The dangers of GPS/GNSS

Feb 2009 | One Comment

The problem is that nothing works 100 %. GPS is very close, but for some users under some circumstances, “very close” is not good enough

Recorded examples of GPS troubles

During the long (and ― don’t forget! ― usually very successful) history of GPS, a number of satellite malfunctions and interference problems have been recorded. Taking the record of April – August of the year 2005 for examples of satellite malfunctions, we fi nd the following:

• SVN37 (PRN7): 3 Apr – Load-shed;

• SVN31 (PRN31): 14 Apr – Baseband reset;

• SVN27 (PRN27): 14 May – Rubidium #1 runoff leads to clock swap;

• SVN26 (PRN26): 9 Jun – Rubidium #1 clock jump;

• SVN15 (PRN15): 22 Jun – Comparator Reference Value Change;

• SVN32 (PRN1): 24 Jul – Load-shed;

• SVN26 (PRN26): 21 Aug – Crypto Variable Upload.

Experienced jamming and other records of intentional interference are (for obvious reasons) less available, but unintentional interference examples are abundant. Let us look at just two of them.

An infamous example is Moss Landing in California. From May 2001 and several months onward, no use of commercial GPS receivers was possible in the whole harbour area out to a distance at sea of at least one kilometer. After the first user reports about GPS unavailability, considerable efforts were launched to find the source of the interfering signal(s). Finally, it was discovered that there were in fact three sources, all of them being active UHF/VHF TV antennas with preamplifi ers onboard pleasure boats.

In December the same year, a GPS jammer caused GPS failures within 180 nautical miles of Mesa, Arizona. Boeing was preparing for upcoming tests and accidentally left a jammer on the L1 frequency, radiating just 0.8 mW. The jammer operated continuously for 4.5 days. There were several impacts to ATC operations during the six days of jamming:

• Aircraft lost GPS 45 nm from Phoenix, performed a 35° turn toward


• NOTAM was not issued until 2nd day;

• numerous pilots reported loss of GPS;

• There were reports of handheld GPS receivers not working.

Time users

Users of GPS as a time and/or frequency reference are an often forgotten or unknown but very important part of the GPS community. Some applications where GPS time is used:

• Navigation;

• Telecommunications;

• Digital broadcasting;

• Power generation and distribution;

• Metrology;

• Meteorology;

• Radar;

• Tests and measurements

• Time tagging (Internet and transport)

• Time-of-day distribution.

Many users of these applications are crucially dependent on GPS for

their systems to work properly.


If satellite signals do not meet requirements, the only thing users can do is to acquire information about the malfunction(s) from other sources as quickly as possible. Such information acquisition is called integrity monitoring. It can be receiver autonomous (RAIM) or received from external wide-area augmentation sources (e.g. WAAS, EGNOS, MSAS, GAGAN) or from local monitors (GBAS = Ground- Based Augmentation Systems).

Integration of GNSS receiving equipment with other sensors (e.g. inertial systems, LORAN) can be very useful in case of satellite signal malfunctioning. Such integration can also be efficient against interference and jamming.

Other countermeasures against all kinds of external interference are fi ltering and advanced signal processing, including adaptive antennas and null-steering. A question which is often asked is whether upcoming systems (Galileo, INRSS, etc.) will solve the problem.

The answer is that they will reduce the problem but not solve it completely.

Satellite navigation problems in cars

Drivers using car navigation equipment experience problems fairly often (Fig. 5). Due to lack of knowledge, these problems are often wrongly ascribed to the satellite system instead of to the real cause, the map system.

(In fact, a lot of users call their car navigator “my GPS”). A summary of these problems might be written:

• Satellite-based car-navigation equipment attracts negative attention for doing a job logically… but unintelligently;

• Expectations for such equipment are high – expected to be smarter than the driver (which-in fact-it often is!);

• Road classifi cation is diffi cult – one person’s farm track is another’s handy shortcut.

Conclusions and recommendations

All GNSS users must evaluate and analyse their own situation:

What would be the consequences in case of GNSS problems?

If the answer is ‘no serious consequences’, then “business as usual”. But if there are possible serious consequences, users must prepare for the unexpected !

Professor Börje Forssell

Norwegian University of Science and Technology, Dept. of Electronics and
Telecommunications, Trondheim, Norway
My coordinates
Mark your calendar
May 09 TO DECEMBER 2009

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  • tunio said:


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