Improving medium frequency R-Mode ranging with GMSK modulation

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Kurzfassung

R-Mode is an upcoming terrestrial navigation system currently under development. Here, it is planned to expand existing maritime services with the possibilities of deriving range information. Services in the Very High Frequency (VHF) and Medium Frequency (MF) bands are currently under discussion. This paper focuses on the extended DGNSS maritime beacon signal on MF frequencies. The legacy DGNSS signal consists of a Minimum Shifted Keyed (MSK) modulated continuous RTCM message stream. To enable ranging, two aiding carriers are placed in the zero crossing of the MSK modulation power spectrum. These are simple sinusoidal signals without modulation. Furthermore, it is defined that the aiding carriers have zero phase at the transmitting antenna, at the full second of the system time. By estimating the phase of the single tone signals on the receiver side, it is possible to derive a propagation time of the subcarrier from the transmitter to the receiver. Considering the frequency of the sinusoidal signal and the propagation path, a distance measure is obtained that can be used to determine the position. However, in previous publications we have shown that the MSK signal causes inter-channel interference with the sub carriers. Therefore, it has been shown that the estimated phase is disturbed. This leads to a reduced accuracy of the distance measurement and consequently to a decrease in the position accuracy. To improve accuracy, we propose in this paper to replace MSK modulation with Gaussian Minimum Shift Keying (GMSK) modulation , a well-known modulation scheme used e.g. in the Global System for Mobile Communications (GSM) standard. By applying a Gaussian slope for the bit transition to the MSK, a higher spectral efficiency is achieved. Therefore, the energy of the modulated signal collected during the estimation of the aiding carrier is reduced and our goal of improved estimation is achieved. GMSK was already under discussion to be used in the maritime beacons during development. It was shown that there is no significant increase of bit error rate when the modulation is changed properly. AlsoFurthermore, because of the similarity of both modulation methods legacy DGNSS receiver would not have to be adapted to the changed modulation from MSK to GMSK on the transmitting side The reduced power within the estimation bandwidth at frequencies, where we expect the continuous waves of the R-Mode system, leads to an improved performance of the estimation. We verify this assumption by simulating three signal combination: the phase estimation for a signal tone, phase estimation of a tone at the fourth zero crossing of a MSK and GMSK signal. Furthermore, we plan to generate real test signals with an arbitrary wave generator. These time-continuous signals can be used to test how a commercial receiver reacts to the changed modulation. In addition, the improvement for RMode with GMSK over MSK is shown by using the R-Mode receiver currently being developed by the German Aerospace Center (DLR). According to the theory, we expect an improvement of the phase estimation accuracy, which we could already show in our simulation.

Verfügbare Versionen dieses Eintrags

• Improving medium frequency R-Mode ranging with GMSK modulation. (deposited 10 Sep 2021 14:18) [Gegenwärtig angezeigt]