NHESSNatural Hazards and Earth System ScienceNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus GmbHGöttingen, Germany10.5194/nhess-15-1243-2015Tracking B-31 iceberg with two aircraft-deployed sensorsJonesD. H.GudmundssonG. H.ghg@bas.ac.ukhttps://orcid.org/0000-0003-4236-5369British Antarctic Survey, High Cross, Madingley Road, Cambridge, UKG. H. Gudmundsson (ghg@bas.ac.uk)16June20151561243125020May201410July201415December20148May2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.nat-hazards-earth-syst-sci.net/15/1243/2015/nhess-15-1243-2015.htmlThe full text article is available as a PDF file from https://www.nat-hazards-earth-syst-sci.net/15/1243/2015/nhess-15-1243-2015.pdf
Icebergs are a natural hazard to maritime operations in polar regions.
Iceberg populations are increasing, as is the demand for access to both
Arctic and Antarctic seas. Soon the ability to reliably track icebergs may
become a necessity for continued operational safety. The temporal and spatial
coverage of remote sensing instruments is limited, and must be supplemented
with in situ measurements. In this paper we describe the design of a tracking
sensor that can be deployed from a fixed-wing aircraft during surveys of
Antarctic icebergs, and detail the results of its first deployment operation
on iceberg B-31.
Introduction
Icebergs represent an environmental hazard to shipping and
fixed marine structures, particularly in the circumpolar Antarctic waters and
the North Atlantic, near to Greenland, where iceberg density is greatest
. Since 1850 there have been 611 recorded collisions
between icebergs and ships . This threat to maritime safety is
expected to worsen as demand for access to these regions increases.
Figure shows that the number of tourists visiting Antarctica by ship
has been rising since records begin in 1992, up until the global economic
crisis in 2007, and then subsequent to the recovery in 2011. Figure
shows that exploration licenses for drilling for petroleum resources off
Greenland have been rapidly increasing, in part due to the diminishing Arctic
sea ice and corresponding effects on ease of access for maritime logistics.
Furthermore global warming and its disproportionate impact on polar regions
have led to increased iceberg populations , though this may in
turn be offset by the increased melt rates of icebergs due to the warming
surface sea temperatures. Thus the threat of icebergs colliding with maritime
infrastructure is rising, and the ability to track icebergs reliably could in
future provide a valuable additional source of information for shipping
operations in polar waters.
Existing monitoring strategies
Satellite-based optical sensors produce high-resolution images of icebergs
that are used for iceberg tracking, but these are unable to penetrate cloud
cover and are dependent on solar illumination. Synthetic array radar (SAR)
satellite performance is independent of solar illumination and generally
unaffected by cloud cover ; however, the spatial coverage of
these sensors is limited, frequently resulting in poor temporal resolution.
As a result, the database of known locations of large icebergs, as maintained
by the US National Ice Center (NIC), is typically updated every 20 days
.
The microwave emissivity of a material is affected by its atomic structure,
and thus can be used to differentiate between sea and ice from satellite
based sensors. Passive microwave radiometry sensors onboard satellites have
been used to track large icebergs and are still used to
track the extent of sea ice e.g. . Another satellite-based
sensor that is still used for iceberg tracking is a microwave scatterometer.
This was first demonstrated in with data from the QuikSCAT
satellite – large icebergs appear as high-backscatter targets surrounded by
lower-backscatter sea water or sea ice. QuikSCAT ceased operations in 2009,
but the technique is still used with data from the Advanced Scatterometer
(ASCAT) satellite and the recently launched OceanSat-2 scatterometer (OSCAT).
This supplements the NIC database with monthly position updates for large
icebergs.
Number of tourists visiting the Antarctic by ship
.
Area of Arctic licensed for oil exploration in Greenland, derived
from NUNAOIL annual report 2012 .
The Polar View website (www.polarview.aq) maintained by the British
Antarctic Survey provides a useful portal for polar operators to access SAR
images from the Sentinel-1 SAR satellite operated by the European Space
Agency. This satellite has been launched recently, and replaces the SAR
coverage provided by Envisat until 2012.
The limited temporal coverage of satellite-based sensors, the dependence of
optical sensors on clear skies and solar illumination, and the inability for
microwave-based scatterometer sensors to resolve small and medium sized
icebergs means that a supplementary method for determining iceberg location
is sometimes necessary.
The Newfoundland and Labrador tourism department uses reported visual
sightings in conjunction with RADARSAT-2 imagery in order to maintain a
separate database of iceberg locations in the region.
An alternative to visual sightings and remote sensing is the use of tracking
buoys.
