1

GPS and Time:Using Clocks in Space for

Accurate Time on the Ground

Dr Bruce WarringtonNational Measurement Institute

Time and frequency: the context

• Accurate time and frequency are essential to modern life:broadcasting, electricity distribution, mobile telephony, high-speed communications, computer networks, satellite navigation, radar speed measurements, electronic transactions and timestamping…

• Australian industry uses a wide variety of measuring instrumentsthat must be accurately calibrated:frequency counters, spectrum analysers, time-interval counters, frequency standards, delay generators, frequency synthesizers, phase meters, oscilloscopes…

2

The Problem

Distributed measurements:various locations and instrumentation

Time-stamping:for registration or legal status

The Solution

A master clock:stable, accurate, high-integrity, legal status…

A means of synchronising clocks:tolerance, separation, interconnections…

3

National Measurement Institute (NMI)

Formed on 1 July 2004, amalgamating:

CSIRO National Measurement LaboratoryAustralian Government Analytical LaboratoriesNational Standards Commission

Physical Metrology Branch• Located in Lindfield, Sydney• Support the Australian National

Measurement System by realising and maintaining Australia’s standards for physical measurement, as required by the National Measurement Act (1960)

Time: the Australian context

The National Measurement Act (1960, amended 2004) explicitly establishes responsibility to ‘maintain… standards of measurement’ including Co-ordinated Universal Time (UTC) in Australia

Dr Barry InglisChief Metrologist

4

Atomic frequency standards

E1

E2

hEEf /)( 120 −=

CCDS, 1967

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Parry and Essen at NPL, 1955

~1 part in 1010

1 s in 300 years~1 part in 1012

1 s in 30 000 years

HP5071A

~1 part in 1015

1 s in 30 000 000 years

NIST F1, 1999

• NMI maintains an ensemble of atomic clocks (caesium clocks, hydrogen masers)

• One clock is designated the Australian realisation of Coordinated Universal Time, or UTC(AUS)

• Commercial standards are accurate to 2 parts in 1012, equivalent to measuring a mass of half a tonne to the nearest microgramme

Primary standards at NMI

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International Atomic Time (TAI)

1 second ‘ticks’>250 clocks worldwide

TAI

International Atomic Time

average

Bureau International des Poids et Mesures, Paris

Co-ordinated Universal Time (UTC) is TAI plus leap seconds, inserted from time to time to account for variations in the rotation rate of the Earth

Averaging of clocks?

1 second

average

time

6

TAI

CircularT

UTC

Bureau International des Poids et Mesures, Paris

>250 atomic clocksworldwide

International timekeeping

MJD – 52900

–350

–450

–550

–6500 200 400 600

UTC

–U

TC(A

US)

[ns]

The Solution

A master clock:stable, accurate, high-integrity, legal status…

A means of synchronising clocks:tolerance, separation, interconnections…

A master clock:stable, accurate, high-integrity, legal status…

A means of synchronising clocks:tolerance, separation, interconnections…

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Principle of Time Transfer: the 1 o'clock gun

• Sound takes 4.7 km/0.35 km/s = 13.4 s to reach timekeeper• Timekeeper notes that his clock reads 13:00:36.7 when he hears the shot• So his clock showed 13:00:36.7 – 13.4 s = 13:00:23.3 as the cannon fired• His clock is 23.3 s fast — this is his clock error, REF–BANG

4.7 km

Correction for the speed of sound required: ~350 m/s or ~3 s/km1 s accuracy requires the propagation distance to be known within 350 m

Principle of Common-View Time Transfer

REFA=01:01:05REFA–BANG=65s fast

REFB=01:00:45REFB–BANG=45s fast

(REFA–BANG)–(REFB–BANG) = REFA–REFB = 20s fast

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TV Sync pulse

Common-View TV Sync Time Transfer

REFA – REFB = (REFA – TV) – (REFB – TV)

TVBA

∆A = REFA – TV ∆B = REFB – TV

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Start

dREF

REF – TV = Counter reading + dREF – range/c – dreceiver – dcable

REF 1 pulse/sec

Measuring REF – TV

dreceiver

dcable

Stop

Separator Receiver Antenna

rang

e (<

100

km)

REFA – REFB = (REFA – GPS) – (REFB – GPS)

