By Douglas Meyerhoff


There is little doubt that the development and wide spread use of the Global Navigation Satellite System (GNSS), of which the Global Position System (GPS) is a part, has been a technological leap that rivals any other navigational advance in the last 100 years.

The introduction of the handheld GPS receiver has given its users an increased sense of confidence in their navigational performance and situational awareness.  I only have to think back to my teenage years while working on my commercial pilots license, to relive the cold prickly feelings of positional uncertainty I felt while flying a Non-Directional Beacon approach during a snowstorm. 

The maritime industry quickly realized the major advantages that GPS navigation provided in the very early stages of its availability.  The emergence of GPS as a reliable and inexpensive navigation aid caused the mariners to embrace it enthusiastically.  The GPS navigation system replaced the existing ground based Loran navigation system as the primary means of long-range navigation amongst most sailors.  Although GPS has enjoyed wide acceptance and popularity, the Loran system has not disappeared entirely.  The reliability of Loran is now being revisited as a land based alternative to GPS in the event of a loss of the satellite capability.  Loran provides accurate position data in 2 dimensions as well as providing an precise time keeping capability.

The use of GPS navigation was helping ships transit narrow channels and harbors long before the aviation industry was able to adopt the GPS service. 

The GPS signals enabling this amazing navigational accuracy are generated from a constellation of satellites owned and operated by the United States Military.  In the early stages of civilian usage, the quality of positional information provided by the US Military was deliberately degraded.  This reduction in accuracy was applied in order to deny any potential enemy the benefit of highly precise position data for targeting or position monitoring.  As the commercial usage of GPS increased the US Military improved the integrity, and guaranteed the availability of the commercial signal. This improvement happened on the 4th of May 2000, when the error tolerance went from hundreds of meters to tens of meters with the flick of a switch.[1]

The increased sophistication of the onboard navigation equipment has allowed more demanding operations to be validated and approved.  In the aviation industry we now regularly see instrument approaches, with vertical guidance, published and authorized by State Regulators.  Some of these approaches are based only on satellite generated positional information supported by a Ground Based Augmentation System (GBAS).  These GNSS based approaches are replacing expensive ground based instrument landing systems while meeting the same weather minimums traditional approaches require. In many cases the availability of the GNSS/GPS approach has provided many airports with a precision approach option where previously one was neither cost effective nor geographically practical.

The GPS satellite network broadcasts a signal that allows a receiving unit to calculate an accurate surface position with remarkable accuracy by comparing the coded signal received from at least 3 different satellites simultaneously.  The reception of a fourth satellite’s signal enables the altitude element of the position to be calculated giving the three dimensional position.

The GPS receiver calculates its position by measuring the time difference in the signal reception from each individual satellite.  The receiving unit then triangulates its position relative to the known position of the broadcasting satellites in space.

The dependability of the GPS signals has been debated for years, as the system was totally controlled by the United States Military. The Military could deny the system either wholly or partially at their whim with little or no notice.  This was not an ideal situation to be in if you were relying on a satellite signal for navigation in poor weather and, with no notice, the signal was withdrawn.

The lack of precision and the total reliance on a military power holding the keys for the system were the major stumbling blocks in enabling the use of GNSS/GPS as a primary source of navigational guidance in commercial aviation.

The increased commercial dependence on the GNSS system however, resulted in a number of different nations and the European Union creating their own independent variations of satellite based navigation to mirror GPS.  Many of the different systems characteristics are similar but control over the entire system no longer rested with one nation.

The Russians have developed and deployed GLONASS, the Europeans are currently deploying Galileo, China is developing the Beidou system, Japan has developed a regional network covering Japan while India have launched a number of satellites destined for the Indian Regional Navigational Satellite System (IRNSS). This is a space based navigation system for the Asian subcontinent.

Each independent constellation is a response to the perceived vulnerability of a system controlled by another military authority retaining the ability to withdraw the service with no notice.  The only GNSS navigation system exclusively controlled by civilians is the European Galileo network and it’s service provider, European Geostationary Navigation Overlay Service (EGNOS). 

The cost of developing and deploying the GNSS systems has been vast, and yet with the multiple GNSS options available there is a vulnerability that has grown in significance with very little acknowledgement.  Although the development of these various independent systems offers hardware redundancy, the major technological choke point is that all GNSS signals are broadcast on the same frequencies and at very low power settings.   

