FINAL REPORT - Stakeholders - Ofcom

The intersection of each line-of-position provides the geographic position of the receiver.
Each line-of-position is the shape of a hyperbola, classifying Loran-C as a hyperbolic
system.
An alternative mode of receiver operation uses only two transmitting stations and an
accurate atomic clock at the receiver to provide actual time of arrival of the transmitted
signals (ranging). This mode is called Rho-Rho navigation. In this mode the lines of
position are circular, and the intersection of the circles gives the geographic position. The
two-position ambiguity of intersecting circles may be removed by other means. Use of this
mode of operation is restricted owing to the high cost of receiving equipment.
Additional secondary factors (ASFs)
Loran-C receivers compute distances from Loran-C transmitting stations using time of
arrival measurements and the propagation velocity of the radio ground wave to determine
position. Small variations in the velocity of propagation between that over sea water and
that over different land masses are known as the Additional Secondary Factors (ASFs).
Corrections may be applied to compensate for such variations. The corrections improve
the accuracy of the Loran-C service at locations where the received Loran-C signal
passes over land on its way from transmitter to receiver.
The values of ASF depend on the conductivity of the earth’s surface along the signal
paths. Sea water has high conductivity, and the ASFs of sea water are by definition zero.
Dry land, mountains, or ice generally have low conductivity and radio signals travel over
them more slowly, giving rise to substantial ASF delays and hence degradation of
absolute accuracy.
ASFs vary little with time, and it is therefore possible to calibrate the Loran-C service by
measuring ASF values throughout the coverage area. NELS currently have a programme
for the mapping of the ASFs in northern Europe, based on a combination of computer
modelling and field measurements. When the data has been collected, it is intended that
these corrections will be available as electronic databases for incorporation in Loran-C
receivers.
Propagation anomalies
The Loran-C system is suitable for many land radio location applications. However,
propagation anomalies may be encountered in urban areas caused by the proximity of
large man-made structures.
Compensation for these anomalies is usually possible either by prior measurement or by
the application of the local ASFs. However, improvement in the accuracy of Loran-C
service may also be achieved by the measurement and broadcast of local corrections if
required.
Service provision in Europe – Northwest European Loran-C System (NELS)
The principal Loran-C activities in Europe are co-ordinated through the Northwest
European Loran-C Agreement, which established the Northwest European Loran-C
System (NELS).
NELS is currently providing a Loran-C service from 8 transmitter stations in Norway,
Denmark, Germany, and France. A map showing the current Loran-C coverage as
predicted by NELS is shown in Figure 3-16. NELS has predicted the additional coverage
that will be obtained when the Loophead station in Ireland becomes operational – shown
by a dotted red line in the figure below. The commissioning of this station is pending the
resolution of disagreements on planning permission and legal issues.
Page 90