Since 1959 indirect phase height measurements based on continuous ground-based records of long frequency radio waves have been carried out at Kühlungsborn.
Essentially, the phase height is determined from an analysis of the local time appearance of minima and maxima of recorded wave amplitudes. The conjecture is, that this interference pattern arises because of a superposition of ground and sky wave.
However, the interpretation of this interference pattern in terms of an ionospheric reflection height relies on the assumption of a constant ground phase as a phase reference. Obviously, it is of great importance to verify this assumption. The used transmitter near Allouis (France, 162 kHz, 1023 km distance) is seeded by a caesium normal, which gives the opportunity to record amplitude, phase and absolute propagation delay. Using a GPS-disciplined Rubidium frequency standard and furthermore a second receiving station 120 km apart from Kühlungsborn a method to clear the nature of wave propagation is established.
Assuming a total reflection, as sketched in the Figure the receiving pattern can be interpreted by a movement in height of the reflecting layer. For a calculation of the virtual height the phase ambiguity has to be removed. This can be done by observing the same transmitter over a different distance.
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