ASF quality assurance for eLoran

Abstract

eLoran has the potential to act as a cross-platform backup radio-navigation system to GPS to provide Position Navigation and Timing. As service providers of differential-eLoran for maritime navigation, the General Lighthouse Authorities of the UK and Ireland (the GLAs) must ensure that the system meets the internationally agreed standards for Accuracy, Availability, Continuity and Integrity. For eLoran to provide accuracy better than 10m (95%), the user must have knowledge of the signal propagation Additional Secondary Factors (ASFs). The GLAs are charged with the measurement, validation and publication of accurate ASF tables for use within users' receivers. To this end an extensive surveying campaign is required in order to measure the ASF tables for the nominated harbours and port approaches for eLoran IOC before 2014. It is important that sufficient survey data is gathered to ensure high-quality ASF tables can be produced, but equally that surveying effort and ship-time are kept to a minimum to reduce costs. Making use of an ASF error-budget, novel processing techniques and an Integrity-based Quality Assurance methodology for ASF surveying have been created. This is designed to give the GLAs a real-time picture of the progress of an on-going survey, so surveying resources can be used effectively to maximise data quality. As an additional benefit, ASF -table Quality figures can be fed into an eLoran Integrity equation to keep the mariner informed of the level of performance they can expect from the eLoran system. This will ensure that eLoran is used with an appropriate degree of trust during the various phases of a voyage for which it is used.