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GPS Backups and Alternatives


Robert Connolly comments on how a return to traditional navigational systems like eLoran can mitigate the security and intelligence vulnerabilities of the GPS system

Robert Connolly comments on how a return to traditional navigational systems like eLoran can mitigate the security and intelligence vulnerabilities of the GPS system and looks at the role of a helicopter in firefighting.


Back in the late 1990s, when Satellite navigation using the US Global Positioning System (GPS) or its Russian equivalent, Global Navigation Satellite System (GLONASS) became available for shipping, there was a serious concern that these systems might be subject to interruption and might not have sufficient position accuracy for safe navigation in busy shipping areas, especially the approaches to busy ports.

Therefore, it was recommended that ships carry a backup navigation system in the form of Loran. In addition, to help ensure more GPS position accuracy, the Differential Global Positioning System (DGPS) was developed and rolled out, using mainly former marine Non-Directional Beacon (NDB) locations because the technology could use existing transmitters and antennas.

Allocated frequencies were 500Hz above the existing NDB frequency for that particular beacon. These DGPS stations transmitted the difference between the positions indicated by the GPS satellite system and known fixed positions.

The system increased position accuracy from 15m to around 10cm.

However, Loran remained the preferred option as a backup navigation system.


Loran and e-Loran

Several European countries, including the UK, began working on a better version of Loran, referred to as ‘enhanced Loran’ or ‘eLoran’. This system relied on a network of transmitters located at various European sites, including a UK site in Cumbria. To improve Loran position accuracy, the UK developed a differential correction Loran system called dLoran that operated on a similar style to the DGPS system.

Unfortunately, some European participants, (Norway and France), took the decision to close their transmitters for this system, due to concerns over operating cost.

This just left the UK transmitter operational. As a result, eLoran closed completely in Europe in 2015, including the UK dLoran transmissions. The UK transmitter remains available for possible future use.

Around the same time, the United States was becoming increasingly worried about the possibility of GPS jamming signals, broadcast intentionally by either a foreign power or by terrorist groups (domestic or foreign).

It was thought that this could be done with cheap jammers that could be purchased on the Internet and used for jamming the in-vehicle GPS signals used by businesses to track their company vehicles.

Trials carried out by the General Lighthouse Authority (GLA) a few years ago demonstrated that these small, low powered, jammers can, indeed, seriously affect the position accuracy of the navigation equipment used by ships.



As a result, the US has been conducting experiments with eLoran. Interestingly, over the past couple of years, the US has closed many of their DGPS correction stations with more to follow this year. However, the US authorities do have a system available that was developed for aeronautical navigation by their Federal Aviation Authority (FAA). It is called the Wide Area Augmentation System (WAAS).

WAAS uses a network of ground-based reference stations in North America and Hawaii to measure small variations in the GPS satellites' signals. Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites, every 5 seconds or more frequently.

The satellites re-broadcast the correction messages back to Earth and any WAAS-enabled GPS receivers can use the corrections when calculating their position to improve accuracy.

WAAS can provide an accuracy of less than 1m laterally.

Whereas it was designed for use by aircraft, the system could easily be used in a maritime environment too.

Europe and Asia are developing similar systems for use with their own satellite navigation programmes.

In recent years, it has become apparent that GPS navigation signals in certain areas of the world have become subject to jamming by foreign powers, sometimes believed to be Russia and North Korea.

Reports of such jamming have come from the Black Sea region and the waters off Korea. Some incidences have also been reported from the Baltic Sea. As a direct result of GPS navigation jamming, South Korea has taken the decision to deploy eLoran and is currently developing a testbed to be ready, by installing a new eLoran transmitter in Gyodong, along with two differential correction stations (dLoran). The latter is expected to be installed in Incheon and Pyeongtaek. The testbed is scheduled to be ready by 2020.

Earlier this year, following the publication of a report on critical infrastructure and its dependence on satellite navigation, the UK Government released an official policy letter stating, “The UK Government is, therefore, supportive of any progress towards initiating and maintaining an operational eLoran network that can provide position, navigation, and timing services and will lend support, where appropriate, to aid its establishment and continued use.”

