Aircraft Approach Guidance Using Relative Loran-C Navigation

Abstract

Precision navigation about a reference point using Loran‐C can be achieved by subtracting the Loran Time Differences measured at the reference point from the received Time Differences, thus reducing by common‐mode cancellation the effect of variations in signal propagation velocity; a potential application of this technique is aircraft final approach guidance. In addition to the classical Geometric Dilution of Precision, performance of Loran‐C in this “relative navigation” mode is limited by reference datum accuracy, by the response of the receiver's tracking loops to noise and vehicle accelerations, and by local deformations in the shape of the Lines of Position grid. Optimization of the receiver's tracking loop bandwidth can result in an expected performance of 45 meters (2‐σ) for a 0 dB signal to noise ratio and typical aircraft low‐approach maneuvering accelerations. Ground‐level measurements at this level of precision indicate the existence of a “grid microdeformation”: localized, repeatable distortions of the Lines of Position over distances much smaller than the carrier wavelength. It is concluded that 2‐σ horizontal accuracies of 100 meters or less are realistically achievable with relative Loran‐C if signal to noise ratio and maneuvering acceleration limits are specified. This accuracy is significantly better than that of VOR or NDB approach systems, but not as good as an ILS Localizer.