Principle of GPS common-view time transfer

‘common view’ GPS time transfer: same satellite tracked from both A and B

BA

∆A = REFA – GPS ∆B = REFB – GPS

GPS satellites transmit:• Timing pulses• Orbital data

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Start

dREF

REF – GPS = Counter reading + dREF – range/c– dant – dant_cable – drx_int – drx_cable

REF 1 pulse/sec

rang

e (~

2200

0 km

)

Measuring REF – GPS

dantdant_cabledrx_int

drx_cable

Stop

GPS ‘Space Segment’

24 satellites in 6 orbital planes

Satellites ~20200 km above Earth’s surface

~12 hour orbits (11 hours 58 minutes)

Each satellite follows the same track in the sky as seen from a point on earth every 23 hours 56 minutes

From NAVSTAR GPS User Equipment Introduction

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GPS satellites

Each GPS satellite:

• Incorporates redundant atomic clocks

• Transmits on two frequencies, L1 (1575.42 MHz) and L2 (1227.6 MHz)

• Transmits timing signals and binary data (almanac, ephemeris, etc) which can be used to determine the precise position of the satellite

Each satellite uses the same frequency; “spread spectrum” signals from individual satellites are distinguished by a unique frequency hopping sequence or code

Link between Position and Time

If a GPS satellite broadcasts a timing signal and we receive it on the ground, we need to make a correction:

Orbital altitude ≈ 20200 km

Speed of light c = 2.998 × 108 metres/second ~ 3 nanoseconds/metre

Signal transit time ≈ 20.2 × 106 m / c ≈ 0.067 s

To make this correction accurately, you need to know:

• Coordinates of satellite• Coordinates of receiver

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Calculation of time from a GPS satellite

Timing signal transmitted at time ts by satellite clock

Arrives at receiver at time (according to receiver)tR = ts + d/c + δR

where δR = receiver clock error

Receiver decodes signal and finds that it left the satellite at ts, and using broadcast ephemeris calculates satellite position at that time: (XS, YS, ZS)

If receiver coordinates (XR, YR, ZR) are known, receiver determines that its clock error δR is

Distance d

( ) ( ) ( )2221RSRSRSsR

sRR

ZZYYXXc

tt

cdtt

−+−+−−−=

−−=δ

(XS, YS, ZS)

(XR, YR, ZR)

GPS spread-spectrum signal structure

http://www.colorado.edu/geography/gcraft/notes/gps/gif/signals.gifNAVSTAR GPS User Equipment Introduction

• C/A code: Frequency “hops” 1023 times each millisecond; pattern repeats every millisecond

• P code: Frequency “hops” 10230 times each millisecond; code is 267 days long (!)

• P code is encrypted to prevent “spoofing”; encrypted P code is called Y code

• Only C/A code is available to civilians

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Timing information in GPS data message• Transmitted at 50 Hz (50 bits per second)

• Contains binary ephemeris and almanac data, and other information needed to compute position and time from the GPS signals

• Full message consists of 25 frames of 1500 bits. Each frame is divided into 5 subframes of 300 bits each (6 seconds long)

Each handover word contains a time of transmission stamp for the first bit of the following subframe. These occur every six seconds.

Example receiver lock sequence (simplified)

Search for visible satellites – look for C/A code lock

When locked, receive ephemeris and other data

Obtain time of transmission ts from handover word and leading bit of following subframeSet receiver clock to approximately ts+70 ms

Latch C/A code phase with sub microsecond resolution, and resolve millisecond ambiguity by counting 1 ms code cycles to next data bit, and then count 20 ms data bits to leading bit of next subframe, and thus find time of reception tR according to receiver clock

Calculate pseudorange c(tR-tS)

Apply satellite clock corrections (from data message)

Calculate satellite coordinates (XS,YS, ZS) at tS (from ephemeris)

Apply corrections for relativistic effects, Earth rotation during signal transit time, L1/L2 phase offset, tropospheric and ionospheric delays (see ICD)

Solve for receiver position and receiver clock offset using measurements from at least four satellites

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Removal of GPS Selective Availability

UTC

(AU

S) –

GP

S (

nano

seco

nds)

Modified Julian Day

Tue May 2 2000~ 2 pm AEST

Start

dREF

REF – GPS = Counter reading + dREF – range/c– dant – dant_cable – drx_int – drx_cable

REF 1 pulse/sec

rang

e (~

2200

0 km

)

Wrinkles (i): Technical details

dantdant_cabledrx_int

drx_cable

Stop

ionospherediono

tropospheredtropo

– diono – dtropo

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Ionospheric delay diono

• GPS satellites orbit about 20200 km above the Earth’s surface• GPS signals must therefore traverse the entire atmosphere• The ionosphere reduces the propagation speed of the GPS signals (extra delay)

The ionospheric delay depends on the density of free electrons along the signal path, which depends strongly on:

• Geographic position• Time of day• Activity of the sun/solar wind

Corrections (of up to 100 ns) are obtained from either models or measurements. Most single frequency receivers use a model, based on parameters broadcast in GPS data.

