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Position fixing

Updated 11 Oct 2019

Stars ship navigation

Traditional means of navigating are dependent upon a magnetic compass to indicate direction and a sextant to fix the position of the ship. Both instruments have limitations but skilled navigators have been using them successfully for centuries before and after the invention of alternatives such as the gyrocompass and GPS.

A sextant is used to measure the angle between the horizon and a celestial body such as the Sun or the Pole Star. Once the angle is determined it can be used to determine the latitude of the ship. Measuring the longitude involves measuring the angle between two celestial bodies at a any specific time. 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 In practice, GPS or any other satellite navigation system has superseded the use of sextants for determining position and the need to determine the exact time no longer exists.

Although there is still a requirement under SOLAS for a ship to have a magnetic compass 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 gyrocompass and the GPS, or a loss of the GPS signal which could result from jamming or GPS satellite malfunctions.

This very short description of celestial navigation may seem dismissive of methods that have been used for centuries by commercial vessels and which is still seen as a desirable skill by leisure navigators and indeed many commercial and military navigators, but it has been superseded on the modern ship by satellite positioning systems.

Satellite Positioning Systems

Navigating is defined as the process or activity of accurately ascertaining one’s position and planning and following a route. Except when in port or sight of land, the first is very difficult and 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.

Before the advent of satellite navigation systems such as GPS, 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 gyrocompasses. 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 with any pedigree is the Russian GLONASS system. The European Galileo system was due to reach Full Operational Capability (FOC) in 2019 but has suffered problems and looks to be delayed. The UK’s impending departure from the EU could also see the UK withdrawing from the project and establishing its own system instead.

The Chinese BeiDou system is another that aims to rival GPS. It was first commissioned on a local basis in 2000 and the first generation system decommissioned at the end of 2012. The second generation of the system, officially called the BeiDou Navigation Satellite System (BDS) and also known as COMPASS or BeiDou-2, became operational in China in December 2011 with a partial constellation of 10 satellites in orbit. Since December 2012, it has been offering services to customers in the Asia-Pacific region.

In 2015, China started the build-up of the third generation BeiDou system (BeiDou-3) for global coverage constellation. The first BDS-3 satellite was launched on 30 March 2015 and there are now 46 satellites in orbit.

Since late 2018, discussions have been taking place between Russia and China with the aim of co-operating on satellite positioning technology. In September 2019, the two countries announced they will soon put in place an agreement involving their respective satellite navigation systems, aiming to promote the compatibility and interoperability of the BeiDou and GLONASS systems. It is said that the resulting service will have more accurate positioning and be more robust than the US GPS system.

Differential Global Positioning System (DGPS)

DGPS is an enhancement to Global Positioning System that provides improved location accuracy, from the 15m nominal GPS accuracy to about 10cm 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 data (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 Vulnerabilities

Satellite navigation systems have transformed navigation at sea as well as on land and in the air but are not infallible. More importantly they are not commercial systems but are controlled by state actors. GPS for example is controlled by the US military and, although it is currently made freely available, the system can be turned off or its accuracy degraded by the US authorities. A similar situation exists with the alternatives from Russia, China and the EU.

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 have not been many examples of this being done maliciously but the possibility cannot be ruled out. It has been claimed that the tanker Stena Impero impounded by Iran in July 2019 was fooled into straying into Iranian waters by interference with its GPS systems.

GPS jamming or spoofing are both factors that many believe should be addressed by manufacturers as the shipping industry addresses concerns over cyberattacks.

A non-malicious and natural 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.

LORAN and eLoran as GPS alternatives

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 harbour 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 latitude/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 20-50m can be achieved. By using Differential eLORAN techniques, the position accuracy can be further improved to the order of 10m.

In early 2013, The General Lighthouse Authorities (GLAs) of the UK and Ireland 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 31 December 2015, eight European LORAN stations in France, Norway, Denmark and Germany were decommissioned although they have been mothballed in case of a rethink of the decision.

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 5m was achieved at sea and in the Rotterdam Europort harbour area. A complete test system has been implemented that includes the eDLORAN reference station and the eDLORAN receiver for the pilots. 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. Its 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.

Elsewhere, regular jamming of GPS signals by North Korea is alleged by the government in South Korea. In 2015 it was announced that this had prompted the South Korean government to implement an eLORAN system that will cover the entire country. The goal of the South Korean system is to provide better than 20m 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 Saudi Arabia are also said to be interested in the technology.

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