Navigation experts are warning about the possibility of decreased GPS accuracy and possible outages over the next months as the sun enters the most active phase of an 11-year solar cycle.

Sunspots and solar flares increase ionising solar radiation, creating more free electrons in the ionosphere. Sudden variations in the density of the ionosphere can cause GPS signals to scintillate and produce delays in signal propagation. The effect is predicted to increase from this month and is likely to last until next year.

It can affect GPS accuracy and in severe conditions can even cause GPS units to fail to lock on to satellites. The effects are worse during the day than during the night. The most likely times affected are right after sunset and latitudes possibly worst affected are about the equator (and in polar cap regions).

Experts are warning of potential in errors of more than 10m. Differential GPS could also be affected, though it is considered ‘generally more resilient’.

Of equal concern is the possibility of outages in GPS coverage. According to a report commissioned by the General Lighthouse Authority. ‘GPS is far from robust. Although reliable and accurate for long periods, it can suddenly and unexpectedly fail. In recent years such failures have been due to a range of vulnerabilities: solar disturbances, space-vehicle failures, unintentional radio interference and, increasingly, deliberate jamming.

‘The consequences have included not only the loss of GPS service but, more seriously, positions and velocities shown on ship’s displays that without warning became incorrect, yet remained plausible.’

With some GPS systems, they say, outages don’t even trigger an alarm. That’s worrying on a yacht and downright scary when you think of it happening on the bridge of a fast-moving ship. The report is ominous about reverting to traditional navigation methods, saying would ‘force the mariner to revert to increasingly unfamiliar fallback navigation methods such as chart, compass and visual bearings.’

You can find up-to-date information of solar weather on the US NOAA site www.swpc.noaa.gov/index.html