SAR for Operational Ice Observation and Analysis

Cheryl Bertoia, Craig Evanego, Chris O'Connors: U.S. National Ice Center

Michael Manore: Canadian Ice Service

Henrik Steen Andersen, Keld Q. Hansen: Danish Meteorological Institute

# OPERATIONAL ICE SERVICES

Operational ice services from more than a dozen nations routinely issue
ice and iceberg bulletins, warnings, analysis charts, and forecasts to
support safe navigation in ice-affected waters. In addition, these ice
analysis products are finding increasing use as a record of ice
conditions to support climate change studies.

The major ice charting nations of the Arctic work in remarkably similar
manners, but with differences in their geographic regions of interest,
user base, and analysis data sets. Most centers now rely heavily on
satellite image data as their primary data source, supplemented by
airborne reconnaissance (visual or with imaging radar), ship reports,
and meteorological inputs. Ice is highly dynamic and requires frequent
and timely data sources for accurate charting. Each operational ice
centre has invested in the infrastructure to receive, analyze and
disseminate large volumes of data in near real-time. Virtually all data
is now processed in digital form and geographic information systems are
used to create products and disseminate them.

# SATELLITE DATA SOURCES

Visible and thermal imagery from the National Oceanic and Atmospheric
Administration (NOAA) -AVHRR (Advanced Very High Resolution Radiometer)
sensor, originally developed for meteorological applications but well
suited to ice monitoring, has been a long-standing workhorse for many
ice centers because of its ready availability and frequent coverage. In
addition, the thermal channels permit imaging of ice and interpretation
of its thickness even in periods of polar darkness. Its 1-km resolution
permits the charting of 'strategic' ice information suitable for making
general ship routing recommendations, but not for close tactical
navigation. Other nations make use of equivalent meteorological
satellites such as Japan's GMS or Russia's Meteor systems. The U.S. NIC
also makes extensive use the Operational Linescan System (OLS) from the
Defense Meteorological Satellite Program (DMSP) satellites which provide
0.5 km visible and thermal imagery. Of course, the main limitation of
optical systems is their susceptibility to cloud cover. The part of the
ice pack of greatest interest to ship traffic - the area near the ice
edge - is typically obscured by cloud or fog about 70% of the time.

Ice services rely on passive microwave imagery from the Special Sensor
Microwave/ Imager (SSM/I) sensor on the U.S. DMSP satellites to provides
near daily, all-weather, multi-channel microwave radiometry over a 1394
km swath. Automatic algorithms for the extraction of ice edge, total ice
concentration and multi-year ice concentration have been developed and
validated over many years. The sensor provides only coarse resolution
sea ice products (12.5 km -- 25 km), but is a reliable data source for
regions where only basic ice edge and ice concentration information is
required for strategic navigation decisions, such as Antarctica. In
addition, the relatively long record of passive microwave data (1972 -
present) and coarse resolution make it a favored data source for
ice-related climate change studies at hemispheric scales.

Because of weather, polar darkness and the desire for higher resolution
imagery the ice services have had a strong interest radar remote sensing
since the technology emerged (Bertoia, et al, 1999). As far back as the
early 1970's airborne real aperture radars (SLARs) were used to map the
ice pack and in the1980's and 1990's airborne SAR (Synthetic Aperture
Radar) and SLAR systems were in operational use. In 1991, the European
Space Agency's ERS-1 provided the first sustained taste of satellite
radar data, and in 1996, wide-swath (500km) SAR data became available
from RADARSAT-1. SAR data offers the advantage of high-resolution
imaging through cloud and polar darkness, and a sensitivity to the
surface roughness and salinity properties of sea ice that help to
distinguish different ice types. RADARSAT data was quickly adopted by
the Canadian and U.S. ice services under national data allocations, and
later by several European ice services on a commercial basis. At the
Danish Meteorological Institute (DMI) and in several other cases, the
purchase of satellite SAR imagery has been made possible through cost
savings by reducing or eliminating aircraft reconnaissance.
Demonstration projects using RADARSAT for ice monitoring in Russia,
Japan and China have been conducted but its use has not yet been adopted
operationally. It is estimated that more than 10,000 scenes per year of
SAR data are currently used for operational ice monitoring.

