A1: The GlobICE processor selects time-separated pairs of SAR or ASAR Wide Swath imagery (at 500m resolution). Once a pair of images has been selected containing the likely start and end point of the motion of a number of tracking points, the displacement vector of each tracking point is computed by correlating regions in the two images. In most cases the time separation between images is approximately 3 days which corresponds to the sub-repeat cycle of the satellite.
GlobICE also uses low resolution (62.5km) SSM/I Quikscat data to seed the high resolution (500m) correlation search.
Q2: Why is the spatial and temporal coverage of some GlobICE products relatively sparse in some locations?
A2: GlobICE takes its main input data from the Envisat ASAR instrument operating in wide swath mode. Envisat was not designed to operate in this mode continuously and frequently switches to other modes of operation. During the Arctic winters of 2007 to 2009 (coincident with the IPY campaigns) the ASAR instrument was commanded into Wide Swath mode more frequently over the Arctic but still with relatively low density towards the Canadian Arctic and the Beaufort Sea.
In order to successfully track sea ice the GlobICE processor requires coincident SAR imagery over grid points every 3-6 days. This is particularly important when attempting to track sea ice over longer periods, ie for Lagrangian (ie seasonal) motion and when processing monthly averages.
The full potential of the GlobICE system could be realised with a continuously operational wide swath SAR instrument such as the forthcoming Sentinel-1 operational SAR mission.
Q3: How are GlobICE products validated?
A3: The primary method of validating GlobICE products is to compare them with in-situ data from Arctic buoys. These buoys drift amongst the sea ice and transmit hourly or daily GPS locations. A number of Arctic buoy campaigns were performed during the main period of GlobICE data (2007-2010). These include the SEDNA and IABP programs. Only a limited number of buoys were coincident with the GlobICE data and can be used for the validation.
Another method of validating GlobICE products is to compare them with the lower resolution (62.5Km) data available from CERSAT/PSI-M-Drift vectors derived from the Quikscat SeaWinds & SSM/I sensor.
Q4: Can GlobICE generate products for the Southern Hemisphere?
A4: The primary goal of the GlobICE project was to develop an operational sea ice motion system for the Arctic, however during Phase-3 of the project, the GlobICE processor was successfully prototyped over the southern hemisphere for a sample period of Aug-Sept 2010. Sample products from this study will be made available shortly.
Q5: Will GlobICE be used with Sentinel-1 Data?
A5: The full potential of the GlobICE system could be realised with a continuously operational wide swath SAR instrument such as the forthcoming Sentinel-1 operational SAR mission. Subject to future funding and a system update, GlobIce is ideally suited for use with Sentinel-1 data.
Q6: Why don't GlobICE gridded products contain associated Latitude and Longitude data?
A6: GlobICE gridded data products contain coordinates in a frequently used Arctic Polar Stereo projection grid, identical to that used for SSM/I products at NSIDC and also to that used by GlobICE's predecessor, the RADARSAT Geophysical Processor System (RGPS) (Kwok et al., 1990). The projection is described in the Product User Guide. A software routine to convert between the GlobICE coordinate system and (lat,lon) is provided in the Software Tools section.
If there are future updates to the system then it is likely that a Latitude and Longitude array will be added in the netCDF gridded products to enhance product usability by data users who are unfamiliar with working with this projection.