Getting a Bearing on ASF Directional Corrections

摘要

The Federal Aviation Administration (FAA) has embarked on a study of Loran-C to evaluate its suitability as an independent radio navigation (RNAV) backup to GPS. Loran’s potential benefit to aviation hinges upon its ability to support Non-Precision Approaches (NPA), which equates to a Required Navigation Performance (RNP) of 0.3 nautical miles. Through FAA sponsoring, the U.S. Coast Guard Academy (USCGA) is responsible for conducting some of the tests and evaluations to help determine whether Loran can provide the accuracy, availability, integrity, and continuity to support NPA’s in the National Air Space (NAS). For maritime usage in Harbor Entrance and Approach (HEA) areas, the goal is to meet the required accuracy of 8-20 meters. A major part of assessing the suitability of Loran is in understanding the nature of Loran ground wave propagation over paths of varying conductivities and terrain. Propagation time adjustments, called “additional secondary factors (ASFs),” are used to adjust receiver times of arrival (TOAs) to account for propagation over non-seawater path(s). These ASFs vary both spatially and temporally, and unless understood and/or modeled, cause a loss in accuracy and/or the ability to guarantee a hazardously misleading information (HMI) probability of less than 1x10-7. The USCGA, as a part of the FAA’s government, industry, and academic team, is striving to improve understanding of both the temporal and spatial variations in (ASF). To mitigate precipitation static issues for aircraft, efforts have focused on H-field loop antennas for the Loran receivers. This paper reports on recent investigations in this program area; these antennas and the additional receiver front end employed to process a pair of loops cause additional variation in the TOA measurements. This paper describes these effects and approaches to mitigate them.