Charles Curry shares his personal perspective on using GPS for precise timing. He discusses how GPS was first used in the mid-1980s in the oil industry for navigation. GPS technology has since evolved greatly, with early commercial GPS receivers costing $50,000 each and only providing positioning accuracy at certain times of day, compared to today's smartphones that have multi-constellation GPS receivers for under $1. GPS is now widely used for disseminating precise timing information via the atomic clocks on GPS satellites. However, increased reliance on GPS for critical applications has also increased vulnerabilities such as jamming and space weather disruptions.

1. CHARLES CURRY TIME FROM THE SKY 15
INFORM NETWORK DEVELOP
Charles Curry first started using
the US-based Global Positioning
System (GPS) in the mid-1980s in
the oil industry.Here he shares
his personal perspective on using
the Global Positioning System
satellites for precise time.
In the mid ’80s I was MD of GSE Rentals Ltd,
a division of Geophysical and Scientific
Equipment Ltd.We hired out geodetic and
hydrographic survey equipment.Amongst
other things (as diverse as seismic streamers
THE JOURNAL TJ
and sonar equipment) this included
navigation equipment,some of which was
based on theTransit satellite system
technology.Transit,which began in 1964,
would ultimately be made obsolete in 1996
by the maturing GPS system.
Back in 1984,we purchased the first
commercially available GPS navigation
receiver – the Magnavox 1502 (see Figure 1)
– to hire out to seismic survey companies
operating in the UK offshore oil exploration
industry.They cost about £50,000 each,
equivalent to roughly £150,000 today.In
those days there were only seven GPS
satellites orbiting and there was an hour at
about 15.00 when you could get a fix in the
North Sea.Compare that price and capability
to today when nearly every smart phone and
tablet has a multi-constellation 30-channel
Global Navigation Satellite System (GNSS)
CHARLES
CURRY
Insights into GPS
TIME FROM THE SKY

2. THE JOURNAL TJ
16 CHARLES CURRY
receiver embedded in the architecture for
less than $1.
It would still be another 10 years before GPS
found its way into use as a precise timing
service. In the UK,BT were the first carrier to
adopt GPS as a timing reference for
frequency stabilising local Rubidium atomic
and quartz oscillators at major switch sites.
Carriers in the US,includingAT&T,did the
same.It should be noted that GPS was not
the primary reference source of frequency
stability for the network; this was and still
remains to this day,a cluster of Caesium
atomic clocks which meet the stability
requirements of ITU G.811 i.e.1 x 10-11.
However,before exploring time from the sky
today,we need to go back another 10 years
to the dawn of GPS.
GPS was conceived in 1973 by the US
Department of Defence; in particular the US
Air Force.A few years ago Colonel Brad
Parkinson told his story,which the author was
fortunate enough to hear,at a Royal Institute
of Navigation conference in Manchester.
There had been a discussion between the
three US forces –Army,Navy andAir Force
about the best candidate for a future Defence
Navigation Satellite System.It was agreed
that,since the USAir Force needed to
navigate in three dimensions,they would be
tasked with conceiving a suitable system.
Col.Parkinson was assigned a project known
as 621B and told to lock himself away with
his best team and not come out until a
system had been conceived!The so called
NAVSTAR program emerged and the first
experimental Block 1 GPS satellite was
launched a few years later in 1978.
As ofAugust 2015 (the 17th,to be precise)
there are 31 operational GPS satellites
orbiting with 24 needed for full coverage.The
GPS architecture is shown in Figure 2.Figure
3 shows a bank of caesium clocks used as a
timing reference for the GPS satellites.But
GPS is not alone.The Russians created
GLONASS (GLObal NAvigation Satellite
System) which is contemporary to GPS and,
although suffering a mid-life crisis through
lack of investment,is a fully funded mature
network with 29 satellites orbiting and 24
needed for full coverage. It then became a
matter of sovereign security and (perhaps)
pride for other major nations with Europe
Volume 10 | Part 1 - 2016
Figure 2: GPS architecture.
Figure 3: Bank of Caesium reference clocks used to provide the timing reference to GPS
satellites. (Photo courtesy of US Naval Observatory.)
Figure 1: Early commercial GPS receiver by
Magnavox – 1502 Geoceiver circa 1984.