You might be interested in reading this interesting report. It demonstrates just how much we rely on satellite GPS data in our everyday lives and points to the potential threats that the system currently faces.

It is available for download at this URL:

It is possible that, over the next few years, we could see eLoran operational again in the UK – possibly even throughout Europe.


Range-Mode GNSS

However, the possible reuse of eLoran is not the only current potential backup navigation system being considered. There is also a relatively ‘new kid on the block’, in the form of Range-mode (R-mode) GNSS (Global Navigation Satellite System).

By adding one or two high-precision continuous-wave (CW) signals, a dGNSS/R-Mode modulator meets the resilient PNT (Positioning, Navigation, and Timing) requirements of e-Navigation.

Currently, evaluations are taking place using MF signals from existing DGPS stations.

Interestingly, the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA), is also evaluating a version of R-mode GNSS.

It is using VHF transmissions by adding some channels; this is possible since some duplex channels have become two simplex channels. An alternative technology is to incorporate the data transmission into the current AIS transmissions on the existing Channels 87 (161.975MHz) and 88 (162.025MHz).

In order for the ranging system to work in the absence of GNSS, only transmitters at known, fixed, locations can be used; for an AIS signal, these would typically be AIS base stations (not ‘mobile’ stations such as ships).

There are two basic options for using the AIS transmissions:

First, existing messages can be used, with data added, in the form of a pulsed signal, a CW signal or two-tone modulation.

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Second, it is also possible to use additional messages. The use of VHF greatly reduces propagation problems, which can affect MF frequencies. The preferred option – under further investigation at the moment – is the use of additional channels with CW signals on one or more channels.

It is felt that using existing AIS channels would lack the required high precision, due to the inability to resolve carrier ambiguities.

Another option, which is explored at the moment, is wide-band transmission across available portions of the VHF marine band.


The Future

With a heightened awareness of the threats to the current GPS navigation system, not just from Earth-based sources (military, terrorist or criminal) but also from solar activity, it is highly likely that, within the next few years, we will see the deployment of a suitable, robust and secure, backup navigation system, be it eLoran, R-mode GNSS or something entirely different.

One thing is for certain, and that is the continued development of digital modes and technologies in the maritime navigation and communications sectors.


Fire-Fighting by Helicopter

Two years ago, the Irish Air Corps brought one of their AW139 helicopters to the Festival of Flight air display, held annually in the coastal town of Newcastle, Co. Down, Northern Ireland. It provided a ‘water-bombing’ fire-fighting display (Fig. 1).

The helicopters’ firefighting role in the Republic of Ireland was developed in order to assist ground fire crews fighting remote wildfires in the country’s national parks. I am sure that, following that display, local residents would never have expected to see a helicopter on a live tasking.

However, during this year’s early hot summer, many areas in the British Isles suffered from severe wildfires, and Northern Ireland was no exception.

One such fire, in South Armagh, was in a very remote and inaccessible location. As a result, Northern Ireland Fire and Rescue Service (NIFRS) requested assistance from the Irish Air Corps to water-bomb the blaze. Their AW139 made the journey over the border and, using water picked up from a local lake, helped to successfully bring the fire under control.

Less than two weeks later, their assistance was once again requested; this time to help NIFRS crews tackle a wildfire that had re-ignited in the hills just behind Newcastle, after being extinguished the previous evening.

Again, it was a difficult area to access by ground crews and their water tankers. In addition to this, this particular fire was threatening a forest that is a major local tourist attraction, along with a holiday home caravan site and some local residential properties.

The AW139 made numerous sorties collecting water from the nearby sea, in the same fashion as they demonstrated there several years previously. In both incidents, the Air Corps placed a crew on the ground to coordinate with NIFRS personnel, who had determined where they wanted the water dropped.


Weather Problems

While on the subject of the Irish Air Corps and the Newcastle, Co. Down, Festival of Flight, one of their AW139 helicopters and Casa maritime reconnaissance aircraft were scheduled to take part in this airshow a couple of days before this column was due to be with our editor.

