Loran-C Signal Stability Study: Saint Marys River

Abstract : The Coast Guard operated an experimental 'Loran-C mini-chain' in the St. Marys River, Michigan from the beginning of 1976 to the end of 1980. The impetus for the experiment was a Congressionally directed Winter Navigation Program and the Great Lakes and the St. Lawrence Seaway. Certain key stretches of these waterways, such as the St. Marys River, have particularly stringent navigation requirements. The mini-chain experiment was an investigation to see if Loran-C could satisfy those requirements. At the same time, the Coast Guard had a mandate to investigate the suitability of Loran-C for use in restricted waterways (harbor and harbor entrance (HHE) areas) throughout the United States. Once the Great Lakes Loran-C chain became operational, the mini-chain was no longer required in the St. Marys River. At the conclusion of the mini-chain experiment, the Coast Guard had the opportunity to explore the final untested concept of the HHE program; the so-called Supplemental LOP. This report documents the results of these tests and shows a Supplemented LOP is a viable concept but not necessary in the St. Marys River. With the existing chain being controlled from a site in the St. Marys River, 25 meter, 99.9% probability performance is possible. The report shows how this appears to be the 'best achievable' Loran-C performance due to a problem known as phase modulation.