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Posted on 3 August 2016

As ship’s increase in size and operating margins in ports become tighter, it is increasingly common for pilots to bring their own portable navigation systems onboard with them. These systems are commonly referred to as Portable Pilot Units (PPUs). While the Pilot brings local knowledge and ship-handling skills, an independent PPU provides extra graphical information, not capable of being displayed on the ship’s own systems, together with known accuracy.

There are many products claiming to be PPUs, but offering widely varying levels of performance, so this article aims to clarify some of the factors to be considered in selecting equipment to assist pilots with navigation and collision avoidance.

While some might consider the PPU as simply being the charting software and display hardware, in reality any PPU must have a data source, and it is the quality of the data source that will determine the accuracy of the position and supporting data displayed.

Though relatively primitive models existed in the 1990s, PPUs have improved significantly in the last decade. There have been major gains in accuracy and functionality, often combined with reductions of size and weight. Most PPUs use position sensors based on Global Navigation Satellite Systems (GNSS) – GPS, GLONASS, Beidou, Galileo, etc., used either singly or in combination.  Display systems are typically based on rugged laptops/tablets or iPads.

PPUs range upwards from a tablet with charting software connected to the ship’s AIS pilot plug, to full-function systems with independent position, heading and rate of turn (ROT) sensors feeding into sophisticated pilot-optimised electronic charting systems (ECS).  The wide range exists partly because different ports have different needs, but also because of the commercial desire to balance cost and convenience against accuracy and independence from ship’s sensors.  

Sensors

Many users employ a light and compact interface that takes its data from the ship’s Pilot Plug; the Pilot Plug provides position, heading, course and speed over the ground (COG and SOG) plus AIS information about other ships.  It is easily the simplest solution to implement, and also the cheapest, but suffers from a number of deficiencies:

  • AIS was not designed to be a source of own-ship positioning. Errors in the displayed position often occur due to factors such as excessive smoothing of the GPS positions, e.g. Cap Blanche grounding in the Fraser River on 25 January 2014 (Reference: The Transportation Safety Board of Canada Marine Investigation Report M14P0014)
  • Heading is only reported as integer values.
  • ROT is frequently absent from the data, or is continuously reported as being present but zero.

Many of these deficiencies can be overcome by using an additional independent position sensor, and several Pilot Plug interfaces are now available which incorporate independent ROT sensors to provide the missing data and to add a decimal point to the heading.

For more complex operations, or ones which are more safety-critical, there is an increasing emphasis on the pilot’s portable system being totally independent of the ship’s sensors; after all, both safety and efficiency are compromised if everyone bases their judgements on the same ship’s data, especially if that data happens to be wrong. At its most basic level, this independence can be achieved using a product such as ‘ChannelPilot’: a compact single unit which provides position, heading, ROT and AIS data direct to the pilot’s display and is completely independent of the pilot plug.

But for more demanding applications, where the PPU will be used to assist with very tight or critical manoeuvers, or with dangerous cargoes such as LNG, or maybe as a docking aid, the position and movement of all critical parts of the vessel needs to be known to within a few centimetres and a few cm/sec. Very accurate positioning is essential, as also is precise, stable heading.  Dual-head PPU sensors are available from a number of manufacturers which provide the high-accuracy data needed for such uses.

Accuracy

Most people think of accuracy only in relation to the position of the GNSS receiver itself.  For high-quality systems, depending on the type of external corrections applied, this could be as good as 2 cm for the position of the main antenna.

In practice, from a purely positional perspective, there is little practical value in knowing the position of all parts of the ship to such fine tolerances; the real benefit comes from the improved accuracy of velocity measurements when random fluctuations of position and heading are eliminated.

The position of the whole ship’s hull is determined by the navigation software in the laptop/tablet, using extra information:

  1. Vessel dimensions (known)
  2. The location of the antenna on the ship (known)
  3. The vessel’s heading

Noting that pilots are often critically interested in what is happening at the far end of the ship (particularly when berthing), the most critical factor in calculating its position and movement is knowing the vessel’s heading – really accurately.  Typically the best systems can offer 0.02° or better.

For a vessel where the bow, say, is 200m from the antenna (on the bridge wing), 0.02° means the total athwartship error of the bow may be in the order of 7-10 cm.  With stable heading, the software can then generate athwartship velocities for that position to an accuracy of ~0.02 kn (or 1 cm/sec) - as good as most laser docking systems.

Software

Although any ECS software could be used by pilots for basic navigational purposes, at least ten software providers have created specialised software which provides pilot-specific features, in particular the ability to:

  • Load different ship parameters easily and save them for future use
  • Adjust depths and limiting depth contours for height of tide
  • Display enhanced chart information such as Bathy ENCs (with enhanced and updated depth information) and Port ENCs (with better positioning and details of wharves, etc)
  • Predict accurate future positions when altering course around bends
  • Show other vessels’ AIS details and predict meeting points when following winding channels
  • Automatically measure and display distances off wharves from different parts of the ship
  • Automatically measure and display closing rates relative to wharves when berthing
  • Record all data for later review

The enhanced capability of the more sophisticated charting systems adds particular benefits when navigating very large ships with variable under keel clearances as a result of squat.  Dynamic Under-Keel Clearance is often used in those situations to maximise the draught (and hence the cargo capacity) for prevailing weather and sea conditions, height of tide and transit speed.  Overlays on the ECS show the precise limits of navigable water.  As an example of using PPUs with this capability, Port Hedland in Western Australia frequently sails up to seven ships in succession – the smallest first – down their single exit channel on the same tide; by using this technology. They regularly export over 1,000,000 tons of iron ore on a single tide!

By making use of the available technology, Pilots are able to further enhance their contribution to the ship’s bridge team and to safe conduct in pilotage waters.  PPUs have proved particularly valuable when environmental conditions deteriorate markedly and unexpectedly, allowing pilotage operations to be completed successfully and with less stress for all concerned.  It all depends on having the right sort of PPU for the type of operation – ‘the right horse for the course’.

About the Author

Paul Stanley has been involved with developing PPUs and training pilots in their use since 2002.  Now largely retired, he is a Director of Navicom Dynamics Ltd, a leading manufacturer of PPUs.