Is GPS 100 per cent reliable?

by • September 21, 2017 • Features, olderComments Off230

On June 22 this year a number of boats off the Russian port of Novorossiysk received spoof GPS signals that gave their position as being several miles inland. (Marinetraffic.com screenshot)

Much has been made in recent years of the possibility of solar flares from sunspot activity wiping out GPS and communication satellites. Less well known is that GPS signals are astoundingly weak. The signal leaving the satellites is in the order of 20 Watts – approximately the power of a light bulb – yet it must travel 20,000 miles to reach the receiver. This leaves the system vulnerable to signals being drowned out by a third party transmitter.

Most concerns have historically centred on the signal being obliterated by a stronger carrier signal of the same frequency – so called GPS Jamming. However, it’s also technically possible to broadcast a false signal to spoof navigation systems into displaying an incorrect position. The first large scale documented incidence of GPS signals being successfully spoofed occurred in the Black Sea in June of this year.

Some 20 ships off the Russian port of Novorossiysk reported their navigation systems were giving a position 25 miles inland, despite reporting positional accuracy within 100m. This is also an issue that could equally affect the Russian GLONASS system that most recent navigation devices (including the latest smartphones) use in parallel to GPS for position fixing.

New Scientist magazine also reports similar incidences in non-nautical settings: “Over the past year, GPS spoofing has been causing chaos for the receivers on phone apps in central Moscow to misbehave. The scale of the problem did not become apparent until people began trying to play Pokemon Go. The fake signal, which seems to centre on the Kremlin, relocates anyone nearby to Vnukovo Airport, 32 km away.”

No one can be 100 per cent sure that the GPS and GLONASS systems will always work flawlessly without third party interference. 

When setting up a yacht for long-distance sailing there’s clearly a benefit in bearing the potential failings of GPS in mind and equipping the vessel appropriately. At the moment this will invariably mean a more old-fashioned approach, with a sextant for use as a back up, even if only to get noon sights to determine latitude. For those voyaging in more coastal waters it points back to diligently maintaining a written log book, with positions and course changes entered more frequently than is commonly the case today.

Looking ahead a technical solution is still possible, using the old Loran C system that was originally developed for use in the Second World War, but which has been decommissioned since 2010. This is based on a fix from strong radio signals emanating from a minimum of three shore stations, each of which has a range of around 8-900 miles. An enhancement of this, dubbed eLoran, gives a positional accuracy of around 20 metres. South Korea is sufficiently worried about the possibilities of GPS spoofing that the country is reported to have commissioned an eLoran network with three beacons that’s scheduled to come online in 2019.

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Photo: No one can be 100 per cent sure that the GPS and GLONASS systems will always work flawlessly without third party interference.

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