Ice tracking buoys
The ability to instrument large expanses of sea ice or iceberg fields from
fixed-wing aircraft has been of interest to military and maritime scientists
for 44 years. In 1970 and 1971 the US coastguard tested an aircraft-deployed
ice penetrator designed to measure ice thickness. The military potential saw
the US Naval Ordnance Laboratory and Sandia labs test larger ice penetrators
in 1973 . The first operational ice tracking sensors were
developed for ice pack drifting experiments and deployed in 1978
. These Air Droppable Remote Access Measurement System
(ADRAMS) buoys were shaped like a 22′′ diameter sphere and adapted for
deployment from a Hercules aircraft. Development of systems subsequent to
ADRAMS has been in response to increasing air safety regulations, the
improvement of battery technologies and the availability of more advanced and
compact electronics.
Current commercial systems are made by Canatec and MetOcean. The MetOcean
Compact Air-Launched Ice Beacon (CALIB) buoy is currently in use by the
Canadian Ice Service and has been used in the past as part of the
International Ice Patrol (IIP).
The CALIB is a commercially available tracking device that can be dropped
from a fixed-wing aircraft. CALIB buoys have been used to track icebergs with
some success.
The IIP first tested them in 2003 and succeeded in tracking an iceberg for
13 days ; however, trials in 2007 (two buoys) and 2011
(one buoy) failed with no data transmissions received .
More success has been had deploying CALIB by hand: a study by the Canadian
Fisheries and Oceans in 2009 deployed 4 CALIBS, each lasted for approximately
lasted for 2 months. A follow-up deployment in 2011 saw 4 CALIBS transmit
data for 4–5 days . The short lifespan of these buoys
may be reflecting the dynamic and unstable nature of their target, although
the deployment profile is not robust to different snow conditions. The CALIB
is designed to partially penetrate the snow pack and stand upright. If
dropped from an aircraft, the depth of penetration depends on the density,
viscosity and depth of the snow coverage, so the CALIB may not penetrate
sufficiently deep to remain vertical, or bury itself too deep such that the
antennas are buried.
In place of iceberg trackers, the IIP now routinely supplements the remote
sensing data sets with measurements from the World Ocean Circulation
Experiment (WOCE) ocean buoys . The IIP typically deploys
12–15 of these buoys into the Labrador Sea each year. These buoys are
deployed from aircraft as part of the iceberg survey missions, or from ship
vessels of opportunity. The trajectory measured by each buoy is then used as
the basis of a model for predicting iceberg trajectories that year.
Helicopters have been used on occasion to instrument icebergs with tracking
devices , see Fig. .
However, their limited range (for instance, the Bell 206 in Fig. has
a maximum range of 702 km, compared to the 1427 km of a Twin Otter) and
payload capacity (635 kg compared to 1940 kg) make them unsuitable for any
operations beyond the proximity of a large supporting infrastructure. There
are also safety concerns when instrumenting smaller, less stable icebergs
.
If a fixed-wing aircraft had the same ability to instrument icebergs, then
the advantages of their increased range, availability and operation costs
will allow significantly more icebergs to be instrumented. Furthermore, it
would be possible to integrate iceberg instrumentation deployment within
existing iceberg survey flights.
Aircraft Deployable Ice Observation System (ADIOS)
Over the last 3 years we have developed and tested an aircraft-deployable
sensor for instrumenting glaciers . This enabled us to
instrument heavily crevassed and otherwise inaccessible glaciers. A
subsequent extension of this programme has been to investigate the
effectiveness of ADIOS for installing tracking devices on icebergs.
Here we briefly discuss the constraints and ultimate design of ADIOS. See
for a more complete description.
Design constraints
In order to minimise costly and time intensive changes to the aircraft
platform, an ADIOS is deployed from a standard sonobuoy launch tube mounted
45∘ to the aircraft floor. This restricts the diameter of the device
at the point of deployment to that of the tube. Also the clearance between
the launch tube and the interior aircraft cabin roof limits the length of any
component of an ADIOS prior to being installed in the launch tube (see
Fig. ).
Helicopter landed on iceberg for tracker deployment. Iceberg between
Makkovik and Hopedale, Canada. Bell 206L helicopter, fuselage length
10.13 m. Image courtesy of S. Prisenberg .
Twin Otter aircraft fitted with sonobuoy launch tube.
As these devices are dropped on otherwise inaccessible icebergs, they have to
be considered disposable, which places constraints on both the cost and the
environmental impact of the design.
The obstacles to installing sensors on icebergs apply equally to the
challenge of retrieving their data locally. Instead an ADIOS must transmit its
data to remote servers via a satellite link. Unlike a sonobuoy, which can
rely on flotation to ensure its communications antenna is vertical and
persists above the surface, this sensor must have a controlled impact angle
and speed in order to set its ultimate orientation and depth within the snow.
