Currently, GNSS is the backbone of most Position, Navigation and Timing (PNT) solutions, in providing 24/7 continuous, all-weather, global coverage to an unlimited number of receiver-equipped users. There is an increasing dependency on the availability and reliability of GNSS. However, GNSS is widely known to be vulnerable to an evolving range of threats (natural and man-made, unintentional and deliberate). This drives the demand for an alternative, complementary and backup system to protect those critical applications that depend on GNSS from a safety, security and/or liability perspective. Harbour to harbour autonomous shipping will especially require robust and reliable PNT solutions with high integrity performance levels, as the availability of a human navigator is removed/decreased.
Recent analysis within the UK has aimed at quantifying the dependency on GNSS. Results estimate that a loss of GNSS for 5 days would generate a loss of £5B in economic activity to the UK. This is the latest information and the authors suggest that this is a conservative estimate. Interesting to note however that this only accounts for a complete denial of GNSS. The report does not account for the impact of disturbances, disruption and confusion caused by degraded conditions to the myriad of applications.
To date, several studies have been undertaken in the maritime sector into the Ranging -mode concept. R-mode is a positioning system concept that involves the transmission of timely synchronised ranging signals using communications channels of existing maritime radio infrastructure.
Two core options have been analysed for R-mode: 1) use of MF beacons (DGNSS), 2) use of AIS/VDES. The second option is understood to be widely preferred due to reasons including MF susceptibility to sky-wave interference, narrow bandwidth and uncertainty over the long-term future of MF beacons. VDES is the planned successor to AIS (Automatic Identification System), intended to address the current issue of over-crowding of AIS. It supports ship-to-ship and ship-to-coast communications as well as a satellite component. R-mode has the potential to be applied to both the terrestrial and satellite components of VDES, which may be referred to as VDE-TER and VDE-SAT respectively.
To this end, a concept named Augmenting GNSS to Prevent Loss of Service (ANGELOS) has been developed, a system capable of providing Positioning, Navigation and Timing services for Maritime applications through a combination of space-based and terrestrial infrastructures. ANGELOS harnesses the satellite component of VDES to deliver a PNT service that will not be restricted to only terrestrial service areas alone. The concept also exploits existing recommendations for the signal of VDE-SAT, driving a standard acceptable to deliver ranging based services. ANGELOS aims to ensure that a VDES R-mode system benefits from the GNSS receiver industry in order to make the solution acceptable to the market and to become a reliable choice for augmentation to GNSS, for those applications that require uninterrupted access to PNT.