The future multi-channel SAR systems (Envisat, ALOS, RADARSAT-2) are
expected to improve the quality of ice information in the imagery but
with the requirement to handle increased data volumes and to develop new
analysis techniques, including the use of multi-polarization and
fully-polarimetric data. Ice centers expect to combine SAR from these
multiple sources in order to resolve ambiguities in ice type
discrimination found when using only one channel of SAR data. Other work
includes improvements in image processing approaches to detect low
concentrations of sea ice and improved detection of icebergs. The
operational centers look forward to a a constellation of SAR satellites
will permit better spatial and temporal coverage, as well as provide
operational redundancy in the event of system failure.

# CASE STUDIES

The following case studies present examples of how the ice centers have
made use of SAR data to provide support to navigation and to create ice
maps in conjunction with other data sources.

## U.S. National Ice Center: Bering Sea Analysis March 27, 2000

The U.S. National Ice Center (NIC) produces a suite of global sea ice
chart, including twice weekly charts of the Alaskan region (available at
[http://www.natice.noaa.gov](http://www.natice.noaa.gov/)) in support of
shipping and fisheries operations and other national intersts in the
Bering, Beaufort and Chukchi Seas. In order to create the Alaskan ice
maps, the NIC receives approximately 6000 images per year from the
Alaska SAR Facility in Fairbanks, Alaska. Each image is typicallly
received at the NIC less than three hours after the image is acquired in
Alaska.

On March 27, 2000, a National Center for Environmental Prediction (NCEP)
surface analysis showed northwesterly winds over the Eastern Bering Sea,
generated by the circulation around a storm system over southeastern
Alaska. These winds dragged cold air (less than 0C) across the ice pack
and out over the relatively warm open water in the southern Bering Sea.
The cold air streaking over the open water produced bands of
stratocumulus clouds, obscuring the marginal ice zone and ice edge.
Clearing skies behind the storm system allowed good visibility of inner
ice pack on the visible AVHRR image (Figure 1). Visible and infrared
imagery from NOAA satellites is a useful adjunct to SAR imagery,
providing useful clues to ice type/thickness.

![](media/image1.png){width="6.5in" height="4.875in"}

Figure 1. NOAA Advanced Very High Resolution Radiometer (AVHRR) visible
image from 0144Z on March 27, 2000. The red box shows the approximate
position of the Radarsat SAR image (Figure 3).

A coincident RADARSAT image (Figure 2) shows belting and stripping of
new, young and first-year ice along the ice edge. Though the marginal
ice zone and ice edge are obscured by streaks of stratocumulus clouds,
this RADARSAT image allowed the ice analyst to accurately analyze the
ice edge and marginal ice zone for the weekly ice analysis (Figure 3).

![](media/image2.png){width="6.481944444444444in"
height="6.481944444444444in"}

Figure 2. ScanSAR Wide RADARSAT image acquired at 0442Z on March 27,
2000. The zoom box show fine belts and strips of ice near the edge.
NIC's SAR imagery is processed to 100-m pixels, then averaged to 500-m
pixel spacing, making the detection of these small features possible,
while eliminating some classification confusion due to image speckle.

![](media/image3.png){width="4.5in" height="6.0in"}

Figure 3. NIC Alaskan regional sea ice analysis chart for XX March 2000.
Sea ice total concentration and ice type (a proxy for ice thickness) are
encoded using the World Meteorological Organizations "egg code"
(described fully at <http://www.natice.noaa.gov/egg.htm>)

NIC analysts have found SAR imagery very useful in analyzing ice
concentration and ice type, particularly in the marginal ice zone and
along the ice edge. Accurate depiction of the ice edge is particularly
important to the large Alaskan snow crab industry in the Bering Sea.
Crab fishermen typically set traps in the vicinity of the ice edge, and
thus require a detailed analysis of conditions at and near the ice edge
for safety of operations. Clouds often obscure the marginal ice zone and
ice edge during the Alaskan winter for months. It is imperative to
include SAR imagery in the ice analysis process for best representation
of ice conditions.