The greatest threat posed to the GNSS signals is created by a variety of portable low powered GPS jammers also known as Personal Privacy Devices (PPD’s).  The presence of these PPD’s poses a genuine threat to the integrity of the GPS signal.  Although the GNSS satellite system is extensive and provides a constant signal, the relative power of the GPS signals received and utilized on the ground is very low. 

Professor Charles Curry, the Founder of Chronos Technology in the United Kingdom, compares the power of the GPS signal utilized in your personal GPS receiver to that of the power a 20 watt light bulb produces 12,000 miles away. 

The PPD jammer operates by simply transmitting a tone on the same frequency as the GPS signal.  As the jamming transmitter is much closer to the GPS receiver, the jammer “drowns out” the genuine GPS signal causing erroneous readings.  Imagine two people speaking to you at the same time on the same frequency. One is 2 miles away speaking in a quiet voice and the other is right beside you.  You expect and believe that you are only receiving input from the ‘transmitter’ 2 miles away and you are relying on those instructions.  You are completely unaware that there is a transmitter next to you. As both transmitters are broadcasting on the same frequency, again you think you are receiving the genuine transmission from two miles away, however, the jammer transmitting next to you is drowning out the genuine transmission.  The jammer will overpower the genuine signal due to the extra power of its signal as a result of it being so much closer to you. 



Major jamming events such as the North Korean attack on South Korea in 2010, 2012 and 2013 were in no way covert or intended to go undetected.  Activity like this would attract government and media attention immediately.

If the jamming attack completely disables the GPS signal, alarms will be raised and appropriate action can be taken to revert to alternate systems.  The attack that raises the most concern is the low power disruption that causes the GPS signal to be distorted and the accuracy of your position information to then be compromised.

Thus far we have been considering the GNSS/GPS system in an exclusive navigation only application.   The assumption, that GNSS/GPS is utilized for navigational purposes only is quite wrong.  Position, Navigation and Time, (PNT) are the three main capabilities of the GPS signal. 

While considering some of the other operational applications of the GNSS/GPS system, I recently came across a superb description of some of the broader applications of the GPS signal and the impact a brief and unexpected interruption of the GPS signal would have on our modern life.  

This description was presented by Don Jewell the Defense Editor of GPS World Magazine and Editor of Defense PNT Newsletter.


I will paraphrase his comments,

Š      You are a marketing Rep for an electronics firm visiting a customer in a small town in the American Midwest.

Š      The small commuter aircraft you are on lands after executing a missed approach because the GPS based approach the pilot was flying failed while the aircraft was on short final.

Š      Because of the missed approach you land late and you rush to the rental car counter.  You are pleased you ordered a car that is fitted with a GPS system to help you find your way to the meeting.

Š      Unfortunately, the rental car company’s computer system is not working so you need to provide your drivers license, proof of insurance while filling out the forms by hand..

Š      You pull out your credit card to pay for the car but the credit card service is down. 

Š      Realizing you are now very late for your sales meeting you pull out your mobile phone to call your client.  There is no cell service.  When have you last seen a working payphone?

Š      Once you finally get underway in the rental car you notice that neither the GPS, nor the mapping app on your mobile phone is working.

Š      5 miles from the airport on a busy highway you run over a board with nails in it and you blow a tire.

Š      You cannot call the Automobile club or the rental agency for assistance because your mobile phone is still not working.

Š      You see a gas station some way away and you walk to it looking for some help.

Š      The gas station cannot help you because their internet-based point of sale computer system is out of service.

Š      The gas attendant refers you to a full service station a couple of blocks away.  The next full service station cannot contact their tow-truck as their radio system is not working.  Besides that they require a credit card for payment and their internet-based point of sake system is out as well.

Š      All is not lost as there is a bank across the street.  You cross the street to withdraw some cash from the ATM but there is a long line of customers waiting for the bank machine to start working.  The ATM has lost synchronization with the bank’s main computer.

Š      You are now incredibly late and you try to call your client’s mobile phone but it doesn’t seem to be working.

Š      You have missed the meeting and lost the sale.


As Don Jewell suggests, all of this disruption is the result of a lone trucker who used a handheld GPS jammer to defeat the GPS locator fitted to his truck, because he didn’t want his boss to know he stops off at a bar for a couple of drinks on his way home everyday.


