The Use of a GPS-Equipped Aircraft as a Mobile Platform to Study the Propagation of Omega Navigation Signals

Abstract

The almost complete constellation of GPS satellites now provides excellent positional information for aircraft, with typical worst-case errors of the order of 100 metres. The low frequency waves used for Omega navigation have wavelengths of the order of 30 kilometres and thus, even in an aircraft moving at varying speeds up to around 500 km per hour, it is now a simple matter to measure VLF phase with a significance at the 1 degree level. This paper describes the use of GPS to provide accurate positional information to VLF receivers located on aircraft of the Royal New Zealand Airforce (RNZAF) en-route to Antarctica. The marked phase irregularities on signals from La Reunion and Argentina, observed north of the icecap in earlier studies using aircraft fitted with inertial navigation systems, have been confirmed and described with much greater precision. Smaller phase anomalies have also been observed on signals from Omega Australia and Omega Hawaii, whilst flying directly over the Antarctic icecap. The latter signal was previously regarded as unperturbed and used in earlier studies as a phase reference. Small-scale periodic variations of phase and amplitude have also been recorded on signals propagating from Omega Australia. These variations are considered to be caused by the reflection of the VLF waves at discontinuities in the height (e.g. mountains) or the lower boundary conductivity of the Earth-ionosphere waveguide. Reflections of smaller amplitude have also been observed flying over the open ocean before reaching Antarctica.