Improving Medium Frequency R-Mode Ranging with GMSK Modulation

The R-Mode terrestrial navigation system evaluates the phase of two carriers next to a modulated signal, from which a range can be derived to allow positioning using different stations. To improve the accuracy of ranging, we propose in this paper to replace the currently used Minimum Shift Keying (MSK) modulation with Gaussian Minimum Shift Keying (GMSK). By applying a Gaussian pulse shape for the bit transition to that of the MSK a higher spectral efficiency is achieved. If the data rate is kept constant, this reduces the energy of the modulated signal within the bandwidth of the phase estimator. This increases the accuracy of the added carrier estimate and achieves our goal of improved distance-estimation accuracy. In this paper, we compare the estimates for an R-Mode signal with MSK and two different GMSK modulations. The increased accuracy is verified using software and hardware in-the-loop simulations.