These PPDs vary widely in power and size.  Some are as small as a golf ball and can be powered from the cigarette lighter in your car.  Others are larger and have the capability of jamming multiple frequencies in the GPS spectrum simultaneously.

The use of GPS tracking and location equipment has met with resistance by individuals or organizations, some of them criminal, who want to avoid being tracked and monitored in this way. 

In response to this desire for anonymity a wide variety of jammers have been developed and marketed.  These devices are readily available over the internet and at very little cost. 

Possession of a GPS jamming device is illegal in the United States. However, possession of the same GPS jammer is legal in the United Kingdom.  Oddly, although possession of a GPS jammer may be legal in the UK, the use of the same jamming device is illegal.  There are a number of influential groups in Europe and North America who are attempting to encourage legislation and strengthen laws associated with possession and use of PPDs/jammers.

The Federal Government of the United States has recognized the significant impact of potential disruption and considers the GPS system “a vital part of the national infrastructure.” 

In the United States 3.3 million jobs are in some way reliant on GPS technology alone.  The European Union identifies 56,000 jobs directly tied to GNSS technology.

The use of GPS data is not limited to position information or precise navigation alone. A large majority of all GPS chips on the market today are dedicated exclusively to precision time synchronization.   

Time synchronization is essential in many industries not associated with transport.  The financial sector requires precise time stamping for all electronic transactions.  The mobile phone networks require precise timing in order to sync the cellular phone signals as they are handed from one cellular phone mast to another while you drive down the highway.  Television transmitters require precise timing synchronization in areas where the signals from 2 different TV transmitters overlap.  Electrical/Nuclear Power companies require precise timing to manage the flow of electricity throughout their electrical distribution networks.

As you can see the range of the GNSS/GPS system usage is extensive.

As a former Air Traffic Controller I regularly experienced instances of Radio Frequency Interference with two aircraft blocking my frequency by trying to transmit at the same time.  I would often get pilots responding with a call of “BLOCKED” when 2 transmitters had broadcast simultaneously.  A loud piercing squeal would be heard on the frequency as a result of two transmitters trying to transmit at once.  That squeal is the result of interference or jamming.  As your GPS receiver “hears” the signal you cannot know that the jamming is happening until the signal is lost.  The same principle applies to GPS jamming because the jammer “blocks” the real GPS signal.

Most of the PPD’s have a relatively low power output and are purchased with the intent of hiding the owner’s activities or physical location.  Often the criminals will use the GPS jammers to defeat location monitoring security devices on expensive cars or cargo of high value. A recent investigation at a major west coast port in the United States revealed 47 instances of shipping containers emitting GPS jamming signals.  Detailed inspections found these containers to be full of stolen high value vehicles.  There is no thought or concern about the additional effect the PPD may have on other GPS receivers within range. 

So potentially a truck driver, attempting to disguise the fact he is using a company vehicle in the evening, goes to the harbor to pick up his friend from his job at the Cargo area of the port.  He leaves the jammer on and in doing so disables the Ground Based augmented satellite approach system. 

The port authority will not be aware of the interference until the first ship, relying on the signal, informs the Harbor Control that the signal has been lost or corrupted.  At this stage there is no way of determining why or how this signal has been compromised.  Is it solar activity interrupting the entire GNSS system, a military exercise targeting the GNSS signals in the area, a deliberate attempt to disrupt a vehicle tracking system or just a random guy using his boss’s truck to help a friend?

In the next article we will examine some of the measures and tools that can be used to quickly detect and locate the source of GNSS radio frequency interference.





Direct-Track Consulting is an industry-leading consultancy providing over 28 years front line aviation experience.  This experience has been gained in both the military and civilian environments of the UK, Canada and the Middle East.  Extensive Air Traffic Management in both the Aerodrome and Approach radar disciplines has been enhanced with particular interest in the GNSS/GPS navigation capabilities and vulnerabilities.  The GNSS/GPS interest has led to strategic relationships with a number of individuals and organizations boasting exceptional qualifications in the research and mitigation of the effects of GPS interference.

Direct-Track Consulting can be reached at for a free initial consultation.





[1] Dr. Todd Humphreys.  Ted Talks 2012.