Occasional errors can render GPS temporarily unreliable

Posted on 01 November 2010 Written by Jim Flannery

n_18_indepthGPS accuracy - its ability to fix a position - occasionally degrades for short periods in a limited number of locations, so it's not a good idea to depend on a GPS/chart plotter alone for navigating, says Bob Markle, president of the Radio Technical Commission for Maritime Services. The commission is an organization of government, commercial and educational entities that helps develop policies and standards for navigation and communications systems.

"We start hanging our hats on [our chart plotters]," he says. "We think they're infallible. That's not the case at all." Markle advises using paper charts as a backup, or using a second independent electronic navigation system. Keeping a proper lookout while under way is also essential. "It's always a good idea to have a second source of information," he says.

Even chart plotter manufacturers acknowledge that this is necessary. "When navigating, carefully compare information displayed on the unit to all available navigation sources, including information from visual sightings, local waterways rules and restrictions, and maps," says a Garmin plotter owners manual. "For safety, always resolve any discrepancies or questions before continuing navigation. Use the electronic chart in the unit only to facilitate, not to replace, the use of authorized government charts. Official government charts and Notices to Mariners contain all information needed to navigate safely."

Markle notes that most chart plotters have an alarm that sounds when the accuracy of the GPS falls below a certain limit, usually set by the operator.

"There are a number of errors which can exist in the GPS itself and the cartography loaded into the navigation aid," reports the team from the Cruising Yacht Club of Australia that investigated the PricewaterhouseCoopers grounding last October. The errors "are not constant and can be minimal with an accuracy of only a few meters, but they can also vary quite quickly and dramatically to the point of not being able to obtain a position fix." In a worst-case scenario, the error could run to several hundred feet, the team says.

Markle agrees. "It sure could," he says.

Markle points out that the Coast Guard and Air Force both try to anticipate degraded GPS performance and whenever possible publish advisories in advance. For example, the Coast Guard Navigation Center (www.navcen.uscg.gov) issued an advisory this spring of "high horizontal dilution of precision" - greater inaccuracy in position readings than normal - in three areas around the world because of satellite maintenance.

The advisory said GPS accuracy would be degraded in a small area around Bass Strait in southeastern Australia for about five minutes, inland of San Francisco to the Canadian border for about four minutes and in eastern Canada for about seven minutes. The advisory didn't say how far off in feet GPS position readings might be, but areas of greatest error are depicted in red and black on the map shown here.

GPS inaccuracies may be due to maintenance of one or more of the 32 GPS satellites, or because some satellites are low on the horizon and subject to more atmospheric distortion, or because the visible satellites are poorly positioned relative to each other - clumped together instead of spread across the sky.

GPS position readings typically require signals from four satellites, their best geometry in the sky being at the four points of the compass in 90-degree steps.

"There are some times when the satellite geometry is less than ideal," says Markle, "but that can be predicted." However, there was no advisory in either Coast Guard or Air Force archives of the two hours of degraded GPS accuracy off Flinders Islet on the night that PricewaterhouseCoopers grounded.

Degradation from the atmosphere and from poor geometry can be more pronounced in higher latitudes - like southeastern Australia - because of the satellites' 55-degree orbits, which take them close to, though never right over, the poles, Markle says.

Other possible sources of GPS inaccuracies include slight shifts in the orbits of satellites due to gravitation, errors in the clocks in the receivers that time the signals from the satellite to the receiver and reflected satellite signals, which bounce off waves, rigging, deck hardware and even crew on deck. Markle says satellite geometry problems should be alleviated as receivers are built that receive signals not only from U.S. GPS satellites but also from satellites in the European Galileo system, which is expected to be operational in 2013.

"As time goes on, you're going to have more GPS-Galileo-combined receivers," he says. "They use a common technology. The more satellites you have access to, the better the service."

This article originally appeared in the November 2010 issue.