Although there was rain forecast for the Saturday afternoon, it was hoped that this would not affect the flying displays, which were to include the Red Arrows, an RAF Typhon and the Battle of Britain Memorial Flight.

Unfortunately, about fifteen minutes before the flying display was scheduled to commence at 01.30pm, the cloud and rain quickly swept down from Slieve Donard, putting visibility well below safety limits for the displaying aircraft. It was hoped that conditions would improve. Unfortunately, they did not, resulting in the cancellation of the entire flying display for the first time ever since the event’s inception nine years ago.

Earlier this year, during the planning stage, the organising council was considering extending this to a two-day event but decided to keep it to one day, owing to costs.

I suspect that they now regret that decision, in the light of the adverse weather conditions on the day.



In my more recent columns, I have been explaining the various systems used to transmit weather data under the Safety of Life at Sea (SOLAS) requirements of the International Maritime Organisation (IMO).

I am sure most of you have listened to the BBC Shipping Forecast at some point or you might have seen gale warnings, transmitted by either voice, RTTY or NAVTEX:

But do you really know what some of the terminologies used actually mean?

For example, a gale warning that is ‘imminent’ – is that in the next hour or over the next few hours?

What kind of time spans are involved here?

Table 1 provides an explanation to some of these terms. The UK Met Office website contains some more explanations of other less common terms.



Finally, the photograph in Fig. 2 shows a Royal National Lifeboat Institution (RNLI) Rescue Support Vehicle (RSV). It was in attendance during last year’s Airwaves airshow in Portrush, Northern Ireland. The RNLI maintains a fleet of twelve of these vehicles, described as a ‘multi-purpose platform’ for supporting rescue, prevention, community education, and in the context of other operational demands.

The four-wheel drive vehicles contain a basic – but effective – command facility, storage, an advanced electrical charging bank, communications equipment, a welfare and changing/privacy area, and also a complete wet- hanging area for the drying out of equipment.

The vehicles are used during flood rescue tasks and for temporary lifeguard cover. They can also be ad-hoc centres for other major tasks and for community events.


Table 1: Terminology used in the BBC Shipping Forecast


Gale Warning Terminology



Winds of at least Beaufort force 8 (34-40 knots) or gusts reaching 43-51 knots

Severe gale

Winds of force 9 (41-47 knots) or gusts reaching 52-60 knots


Winds of force 10 (48-55 knots) or gusts reaching 61-68 knots

Violent storm

Winds of force 11 (56-63 knots) or gusts of 69 knots or more

Hurricane force*

Winds of force 12 (64 knots or more)

Gale Warning Timing



Expected within six hours of the time of issue


Expected within six to 12 hours of the time of issue


Expected more than 12 hours from time of issue

Perhaps* Later

Used when a gale is considered possible in the "later" period, but the forecaster is not sufficiently sure to issue a warning.



Very poor or Fog

Visibility less than 1,000 metres


Visibility between 1,000 metres and 2 nautical miles


Visibility between 2 and 5 nautical miles


Visibility more than 5 nautical miles

Pressure System Movement



Moving at less than 15 knots


Moving at 15 to 25 knots

Rather quickly

Moving at 25 to 35 knots


Moving at 35 to 45 knots

Very rapidly

Moving at more than 45 knots

Wind Direction Change


Wind direction

Indicates the direction from which the wind is blowing

Becoming cyclonic

Indicates that there will be considerable change in wind direction across the path of a depression within the forecast area


The changing of the wind direction clockwise, e.g. SW to W


The changing of the wind in the opposite direction to veering (anticlockwise), e.g. SE to E

Sea State



Wave height less than 0.5 m


Wave height of 0.5 to 1.25 m


Wave height of 1.25 to 2.5 m


Wave height of 2.5 to 4.0 m

Very rough

Wave height of 4.0 to 6.0 m


Wave height of 6.0 to 9.0 m


This article was featured in the October 2018 issue of Radio User