Group-velocity variations of v.l.f. signals

Some experimental transmissions for the Omega navigation system were monitored over an 8500km path to determine the phase variations of the difference frequency between 10.2kc/s and 13.6kc/s. These results are interpreted in terms of the group velocity at the mean frequency of 11.9kc/s and are compared with calculations of group velocity based on first-waveguide-mode phase velocities due to Wait. The significance of these results in the design of a ground-based radionavigation aid is discussed.