What are Additional Secondary Factors (ASFs)?
The aim of an eLoran receiver is to measure the time it takes signals to propagate from each of the transmitters to the receiver’s antenna. The 100kHz groundwave radio signal propagates over the surface of the earth. The speed of propagation depends on the electrical conductivity of the surface over which it propagates. The signal will travel most quickly over sea-water, a little more slowly over good farming land, and even more slowly over rock, mountains, desert and ice.
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.
What is differential-Loran?
An ASF on an individual transmission manifests itself as a timing offset that needs to be “calibrated” out. For maritime harbour entrance and approach, the aim is to survey the ASFs for major harbours and publish them to mariners. Of course ASFs are only measured once and for all and then fixed in the receiver. But they vary slightly over time due to propagation changes, so called temporal variations, for example rain soaking into the ground, freeze-thaw paths etc.
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.
How do I transmit information over Eurofix?
In order to send data over Eurofix, the eLoran data channel in Europe, you need to be able to send the messages you require up to the Eurofix Reference Station at the eLoran station. This is achieved using a Virtual Private Network (VPN) over the Internet. In order for a third party to send data over Eurofix they will need a Eurofix Data Client. The Data Client is effectively a PC with an attached eLoran receiver used for message integrity checking.
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
What are the expected minimum and maximum ranges of eLORAN stations?This depends on transmitted power and terrain, but 1000km is typical.
Is it possible to get an independent position from only one eLORAN station?
No. Two are required if user equipment is synchronised (range-range mode), three if unsynchronised, or in hyperbolic mode.
How many eLoran stations do I need to cover an area such as….?
As above, two in range-range mode, otherwise three, but just to cover a limited area, short range stations might suffice. Coverage prediction should be used to optimise the position of the stations. The GLAs sponsor a PhD student at the University of Bath, who is further developing the GLA’s coverage prediction software.
What is the susceptibility of LF against intentional/unintentional jamming?
All things are possible! Of course it is possible to jam eLoran, but it is very difficult, requiring high powers, large antennas and a large truck like vehicle to caryy the equipment.
How does accuracy degrade as one moves away from an eLORAN station?
This depends on the geometry of the stations and nature of the terrain (poor conductivity land attenuates and delays the signal more than good conductivity land). In general the lower the signal-to-noise ratio from a station the more variation you will see in the time of arrival measurements of the eLoran signals and therefore the lower will be the accuracy. ASFs correct for fixed propagation errors and can be accounted for by tables of data in receivers. Differential-Loran can improve accuracy performance and reduce the effect of the changing conditions that cause such variations.
What is the GLAs' proposed methodology for determining area ASF within the sea areas for which they have responsibility?
The maritime navigation application that demands the highest accuracy from eLoran is that of Harbour Entrance and Approach (or Port Approach in IMO terms). It is the aim that accuracy approaching 10m(95%) will be met through the provision of accurate and high integrity ASF data measured at all of the major ports and harbours in the service areas for which the GLAs are responsible.
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.
Assuming that there is no alternative method to taking eLoran equipped vessels thoughout the relevant UK / Irish waters, and comparing observed readings with GNSS, how would you see this survey being carried out in practice?
The process of ASF measurement is to traverse the entire harbour, or approach, in such a manner that a grid of ASFs is built up, with each ASF data point separated by 500m or so in latitude and longitude. This grid would be formed by making a series of runs along parallel paths separated by 500m all over the coverage area of the port. ASFs can be interpolated between grid points, and to a certain extent extrapolated outside the survey area. Several repeat runs would be performed to ensure the integrity of the measurements. ASF measurements made on land near to the harbour approach would serve as useful calibration points, that could be visited by our Inspector of Lights during the performance of his “day job”.
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.
What is re-radiation and how does it affect navigation with eLoran?
Large metal structures like bridges, overhead electric cables and metal framed buildings act as antennas that receive the Loran signal and then re-radiate it. The effect is very similar to the multipath phenomenon seen in GPS, but the physics is somewhat different. The correct term for eLoran is “re-radiation” not multipath. It results in interference with a slightly delayed version of the wanted signal. The effect is most noticeable within about 500m from the structure. Re-radiation is repeatable around fixed structures, but it is NOT ASF and should not be considered to be lumped in with the ASF effect, since re-radiation is an effect that is highly directional when using H-field antennas. ASF is a specific effect due to ground conductivity of land and terrain elevation.
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.
What is the difference between Loran-C and eLoran?
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
Where can I find FAQs on eNavigation?
- Visual Signals
Where can I find information on marine AtoN lights and lighthouses?
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/.
- Visual Signals
How do I know what intensity a light has to be to meet a certain range?
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.
- Visual Signals
Why do some lighthouses have rotating optics and others have fixed optics?
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.
- Visual Signals
Why do LED buoy lights appear more conspicuous than ones with incandescent lamps?
There are many reasons why some lights appear brighter or more conspicuous than others. Generally speaking, LEDs are efficient generators of coloured light and the colour produced is less saturated than that produced by incandescent lamps with coloured filters. White LEDs usually produce a bluer white than incandescent filament lamps, which can appear quite yellow. This blue enhancement can be very useful when trying to detect a buoy light against a background of yellow sodium street lights. Furthermore, the flashing character produced by an LED light has a faster on-off transition than a filament lamp. Filament lamps take time to warm up due to the thermal mass of the filament, whereas LEDs have virtually no warm up time. The fast on-off time of an LED is more noticeable to the eye than the slow warm up and cool down of a filament lamp.