Understanding GNSS Availability and How it Impacts Maritime Safety

A. Grant, P. Williams, G. Shaw, M. De Voy, N. Ward

Abstract:	The General Lighthouse Authorities of the United Kingdom and Ireland (GLAs) provide marine aids-to-navigation (AtoNs) for the benefit and safety of all mariners within their waters. These AtoNs range from traditional lighthouses and buoys through to radionavigation systems, such as marine radiobeacon DGPS. It is widely recognised that Global Navigation Satellite Systems (GNSS), particularly GPS, have become the primary means of obtaining position, navigation and timing information at sea. Mariners have been conditioned to believe that GPS is infallible, yet this is not the case. This paper reports on recent investigations by the GLAs on three specific threats to GNSS availability; namely the effects of intentional interference and GPS jamming, the impact of a reduced number of available satellites and finally the effect of space weather. Over the past few years the GLAs have conducted a number of GPS jamming trials, investigating the effect of GPS service denial on a number of GLA vessels, their bridge systems and also on the GLAs’ own AtoNs. The implications of jamming can be severe, particularly when the strength of the jamming signal is comparable to the true GPS signal. During such conditions, hazardously misleading information (HMI) has been observed. Moreover, GPS service denial results in multiple alarms and the simultaneous failure of many bridge systems including the ship’s radar, gyro-compass and Automatic Identification System (AIS). It is nowadays taken for granted by mariners that these systems provide accurate situational awareness, and are often regarded as the backup upon which to rely in the event of GNSS failure, and yet they are themselves vulnerable to the failure of GNSS. This paper also considers the impact of the 2009 US Government Accountability Office (GAO) report which predicts ‘significant challenges in sustaining and upgrading widely used capabilities’ of GPS due to the struggle to meet launch schedules for GPS IIR replacement and IIF follow-on satellites. The GAO report concluded that the probability of maintaining a constellation of at least 24 useable GPS satellites would fall below 80% by 2011 and that this probability would not return consistently above 95% until 2015. More recent GAO predictions in September 2010 indicate (with 95% confidence) that the number of satellites should not fall below 24 in the medium term, but concerns remain over the effects of satellite ageing and the rate of replacement. The paper reports on the results of initial investigations into the effects of a reduced GPS constellation on maritime GPS-based navigation. Navigation accuracy performance may be significantly impaired, with the receiver output position being lost or, observed to freeze during prolonged GPS outages. In such conditions there is again the possibility of hazardously misleading information being produced by the ship’s navigation system and this could pose a real threat to maritime navigation. In a time when GPS jamming units are becoming more common, when satellite constellations are changing and as we approach the solar maximum in 2013, understanding GNSS availability and its effects is crucial to maintaining safe navigation.
Published in:	Proceedings of the 2011 International Technical Meeting of The Institute of Navigation January 24 - 26, 2011 Catamaran Resort Hotel San Diego, CA
Pages:	687 - 695
Cite this article:	Grant, A., Williams, P., Shaw, G., De Voy, M., Ward, N., "Understanding GNSS Availability and How it Impacts Maritime Safety," Proceedings of the 2011 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2011, pp. 687-695.
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