3. TIME FROM THE SKY 17
INFORM NETWORK DEVELOP
(Galileo),China (Beidou/Compass),Japan
(QZSS) and India (IRNSS) developing GNSS
systems. China has the most developed
system with 15 satellites operational and 20
more planned.Galileo has launched 8
satellites for trial purposes (not all are
working at full operational capability_ and 22
are planned.India and Japan both plan
localised systems covering their own
countries using a combination of
geostationary and geosynchronous satellites.
Frequency allocations span the L1 to L5
bands from approximately 1175MHz to
1600MHz with GPS occupying the L1 band at
1575.42MHz.These are detailed by
Navipedia1
.
So why are global navigation satellite
systems suitable for disseminating precise
time?All GNSS satellites have atomic clocks
on board.Depending on the system and
generation,these variously include high
performance rubidium,caesium and
hydrogen masers.These are continuously
monitored from networks of ground stations
and provide the necessary triangulation
capability and accuracy for navigation and
positioning to within a few metres.Consider
that light travels approximately 30cm in a
nanosecond and then one has the basis for
slaving a local oscillator to visible GNSS
satellites.Then a system can be created
which outputs frequencies such as
2.048MHz,10MHz and 1 pulse per second.
This is discussed in more detail in Panel 1.
Evolution of GPS receivers
In the early days,timing receivers were
relatively large 19-inch rack-mount
constructions.They used high performance
quartz crystal oscillators or rubidium atomic
oscillators to smooth the effects of
atmospheric variations and selective
availability.GPS signals were broadcast on
two frequencies,L1 and L2.L2 was reserved
for military applications; L1 was the civilian
band at 1575.42MHz and is freely available.
However.it was deliberately degraded using
a technique called‘selective availability’.
After a combination of events during the
1990s,including the oil survey industry
developing techniques to mitigate this
degradation,selective availability was
switched off in May 2000.This improved the
accuracy of the signal and led to the
development of a new generation of chip-
level receivers.This also meant that,unless
significant holdover performance2
was
required,lower cost oscillators such as
temperature-compensated crystal oscillators
and miniature oven controlled crystal
oscillators (OCXO) could be used.GPS-based
timing receivers can be produced today with
a bill of materials cost of less than £20 and,
apart from holdover performance,compare in
stability and accuracy to the early 1996 era
units which cost in excess of £5000 (or
£8500 in today’s money).This comparison is
illustrated in Figure 4.
Timing performance
Timing performance can be measured quite
easily and displayed using an ITU-
standardised metric known as MaximumTime
Interval Error (MTIE).MTIE looks at the time
interval error (TIE) measured in nanoseconds
over varying observation periods and relative
to a higher stability reference.It takes a long
TIE dataset and reduces it to about 20 points
on a log-log graph.Each point is the
maximumTIE in an observation window of
data-points.For example,the 10-second MTIE
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1
See: http://www.navipedia.net/images/1/1c/Galileo_Signals_in_Space.png
2
Holdover performance is the degree to which a clock can maintain its accuracy after losing its traceability to a more accurate source of timing.
PANEL 1
Every GPS satellite has a highly stable atomic clock that is continuously corrected from the
ground control segment to remove any drift in its time.Each satellite transmits its own
pseudorandom noise code that utilises the on-board atomic clock as the time base,and
any ground-based GPS receiver needs to lock onto the received pseudorandom noise
codes and recover each satellite’s time.Once locked to multiple satellite pseudorandom
noise codes,the GPS receiver can calculate a 3-dimensional position and extract week
number (WN),time of week (TOW) and Coordinated UniversalTime (UTC) offset from the
satellite navigation message to enable calculation of an accurate time of day (TOD) relative
to UTC.
In addition to knowingWN,TOW and UTC offset,the GPS receiver can determine its local
oscillator clock bias and clock drift relative to the received satellite signals and can then
calculate the precise“top of second”relative to UTC to generate a one pulse per second
(1pps) signal that represents each second of UTC.Modern GPS receivers that are designed
for timing applications can achieve 1pps accuracies of sub 20ns rms relative to UTC.
To create a source of GPS derived frequency,it is possible to steer an external oscillator that
is selected to provide the required performance in terms of phase noise and holdover
stability for the particular intended application.The external oscillator is typically steered by
locking it to the GPS receiver 1pps signal via a phase locked loop (PLL) as shown in Figure
A.
