Signals from space are subject to interference
Issue 75 : Nov/Dec 2010
The satellite-based Global Positioning System (GPS) provides very accurate navigational information, but everyone who uses it should be aware that it has vulnerabilities. These include susceptibility to radiation caused by solar storms, sunspots, and solar fl ares and to human interference by hacking and jamming. Individual satellites occasionally malfunction and some older GPS receivers have compatibility problems when newer satellites with more modern programs are inserted into orbit. Users of GPS receivers can also introduce errors that result in inaccurate data being displayed.
Solar interference
Earth’s magnetic field protects its surface from most of the harmful radiation emanating from the sun, but GPS satellites orbit thousands of miles beyond it where they are exposed to the solar wind — protons and electrons streaming from the sun at close to the speed of light. They are also vulnerable to the the sun’s magnetic storms, a powerful one of which could disrupt, or even wipe out, the electric power-grid systems on earth.
During periods of increased sunspot activity, the solar wind can become so intense as to disrupt a satellite’s signals. If the solar event is too violent, control stations on the ground can shut down the satellites to protect them from damage.
Solar flares are fierce explosions on the sun that release enormous amounts of energy. They emit radiation across the entire electromagnetic spectrum that can seriously affect radio communications on earth as well as GPS satellites. When the sun is active, at the height of the 11-year sunspot cycle, several solar flares may be erupting simultaneously; when it’s quiet, a week may go by without a single one. One solar storm wiped out GPS reception on the sunny side of the earth for 14 hours, and many shorter failures have been caused by solar activity.
The 11-year sunspot cycle will peak during 2011 and GPS failures are expected to increase.
Terrestrial jamming
The signal strength of a GPS radio wave is extremely small — one ten-billionth that of a cell phone signal. This makes the GPS signal very vulnerable to jamming.
Unintentional jamming can occur if a signal or noise masks the GPS signal. Not long ago, it was discovered that a popular brand of onboard TV antenna emitted a signal that could wipe out GPS reception for boats within a half-mile radius.
Governments unfriendly to the U.S. have systems that can be used to intentionally jam signals. U.S. military testing can also disrupt GPS. Furthermore, a hacker using a simple one-watt jammer can wipe out GPS reception for thousands of square miles, as happened during the 1997 Moscow air show.
GPS receivers may not be equipped to sense or indicate when the signal is being jammed and may continue to display a position, but one that is wildly inaccurate.
Electronic anomalies
The GPS system was originally created with 24 satellites but now has more than 32 in operation. The greater number improves accuracy and availability, but the new satellites inserted into the GPS constellation can cause problems with receivers.
Several models of GPS receiver give erroneous readings when they see more than the original 24 satellites. They show a speed over the ground (SOG) of up to 800 knots and produce random erroneous fixes. If a boat were being steered by an autopilot that accessed such a GPS receiver, the result could be disastrous. Since each GPS satellite is above the horizon for about 8 of every 24 hours, the error introduced by a high-numbered satellite appeared to be intermittent.
One updated WAAS satellite caused a popular GPS model to crash. The only way it could be turned back on was by removing the battery, moving the device into an area where it could not access a WAAS signal, reinserting the battery, and reprogramming the GPS.
Recently, aircraft and ships in the North Atlantic found that GPS was placing them about 75 miles from where they thought they were. This was due to an atomic clock failure in a GPS satellite. The offending satellite was finally shut down by the ground control station.
The need for reliable backup aboard was dramatically illustrated on July 17, 2004, when all units of a particular model of GPS chartplotter crashed simultaneously. Engineers at the parent company were stumped. It took a month to solve the problem and every affected unit had to be returned for an upgrade. Boaters using this particular chartplotter who had no other backup system were forced to go back to basics.
User errors
Most GPS users are unaware that a hundred or more different chart datums are available within a GPS receiver. Most units default to the geodetic datum WGS84/NAD83, which is the datum used for all current U.S. charts. However, when navigating in foreign waters and using foreign charts, be sure to set your GPS to the datum to which the chart is referenced. If you don’t, your indicated position could be miles off.
Carry alternatives
Because of the very real potential for loss of GPS signals due to solar activity, jamming, or electronic problems, every sailor should have one or more alternative ways of navigating.
Don Launer, a Good Old Boat contributing editor, has held a USCG captain’s license for more than 33 years and has sailed the East Coast from Canada to the Caribbean. In recognition of his lifetime achievements as a sailor, and as an author and lecturer on maritime topics, he was recently inducted into the Barnegat Bay Sailing Hall of Fame.
Thank you to Sailrite Enterprises, Inc., for providing free access to back issues of Good Old Boat through intellectual property rights. Sailrite.com
