How to fix the position of a ship

Malcolm Latarche
Malcolm Latarche

12 May 2017

Without knowing the exact position of a ship to begin with, navigation is impossible beyond sailing in a general direction. Accurate navigation requires a means of determining a ship’s exact position and direction at all times and under all conditions.

In times past this would have been dependent upon a magnetic compass and a sextant. Both instruments have limitations but skilled navigators have been using them successfully for centuries before and after the invention of alternatives such as the gyro compass and GPS.

There is still a requirement under SOLAS for a ship to have a magnetic compass but there is no longer a mention of a sextant. There is however a requirement under STCW for navigators to be able to perform celestial navigation which would most definitely involve the use of a sextant. Many navigators and some ship operators will ensure that a sextant is available on board for use in emergencies such as a power failure which would take out the gyro compass and the GPS, or a loss of the GPS signal which could result from jamming or GPS satellite malfunctions.

Modern equipment

Before the advent of modern systems such as GPS, the use of sextant depended upon a means of determining the exact time. Until marine chronometers were developed in the 18th century there was no accurate means of determining time and therefore the longitude of the ship was something of guesswork. A chronometer capable of receiving time signals is still required equipment on ship and in the IMO’s guidelines for bridge layout it is suggested that it be at the documentation and planning workstation on the bridge. In practice, GPS or any other satellite navigation system has superseded the use of sextants for determining position and the need to determine the exact time no longer exists.

A magnetic compass is of course only able to indicate magnetic North which is not a fixed point in any case and local geo magnetic conditions can cause it to be in error as can the metallic structure of the ship itself (especially if there have been changes to the superstructure or after drydockings) or the cargo the ship is carrying. The strength of the earth’s magnetic field has reduced noticeably in recent times and the movement of the magnetic north pole has accelerated. It is even considered possible that the earth’s magnetic core could flip reversing its natural polarity making magnetic compasses point South instead of North.

Even without natural changes, over time the accuracy of a magnetic compass will deviate and it will be necessary to correct the compass and record the deviation. This is done by a process known as swinging the ship which should be carried out in open water at regular intervals. The compass is checked using a reference point such as the sun or a visible landmark on a known bearing. Deviations will be checked with the ship on all eight of the main headings and corrections made by repositioning the corrective elements that are located around the binnacle.

The compass card is isolated from movement as much as possible by suspending the card on a jewelled mounting and in a liquid filled housing. Both of the damping means can become defective, the mounting by wear and the fluid by leakage or appearance of air bubbles. Tests can be done using a magnet to deflect the card through 90º and then releasing it and timing how long before the card returns to showing North. If the time is excessive, the compass may need to be calibrated or repaired.

All vessels should have their compass swung/adjusted and a new deviation card issued at maximum two yearly intervals. When a new vessel is commissioned, compass deviation on any heading should be no more than 3°. Thereafter, deviation on any heading should be 5° or less.

Vessels transiting the Panama Canal are required by the canal authorities to have had a valid compass deviation card issued within the previous 12 months. Some flag states and many shipowners will stipulate that the magnetic compass is to be swung and adjusted annually.

The limitations of the magnetic compass were a driving factor for the development of the Gyro compass in the early years of the 20th Century. Invention of the device is usually credited to Raytheon Anschutz but there were earlier variants in its evolution. A gyro compass makes use of gyroscopic principals and the earth’s rotation to give a bearing that remains aligned to true North once the initial heading is set and the gyro put in motion. Unlike a magnetic compass, the gyro compass is not hampered by external magnetic fields but can be affected by rapid changes in the orientation and attitude of the ship.

Before the advent of GPS, a magnetic compass would be used to set the gyro compass to the correct heading. On most modern ships, the GPS or other navigational aids feed data to the gyrocompass allowing a small computer to apply a correction. Alternatively, a design based on an orthogonal triad of fibre optic gyroscope or ring laser gyroscopes will eliminate these errors, as they do not depend upon mechanical parts.

The fibre optic gyrocompass is a complete unit, which unlike a conventional compass, has no rotating or other moving parts. It uses a series of fibre optic gyroscope sensors and computers to locate north. It has very high reliability and requires little maintenance during its service life. The system usually includes a sensor unit, a control and display unit, and an interface and power supply unit. It is often linked with the ship’s other navigational devices including GPS.

The gyro compass does not need to display the heading mechanically on a single display because it uses a sensor and the information can be sent to repeater units which would be located at the steering station, in the emergency steering room and on the bridge wings. The exact number and location of repeaters is governed by SOLAS and will depend upon the size of the vessel.

Although a gyro compass is unaffected by the magnetic interference from the ship or surrounding equipment, it is reliant on a stabilised power source and in some instances on a GPS input to remain functioning.

Satellite positioning systems

Before the advent of satellite navigation systems, most vessels were required by SOLAS to be equipped with a radio direction finder (RDF) for determining exact position at sea. RDF systems such as Decca Navigator and LORAN-C make use of radio signals transmitted from a series of shore-based radio stations. The signals would be on different frequencies allowing triangulation to be used when two or more (preferably three) signals were received and identified by the equipment on board ship.

Unlike the satellite systems that replaced them, RDF technology was not available outside of the radio range of the transmitters. However, since precise location and direction is needed most during approach to land and ports this was not a major shortcoming, except perhaps for giving the vessel’s position during emergencies.

