The stringent accuracy demands of the Harbour Entrance and Approach (HEA) maritime application (8-20m, 95%) requires precise knowledge of signal propagation variations along the signal paths between each Loran station and the user's receiver.
Propagation variations include:
- Primary Factor (PF) variations - due to the changing refractive index of the atmosphere as weather fronts pass across the propagation path.
- Additional Secondary Factor (ASF) variations - due to changes in the electrical conductivity of the surface of the earth upon which signal propagation speed depends.
- Transmission time variations (jitter)
Grids of Additional Secondary Factor data will be stored within a user's receiver following measurement and publication. A grid will be provided for each harbour or port. These ASFs are measured once and for all on a particular day of the year. However, they are not constant values, they vary slightly over time.
These temporal variations can be mitigated by providing differential-Loran (dLoran) corrections to the user at each harbour or port.
A prototype dLoran reference station is currently operating at Harwich port, broadcasting corrections using the data channel (Eurofix) implemented on the eLoran signal from Anthorn.
