Location, Location, Location ( With Apologies to Kirsty & Phil)

There is a general misconception that ECDIS is entirely dependent upon the satellite Global Positioning System (GPS) to function but this is not the case since every ECDIS must be capable of being used to plot positions from any source be it visual bearings, radar or even stellar observations. The major problem to date is that on the majority of ECDIS currently on the market the manual option is not obviously available and not always user friendly and I have certainly yet to come across any navigator who has managed to plot a stellar observation on an ECDIS! Mind you it’s becoming increasingly rare to find any navigator who has plotted a position from a sun sight or stellar fix on a paper chart!

GPS has been fully operational with its constellation of 24 satellites for nearly 20 years and the developed world’s infrastructure is now almost totally dependent upon its constant availability so why would anyone bother trying to navigate without it? The answer is that GPS is so vulnerable that in many ways it is pretty miraculous that it works at all!

The signal strength from the satellites has been likened to someone in New York shining a 60 watt light bulb and someone in London having to see it, so it is not surprising that many are deeply concerned about developing back up systems to take over in the event of a GPS outage.

What might cause GPS outage?

There are several potential causes for GPS outage and the two most likely to affect shipboard GPS are sunspot activity and jamming.

Solar activity

The sun is always active in producing electromagnetic emissions which have the potential to disrupt GPS and historically this activity peaks and troughs in 11 year cycles. 2008 was the quiet trough and the activity is currently increasing again with the next peak anticipated in 2013. Alarmist stories of satellites being totally knocked out resulting in Armageddon for the developed world seem to be unfounded but, given the number of articles raising concerns over the effects of solar activity on GPS, it is evident that a quantifiable risk exists. GPS outage problems resulting from increased solar activity were anticipated for June but although I have read some reports of relatively minor positional errors of around 5m it seems that not all satellites are affected equally and I have been unable to find any reports of problems sufficiently severe to render GPS unusable so it is possible that the effects may not be as great as feared. Additionally, advances in receiver technology using the dual frequencies transmitted from the GPS satellites are also reducing the possibility of severe disruption. Only time will tell if this is really a serious threat but if it is then it is probable that the effects on the world’s shipping will be well down the impact list!

GPS jamming

In contrast to the solar activity, the jamming of the satellite signal is a real and very significant problem in that it generally results in the total failure of the GPS receiver. Jamming can be unintentional or deliberate but in either case it is always serious. An example of unintentional jamming occurred in  Moss landing, California where a faulty TV aerial amplifier blocked out GPS over the whole harbour area and there are other examples of faulty electrical equipment having a similar effect on GPS.

Currently in the USA there is a major scandal arising out of a start up broadband company called “Lightsquared” which has been granted a license to create a high power cellphone network to bring high speed broadband and mobile phone coverage to remote areas. The problem is that since its initial application for a license there were major concerns that the $14bn network used frequencies too close to the GPS frequencies which would interfere with the GPS signal. The license was granted on the condition that no such interference would be detected. However, the Government body responsible for granting the license (FCC) commissioned a test which revealed that GPS was seriously affected as per the following extract from a report on the trials:

“Last month, the National Executive Committee for National Space-Based Positioning, Navigation & Timing, and the Federal Aviation Administration tested the LightSquared systems and found them disrupting the signal strength to all GPS devices in the test area”.

In some tests, all GPS-based receivers including those used by the U. S. Coast Guard lost their ability to navigate. Some GPS systems used by space agency NASA for scientific use were also seriously impacted due to LightSquared’s service.

So, that would appear to be conclusive, or is it? The FCC hasn’t withdrawn the license ( this is a massive infrastructure project) but seems to be accepting assurances from Lightsquared that the problem can be resolved by technological fixes. The large opposition lobby group, the Coalition To Save Our GPS, has countered that the technical fixes haven’t yet been invented. At the time of writing this article the mess hasn’t been resolved and, as can be imagined, the “blogosphere” is running red hot over the issue. The lawyers are no doubt rubbing their hands with glee as well!

Deliberate GPS jamming

This is where the greatest potential for GPS disruption lies, especially in port and near coastal waters. The use of such jammers is mainly for criminal activity such as car and lorry theft and for avoiding road tolls but they can also be used to provide privacy in an office environment. Currently the laws are being updated all the time but generally such units are illegal to buy in the UK or to use but it isn’t actually against the law to own one! The fact is that these units are readily available and can be purchased for around £25 for an in-car unit with a declared range of 5 – 10 metres and around £100 for a high power lorry unit with a range of around 100 m. However, these ranges are misleading since such units can cause severe disruption over a much wide area.