The 1pps signal provides the
excellent long term stability
and the oscillator can provide
good short term stability if an
oven-controlled oscillator is
used.The frequency of the
oscillator is therefore steered
by GPS and can be used as a
highly stable system clock.
Figure 4: Change in size of GPS timing receivers
between 1995 and 2015.HP55300 – still in use
with a major telecom operator.
Figure A: Deriving
frequency from
GPS.

4. THE JOURNAL TJ
18 CHARLES CURRY
is the maximumTIE as a 10-second window
is moved through the dataset.If the MTIE line
sits below the‘Standard’,in this case an ITU
G.8272 Primary ReferenceTime Clock,then all
is good.
Increased expectations of GPS
GPS has indeed become one of the greatest
technological achievements and successes of
the 20th century.Ranking alongside the PC
and mobile phones,nearly every smart phone
has an embedded GPS receiver.However its
success is also one of its great weaknesses.
It’s cheap,it’s everywhere,critical
applications rely on it but little thought is
invested in embodying mitigation solutions.
What are some of these issues?
Back in 1996,GPS was just becoming
accepted as a core timing technology for
frequency stability in telecom networks.The
time/phase aspect had yet to emerge,so the
edge of a 1pps pulse was not aligned to
Coordinated UniversalTime (UTC) with any
high degree of precision.The GPS timing
signal was stabilised with a high performance
OCXO and used to stabilise a pair of
oscillators,usually rubidium backed up with
high stability quartz.Multiple frequency
references were created to distribute timing
to all local infrastructure in the exchange. The
whole system was christened a
Synchronisation Source Utility in the ITU
Standards and provided synchronisation for
both fixed line networks and the emerging
wireless networks.
Second generation (2G) mobile phone
technology began to emerge in the early
2000s; Code Division MultipleAccess (CDMA)
in the USA,Global System for Mobile
Communications (GSM) in Europe and a
mixture around the world.CDMA needed
precise time at the base station using aTime
Division Duplex modulation concept. This
was achieved using early low cost GPS
receivers and OCXO technology; holdover
was compromised to save cost.GSM on the
other hand,despite later standardisingTime
Division Duplex modulation through the ETSI
3GPP Standards,adopted Frequency Division
Duplex modulation and delivered frequency
stability to one part per billion over the access
layer from the core Synchronisation Source
Utility architecture.
The drive for more capacity over mobile
networks – from 3G to 4G and 5G in the
future – means that clusters of small cells
need to communicate with each other
(amongst other features) in a synchronous
manner.This requires precise time at the
edge,just as it was in CDMA.This has
coincided with a revolution in fixed line core
transport networks with the transition from
Synchronous Digital Hierarchy to Carrier
Ethernet-based networks. Suddenly that nice
synchronisation-friendly route back to the
core clocks has been taken away,just when
the mobile networks needed it.The
Standards bodies realised this over 10 years
ago and adopted IEEE-1588 PrecisionTiming
Protocol as the mechanism for transporting
UTC aligned time (or‘Phase’ as it became
known) and frequency over Ethernet
networks.This has proved a successful
technology for frequency but not quite so
effective for Phase as network phenomena
such as packet delay variation disrupt the
way PrecisionTiming Protocol works and
these new networks are likely to need better
than 500ns.So back to the drawing board
perhaps,or do what CDMA did and deploy
GPS at the edge.
GPS at the edge
GPSatthe edge seemstheidealsolution; itis
amaturelow cost technology capableof
delivering precisetime,phase aligned toUTC
Volume 10 | Part 1 - 2016
Figure 5: GPS antenna on a tall building in Bangkok

5. TIME FROM THE SKY 19
INFORM NETWORK DEVELOP
towithinafew10sofnanoseconds.But there
areanumberofissuesasidentified below.
Cost of deploying roof antennas
The cost of deploying roof antennas,such as
that shown in Figure 5,is a major concern.A
typical deployment will require a site survey,
the writing-up of a suitable method statement
so that buildings facilities management are
happy from a health and safety perspective;
obtaining roof access permissions especially
if the roof space is owned or leased by a third
party; two fully trained operatives with the
necessary working-at-height certification and
local roof space access training for up to two
days. Then the cable run can be quite long
especially in an urban canyon environment.
Hiring roof access equipment may be
required.Fitting lightning arrestor technology
is essential to protect the GPS receiver from
damage.Basically a £50 receiver can cost
anything from £5000 to £10,000 to install if
all the costs are included.
