Navigating Harbors at High Accuracy Without GPS: eLoran Proof-of-Concept on the Thames River

摘要

The U.S. Coast Guard has operated the Loran-C system in the United States since the 1970’s. The current system of 24 Loran stations provides a stated absolute accuracy in navigation position of 0.25 NM. There has been interest in recent years in using Loran-C as a back-up system in case of the loss of GPS; especially in Harbor Entrance and Approach (HEA) areas. The Coast Guard Academy in conjunction with the Coast Guard Loran Support Unit (LSU) has conducted a proof-of-concept demonstration of the ability of Loran-C to support the HEA navigational requirements. The purpose is to demonstrate that Loran-C has the capability to meet the required accuracy of 8-20 meters. In order to meet these accuracy requirements user receivers must use Additional Secondary Factors (ASFs) in calculating the user position. ASFs are propagation time adjustments that are added to the receiver’s times of arrival (TOAs) to account for propagation over nonseawater paths. These ASFs vary both spatially and temporally and both variations need to be accounted for to meet the accuracy targets. As one component of the eLoran system, a reference station located nearby the harbor will be used to estimate the temporal changes in the ASFs relative to the published spatial grid; these differences will be broadcast using the Loran Data Channel (9th pulse) to the user receiver. The spatial variations will be accounted for through the use of a grid of ASF values that is known by the receiver a priori. This general method to HEA navigation was discussed by the authors in 2003 (ION AM 2003). One of the considerations in the development of the spatial grid is the appropriate choice of grid spacing. This was investigated by the authors using predicted ASF values in 2004 (ION AM 2004). More recently (ION GNSS 2006) we developed a technique to process survey data into a harbor grid. In this paper we will describe a complete mapping of the Thames River, New London harbor, and approaches and the development of the ASF spatial grid using the “inverse interpolation” technique that we discussed in ION GNSS 2006. This real world data will also be used to examine the required grid density. This grid will be supplied to a user receiver for a demonstration of eLoran. In addition, differential corrections from a reference station at the Coast Guard Academy will be broadcast live from Seneca. This paper will discuss the development of the proof-of-concept eLoran receiver used to navigate the harbor in real-time with sub-20m accuracy using Loran TOAs, the ASF grid, and demodulated differential corrections.