In the USA in 2009 such a unit wreaked havoc twice per day with.. a two hour disruption to air traffic controllers’ monitors, failure of doctors’ emergency pagers, ATMs refusing to dispense cash, confused maritime traffic management and a cell phone blackout. It took two months to identify the source which was a lorry driver using a cheap jamming unit to avoid tolls on the New Jersey turnpike.

Although there haven’t been any such disruptions recorded so far here in the UK, many are concerned that as the tracking and monitoring of road users increases it is inevitable that such devices will be increasingly used. More serious is the potential use of jammers in a terrorist attack and consequently both the US and British Governments have conducted jamming trials to assess the potential disruption and this has included specific maritime trials.

Here in the UK two major trials have been carried out by the General Lighthouse Authorities (GLA’s) using equipment provided by the Ministry of Defence (MoD)

for the first trial the NLV Pole Star was used to monitor the effects of a directional jammer operating at 1.5watts placed on Flamborough head under strictly controlled conditions. The effects of this trial were dramatic resulting in:

Numerous alarms on the bridge

Erroneous GPS positions

Failure of GPS fed equipment

Erroneous information presented on the vessels ECDIS

Misleading information presented by the vessels AIS

Reduced situational awareness.

GPS track during the jamming trial. All GPS devices failed.        Image courtesy of Dr Alan Grant, GLA

One important aspect of this trial was to ascertain the effects of GPS jamming on a back up navigation system known as e-Loran which the GLA’s have been developing during the last decade. As anticipated, the e-loran input was unaffected by the GPS jamming so a further trial involving a wider group, including UK and EU Government representatives, was arranged off the Tyne in December 2009 using the THV Galatea. These trials confirmed the findings of the Pole Star trials with both the on-board GPS and the hand held portable devices carried by the visitors being rendered useless. The on board ECDIS had been set up to receive the  e-loran signals and again this system was unaffected with a positional accuracy of within 9 metres being achieved throughout the trial.

The e-LORAN track during the jamming trial.           Image courtesy of Dr Alan Grant GLA


All of you will be familiar with the Long Range Navigation (LORAN) system that was developed in the USA during WW2 based on the British GEE radio navigation principle. Using long wave transmitters LORAN had a range of around 1200 miles but whilst useful in open ocean it was never sufficiently accurate for reliable position fixing in coastal waters. The arrival of satellite navigation in the 1980’s saw a gradual decline in LORAN usage and the arrival of GPS in the 1990’s basically rendered it, along with the Decca navigation system, obsolete. The Decca system was shut down in 2000 but the LORAN chains were maintained.

Recognising the importance of a totally independent back up for GPS, work was undertaken to make LORAN sufficiently accurate to provide a back up for GPS and the result was an Enhanced LORAN or e-Loran. In the UK the importance of a system independent of GPS was recognised by the Government and in 2007 the DfT awarded the GLA’s a 15 year contract to provide and maintain e-LORAN. Other EU countries along with Russia and a few Far East countries have also agreed to maintain funding and there are currently 16 transmitters with coverage being particularly comprehensive in the seas around Europe and the Mediterranean.

In contrast to the UK & Europe, in the USA, somewhat surprisingly, the arguments in favour of e-LORAN have been rejected and in 2010 the USA chains were shut down and are being dismantled, all to save just $20m per annum! The US Defence Department’s solution to the problem of jamming is apparently to create a smartphone “app” that will detect GPS jammers which the public will be asked to download and leave running. It is anticipated that this will create a high density network to quickly identify the jammers! I did check to see if the article reporting this was dated 1st April because surely if  GPS is jammed a smartphone won’t work?

Although the UK and Europe have been supporting e-LORAN, so far there aren’t many combined GPS/ eLORAN receivers available and so far as I am aware no ECDIS manufacturer is offering e-LORAN as part of their installation.

ECDIS & Radar alignment

In part 3 ( issue 302), I suggested that, where a radar overlay is incorporated into ECDIS it should be perfectly feasible in the event of the failure of GPS input for an “intelligent” ECDIS to recognise a coastline’s features and align the ENC to the radar input. Although I am not aware of any systems that can perform this automatically, it has been pointed out to me that manual alignment of the ECDIS to a navigational feature such as that provided by radar overlay  is actually a requirement contained within the ECDIS performance standards and I understand that at least one manufacturer is working on automating this function.

