LORAN - LORAN Charts

LORAN Charts

Where available, common marine nautical charts include visible representations of TD lines at regular intervals over water areas. The TD lines representing a given primary-secondary pairing are printed with distinct colors, and note the specific time difference indicated by each line. On a nautical chart, the denotation for each Line of Position from a receiver, relative to axis and color, can be found at the bottom of the chart. The color on official charts for stations and the timed-lines of position follow no specific conformance for the purpose of the International Hydrographic Organization (IHO). However, local chart producers may color these in a specific conformance to their standard. Always consult the chart notes, administrations Chart1 reference, and information given on the chart for the most accurate information regarding surveys, datum, and reliability.

There are three major factors when considering signal delay and propagation in relation to LORAN-C:

1. Primary Phase Factor (PF) – This allows for the fact that the speed of the propagated signal in the atmosphere is slightly lower than in a vacuum.
2. Secondary Phase Factor (SF) – This allows for the fact that the speed of propagation of the signal is slowed when traveling over the seawater because of the lower conductivity of seawater compared to land.
3. Additional Secondary Factors (ASF) – Because LORAN-C transmitters are mainly land based, the signal will travel partly over land and partly over seawater. ASF may be treated as land and water segments, each with a uniform conductivity depending on whether the path is over land or water.

The chart notes should indicate whether ASF corrections have been made (Canadian Hydrographic Service (CHS) charts, for example, include them). Otherwise, the appropriate correction factors must be obtained before use.

Due to interference and propagation issues suffered from land features and artificial structures such as tall buildings, the accuracy of the LORAN signal can be degraded considerably in inland areas (see Limitations). As a result, nautical charts will not show TD lines in those areas, to prevent reliance on LORAN-C for navigation.

Traditional LORAN receivers display the time difference between each pairing of the primary and one of the two selected secondary stations, which is then used to find the appropriate TD line on the chart. Modern LORAN receivers display latitude and longitude coordinates instead of time differences, and, with the advent of time difference comparison and electronics, provide improved accuracy and better position fixing, allowing the observer to plot their position on a nautical chart more easily. When using such coordinates, the datum used by the receiver (usually WGS84) must match that of the chart, or manual conversion calculations must be performed before the coordinates can be used.