FAQs

A briefing note comparing Loran-C and eLoran has been prepared by the Research and Radionavigation Directorate of the General Lighthouse Authorities of the United Kingdom and Ireland. The briefing note is available for download and compares different generations of Loran in terms of their capability, performance and functionality

An extensive list of eNavigation FAQs can be found on the IALA website and are available for download as a pdf. 

The International Association of Lighthouse Authorities and Marine Aids to Navigation (IALA) publishes recommendations and guidelines on marine aids to navigation, many of which are available free at http://www.iala-aism.org/.

Range /Intensity charts for marine aids to navigation are published in IALA Recommendation E-200 part 2 “on Marine Aid to Navigation Signal Lights”. The recommendation can be downloaded at http://www.iala-aism.org/. Click on the “Publications” tab and then select “Recommendations”. Click on Eng for a free English download.

Rotating optics usually have a number of lens panels arranged around the centre of a rotating table with a light source (e.g. a lamp) continuously burning at the centre. Each lens panel has the light source at its focal point and this forms a “pencil” beam, which is a narrow shaft of light of high intensity. The several lenses form several beams arranged like the spokes of a wheel. As the table rotates, each beam passes the eye of a distant observer giving the appearance of a flashing light. The rhythmic character and timing is determined by the displacement of the lens panels around the rotating table and the rotational speed. The beams all point towards the horizon.

Fixed optics are cylindrical in form and the light source at the centre forms a “fan” beam. This is a disc of light directed at the horizon. Fan beams are typically less intense than pencil beams but they can be used to form different coloured sectors with sharp transition between the colours. The rhythmic character and timing is provided by switching the light source on and off.

A Loran receiver computes its position solution in two stages. The first stage assumes that the entire earth’s surface is made of sea-water. A receiver has a very accurate model of 100kHz groundwave propagation delay over the sea-water surface built into its firmware.

The second stage is to take into account the extra delays from each of the signals due to propagation over any land along the propagation path. A receiver cannot possibly know about the land along the propagation paths between it’s location and the locations of the eLoran transmitters, so we need to tell the receiver about the effect of that land.

So a receiver has a built in table of Additional Secondary Factors (ASFs). This is a lookup table referenced by the position solution computed using the sea-water only assumption. Once the time of arrival measurements (pseudoranges) have been adjusted for the ASFs another position computation is made, which is much more accurate. Without ASFs there would be a position offset of maybe up to several kilometers from where the receiver actually is (in the UK anyway).

Off the East Anglia coast, where the land is flat and of good conductivity, ASFs will vary smoothly and sufficiently far from the coast will be uniform; in the sea lochs of the West of Scotland, where the land is mountainous and of poor conductivity, ASFs will vary rapidly. As such the density of the ASF grid required will also vary. For example, far from the coast the ASF will flatten out and you could end up only needing a single ASF value (one for each transmitter) to cover a fairly large area.

To cope with those variations we use differential-Loran, with a Reference Station located at a precise position close to the harbour. The Reference Station can determine the ASF temporal variations and send those corrections out over the eLoran signal itself (Eurofix or the Loran Data Channel) so that the mariner can receive the corrections and adjust the measured pseudoranges before computing a position.

The Data Client is assigned a fixed IP address on the VPN and the software sends RSIM messages (a message type developed by the RTCM for controlling differential-GPS Reference Stations and Integrity Monitors) over the VPN. RSIM is conveniently used for eLoran in this case too.

You cannot simply connect to a wireless data stream and send data to the station. Not withstanding the bandwidth priority considerations (25 to 50 bps for everything!), you will need a dedicated system to send your messages.

If you need a Data Client, talk to Reelektronika about your project. They developed the Eurofix equipment and maintain the VPN upon which Anthorn and our differential-Loran Reference Station sits. No doubt they can provide you with a description of Eurofix and and some background information. They could also develop an experimental software suite for you as part of the existing VPN.

Reelektronika’s website is at www.reelektronika.nl

Sitex Marine offer a commercial integrated GPS/eLoran receiver - http://www.si-tex.com

Crossrate Technology have recently released their eLGPS 1110 - http://www.si-tex.com

Reelektronika provide several high-end research and development receivers – http://www.reelektronika.nl

These measurements will typically be made once and for all on a particular day of the year. Any seasonal/weather related variations in the ASF data will be compensated for by the provision of one or more differential-Loran Reference Stations situated near each of the major ports. So in the first instance, ASFs will need to be measured at each of these ports. We estimate that this initial survey will be achieved through the use of a locally hired rigid inflatable boat (RIB) at each of the harbours. Alternatively, the local harbour master may be in a position to offer assistance. However, the use of such small boats may be limited to close in to the harbour. Approach channels that stretch further out to sea may be measured using larger vessels as appropriate.

ASF measurements are made using highly specialised, accurate and expensive equipment. It is NOT simply a matter of comparing GPS against eLoran and saving the residuals. ASFs are precisely measured against UTC and the known arrival times of the eLoran signals, based on sea-water propagation, and the expected transmission time of the Loran signals. A loran simulator is built into the measurement unit to compensate for channel delays and temperature variations in the antenna, making the equipment much more sophisticated than a standard eLoran receiver/GNSS receiver combination. GPS is used to maintain accurate time of the time-tag marker that is used to measure the time of arrivals of the Loran signals, and also to act as a ground truth position system.

Harbour ASFs are very much the priority for the GLAs at the moment, with coastal phase ASFs a potential project for the future. In the future, ASFs around the rest of the coastline might best be measured using "vessels of convenience". ASF measurement equipment could be installed aboard the vessels and data removed from the units when they dock. Alternatively, remote access to the measurement systems could be employed where an engineer can remove the data via a broadband Internet connection.

Once the ASF data has been measured, published and stored within a user's receiver, ongoing validation of the data would be required. This validation could be performed using a standard eLoran receiver loaded with ASFs and comparing Loran against GPS aboard the vessel. This would require equipment that is more simple than an ASF measurement suite. The GLAs are running a project to install eLoran monitoring equipment (receiver plus PC in a single box) aboard each of their vessels, and those systems would be vital in proving (or disproving) eLoran performance around UK and Irish waters, in addition to performing an ASF validation role.

We are sponsoring a PhD programme of work at the University of Bath, with the student developing a full eLoran service volume coverage prediction software suite. This is a continuation of work that was started at the University of Wales, Bangor, several years ago. Coverage prediction includes station geometry, interference analysis, radio atmospheric noise, groundwave and skywave field strength and delay predictions, receiver processing capabilities etc. We are a couple of years away from having that full capability at the GLAs through the studies at Bath.

Even more useful coverage plotting software will show you availability performance figures as well as accuracy.

For something like a bridge, or a large building, the spatial effect is very repeatable (Terry Moore at Nottingham has an MSc student who has seen such effects at road junctions, most likely to be the electrical infrastructure contained within pedestrian crossings), so it could be compensated for in tabular form. One possible problem for maritime eLoran is when a vessel navigates into a harbour. The configuration of the harbour's re-radiation environment will change with the presence or absence of other large vessels. This effect may be something that the GLAs should get involved in investigating.

Wouter Pelgrum (ex. Reelektronika) did some work on re-radiation in his PhD thesis.