Marine polar navigation
An issue that has come in for particular attention in recent years is navigation in high latitudes and especially the use of the northern sea routes for commercial traffic and oil and gas exploration.
Ships have always navigated through ice-infested waters but the conditions found in the Baltic, Black Sea and other areas that freeze, although harsh and damaging to ships, are quite benign compared to conditions nearer the poles. The interest shown in polar navigation has led the IMO to undertake the development of a Polar Code that places new requirements on ships operating in such regions.
As part of the work, the IMO first adopted voluntary guidelines for ships that have since evolved into the IMO’s mandatory Polar Code which has now been adopted and which came into effect for new vessels on 1 January 2017 and for existing ships a year after that.
The Polar Code is less extensive in many ways than the earlier guidelines with three chapters (9-11) covering navigation-related aspects including equipment, communications and procedures.
With regard to functionality and the type of equipment, almost nothing is said in the code, leaving the guidelines as the most comprehensive source of information. These are laid out in IMO document A 26/Res.1024 Guidelines for ships operating in polar waters. The document was published in March 2010 and flag states have been ‘invited’ to apply it to ships built after January 2011 and ‘encouraged’ to apply it to older vessels as far as practical.
The guidelines cover a number of areas with Chapters 1 and 12 being of particular interest to those involved in navigation.
Chapter 1 deals with the requirement for special ice navigators and refers to a later chapter as regards their qualification. In some parts of the world – Canada is a good example – ships were obliged to have an accredited ice navigator on board when operating in ice even before the guidelines were adopted and published. In recent years, the number of courses developed to teach ice recognition (there are more than 30 different types of recognised ice formations) and ice navigation has multiplied and there are even simulator courses available in some locations.
The additional requirements of Chapter 12 of the guidelines are not particularly onerous but the final requirement is one that equipment makers have responded to in a number of ways. Conventional marine radars are inadequate for ice navigation (except when following an icebreaker) because they make use of echo stretching, or expansion. This technique stretches’ a radar echo to enable the target to be determined easily against background clutter. It is useful in high seas where the only high-intensity radar echoes are those from vessels, land or weather clutter but when used in ice the resultant radar image is at such a consistently high intensity that the radar operator must make adjustments to reduce the number of echoes – invariably removing many of the ice echoes.
With the prospect of extended navigation in Arctic waters, several leading radar makers have developed systems specifically designed for use in ice-infested waters. The technologies used by the companies to enhance their radar systems vary. Some prefer to make use of standard 9GHz X-band navigation radar with special software being used to enhance the image. Kelvin Hughes’ ice version of its ETD radar systems, called MDICE, is available as an upgrade. MDICE uses a scan-to-scan correlation technique which integrates the returns from a large number of scans to improve target detection. Advanced image processing techniques enhance the visual quality of these returns, allowing clearer target differentiation via a quasi-3D representation. Adjustments are possible to fine-tune the system to suit prevailing conditions.
The Simrad ARGUS system also uses enhanced software that can display different types of ice in different colours. This allows navigators to distinguish softer younger ice from older and more dangerous hard ice and solid objects. In order to gain the best image of the ice, Simrad advocates having a dedicated X-band ice radar with its antenna sited a little lower than the main S-band radar and says it is better still to have two ice radars located at a distance from each other. Coupled with the software this can produce an almost stereoscopic image and having two ice radars also adds a degree of redundancy. The software needed can be pre-loaded into the radars but will only be activated if an upgrade key is purchased.
An alternative method adopted by other system makers is to split the signal feed from the X-band antennae into two, with one branch going to the conventional display and the other to the ice radar display by way of a processor module containing the necessary software. Rutter’s S6 radar is one such system but its display is 12-bit as opposed to the normal 4-bit maximum systems used by most vessels. This allows for a display with 256 intensity levels and a much higher definition. Rutter also plans to incorporate wave and current information into its products to generate more information for end users.
Furuno’s FICE-100 ice radar is another hybrid device and when installed is connected to Furuno FAR 2xx7ARPA navigation radar without affecting any of that device’s properties or performance. Furuno says in its product description that the ice radar’s principle of operation is the opposite of the navigation radar, so it is not suitable to the actual navigation. It requires its own processor and device in order to be efficient due its different calculation algorithms.
Ice under the spotlight
Complementing radar with other methods of ice detection is a comparatively new idea. One means that has been tested by Kelvin Hughes is the use of thermal imaging cameras. The company the Kelvin Hughes worked with – FLIR – has also conducted its own tests in Greenland. FLIR claims its equipment is of particular use for detecting smaller pieces of ice.
A good lookout can spot these during daytime but at night the combination of darkness and fog or snow can limit the capability of regular eyesight to detect ice hazards. Thermal imaging cameras record the intensity of electromagnetic radiation in the infrared spectrum. All matter emits infrared radiation, even cold objects such as ice. In a thermal imaging camera, this infrared radiation is focused by a lens onto a detector and the intensity of the recorded infrared radiation is translated into a visual image.
Because thermal imaging cameras rely on thermal contrast instead of colour contrast they do not need lighting to produce crisp images during the night. They provide a good overview of the situation giving a much better idea of the surroundings than the narrow beam of a searchlight.
During tests in Greenland thermal imaging cameras were successfully used to detect pieces of ice of different sizes and shapes. These are generally divided into three categories: icebergs, bergy bits and growlers. Icebergs are floating chunks of ice with more than 5m of their height exposed above sea level. Bergy bits are pieces of icebergs showing 1m-1.5m above sea level. Growlers are pieces of icebergs showing less than 1m above sea level. With the thermal imaging camera all of those three categories were detected.
Due to their size, icebergs are usually relatively easy to detect by radar. On most occasions, using radar should suffice in detecting them but bergy bits are smaller than full-grown icebergs, making them harder to detect, both by radar and visually. Even the large bergy bits can be difficult to detect using marine radar, due to their shape: their sides are often oriented in such a way that radar energy is deflected away from the antennae. Combined with sea clutter, this characteristic can make it very difficult to spot them on the radar. During the test, many bergy bits were observed with the thermal imaging camera and they showed up very clearly in its image.
Growlers, being the smallest category, are the most difficult form of ice to detect both visually and on radar. Though small, growlers can still pose a serious threat even for ice-strengthened vessels. Growlers made out of ice less than one year old should not cause much damage to such vessels, if they maintain a safe speed. But due to its pressurised environment, ice from glaciers and multi-year sea ice can have a much higher density, so growlers made of multi-year ice can be a lot heavier than those made out of the less dense younger ice.
Source : https://shipinsight.com/guides/polar-navigation
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