LOP [line of position] distortion of LORAN-C observed at Sagami Bay [Japan]

"LOP (Line of Position) distortion of LORAN-C (Long Rang Navigation System-C) Northwest Pacific Chain (8930 chain) was investigated at Sagami Bay in 1996. The data of each LORAN-C station of W (Okinawa Gesasi), X (Minamitorisima), Y (Hokkaido Tokachibuto) and Z (Korea Pohang) were measured onboard a research ship under her cruising and at a fixed point in Tokyo. Though the LOPs shown in LORAN-C nautical chart are derived theoretically from assuming the ground wave propagation along the sea surface, if land is contained in the paropagaiton path like coastal sea area, the delay of propagation time called secondary factor (SF) or additional SF (ASF) occurs due to the change of earth impedance and this leads to the LOP distortion. As the results of observation, the high repetition accuracy of 0.1 micro s of LOP was observed at the fixed point, and it was cleared that the large local distortion, which is caused by long land propagation and it attains to some hundred meters, are existing in Y and Z station's LOP at Sagami Bay. In this paper, the influence of distortion for LORAN-C positioning accuracy is discussed and the proposal to improve the positioning accuracy is made."