Reliability and continuity
Reliability and continuity are critical for mobile
networks. However,in order to reduce the
price of GPS receivers to meet the edge of
network cost model,holdover stability is the
first casualty,resiliency the second.High
stability oscillators are expensive compared
to temperature compensated crystal
oscillators and low cost OCXOs and,unless
one uses the emerging Chip ScaleAtomic
Clock technology,extremely power hungry.
Also Phase/Time accuracy is far more
critically dependent on oscillator holdover
stability than frequency [1] so this is a double
“gotcha”.Resiliency can be met by using
multiple GNSS systems.This can be a benefit
in the so-called‘urban canyon’ environment
where a 360 degree sky view is
compromised by buildings where more
satellites would be in view.Continuity can be
met by using ephemeris assist3
,but will not
be a solution in deep indoor locations and will
struggle to regain lock in locations with poor
sky view.
Jamming
There is another emerging threat from low-
cost GPS jammers such as that shown in
Figure 6. The GPS signal from the sky is
incredibly weak,equivalent to a 20W light
bulb 12,000 miles away.Low power jammers
are readily available over the Internet and can
wreak havoc.They range from simple GPS-
only devices to multiband GNSS devices
which will take out all GNSS frequencies
including the Galileo bands and multi-channel
devices which also block the cellular
frequencies.They are not illegal to own,but
their use contravenes theWirelessTelegraphy
Act 2006 and can result in imprisonment and
a serious fine.Despite this,their uses range
from personal privacy to organised crime,
terrorists and state aggression.So whilst the
WirelessTelegraphyAct might ultimately deter
the disgruntled individual,it is unlikely to deter
the organised crime gangs or ideologically
motivated terrorists.
Currently as many as 10 events per day on
sensors located near busy roads and
motorways are being observed.Whilst these
are of little consequence if there is some
holdover mitigation,they may show up as an
alarm and ultimately localised inter-cell de-
synchronisation.Too many alarms or localised
network failure may result in operations
centre personnel removing the alarm visibility
increasing the threat from a prolonged GNSS
outage.The jamming threat is well
documented by the RoyalAcademy of
Engineering [2].
Space weather
Space weather can also disrupt the service
with unpredictable results. A major
broadcasting service was recently
compromised by a minor space weather
event.This resulted in many GPS receivers
across a national digital broadcast network
losing lock; the low cost OCXO went into
holdover causing inter-transmitter interference
and significant loss of service at the
boundaries between transmitter coverage.The
impact of space weather is covered in another
RoyalAcademy of Engineering report [3].
Spoofing
Spoofing is an entirely different and very
aggressive targeted cyber-physical attack
concept.It is possible to rebroadcast the GPS
signal with different time and position
information.One could steer a target receiver
to a different time.This would cause major
disruption for time stamping in,for example,
the financial services sector which is being
driven towards very high standards of time
accuracy by a new EU regulation called MIFID
2.This concept could also cause equipment to
fail if time authentication with a remote server
was required as part of continuous operations.
Politics
Politics plays its part in the application of
GNSS signals.Russia for example has
mandated the use of GLONASS for certain
applications including in mobile phones.The
EU is looking to mandate Galileo (particularly
the public regulated service) for critical
national infrastructure (CNI) applications such
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“
”
Figure 6: Multiband GPS jammer covering L1,L2,
L3,L4 and L5 at 500mW per channel.
Based on concepts originally used before Transit
in WW2, eLoran offers time stability traceable to UTC.
It works indoors and is not vulnerable to the same
jamming and spoofing threat as GNSS. It does,
however, have geographical limitations.
3
Rather than rely on the satellite’s transmission of its orbital position (ephemeris), this data can be derived via wireless networks or the Internet.

6. ITPINSIGHT CALL
Want to talk to the authors?
To discuss this article and its content,
join in the ITP Insight Call on 17 May.
