Multipath VLF propagation effects on correlation receivers

The effect of electromagnetic propagation at very low frequency in the Earth ionosphere waveguide can distort wideband communication waveforms and produce a splitting of the correlation peak of the received signal at a modal interference null where multipath signals arrive out of phase. Propagation from an inclined dipole antenna trailed behind an orbiting aircraft can cause the split correlation pattern to fluctuate throughout the orbit period. For the case of an MSK (minimum shift keying) signal format, expressions and graphs are given for the correlation vector, as a function of receiver synchronization time and aircraft flight direction.