Back to the plot!

As previously mentioned, all ECDIS can be used for plotting bearings taken visually or from radar but there are three major problems associated with this.

Firstly, the vast range of ECDIS operating systems means that there isn’t a standardised procedure for manual plotting so instead of a simple button on the screen marked “manual plotting” that would bring up a set of standardised and familiar plotting tools, the facility is generally hidden in a sub menu and even once found may not be logical to use. The good news though is that once located and understood, manual plotting on an ECDIS is far quicker than on a paper chart. Taking the manual plotting facility further, ECDIS would provide an ideal platform for including different chartwork tools such as a vertical sextant angle. The ECDIS database knows the height of all objects and the height of tide at the time of the observation. The navigator takes the vertical sextant angle, enters it in and “click” a range circle appears around the object and an ECDIS could also easily combine that with the echo sounding to highlight an area of position probability. Simple, it’s what computers can do!

The small screen size means that the navigator may have to scroll and zoom the display to find the land or sea marks being used for the position fix, a factor that I found particularly frustrating when I did the ECDIS course. Zoom out and the names, or even the marks themselves might disappear, zoom in and you have to hunt around to find them. The good news here is that large chart displays are gradually appearing on the market although the bad news is that many ECDIS only vessels have dispensed with the chart room so there’s now nowhere to locate a large horizontal display!

Conrac’s digital chart table

The third and probably most crucial factor is mindset and experience. Most of the traditional chartwork skills have been lost even with paper charts since the advent of GPS with many of the latest generation of navigators having got into the bad habit of just plotting the latitude & longitude from the GPS display onto the chart. ECDIS compounds this problem by showing a reassuring dot where the ship is and when the GPS signal is lost the ship will continue to update its position in Dead Reckoning (DR) mode and there is a recognised problem of a reluctance by some navigators to accept that this may not be where the ship actually is! This factor is compounded by the failure of the alarm systems on ECDIS to seriously alert the navigator that the GPS input has failed. The GPS failure alarm sounds the same as any other ECDIS alerts so it is very easy for a navigator to merely accept the alarm and take no further action. What I believe is required is a large red warning to be placed on the screen stating that the GPS has been lost and that the ECDIS is operating in DR mode which, even when acknowledged will reappear on the screen at regular intervals. Another alert could be set to appear if no manual position has been input at intervals determined by whether the vessel is in open or coastal waters or even better, have the magic ship position circle replaced by a flashing red “lost position” symbol similar to the lost target function on the radar.


The myth that if GPS fails the ECDIS becomes unusable has largely grown out of the manufacturers’ concentrating on GPS as the input and failing to provide the navigator with simple, easily accessible alternative positioning options However, with a back up such as e-LORAN, the potential need to for resorting to manual plotting will be reduced and further non GPS positional accuracy could be achieved by including the increasingly available Inertial Navigation Systems into ECDIS.

How missiles do it!!

Whilst researching this article I came across the following explanation of missile guidance from the US Airforce which I couldn’t resist including!

The missile knows where it is at all times. It knows this because it knows where it isn’t. By subtracting where it is from where it isn’t, or where it isn’t from where it is), it obtains a deviation. The guidance subsystem uses deviations to generate corrective commands to drive the missile from a position where it is to a position where it isn’t, and arriving at a position where it wasn’t, it now is. Consequently, the position where it is, is now the position that it wasn’t, and it follows that the position that it was, is now the position that it isn’t.

In the event that the position that it is in is not the position that it wasn’t, the system has acquired a variation, the variation being the difference between where the missile is, and where it wasn’t. If variation is considered to be a significant factor, it too may be corrected. However, the missile must also know where it was.

The missile guidance computer scenario works as follows. Because a variation has modified some of the information the missile has obtained, it is not sure just where it is. However, it is sure where it isn’t, within reason, and it knows where it was. It now subtracts where it should be from where it wasn’t, or vice-versa, and by differentiating this from the algebraic sum of where it shouldn’t be, and where it was, it is able to obtain the deviation and its variation, which is called error.

Hmm! I wonder what Captain Cook would have made of that?          JCB


One Response to “ECDIS PART 4”

Scott Humphrey
January 2nd, 2013 at 19:00

Outstanding article!
Scott Humphrey,Training Director
Vessel Traffic Service San Francisco


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