Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Time from the Sky

219 views

Published on

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.

*** Shared with Permission from ITP Journal Volume 10 | Part 1 - 2016 ***

Published in: Technology
  • Be the first to comment

  • Be the first to like this

Time from the Sky

  1. 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. 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. 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 THE JOURNAL TJ 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. 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. 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 THE JOURNAL TJ “ ” 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. 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 THE JOURNAL TJ 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/

×