Modulation Schemes for the Eurofix Datalink

Abstract

In the Eurofix system DGPS corrections are transmitted using the Loran-C signal as a datalink by time shifting the Loran-C pulses. The performance of the datalink, however, is bounded by restrictions of both the Loran-C system and the DGPS system. First, the normal Loran-C performance should not degrade noticeably due to the extra data modulation. Second, the DGPS corrections should be regularly updated with a low message error probability. The use of error correcting code reduces the effective data rate but is necessary to achieve a low message error probability In this report the properties of the Loran-C datalink are discussed and possible modulation schemes for the Loran-C datalink are derived. The design of the modulation schemes is restricted by the balancing requirement imposed by the normal Loran-C performance. The degradation of the normal Loran-C performance is simulated as a function of the signal-to-noise ratio for the different balanced modulation schemes. The balancing requirement reduces the data channel capacity significantly. Also, the normal Loran-C receiver is not guaranteed a bias-free tracking signal with balanced modulation as the Loran-C performance degradation depends on the alignment of the receiver window with respect to the balancing block. Therefore, simulations have been done to investigate whether unbalanced data transmission yields tolerable Loran-C performance degradation. A coding scheme based on nearly balanced modulation yields great improvement in the properties of the data channel. The normal Loran-C performance degradation is comparable with the degradation due to balanced modulation.