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Positioning of ships

During audits and inspections on board of our vessels the position of the vessel is not maintained as required. Almost all vessels are relying on the (D)GPS position.

Positioning is essential for navigation: to ensure you have situational awareness; know where you are in relation to navigational hazards and, of course, to enable effective commercial operations. At this time, there is no one perfect means of fixing your position. A wide range of options exists, and professional mariners must know when to use each, based on their strengths and weaknesses. Of course, the professional navigator will always employ as many means as possible and will never rely on any one alone. All positions to be entered in the deck logbook. Knowing which methods to use when is key to success as a navigator.

Positions, please !

Visual Observations

Visual observations are an excellent way of maintaining situational awareness. This can entail everything from plotting compass

bearings, following leading lights, to the noting of beam bearings.

Strengths:

-Excellent for maintaining situational awareness

-Leading lights and sector lights are valuable in pilotage areas

-Should be used in conjunction with ECDIS and radar for checking and back-up

-Helps when interpreting a radar picture

Weaknesses:

-Reliant on good visibility

-Must be within visual range

-Object of bearing must be clearly identified

Dead Reckoning and Estimated Position

Dead reckoning (DR) is predicting where you will be, based upon speed and course. An ‘estimated position’ (EP) refines that prediction further by applying set and drift. Estimating where you should be at what time was essential in the past, but still very useful in modern times for aiding situational awareness

and giving advanced warning of any anomalies in other means of positioning.

Strengths

-Proven traditional technique

-‘Automated’ systems embedded within ECDIS

-Uses existing sensors - log, gyrocompass, etc.

-Potentially gives rapid alerts of loss of GNSS accuracy

-Reasonable accuracy over shorter periods

Weaknesses

-Currently, poor accuracy over long periods

-Some ECDIS equipment have poor embedded implementations of DR/EP

Radar

The use of radar is time-tested for positioning and can be highly accurate. It is fully independent of external sources. Radar is highly effective when using parallel indexing, and can supplement GNSS particularly when overlaid on ECDIS.

Strengths:

-Self-contained positioning system, can be set to require no external support

-Racons provide clearly identified important objects and aids to navigation

-Radar and target tracking functionality (ARPA/AIS) provide a range of effective tools and

alarms to support decision making, e.g. target notification, CPA notification

-Improves situational awareness in restricted visibility

Weaknesses:

-Objects must be within radar visibility

-Subject to interference from heavy seas and weather

-Reliant in some modes on accurate gyro input

-At times a Radar picture can be difficult to interpret

Global Navigational Satellite Systems

GNSS, and more specifically GPS, have been rapidly integrated into modern navigation and can be particularly effective when integrated with ECDIS. These systems give real-time global coverage and historically, have been highly reliable.

Strengths:

-Generally very accurate and reliable position fixes

-Global coverage with 24/7 availability

-Increased accuracy and reliability when used with differential systems, including Satellite Based Augmentation Systems (SBAS)

-Easily integrated into a wide range of onboard systems

-Position can be clearly displayed on ECDIS

Weaknesses:

-Easily susceptible to intentional or non-intentional jamming or spoofing

-Complacency from ease of use can lead to over reliance on a single system

Echo Sounder:

Excellent complementary tool to confirm when you come across anticipated depth areas, but can rarely give an independent position. Vessel can follow a depth contour as a crude form of indexing.

TAKE 10

1 Aware and alert

Professional navigators need constant situational awareness to ensure the safety of lives, the vessel, her cargo, the environment and to ensure commercial effectiveness.

2 Safety in numbers

Never rely on a single means of fixing the ship’s position.

3 Check, check and check again

Good situational awareness requires the continual checking

of complementary positioning systems and the intelligent

application of common sense.

4 Judging the jamming risk

Although the coordinated use of multiple GNSS, such as GPS and GLONASS, improves reliability, all GNSS share a common weakness and therefore are equally susceptible to intentional or unintentional jamming.

5 Multiple choice

There are many methods of positioning available for the navigator, some based on traditional techniques and others on modern technology. Each has strengths and weakness and no one system has proven to be good enough to use on its own.

6 The Human Element

Although all these positioning methods may be complementary, they are not always automatically integrated. The professional mariner needs to be the human integrator of these systems and this skill require training and practice.

7 Prepare for failure

The loss of GPS is a real risk, and should be identified as such, with clear procedures for identifying failure, contingency plans and drills for dealing with the loss.

8 Avoiding over-reliance

Over-reliance on GPS, particularly when integrated into ECDIS, can lead to complacency and poor decisions. The use of GPS with ECDIS has revolutionised navigation and all shipping companies and crews should assess how this impacts on navigational practices.

9 Pole positioning

Training for the use of electronic positioning systems should not just address how to use the knobs and

buttons (‘knobology’), but most importantly, how to use technology to support good decisions with full awareness of inherent weaknesses.

10 Share your knowledge!

Mentoring is key. Experienced mariners should take time to help fellow mariners master positioning techniques. This may be Masters mentoring in the use of the sextant, or juniors helping the older generation understand the application of technology.

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