The requirement to install RDF systems was dropped from SOLAS at the turn of the century but there are moves to re-instate the technology as a standby in case of satellite system malfunctions. Satellite navigation has caused a revolution in marine navigation and feeds in to so many modern systems including ECDIS, AIS and the latest gyro compasses. It is also an essential technology in making dynamic position possible. The most commonly used satellite system is GPS but there are alternatives. Currently the only functioning alternative is the Russian GLONASS system with the European Galileo system due to come on stream imminently following successful launchings of satellites in November 2016 and the Chinese BeiDou system expected to be operational throughout Asia in 2018 and globally by 2020.

Ships that are not obliged to carry a gyro compass are required to have a transmitting heading device (THD) that shows the ship’s true heading. Most THDs in use today are commonly known as a GPS Compass and make use of an antenna with two or three GPS sensors. Where there are two they will be placed one either side of the vessel’s centre line and at equidistance from it so as to be able to calculate the orientation of the vessel. The sensors can also be used to calculate pitch and roll or trim.

Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations. DGPS uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.

The first reference stations were established by US and Canadian authorities but a number of commercial DGPS services are now available. These services sell their signal (or receivers for it) to users who require better accuracy than GPS offers. Almost all commercial GPS units, even hand-held units, now offer DGPS data inputs.

GPS is controlled by the US military and although it is currently made freely available, the system can be turned off or its accuracy degraded is determined by the US authorities. The system is also not immune from jamming and atmospheric interference.

The issue of jamming has become very topical with the discovery that readily available $10 gadgets can be used to disrupt the GPS system over localised areas. So far there has not been an example of this being done maliciously but the possibility cannot be ruled out. More of a danger to the safety of navigation is the fact that solar flares and mass corona ejections could knock out many of the satellites needed for GPS and other systems to function.


In recognition of the potential problems with GPS some countries are re-introducing an improved LORAN system. Enhanced Long Range Navigation (eLORAN), is the next generation of LORAN and has a reported accuracy near that of conventional GPS positioning in coast-wise and harbor applications, and uses the infrastructure that is already in place.

Its effectiveness is a result of solid-state transmitters, advanced software applications and uninterruptible power sources, along with a new generation of shipboard receivers. Because the signal is much more powerful than GPS, eLORAN is not nearly as susceptible to jamming.
An eLORAN receiver gets its location information by measuring the arrival times of the pulses of at least three eLORAN stations. Using the measured timing values, the user is able to determine latitude and longitude of the receiver’s position, and its time with reference to universal time (UTC).

With conventional LORAN the absolute accuracy of the lattitude/longitude position is a function of effects to the signal passing over irregular terrain. These deviations from modelled propagation are known as Additional Secondary Factors (ASFs). By measuring and mapping the ASFs, and adjusting the measured arrival times accordingly, the user’s position accuracy of 20 to 50 metres can be achieved. By using Differential eLORAN techniques,
the position accuracy can be further improved to the order of 10 metres.

In early 2013, The General Lighthouse Authorities of the UK and Ireland (GLA) announced that ships in the Port of Dover, its approaches and part of the Dover Strait can now use eLORAN technology as a backup to GPS. The ground based eLORAN system provides alternative position and timing signals for improved navigational safety.

The GLAs’ plan was to extend their current trials and to continue building a European consensus in favour of eLORAN and to prepare for the introduction of eLORAN services in 2018. However, it would appear that potential European partners are less keen and on the 31st of December, 2015, eight European LORAN stations in France, Norway, Denmark and Germany were decommissioned.

However, in the Netherlands, a local company Reelektronika has, on request of the Dutch Pilots Corporation, developed and successfully tested Enhanced Differential Loran (eDLoran). The company said in January 2014 an accuracy of 5 metres was achieved at sea and in the Rotterdam Europort harbour area. A complete test system has been implemented.

Which includes the e-Loran reference station and the eDLoran receiver for the pilots. This small and lightweight receiver can link using wi-fi with the standard software of the pilot’s GPS-RTK equipment. New equipment for vessels using the system has been developed and released in 2017.

The US also has most of the infrastructure in place to initiate eLORAN without much delay and Russia and China also have LORAN systems that can be upgraded. The Russian system is commonly referred to as Chayka rather than LORAN but operates on the same principles. In December 2014, the US Department of Defense (DoD) decided to investigate future possibilities and in January 2015 invited tenders for possible supply of some 50,000 eLORAN receivers. The DoD is looking at both stand-alone eLORAN receivers and receivers that integrate eLORAN and GPS. More specifically they are looking for data on the size, weight, power, and cost of eLORAN receivers designed for maritime, aviation, vehicular, and timing applications.

In June 2015, a US Coast Guard Loran mast in Wildwood NJ was reactivated for a year-long demonstration and research eLORAN project. The signal is receivable at distances of up to 1,000 miles. As well as maritime uses, the US believes that eLORAN can provide navigation for drones in its airspace. The project involves two engineering companies, UrsaNav, a supplier of eLORAN technology, equipment, and services, and Harris (which recently acquired Exelis), provide funding and technology for the tests supported by the USCG, Department of Defense, Department of Homeland Security and other federal agencies under a Cooperative Research and Development Agreement. This year (2017) a bill is passing through the US judicial system that could see the project extended and expanded to a full commercial service.

Elsewhere, regular jamming of GPS signals by North Korea is alleged by the government in South Korea. In 2015 it was announced that this has prompted the South Korean government to implement an eLORAN system that will cover the entire country by 2018. The goal of the South Korean system is to provide better than 20-metre positioning and navigation accuracy over the country. The South Korean government hopes to expand coverage to the entire Northeast Asia in close collaboration with Russia and China in the near future. India and Saudia Arabia are also said to be interested in the technology.