WAYPOINT - How do you solve a problem like positioning?

01 Oct 2013 The Navigator

Dr Andy Norris, an active Fellow of The Nautical Institute and the Royal Institute of Navigation, asks why positioning is still such a problem to the modern navigator.

The introduction of a full GPS service in the 1990s seemed to suggest that global navigation satellite systems (GNSS) were the final answer to a vessel’s navigational needs. This standpoint was accentuated when President Clinton announced in 2000 that the artificial degradation of accuracy known as Selective Availability would be immediately removed. This improved GPS accuracy for non-defence users from 100 to more like 10 metres.
Users had seen full coverage of GPS steadily emerge, and soon realised that the most probable error that they would encounter would be a local failure with the vessel’s GPS receiver. This led to two or more receivers being fitted to most vessels, making the total loss of position a very rare event. Early GNSS-related accidents, not least that of the Royal Majesty, also led to improved alert standards for new equipment.

Radio interference
The availability of differential GPS (DGPS) in many coastal regions further increased positional accuracy and integrity – and the reliance on GPS. Underneath, a few problems were emerging. For instance, in some areas it had become evident that local TV stations were causing interference to many receivers; electronic failures in other radio equipment could occasionally cause massive interference; some naval radars were capable of blanking out GNSS over a wide area.
In fact, these problems were not being encountered by most users, perhaps leading many into undue complacency about the continuing availability and accuracy of GPS. Even the widely publicised fears about jamming and other potential problems have, to date, been rarely seen at sea.
In world politics terms, the lack of any significant incidents has perhaps also meant that the potential issues have failed to get any immediate resolution. In the past 10 years, governments have mainly acted to reduce the reliance on the sole use of GPS by setting up alternative satellitebased systems.

Unjammable technology
GLONASS has become a good, publicly available GNSS and other systems are planned to become global too, such as Galileo and BeiDou. This all contributes to improved integrity. Unfortunately, they all work on similar frequencies with similar signals, which makes them equally susceptible to jamming and interference.
Unfortunately, the military – traditionally the monetary sponsors of positioning systems – are generally not interested in systems such as eLoran, because all radio frequency based systems are jammable; at least by an opposing military force. Instead, they have an ever-growing interest in inertial-based positioning systems, which are effectively unjammable. There is a lot of research taking place into making such systems more accurate, smaller, and less expensive. One day, the maritime industry will undoubtedly benefit from such technology.

OOWs must be experts on the potential problems with GNSS and how such problems can be quickly identified. Not least, the knowledge and skills of how to safely navigate without GNSS must be maintained and updated

Good seamanship
eLoran is a proven technical solution for merchant vessels, but its effective implementation over wide areas needs relatively complex coordination by multiple countries. High power transmitting ground stations with very tall antennas are required, typically spaced at about 1,000 kilometres. Also, to match the accuracy of GPS, local differential stations are needed close to ports and other critical areas. This all requires good inter-government resolve, interaction and funding.
As pointed out throughout this edition of The Navigator, the potential for major problems on the over-reliance on GNSS is real and growing. However, because of the complexity of getting agreement for secondary systems, it could be many years before good solutions for vessels are both available and globally effective.
In the meantime, we have to rely on good seamanship. OOWs must be experts on the potential problems with GNSS and how such problems can be quickly identified. Not least, the knowledge and skills of how to safely navigate without GNSS must be maintained and updated – as has effectively been the case since the earliest days of navigation.

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