GPS/GNSS Navigation IS IT ALL CLEAR
SKIES?
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 satelliteÕs
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 itÕs 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 (PPDÕs). The presence of these PPDÕs 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 Òdrowns outÓ 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 ÔtransmitterÕ 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.
https://youtu.be/wR_pG9AplDE?t=3m
I
will paraphrase his comments,
á
You are a marketing Rep for an electronics firm visiting a
customer in a small town in the American Midwest.
á
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.
á
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.
á
Unfortunately, the rental car companyÕs computer system is
not working so you need to provide your drivers license, proof of insurance
while filling out the forms by hand..
á
You pull out your credit card to pay for the car but the
credit card service is down.
á
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?
á
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.
á
5 miles from the airport on a busy highway you run over a
board with nails in it and you blow a tire.
á
You cannot call the Automobile club or the rental agency for
assistance because your mobile phone is still not working.
á
You see a gas station some way away and you walk to it
looking for some help.
á
The gas station cannot help you because their internet-based
point of sale computer system is out of service.
á
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.
á
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 bankÕs main computer.
á
You are now incredibly late and you try to call your clientÕs
mobile phone but it doesnÕt seem to be working.
á
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 didnÕt 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 Òa vital part of the national
infrastructure.Ó
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 ÒBLOCKEDÓ 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 ÒhearsÓ 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 ÒblocksÓ the real GPS signal.
Most
of the PPDÕs have a relatively low power output and are purchased with the
intent of hiding the ownerÕs 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 bossÕs 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 info@Direct-Track.com for a free
initial consultation.