A real-time correction technique for transionospheric ranging error

For a signal traversing the ionosphere, the second difference of phase shift versus frequency is proportional to the integral of electron density along the path and, therefore, to excess ionospheric group delay. By measuring this second difference at the operating frequency in order to make range corrections, the performance of very high frequency (VHF) ranging satellite navigation systems may be made to closely approach that of L-band systems. The application of the technique only requires an easily generated triple of continuous wave (CW) signals. The signal-to-noise and phase-stability requirements are relatively modest. A comparison with other possible techniques shows that the second-difference-of-phase system is probably the most attractive one.