These criteria, in conjunction with local snow accumulation rates, will
determine the upper limit of the lifetime of the ADIOS.
The aforementioned size constraints also limit the power source. An
effective solar panel or wind turbine will not fit through the launch tube,
so the payload has to be powered by a primary battery. In turn this restricts
the electronics to consist of only low-power components. The capacity of the
power source and the power consumption of the payload will be a limit on the
effective duration of the operation of the device.
The ADIOS is designed to partially penetrate the snow, leaving a mast protruding
vertically from the surface. The device needs to impact the iceberg with
sufficient force so as to partially bury itself even in dense snow
conditions. This in turn means the ADIOS will rapidly decelerate after impact.
The payload has to be resilient to large deceleration forces and survive the
impact intact.
In order to ensure the ADIOS is safe to deploy from an airborne platform, the
trajectory of the device after deployment needs to maximise separation from
the aircraft as fast as possible. The slowest operational speed of the
aircraft we use in this programme is 50 ms-1, meaning the device
is dropped into an airstream of an equivalent velocity. Thus, whilst the
device is within proximity of the aircraft it has to have a small aerodynamic
profile in order to prevent the airstream, or turbulence under the aircraft,
from deflecting the ADIOS back towards the aircraft.
ADIOS design
The ADIOS is 2.5 m long and consists of a slender 1.5 m mast, a wider
payload compartment and a solid aluminium nose cone (see Fig. 5). The mast
and payload compartment are manufactured from poly-propylene, chosen for its
impact strength in cold environments. The remaining components are
manufactured from aluminium.
In order to ensure that, after impacting with the snow, the payload
compartment is subsurface whilst leaving the antenna mast protruding above
the surface, four snow brakes are mounted at the top of the compartment. Once
the device is buried to a depth of 1 m, these snow brakes effectively
increase the surface area by a factor of 4, and correspondingly its drag
in the snow. These snow brakes fold forward and fasten closed during
deployment, so as to fit through the launch tube and minimise their
aerodynamic effects whilst in proximity to the aircraft. When the device is
clear of the aircraft they are released and locked open. The size and shape
of these brakes is a tradeoff between their adverse aerodynamic qualities and
their ability to stop the device burying to too great a depth.
Without some form of parachute to provide stabilising drag, during free fall
the ADIOS will oscillate about its centre of pressure and the horizontal velocity
of the device will be largely sustained. Both effects prevent the device from
impacting with the ground at 90∘. However, parachutes can also
introduce payload oscillations due to the irregular and fluctuating airflow
conditions around and through the surface of the canopy. In the case of solid
flat circular parachutes, the airflow separates from the leading edge of the
hemisphere in alternating vortices. Dynamic stability is achieved by
controlling this airflow with a more advanced canopy shape adapted from the
Mars Viking lander parachute .
ADIOS design.
Design testing
Over the last 2 years we have conducted design trials in a vertical wind
tunnel and from flights local to two Antarctic stations. These trials were
used primarily to improve the design stability and the depth to which each
ADIOS unit buried itself. By refining the parachute design, snow brake design
and the centre of gravity we were able to ensure each ADIOS unit impacts with the
surface within 10∘ of vertical and 20 cm of the specified 1 m
depth. The final design used a parachute size that set the terminal velocity
of the ADIOS at 42 ms-1.
Limitations
The ADIOS is designed to stand upright within a snow pack at least 1 m deep. The
majority of Antarctic icebergs travel counter-clockwise around the perimeter
of the continent, and accumulate in the Weddell Sea. They are then typically
propelled into the Scotia Sea along a northward corridor, until they enter
the Antarctic Circumpolar Current . Until they cross
66∘ S the average iceberg surface mass balance is positive. Thus
Antarctic tabular icebergs are likely to have sufficient snowpack for
instrumentation by the ADIOS.
The trajectory of the ADIOS is predictable, as is the effects of any wind acting
on it during descent. As a result, during the trial deployments we were able
to consistently drop an ADIOS into a 10 m square area – icebergs with a
smaller surface area are not suitable targets for instrumentation by an ADIOS.
Further improvements are hoped to be gained by means of an electronic
targeting display undergoing trials in 2014/15.
The majority of Arctic icebergs form from glaciers on the north-west and
south-east quadrants of Greenland. Here snow accumulates between September
and May, but then rapidly ablates between June and August .
As a result there is typically little surface snowpack on Arctic icebergs,
so these are less appropriate for instrumenting with the ADIOS.
Case study: tracking B-31
In the following section we demonstrate the capability of the ADIOS for
tracking icebergs by presenting data collected by two ADIOS units deployed on
iceberg B-31 (see Fig. 7).