Wrinkles (ii): Traceability“The property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties”

from Australian Standard 3807-1998Vocabulary of basic and general terms in metrology

Advantages• Greatly facilitates legal acceptance – eliminates spurious and costly

technical arguments• Associates the credibility of NMI with the measurements• Third party organisations (eg NATA) are available to certify compliance

using internationally accepted protocols and standards (eg ISO 17025)

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Traceability of GPS Time to UTC(AUS)GPS satellite

GPS TimeUTC ± 1µs; no leap seconds

UTC(AUS)International Bureau of Weights

and Measures (BIPM)

United StatesNaval Observatory (USNO)

GPS Master Control Centre

UTC

UTC(USNO)National Measurement Institute

Lindfield, Sydney

GPS common-viewtime-transfer system

UTC(AUS) – GPS Timeftp://time1.tip.csiro.au/pub/timedata

NMI GPS time-transfer system

LINUX PCData logging and processing

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NMI

Remote calibration

LINUX PCdataco

ntro

l

A Calibration Laboratory

Time transfer: comparing clocks using GPS

• Systems developed at NMI using custom hardware and software

• Used within Australia to deliver remote calibration to client laboratories

• Used by national measurement institutes throughout the Asia-Pacific

• Key contribution to NMI’s reputation in the region and around the world

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NMI

NTP server

LINUX PCdataco

ntro

lRb

NTP

121 ms62 ms

40 ms

74 ms40 ms

Disseminating time around Australia

One example:

• Network Time Protocol (NTP) used to synchronize clocks over a network

• But Australia is a big country; delays can easily reach a tenth of a second or longer

• NTP servers maintained across the country, to minimise latency

• Many registered users of this service

Another example:

• Speaking clock service

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Joint project with Geoscience AustraliaKey linkage between timing and geodesyContribution to Australia’s reference networkHigh-quality installation for time transfer

GPS geodetic station

Monitoring stations in the global network submitting geodetic data to the IGS

Did the Earth move?

Plate tectonic movement deduced from GPS observations at nodes of

the Australian reference network

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Two-Way Satellite Time Transfer

A – B = ½(∆A – ∆B) if delay A = delay B

two-way communication allows direct measurement of propagation delay

BA

∆B = B – (A+delay A)A + delay A

∆A = A – (B+delay B)

B + de

lay B

NIST, Fort CollinsTL, Taoyuan

NMI Sydney:4.6 m antenna20 W transmit powerMITREX modem

eg NSS 5, 183º E, C band (4/6 GHz)

Antenna platform at NMI Lindfield

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Developing tomorrow’s standards• Based on 171Yb+ ions held in an

electromagnetic trap and cooled with laser light

• Working towards an accuracy of a few parts in 1015, equivalent to measuring the distance to the moon with an uncertainty less than the width of a human hair

• Research like this can only be undertaken at national standards laboratories

• A key contribution to international metrology and fundamental science

ACES mission: atomic clocks in space (2010?)

International Space Station Columbus (ESA)

• Microgravity environment gives highest accuracy

• New joint project between NMI and the University

of Western Australia

• Participation showcases Australian technology

and expertise to the international community

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E1

E2

hEEf /)( 120 −=

An argument for ‘pure’ research

Atomic and molecular beam resonance experiments

New York Times Jan 21, 1945

Parry and Essen at NPL, 1955

Summary• Accurate standards of measurement underpin modern life, and demand is

continually growing

• Time is a special case, with particular challenges but a wide variety of applications

• GPS is a key technology, not only for providing one-way time but also for allowing common-view synchronisation over large distances

• Advances at the frontier of measurement enable novel applications

• NMI maintains and develops standards to meet Australia’s current and future needs, and our reputation for excellence in measurement contributes to Australia’s impact internationally