Can You Trust Your GPS?

 GPS, Safety Alerts, safety flash  Comments Off on Can You Trust Your GPS?
Jan 202016

Do you know what equipment relies upon the U.S. Global Positioning System (GPS) signal?  How would you respond if you lost the signal? asks an advisory from the US Coast Guard.

This past summer, multiple outbound vessels from a non-U.S. port suddenly lost GPS signal reception.  The net effect was various alarms and a loss of GPS input to the ship’s surface search radar, gyro units and Electronic Chart Display & Information System ECDIS, resulting in no GPS data for position fixing, radar over ground speed inputs, gyro speed input and loss of collision avoidance capabilities on the radar display.

“Fortunately, the vessels were able to safely continue their voyage using radar in heads up display, magnetic compass and terrestrial navigation.  Approximately 6nm later, the vessels’ GPS units resumed operation.  Although the vessels had back-up systems to allow a safe transit, the consequences could have been severe.”
These types of events highlight the potential detrimental impact to navigation caused by GPS interference or jamming and the importance in understanding how your vessel’s or facility’s equipment could be impacted by a loss of GPS signal.

Positioning signals may be impacted by interference from both natural and human-made sources.  The most common types of interference are reception issues, usually due to bad installations, poor antenna positioning or faulty equipment.  Jamming devices (, while illegal in the U.S. and a threat to safety, have been used for nefarious or deceptive purposes.

Interference can also be unintentionally caused when operating GNSS in close proximity to other radiating devices,
such as amplified TV antennas (see our Safety Alert 11-02).  Therefore, it is important to remember to use all available means for navigation and maintain proficiency so you can still navigate should your primary GPS fail.

Indicators of positioning systems interference include an intermittent signal, no signal, or an incorrect signal. Suspected or suspicious disruptions should be reported immediately.  Critical information to take note of during a disruption event includes location, time, and period of outage.

Commercial operators are reminded, should your navigation or other equipment onboard, such as AIS, be impaired as a result of a disruption or interference, this should be reported to the nearest U.S. Coast Guard Captain of the Port,
District Commander or Vessel Traffic Center as soon as possible; and, await
further directions (per 33 CFR 164.53).

All operators should be aware, vigilant, and immediately report GPS disruptions to the U.S. Coast Guard Navigation Center (NAVCEN).  The report will be disseminated to the U.S. Air Force GPS Operations Center and the Federal Aviation Administration in an attempt to identify the problem and correlate with any other GPS incidents in the same general geographic location.  Depending on the severity of the report, NAVCEN may refer it to
law enforcement and/or other federal agencies for further investigation.

Download safety advisory

See Also

Jun 112008

Comfortable that having eight GPS receivers from four different manufactures aboard will make sure you know where you are? Don’t be complacent is the message in the latest safety flash from the International Maritime Contractors association, IMCA.

One would expect a GPS antenna failure to just result in a lost signal, but it could result in the receiver becoming a transmitter and knocking out the other GPS units. An IMCA member reports that when approaching the work area, which was close to the shore, the vessel experienced a complete failure of all its GPS receivers (eight receivers from four different manufacturers/suppliers).

Following extensive fault-finding which isolated the problem to the area of the vessel bridge, it was discovered that one GPS unit was causing the failure of all the other units. The receiving antenna for the unit had failed and had become a transmitter, the signal from which subsequently blocked all the other GPS receivers mounted nearby. The close proximity (<1m) of this damaged antenna unit to two other GPS antenna units may also have caused damage to the internal electronics of these units.

The seriousness of the failure is self-evident. Be aware of the potential hazard posed by a faulty GPS antenna and that all GPS antennae should be separated by at least two metres where possible.