***B. Canadian Ice Service: Canadian Archipelgo February 1-4, 2001***

The Canadian Archipelago hosts a diversity of ice regimes composed of
multi-year, first-year, and new ice types, as well as icebergs. Ice
growth typically starts in late-September and continues throughout the
Arctic winter until early melt begins in May and June. Wintertime
conditions are extremely cold (-30C to -40C) which supports rapid and
thick sea ice growth. First-year ice will reach thickness of 1.5m --
2.0m, while multi-year ice may achieve thicknesses of 5m or more.
Navigation in this region is limited to the brief Arctic summer
(July-September) when the ice is weaker due to melt and decay and some
leads are present. Hard, thick multi-year ice poses the greatest hazard
to ships operation in this region so its detection and mapping is
particularly important.

Figure X is a mosaic of multiple RADARSAT-1 orbits collected February
1-4, 2001. Figure Y is the resulting ice analysis chart derived from the
mosaic and published in the Canadian Ice Service's annual Arctic Ice
Atlas. The imagery illustrates typical wintertime, cold-weather ice
signatures. Multi-year ice floes are clearly distinguishable by their
bright signature, rounded shape and mottled texture (A). The edge of the
multi-year pack ice in the Beaufort Sea is clearly visible because of
this bright signature (B). Undeformed first-year ice is dark in tone due
to specular reflection from its saline (lossy), smooth surface (C).
Deformed ice has a bright signature caused by high surface roughness and
multi-bounce scattering (D). First-year ice generated under moderate
wind and wave conditions may also exhibit bright signatures due to the
formation of rough ice 'pancakes' with raised, angular edges (E). Cold,
fresh water snow overlying the ice is virtually transparent at C-band so
all returns are controlled by the surface (first-year) or volume
(multi-year) scattering characteristics of the underlying ice.

Figure Ya and Yb are subscenes of RADARSAT images from (site) in the
Central Archipelago. Image Ya was acquired May 12, 1999 under cold
conditions and exhibits signatures similar to those in Figure X. Figure
Yb, however, was acquired July 30 and illustrates the radical signature
change that occurs under wet snow conditions during the melt season. In
the summer the overyling snow pack becomes saturated with liquid water
and becomes the main contributor to scattering. This has the effect of
decreasing contrast between ice types by masking the returns from the
underlying ice. Multi-year ice that was easily interpreted under cold
conditions is now difficult to distinguish from the surrounding
first-year ice. As most navigation in the Arctic takes place during the
summer, this poses a significant operational challenge. In regions with
no or little ice motion winter imagery is often used in combination with
current imagery to aid in multi-year ice detection and avoidance.

![](media/image4.jpeg){width="6.151388888888889in"
height="4.613888888888889in"}

Fig X.( caption to follow)

![](media/image5.png){width="4.95in" height="3.7743055555555554in"}

Fig Y. (caption to follow)

![](media/image6.png){width="3.6980314960629923in"
height="2.396456692913386in"}

Fig Z. (caption to follow)

***C. Danish Meteorological Service***

For the safety of ship navigation the Danish Meteorological Institute
produces satellite-based ice charts for Greenland waters on a routine
basis. All areas around Greenland are charted as required by shipping
traffic, but the Cape Farewell area, through which almost all ships to
and from Greenland have to pass, is charted throughout the year.

The waters off the southernmost part of Greenland, the Cape Farewell
area, belong to some of the most hazardous to navigation in world due to
the combination frequent severe storms, low visibility, multi-year sea
ice and icebergs. Sea ice is present in the Cape Farewell area from
December to August but icebergs from the east coast glaciers occur
throughout the year.

The distribution of sea ice in the Cape Farewell area is strongly
affected by the position and intensity of weather systems and large
variations on short time scales may result. In general sea ice drift
approximately equals 1 km/hour, however larger drift speeds frequently
occur.

Sea ice several meters thick drifts into the Greenland Sea from the
Arctic Basin via the Fram Strait. This multi-year ice combines with
locally formed first year ice and icebergs as it reaches the Cape
Farewell area, then continues around the southern tip of Greenland. The
ice pack often drifts several hundred kilometres north along the west
coast of Greenland.