To book onto the call visit:
https://www.theitp.org/calendar/
ABOUT THE AUTHOR
Prof Charles Curry,BEng,
CEng,MITP,FIET
CharlesisManaging Directorof Chronos
Technology.He graduated in Electronics from
Liverpool Universityin1973,andstarted his
careerat GECHirstResearch Centre in silicon
MOSboundaryresearch,progressing toRacal
Instrumentswhere hewasresponsiblefor
salesofspecialist frequencyand timeproducts
(LoranC) andCaesiumstandards.Duringthe
early 1980sat GSERentalsCharleswas
involvedwith some of thefirst civil GPS and
laptop PCdeploymentsintotheNorthSea
offshoreoilexplorationindustry.He founded
Chronosin1986,aleadingservicesolutions
providerand manufacturerof synchronisation,
timing,GNSSand GPSjamming detection
products.Charlesfounded theInternational
Timing &Sync Forum (ITSF) in2001 and isa
memberoftheUSWorkshopon
SynchronizationTelecommunicationsSystems
(WSTS)SteeringGroup.Charleshas Honorary
Professorshipsatthe UniversitiesofBath and
Liverpool.
REFERENCES
1. Curry,C.ChronosTechnology white
paper:Dependency of
Communications Systems on PNT
Technology. Mar 2010.Available at
http://tinyurl.com/jecxc5n
2. The RoyalAcademy of Engineering.
Global Navigation Space Systems:
reliance and vulnerabilities. Mar
2011.
3. The RoyalAcademy of Engineering.
Extreme space weather:impacts on
engineered systems and
infrastructure.Feb 2013.
ABBREVIATIONS
CDMA Code Division Multiple
Access
CNI Critical National Infrastructure
GLONASS GLObal NAvigation Satellite
System
GNSS Global Navigation Satellite
System
GPS Global Positioning System
GSM Global System for Mobile
Communications
MTIE MaximumTime Interval Error
OCXO Oven Controlled Crystal
Oscillator
PNT Positioning Navigation and
Timing
UTC Coordinated UniversalTime
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20 CHARLES CURRY
as Utilities,Telecoms and Financial Services
applications.The bizarre aspect of this is that
the jammers can disrupt all GNSS
frequencies.
eLoran
Does all this mean that we have designed the
perfect system – but with a significant
vulnerability? Not really.The probability of
being in the same place as jamming for long
enough to cause damage (unless very
powerful) is extremely low.For fixed CNI,help
is at hand in the form of high power terrestrial
transmissions of a complementary
Positioning Navigation andTiming (PNT)
service known as eLoran.
Based on concepts originally used before
Transit inWW2,eLoran offers time stability
traceable to UTC.It works indoors and is not
vulnerable to the same jamming and
spoofing threat as GNSS. It does,however,
have geographical limitations – needing to
see at least two transmitters for a resilient
service and knowledge of local differential
timing corrections to provide UTC traceability
and accuracies to the same order of
magnitude as GNSS.eLoran is at a different
technology readiness level to GPS.
Transmissions are fit for purpose but at the
moment receivers are too expensive and the
system is mired in inter-EU state politics.In
fact,since beginning to draft this article
Loran-C stations in France,Norway,Denmark
and Germany have switched off. Anthorn in
Cumbria,UK remains the only Loran station
broadcasting in the EU in 2016 with the
modernised eLoran transmissions.This will
be preserved as a timing and data service
only until more transmitters can be brought
back into service. The USA switched off their
Loran-C transmissions a few years ago but
are now seriously considering bringing
eLoran back for precise timing applications in
the first instance.
AUTHOR’S CONCLUSIONS
This story started in 1973 with the creation of
GPS,stepped forward to 1996 with the first
significant timing application in telecom
networks,had a boost in 2004 with the
switching off of selective availability and now,
in 2016,has a major demand based on next
generation mobile network applications and
new stringent financial services sector rules.
Where will we be in another 10 or 20 years?
If the lessons of the past including cost
reduction,miniaturisation and technology
hybridisation are to be learned we will have
multi PNT technology embedded in all CNI
applications.We won’t have to worry about
roof antenna installations and the whole thing
will be less than a few dollars.
Volume 10 | Part 1 - 2016
AFTERWORD
SVN23 Timing Anomaly
Little did I think that a GPS timing fault of
unprecedented scale and global impact
would occur whilst this article was in
preparation.GPS SatelliteVehicle Number
(SVN) 23 launched in 1990 was retired
from service on January 25th 2016.
Shortly afterwards a 13 microsecond time
error started to be broadcast from 15 of
the GPS satellites.This was in the UTC
offset message and was picked up by
GPS timing receivers all over the world.
The problem lasted for over 12 hours and
makes some of the comments about GPS
vulnerability particularly prescient.Read
how it impacted the Chronos support
team here.http://www.chronos.co.uk
/index.php/en/