2.1.3 Section summary
The use of integrated buoy telemetry with near-real time processing of
RADARSAT imagery proved an invaluable tool for determining the best locations to deploy the inner and outer hexagonal array as well as identifying strategic locations to place 5 stress sensors in both first year and multi-year ice. As seen in Figure 2.5, we were able to target a very active zone and span that zone with the buoy array through careful coordination of RADARSAT motion products and buoy telemetry to maximize positioning and minimize deployment time.
2.2 MODIS
Nick Hughes, SAMS
2.2.1 Summary
This is a report on MODIS satellite image acquisitions for the SEDNA project which covered the period between the surfacing of the submarine HMS Tireless in the area on 16 March 2007 through to the end of the ice camp on 15 April.
MODIS provides a medium resolution visible image suitable for providing sea ice information during cloud-free periods. It can also be processed to yield data on surface temperature and albedo. This data is particularly useful when used in conjunction with images from SAR satellite sensors.
2.2.2 Background
MODIS (or Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra's orbit around the Earth is timed so that it passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon.
Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1 to 2 days, acquiring data in 36 spectral bands, or groups of wavelengths (see MODIS Technical Specifications). These data will improve our understanding of global dynamics and processes occurring on the land, in the oceans, and in the lower atmosphere. MODIS is playing a vital role in the development of validated, global, interactive Earth system models able to predict global change accurately enough to assist policy makers in making sound decisions concerning the protection of our environment.
2.2.3 Processing
Daily raw MODIS data files (Level 1-B) in Hierarchical Data Format (HDF) were acquired for the SEDNA project for the period 15 March through to 16 April 2007.
This allowed generation of quick-look images from a selection of the 36 channels available on MODIS and will allow further processing by sea ice, oceanographic and atmospheric parameter retrieval algorithms later in the SEDNA project. The data was ordered through NASA Goddard Space Flight Center (GSFC) Level 1 and Atmosphere Archive and Distribution System (LAADS Web)
(http://ladsweb.nascom.nasa.gov/). The data period covers the period when the area was visited by the Royal Navy submarine HMS Tireless as part of ICEX-07 through to the end of the APLIS ice camp.
The initial stage of processing was to generate single channel geo-referenced images to provide a consistent daily coverage. The projection used for SEDNA MODIS images is Polar Stereographic with a central longitude at 145°W and latitude of true scale at 90°N on the WGS84 datum. Resolution was increased to 100 metres, from the 250 metres maximum acquired by MODIS, by cubic
convolution interpolation to aid comparison with the Envisat ASAR wide swath images also acquired for SEDNA. The software used was the MODIS Swath Reprojection Tool (MRT Swath) supplied by the NASA/USGS Land Processes
Distributed Active Archive Center (LP DAAC)
(http://edcdaac.usgs.gov/landdaac/tools/mrtswath/). This takes the raw data from the HDF files and outputs single channel geo-referenced images in GeoTIFF format. This format is the result of an effort by over 160 different remote sensing, GIS, cartographic, and surveying related companies and organisations to establish an interchange format for geo-referenced raster imagery based on the common Tag Image File Format (TIFF). Further information can be found at http://remotesensing.org/geotiff/geotiff.html.
GeoTIFF format was then used for all further image processing and archiving.
Generation of quick-look images was performed using OpenEV software. This is an open source software library and application for viewing and analysing raster and vector geospatial data. More information on OpenEV can be found, and the software downloaded, at http://openev.sourceforge.net/. However the version used for the SEDNA MODIS images was supplied as part of the FWTools open source GIS binary kit (http://fwtools.maptools.org/) which also includes other free applications including the Geospatial Data Abstraction Library (GDAL) and the PROJ.4 cartographic projections library. The individual channel images in
GeoTIFF format were loaded into OpenEV. This allows the generation of a multi-channel image through the ‘Compose’ option on the ‘Image’ menu. Three
images corresponding to different channels were then selected to produce an RGB (red-green-blue) colour image corresponding to either a visible or false colour composite (FCC) quick-look image.
Visible images were created using reflective channels 1, 4 and 3. These correspond to 620-670 (red), 545-565 (green), 459-479 (blue) nm (nanometre) light bandwidths.
False colour composite images were created using channels 31, 2 and 3. These provide a low resolution (1,000 metre) thermal infrared image at 10.78-11.28 µm (micrometre), a high resolution (250 metre) 841-876 nm near-infrared image, and the medium resolution (500 metre) 459-479 nm (blue) visible image. This follows a method used by [Schneider and Budéus 1997] for Landsat images to improve discrimination of sea ice from open water. Cold snow and ice surfaces appear as blue and the relatively warm, thermally emitting, open water is bright red.
After composing the image in OpenEV it was exported to a GeoTIFF file.
As OpenEV does not apply compression to an image this was done using the gdal_translate utility from GDAL.Image Assessment
A full list of the images acquired is shown in appendix 1. A selection of some of the clearer images is presented here with a brief initial evaluation of the main features. The visible image is on the left and false colour composite on the right.
23 March 2007 20:50 UTC
Although there is some thin cloud cover large ice floes, especially thick multi-year, are visible. Open leads appear to be fairly random in their distribution.
25 March 2007 22:15 UTC
After a couple of days there was the initial development of a shore lead along the coast of Alaska. Some leads are visible in the ice cover offshore through a
patchy cloud cover. These still appear to have no particular orientation.
26 March 2007 21:20 UTC
The following day a large lead system, trending north-west to south-east, has developed running parallel to the Alaskan shore.
28 March 2007 20:50 UTC
The shore lead along the Alaska coast is now fully developed. Apart from the north-west to south-east lead system, no clear leads are visible in the offshore ice.
31 March 2007 21:20 UTC
A number of lead systems have developed. These run north-south in the northern part of the image and then trend towards the west as they run towards the shore lead.
10 April 2007 23:35 UTC
After a number of days in which cloud obscured the ice it was visible again on 8 April 2007 22:10 UTC image. During this time the ice cover continued to break up with a multitude of small leads fracturing the cover. Around Point Barrow the ice cover has broken away to start forming an embayment. Leads running north-north-west to south-south-east are dominant. These are crossed by smaller leads running north-west to south-east forming a lattice pattern. The shore lead east of Point Barrow appears to be closed
15 April 2007 22:15 UTC
The ice cover break-up continues with the north-west to south-east leads
becoming dominant. As these run southward they curve back westward towards Point Barrow. Cloud obscured the Alaskan coastline making visual observation of any shore lead impossible.