GPS and Time: Using Clocks in Space for Accurate Time on the Ground

Transcription

1 GPS and Time: Using Clocks in Space for Accurate Time on the Ground Dr Bruce Warrington National 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 instruments that must be accurately calibrated: frequency counters, spectrum analysers, time-interval counters, frequency standards, delay generators, frequency synthesizers, phase meters, oscilloscopes 1

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 2

3 National Measurement Institute (NMI) Formed on 1 July 2004, amalgamating: CSIRO National Measurement Laboratory Australian Government Analytical Laboratories National 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 Inglis Chief Metrologist 3

4 Atomic frequency standards The second is the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. CCDS, 1967 E 2 f = ( E E1) / h 0 2 E 1 Parry and Essen at NPL, 1955 ~1 part in s in 300 years HP5071A ~1 part in s in years NIST F1, 1999 ~1 part in s in years Primary standards at NMI 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 10 12, equivalent to measuring a mass of half a tonne to the nearest microgramme 4

5 International Atomic Time (TAI) average TAI 1 second ticks >250 clocks worldwide Bureau International des Poids et Mesures, Paris International Atomic Time 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 time average 5

6 International timekeeping Bureau International des Poids et Mesures, Paris 350 >250 atomic clocks worldwide TAI UTC UTC UTC(AUS) [ns] Circular T MJD The Solution A master clock: stable, accurate, high-integrity, legal status A means of synchronising clocks: tolerance, separation, interconnections 6

7 Principle of Time Transfer: the 1 o'clock gun 4.7 km Correction for the speed of sound required: ~350 m/s or ~3 s/km 1 s accuracy requires the propagation distance to be known within 350 m 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: s = 13:00:23.3 as the cannon fired His clock is 23.3 s fast this is his clock error, REF BANG Principle of Common-View Time Transfer REF A =01:01:05 REF A BANG=65s fast REF B =01:00:45 REF B BANG=45s fast (REF A BANG) (REF B BANG) = REF A REF B = 20s fast 7

8 TV Sync pulse Common-View TV Sync Time Transfer A TV B A = REF A TV B = REF B TV REF A REF B = (REF A TV) (REF B TV) 8

9 Measuring REF TV REF TV = Counter reading + d REF range/c d receiver d cable Start d REF Stop d cable range (<100 km) Separator Receiver Antenna REF 1 pulse/sec d receiver Principle of GPS common-view time transfer GPS satellites transmit: Timing pulses Orbital data A B A = REF A GPS B = REF B GPS REF A REF B = (REF A GPS) (REF B GPS) common view GPS time transfer: same satellite tracked from both A and B 9

10 Measuring REF GPS REF GPS = Counter reading + d REF range/c d ant d ant_cable d rx_int d rx_cable Start d REF Stop d rx_cable range (~22000 km) REF 1 pulse/sec d rx_int d ant_cable d ant GPS Space Segment 24 satellites in 6 orbital planes Satellites ~20200 km above Earth s surface ~12 hour orbits (11 hours 58 minutes) From NAVSTAR GPS User Equipment Introduction Each satellite follows the same track in the sky as seen from a point on earth every 23 hours 56 minutes 10

11 GPS satellites Each GPS satellite: Incorporates redundant atomic clocks Transmits on two frequencies, L1 ( MHz) and L2 ( 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 Speed of light c km = metres/second ~ 3 nanoseconds/metre Signal transit time m / c s To make this correction accurately, you need to know: Coordinates of satellite Coordinates of receiver 11

12 Calculation of time from a GPS satellite (X S, Y S, Z S ) Timing signal transmitted at time t s by satellite clock Arrives at receiver at time (according to receiver) t R = t s + d/c + δ R where δ R = receiver clock error Distance d Receiver decodes signal and finds that it left the satellite at t s, and using broadcast ephemeris calculates satellite position at that time: (X S, Y S, Z S ) If receiver coordinates (X R, Y R, Z R ) are known, receiver determines that its clock error δ R is (X R, Y R, Z R ) d δ R = tr ts c 1 = tr ts c ( X X ) 2 + ( Y Y ) 2 + ( Z Z ) 2 S R S R S R GPS spread-spectrum signal structure C/A code: Frequency hops 1023 times each millisecond; pattern repeats every millisecond P code: Frequency hops 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 NAVSTAR GPS User Equipment Introduction 12

13 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 t s from handover word and leading bit of following subframe Set receiver clock to approximately t s +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 t R according to receiver clock Calculate pseudorange c(t R -t S ) Apply satellite clock corrections (from data message) Calculate satellite coordinates (X S,Y S, Z S ) at t S (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 13

14 Removal of GPS Selective Availability UTC(AUS) GPS (nanoseconds) Tue May ~ 2 pm AEST Modified Julian Day Wrinkles (i): Technical details REF GPS = Counter reading + d REF range/c d iono d tropo d ant d ant_cable d rx_int d rx_cable Start d REF Stop d rx_cable ionosphere d iono troposphere d tropo range (~22000 km) REF 1 pulse/sec d rx_int d ant_cable d ant 14

15 Ionospheric delay d iono GPS satellites orbit about 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 Vocabulary 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) 15

16 Traceability of GPS Time to UTC(AUS) GPS Time UTC ± 1µs; no leap seconds GPS satellite GPS Master Control Centre UTC(USNO) United States Naval Observatory (USNO) National Measurement Institute Lindfield, Sydney GPS common-view time-transfer system UTC International Bureau of Weights and Measures (BIPM) UTC(AUS) UTC(AUS) GPS Time ftp://time1.tip.csiro.au/pub/timedata NMI GPS time-transfer system LINUX PC Data logging and processing 16

17 Remote calibration NMI A Calibration Laboratory control data LINUX PC 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 17

18 NTP server NMI control data NTP LINUX PC Rb Disseminating time around Australia One example: Network Time Protocol (NTP) used to synchronize clocks over a network 121 ms 62 ms 40 ms 74 ms 40 ms 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 18

19 GPS geodetic station Joint project with Geoscience Australia Key linkage between timing and geodesy Contribution to Australia s reference network High-quality installation for time transfer 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 19

20 Two-Way Satellite Time Transfer B + delay B A + delay A A B A = A (B+delay B) B = B (A+delay A) A B = ½( A B ) if delay A = delay B two-way communication allows direct measurement of propagation delay eg NSS 5, 183º E, C band (4/6 GHz) TL, Taoyuan NIST, Fort Collins NMI Sydney: 4.6 m antenna 20 W transmit power MITREX modem Antenna platform at NMI Lindfield 20

21 Developing tomorrow s standards Based on 171 Yb + ions held in an electromagnetic trap and cooled with laser light Working towards an accuracy of a few parts in 10 15, 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 21

22 An argument for pure research E 2 f = ( E E1) / h 0 2 E 1 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 22