THE USE OF A GPS-EQUIPPED AIRCRAFT AS A MOBILE PLATFORM TO STUDY THE PROPAGATION OF OMEGA-NAVIGATION SIGNALS

The almost complete constellation Of GPS satellites now provides excel lent positional information for aircraft, with typical worst-case erro rs of the order of 100 metres. The low frequency waves used for Omega navigation have wavelengths of the order of 30 kilometres and thus, ev en in an aircraft moving at varying speeds up to around 500 km per hou r, 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 o f the Royal New Zealand Airforce (RNZAF) en-route to Antarctica. The m arked phase irregularities on signals from La Reunion and Argentina, o bserved north of the icecap in earlier studies using aircraft fitted w ith inertial navigation systems, have been confirmed and described wit h much greater precision. Smaller phase anomalies have also been obser ved on signals from Omega Australia and Omega Hawaii, whilst flying di rectly over the Antarctic icecap. The latter signal was previously reg arded 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 variation s are considered to be caused by the reflection of the VLF waves at di scontinuities in the height (e.g. mountains) or the lower boundary con ductivity of the Earth-ionosphere waveguide. Reflections of smaller am plitude have also been observed flying over the open ocean before reac hing Antarctica.