Initial Results of MF-DGNSS R-Mode as an Alternative Position Navigation and Timing Service

Position, Navigation, and Timing (PNT) is part of the critical infrastructure necessary for the safety and efficiency of vessel movements, especially in congested areas such as the North Sea. Global Navigation Satellite Systems (GNSS), especially the U.S. Global Positioning System (GPS), have become the primary PNT sources for maritime operations. The GNSS position is used both for vessel navigation and as the position and timing source for other systems such as Automatic Identification System (AIS). Unfortunately, GNSS is vulnerable to jamming and interference, intentional or not, which can lead to the loss of positioning information or, even worse, to incorrect positioning information. The user requirement is for dependable PNT information at all times. One potential source of resilient PNT services is Ranging Mode (R-Mode), an alternative PNT concept related to Signals of OPportunity (SoOP) PNT, which uses signals independent of GNSS. In 2013 the German Federal Waterways and Shipping Administration contracted for a feasibility study of R-Mode using medium frequency (MF) Differential GNSS (DGNSS) and very high frequency (VHF) AIS signals as well as those signals in combination and in combination with eLoran. At ION GNSS+ 2014 some of the authors presented the results from that feasibility study and showed the projected performance using the signals individually and in combination. In most of the shipping lanes on the North Sea it appeared that 10m or better performance could be achieved. Following up on that work, prototypes of a transmitter and receiver for MF-DGNSS R-Mode have been developed. The transmitter was installed in IJmuiden (Netherlands) and the receiver deployed along the Dutch coast to the south of the transmitter for initial on-air testing of the R-Mode concept; both were synchronized to UTC via GPS. While positioning is not possible with only one R-Mode transmitter, the combination of a synchronized transmitter and receiver pair allows for useful testing of the R-Mode concept. Specifically, the receiver could estimate a true range (rather than a pseudorange); hence, the stability of the range with environmental variations (e.g. weather, day/night skywave effects, etc.) can be studied. To further study skywave effects, the R-Mode modulator was relocated to a more powerful transmitter at Heligoland (Germany) and the receiver moved to a location near the Kiel Canal (Germany). A second receiver was installed at a similar distance from the transmitter to enable simultaneous comparison of two different propagation paths. Later a third receiver was added in order to provide three paths of different lengths. The original R-Mode feasibility study also examined the number of stations that it would be possible to receive in the North Sea area to ensure there were sufficient stations for positioning. However, it did not look at the converse question; how would a large number of R-Mode signals impact legacy users. As part of the prototype development, very limited experiments were performed to assess the impact of the new signals on legacy DGNSS receivers. Specifically, during the on-air testing mentioned above, a commercial DGNSS receiver was able to accurately demodulate and decode the on-air transmissions from the single prototype R-Mode transmitter; the impact of multiple signals was not examined. A new study, funded by the General Lighthouse Authorities of the UK and Ireland, has just been initiated to analyze how legacy equipment would respond to multiple R-Mode signals at different frequencies, both in-band and out-of-band. This paper presents details of the prototype transmitter and receiver and includes statistical analyses of the range estimates recorded to date and the impact of skywave interference at night using data from the German test sites. Additionally, this paper includes preliminary results of the R-Mode signal interference study. INTRODUCTION Position, Navigation, and Timing (PNT) is part of the critical infrastructure necessary for the safety and efficiency of vessel movements, especially in congested areas such as the North Sea. Global Navigation Satellite Systems (GNSS), especially the U.S. Global Positioning System (GPS), have become the primary PNT sources for maritime operations. The GNSS position is used both for vessel navigation and as the position and timing source for other bridge systems, such as Automatic Identification System (AIS). Unfortunately, GNSS is vulnerable to jamming and interference, whether intentional or not, which can lead to the loss of positioning information or, even worse, to incorrect positioning information. The user requirement is for dependable PNT information at all times, even under GNSS jamming conditions. A variety of technological solutions to an alternative PNT system are possible; in the radio frequency (RF) domain we have the so-called “Signals of OPportunity” (SoOP) approach (e.g. [1]). This