In October 2011 a survey flight as part of Operation IceBridge
discovered a newly formed rift that appeared to span the
entire Pine Island Glacier ice shelf. Subsequent flights showed that the rift
was not quite complete, but estimated that a complete separation could occur
within months . It would eventually separate to form iceberg
B-31 in November 2013, but 11 months before its birth we had an opportunity
to deploy two ADIOS units on it.
During the Austral season 2012/13 we deployed 37 ADIOS units on Pine Island
Glacier, two of which were west of the rift. This was a unique opportunity to
study the birth of an iceberg as well as to evaluate the potential of ADIOS
units for iceberg tracking.
The ADIOS units we deployed were fitted with a low-power single-band GPS
receiver. Each unit takes a position fix six times a day, then combines this
data with measurements of the GPS accuracy, the unit temperature and the
battery voltage. Once a day the data packet is compressed and transmitted
over the iridium satellite network. When the available battery power
decreases, or GPS reception is no longer possible, the unit enters a low-power mode. In this mode the unit intermittently attempts to transmit the
last recorded GPS position. The doppler shift in the iridium transmission,
measured by the receiving satellite, makes it possible to determine an
approximate location in the event that the GPS is no longer operational. One
of the deployed ADIOS units was dropped in a position known to be static. This has
been used to calculate the position accuracy in the different operating
modes, see Fig. . This shows that the GPS position accuracy is
in the order of metres, whereas the position accuracy calculated by the
iridium transmission is in the order of kilometres.
Accuracy of position measurements in GPS and low-power operating
modes.
Separation of B31 iceberg from Pine Island Glacier, USGS/NASA.
LANDSAT image, 13 November 2013.
Operational performance of the two ADIOS iceberg tracker units
located on iceberg B-31, which calved from the Pine Island Ice Shelf in
November 2013.
Battery voltage and satellite reception performance of the two ADIOS
iceberg tracker units located on iceberg B-31.
Distance separating each ADIOS unit.
Tracks generated from ADIOS units situated on B-31 iceberg. Iceberg
outlines derived from Radarsat2 SAR (courtesy of MacDonald Dettwiler and
Associates – MDA Corporation) and MODIS optical (courtesy of NASA) satellite
imagery.
Since January 2013 we have recorded 4152 position reports from two ADIOS
sensors over a period of 406 days (see Fig. ).
The first 10 months of this data set show B-31 calving from the Pine Island
Glacier ice shelf (see Fig. 11). Shortly after its birth, we saw a small part
of B-31 (which happened to have an ADIOS unit on it) break off and separate
from B-31. This can be seen in the increasing separation between the two
ADIOS units (see Fig. ). The second, smaller iceberg has
become part of the ice mélange surrounding B-31. From the deployment of
the ADIOS until the carving of B-31, we recorded 91 % of the expected
daily GPS transmissions. The lost transmissions are most likely due to there
being a sub-optimal Iridium satellite constellation during the time the ADIOS
is trying to transmit.
After the calving of B-31 (November 2013, see Fig. ) both ADIOS
units started to operate intermittently in GPS and low-power operating modes.
Figure shows the reported battery voltage and the daily average
number of GPS satellites seen by one of the ADIOS units. The sustained battery
voltage and continued transmissions suggest that neither the battery or the
electronics were damaged. Instead, the drop in the number of GPS satellites
seen shortly after indicates that the ADIOS could have become partially
buried, tilted, or fallen into a crevasse. Despite this both ADIOS units have
continued to intermittently achieve a position fix and transmit it. After the
calving event we recorded 29.4 % of the expected daily GPS transmissions,
and 21.2 % of the low-power daily transmissions.
Conclusions
The threat icebergs pose to ships and fixed maritime structures
is rising in line with demand for access to Arctic and Antarctic waters. This
threat can only be partially mitigated by satellite tracking of icebergs, so
there is an increasing demand for the ability to track icebergs with in situ
tracking devices.
The Aircraft Deployable Ice Observation System (ADIOS) is particularly
appropriate for instrumenting Antarctic icebergs, where there is typically
sufficient surface snow for the ADIOS to stand upright in, and can be deployed
from fixed-wing aircraft as part of larger iceberg survey missions. This has
been demonstrated with the successful tracking of the B-31 iceberg with two
ADIOS instruments. The location data these instruments transmitted provided
operational support to the I-STAR C expedition during the 2013/14 cruise in
the Amundsen sea.
Acknowledgements
This study is part of the British Antarctic Survey Polar Science for Planet
Earth Programme. It was funded by The Natural Environment Research
Council (NE/I007156/1).
Edited by: B. D. Malamud
Reviewed by: two anonymous referees
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