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Position, navigation, and timing information are critical to today's infrastructures; as a result, the possibility of estimating ranges is being explored in more and more radio systems. One way to achieve this is to extend the modulation with time-synchronised aiding carriers and to estimate their phase at the receiver side. In this paper, we present two ways to minimise the negative influence of the modulation on the phase estimation. We show that the classical maximum likelihood estimator is still an efficient estimator for our problem, using a medium-frequency R-Mode signal as an example, and is therefore used in receiver designs. We then describe two possible ways to precondition the signal to increase the accuracy for short observations. As a first approach, we describe how window functions can positively change the signal-to-noise ratio for our estimation. As a second approach, we show the use of a narrowband bandpass filter. Finally, we show that these approaches, applied to real measurements, improve the variance of the estimate by up to two orders of magnitude.

The R-Mode system is a terrestrial navigation system currently under development, which exploits existing means of medium frequency radio transmission. The positioning and timing performance depends on the estimation of the signals’ phase offset, from which the ranging information is derived. For an analogous problem such as the single-tone phase estimation, the Cramér-Rao bound (CRB) describes the minimal achievable performance in the mean squared error sense. For R-Mode, the problem involves the estimation of the phase offset for a beat signal, which can be described as the difference of phase estimation for the two aiding carriers next to the signal. This estimates are not statistically independent for finite observation, as we show in this paper. The effect becomes stronger for short observation times, which are important for a near real time application. In this contribution, we are interested in phase offset estimation for the signal models relevant to R-Mode: a beat signal and a beat signal combined with an MSK signal. A closed-form lower CRB is proposed for the aforementioned signal models phase estimation, as well as a generalization of the bound for the phase-difference estimation. Based on this derivation, optimized bit sequences are shown to improve performance of the estimates. The validity of the proposal is verified based on a simulation setup. Measurements acquired during a measurement campaign serve to further justify the usefulness of the bound. Some possible applications of such a bound are R-Mode coverage prediction and the associated phase estimators’ performance.

R-Mode is a terrestrial navigation system under development for the maritime domain that provides backup in case of a GNSS outage. This paper describes the first test results for real-time positioning on board a ship using medium frequency R-Mode signals. The estimation and positioning algorithms used are described in detail and it is shown how they are integrated into the R-Mode receiver developed by the German Aerospace Center. Moreover, during two daytime experiments with lower and higher dynamic movements of a ship in the Baltic Sea, we were able to achieve a 95% horizontal positioning accuracy of better then 12 m in the center of three R-Mode transmitters. This demonstrates the first time that the medium frequency R-Mode has provided positioning at sea.

Die Kenntnis der genauen Position auf dem Meer ist seit jeher eine große Herausforderung. Die Leuchtfeuer früherer Zeiten sind heute durch Funkfeuer ersetzt worden. Doch wie bestimmt man den Abstand zwischen einem Funkfeuer und einem Schiff und welche Systeme werden dazu aktuell eingesetzt? Auf diese Fragen wird der Vortrag Antworten geben.

Trading in the modern world strongly depends on merchant shipping. The ongoing globalization is accompanied with an increasing vessel sizes and traffic density in ecological and economical sensitive areas. To prevent collisions and groundings Global Navigation Satellite Systems (GNSS) are used today as the primary source for position, navigation and timing information. Because of GNSS vulnerability the maritime world is looking for a backup system. R(anging)-Mode, a new system which utilizes existing maritime signals of opportunity is one candidate to provide absolute positioning and timing. Currently a R-Mode reference implementation on maritime radio beacons, which operate in the 300 kHz band, is broadcasted. Here, the legacy signal-, Minimum Shift Keying (MSK) modulated information, is extended by two continuous wave (CW) signals. The paper shows how the phase is estimated using time synchronous measurements of a Software Defined Radio with external time synchronization. In the first part the measurement setup is described, with special attention given to time synchronization using a GNSS stabilized rubidium clock, which is a key factor for good estimation results. Furthermore, a maximum likelihood estimator of the phase and a Hilbert transform based estimator are introduced. In the end the outcome is presented when applying these methods on recorded signals from German radio beacons located in Helgoland and Zeven.

Signals from Global Navigation Satellite Systems are the primary sources for Position, Navigation and Time (PNT) information on-board any vessel today. Because these signals are prone to interferences, a maritime backup system is needed to provide reliable PNT data. R(anging)-Mode is such a system. It utilizes existing maritime radio beacons or base stations of the Automatic Identification System (AIS) by adding ranging components to the legacy signals. The first modified radio beacons transmit medium frequency (MF) R-Mode signals in the north of Germany. This paper describes the current state of our research and development activities at the receiver-level for MF R-Mode signals. It introduces the receiver platform which is based on off-the-shelf components and explains the implemented algorithms for distance estimation. Furthermore, the results of first ranging measurements are presented which show the general suitability of R-Mode technology as a source for maritime positioning and timing data.