GPS and Time - Engineers Australia · • GPS satellites orbit about 20200 km above the Earth’s...
Transcript of GPS and Time - Engineers Australia · • GPS satellites orbit about 20200 km above the Earth’s...
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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…
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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…
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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
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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
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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