term refers to the opportunistic use of RF signals, typically communications signals, which exist in the geographical area of the receiver. While these signals are not primarily intended for positioning, a SoOP navigation receiver attempts to exploit them as such. Specifically, if each SoOP can provide a (pseudo) range to the receiver from a known location, a trilateration position solution is possible. Usually, there is no alteration of the SoOP signal. In some instances, minor improvements are initiated to improve the signal’s characteristics; for example, synchronizing the signal to a known (and GNSS independent) source of UTC. So called “Ranging Mode” (or RMode) is one such example. In 2013 the German Federal Waterways and Shipping Administration contracted for a feasibility study of R-Mode using medium frequency (MF) differential GNSS (DGNSS) and very high frequency (VHF) AIS signals as well as those signals in combination and in combination with eLoran [2]. This study was part of the European ACCSEAS project. At ION GNSS+ 2014 some of the authors presented the results from that feasibility study and showed the projected performance using the signals individually and in combination [3]. The study stated that the AIS R-Mode was feasible with the existing signal structure (beyond synchronizing the broadcasts and, perhaps, adding a fixed ranging message), but that the DGNSS R-Mode would be greatly improved by actually modifying the signal, adding one or two continuous wave (CW) signals to the broadcast (i.e. not be a pure SoOP, but a hybrid version). In most of the shipping lanes of the North Sea it appeared that 10meter or better performance could be achieved via R-Mode. Rather than define desired performance up front, the approach of the ACCSEAS project was to try to identify what level of performance was possible and identify what limited the system performance. The decision could then be made as to whether the solution was worth pursuing. To continue that work, prototypes of an MF-DGNSS R-Mode modulator and receiver were developed; both are reviewed below. The modulator was installed at a transmission site in Ijmuiden (Netherlands) and the receiver deployed to the south along the Dutch coast for initial on-air testing of the R-Mode concept. While positioning is not possible with only one RMode transmitter, the combination of a synchronized transmitter and receiver pair allowed for useful testing of the R-Mode concept. Specifically, having both the transmitter and receiver clocks synchronized to a common time source (UTC via GPS in this case, noting that the final full system would be GNSS independent) allows the receiver to estimate a true range (rather than a pseudorange); hence, the stability of the range with environmental variations (e.g. weather, day/night skywave effects, etc.) could be studied. To further study propagation and skywave effects, the R-Mode modulator was relocated to a more powerful transmitter at Heligoland (Germany) in the North Sea and the receiver moved to a location near the Kiel Canal (Germany east of the transmitter). A second receiver was installed to the west of the transmitter to enable simultaneous comparison of two different propagation paths. A third receiver site, nearly along the signal propagation path to the Kiel Canal, was added most recently. After a brief description of both the modulator and receiver below, experimental results from these receivers are presented. The R-Mode feasibility study also examined the number of DGNSS transmitters that it would be possible to receive in the North Sea area to ensure there were sufficient stations for positioning [3]. However, it did not look at the related issue; how a large number of R-Mode signals would impact legacy users. As part of the prototype modulator development, very limited experiments were performed to assess the impact of the new signals on legacy DGNSS receivers. Specifically, during all of the on-air testing mentioned above, a commercial DGNSS receiver was able to accurately demodulate and decode the on-air transmissions from the single prototype R-Mode transmitter; however, the impact of multiple signals was not examined. A new study has just been initiated by the General Lighthouse Authorities of the United Kingdom and Ireland to analyze how legacy equipment would respond to multiple R-Mode signals at different frequencies, both in-band and out-of-band. Preliminary results from that work are also reviewed herein. BACKGROUND ON MF DGNSS BROADCASTS The MF DGNSS system transmits its information via a binary modulation method known as Minimum Shift Keying (MSK) [4]. Assuming that the MSK transmission is controlled by a precise time/frequency source, both the times of the bit transitions (potentially once every 10 milliseconds) and the underlying phase of the transmitted signal (a sinusoid at approximately 300 kHz) could be exploited to estimate the time of arrival (TOA) for ranging applications. The R-Mode feasibility report examined the potential perf