The ice floes in the Fram Strait are often 50 km across or more. During
their southward drift along the Greenland coast, the floes break into
smaller pieces and are mixed with first year sea ice, particularly at
the ice edge. South of Scoresby Sound the floe sizes seldom exceed 5 km.
By the time the ice reaches the Cape Farewell area the floe sizes are
usually less than 100 meters. These very small floes are mostly
multi-year floes with thicknesses greater than 2 meters and are a great
danger to shipping.

The backscatter signatures from open water and sea ice in this region
are not unique. The backscatter from the water is dominated by the local
wind conditions and that from the ice is dependent on ice type, ice
concentration, surface roughness of the individual floes and level of
surface melting. The backscatter signals are also critically dependent
on the radar incidence angles (far / near range effects). For example it
is quite common that belts of ice appear nearly white in the far range
and nearly black in the near range when viewed against the background
sea clutter.

The present case illustrates a typical situation during the Cape
Farewell sea ice season. To an experienced ice analyst, the available
RADARSAT ScanSAR Wide image is quite straightforward to interpret. This
may leave the impression that SAR imagery is the ultimate ice charting
tool for ice services and indeed this is true in many cases. However, in
some cases SAR imagery is extremely difficult to analyze. This is
particularly true when low concentrations sea ice covered with surface
melt water are found in the near range part of the image.

![](media/image8.png){width="7.0in"
height="5.764583333333333in"}Figure 1. A RADARSAT ScanSAR Wide scene
from May 12 2000 20:48 UTC covering part of the Cape Farewell area. The
scene was received by the Gatinau receiving station. The pixel size is
100 meters. The satellite orbit is ascending and consequently the left
part of the image is in near range and the far range is to the right.
Often the far range part of the image provides better possibilities for
discrimination between sea ice and open sea. The beam seams is clearly
visible going from the top the bottom, however the bright line parallel
to the beam seam is caused by a processing error.

![](media/image9.png){width="5.666141732283465in"
height="4.53740157480315in"}Figure 2. A subsection of the image shown in
figure 1. The sea surface roughness may be dampened as a consequence of
calm winds or the presence of sea ice and consequently the backscatter
signal is reduced. In the areas marked (A) the dampening effect of
smooth sea can be seen due to the presence of 10 -- 30 % sea ice. The
wind arrow indicates the Northwest direction of the wind (7-8 m/s) and
in area (B) lee effects are clearly visible.

![](media/image10.png){width="6.082677165354331in"
height="4.508267716535433in"}Figure 3. A subsection of the image shown
in figure 1. This image illustrates different sea ice concentration
levels. In all cases the sea ice type is predominately multi-year ice
with floe sixes ranging from 10 to 100 meter. (A) Icebergs, (B) Icebergs
and less than 10 % sea ice, (C) less than 10 % sea ice, (D) from 10 to
30 % sea ice and belts. (E) 40 to 60 % sea ice, (F) 90 to 100 % sea ice.

![](media/image11.png){width="7.266666666666667in"
height="5.955555555555556in"}Figure 4. The resulting ice analysis
resulting from a visual interpretation. The ice analyst always tries to
generalise the information available in the image to produce a simple
yet elaborate chart, which can be used for navigation. The ice edge is
in magenta and the inner boundaries are in cyan. The special symbols
indicate the presence of icebergs and belts of ice.

![](media/image12.wmf){width="7.213888888888889in"
height="6.118055555555555in"}Figure 5. The resulting ice chart based on
the ice analysis. The different ice areas are described using the
standard WMO ice egg symbols. On an annual basis DMI produces in NRT
about 180 navigation ice charts for the Cape Farewell waters and about
150 charts to ships operating along the West and East coast of
Greenland. One a week an ice analysis covering all Greenland waters is
produced based on all available satellite data and auxiliary background
information.

### D. National Ice Center and Canadian Ice Service: Great Lakes Analysis: February 22, 2001

# 

This case study highlights the shared responsibility between the U.S.
and Canada for ice charting in the Great Lakes. The Lakes are an area of
common operational interest and the collection of satellite imagery and
the production of charts is coordinated. The NIC issues an NIC-generated
chart of all five Great Lakes weekly and also issues the CIS produced
five lake composite weekly. In addition to a weekly composite chart, the
CIS issues daily charts of a sub-set of lakes as required to support
navigation and also issues the NIC generated composite chart weekly.
Both ice services share satellite data sets and analyses for the
generation of their respective products.

![](media/image13.jpeg){width="5.192361111111111in"
height="4.722222222222222in"}

Figure X. A mosaic of two RADARSAT-1 ScanSAR Wide-A images covering the
central Great Lakes, acquired February 22, 2001 at approximately 23 UTC.

In this image, high concentrations of ice are clearly evident in Lake
Erie. The ice concentration, ice deformation and therefore ice thickness
increase towards the eastern end of the lake under the influence of
seasonally predominant westerly winds that funnel the ice into the
narrower eastern end of the Lake. The bright returns seen in this area
indicate strong deformation conditions. Low ice concentration, thinner
ice types, and less deformed ice are present in the western portion of
the Lake, which is typically an area of new ice formation in the latter
half of the season. Wind conditions at the time of the image acquisition
were SW at 10-15Kts, and the temperatures ranged from --8 to --5
Celsius.

In Lake Huron, low-to-moderate concentrations of ice are evident around
the southwestern and southeastern shores. Higher concentrations and
thickness types are evident in the Saginaw Bay and along the
northeastern shores because of their shallow depth and relatively
sheltered topography. A large area of open water is clearly visible in
the central portion of the lake. The high return of the younger ice
types in the southern portion of the lake result from the predominant
westerly wind compacting ice along the eastern shoreline, creating
fields of brash ice. At the time of the image the winds were from the
south at 5 to 10 Kts, and sections of brash ice are visible drifting
away from the southern coastline.

In the small sections of Lakes Superior and Michigan covered by the SAR
images, ice is located only along the eastern margins. Interesting to
note is the relatively high backscatter over open water (compared to the
other lakes) caused by the small incidence angle in the far range.
Again, predominant westerly winds have compacted the ice along the
eastern shores. In northeastern Lake Michigan, a bright linear feature
between the open water and the area of ice packed into the Straits is
visible. This is a typical signature in SAR imagery that indicates that
ice is under compaction. A linear region of high return results from the
interaction of the wind and waves with the ice pack, which pile ice up
along the ice edge. This feature is very useful in edge identification.
A similar signature can be seen were fast ice boundaries interact with
open water or pack ice areas.

Ice charts resulting from the analysis of a RADARSAT scene (Figure X)
are presented in Figures Ya and Yb. As the Great Lakes are fresh water,
the radar signatures differ from those for sea ice and are driven by a
combination of both surface and volume scattering in all ice thickness
types.

![](media/image14.png){width="6.495833333333334in"
height="4.957638888888889in"}

Figure Ya. CIS Great Lakes daily chart, based specifically on images
aquired that day and shown in Figure X.

![](media/image15.jpeg){width="6.340972222222222in"
height="8.294444444444444in"}

Figure Yb. NIC Western Great Lakes weekly composite, based on imagery
acquired immediately prior to the images shown in Figure X. This chart
nicely complements Figure Ya, giving mariners a complete picture of ice
navigation conditions.

III\. Possible conclusions section or just end with final case,
depending on how long cases are. If we need, we can use some stuff about
future directions, developmental products.

For operational ice services, continuity is of cause a major concern and
the present configuration with only one operational satellite that can
provide imagery with sufficient coverage is not optimal. The future
situation with both RADARSAT and ENVISAT present will without doubt
increase data coverage and make SAR data even more useful for ice
services around the world.

**References:**

Bertoia, Cheryl and J. Falkingham, F.Fetterer, 1998. Polar SAR Data for
Operational Sea Ice Mapping, in R. Kwok and C. Tsatsoulis (Eds.).
*Recent Advances in the Analysis of SAR Data of the Polar Oceans*,
Springer Verlag: Berlin, pp. 201-234.