Review and Preview of Two-way Time Transfer for UTC- from TWSTT

1 to TWOTT

2

Bi-directional method is widely used in metrological measurements. By the symmetric principle, systematical errors are largely cancelled.

TWSTFT uses the reciprocated radio signals emitted by two ground laboratories through geo-satellite, the atmosphere delay effects are greatly reduced.

TWOTFT is also based on the reciprocity in both the signals to balance the propagation path delays in the fibres.

By investigating the historical documents and the latest developments, we briefly review and preview the two-way for UTC time transfers.

Key words: UTC, TWSTFT, TWOTFT

ID 520 EFTF2013 July Prague

-Ongoing-1999 – present [2,3]

-Oncoming-In the coming 5-10 years [3,4]

• Long-term goal: Compare the optical clocks ~10-18@one day• More than 14 UTC laboratories active• Already operational UTC(AOS)-UTC(PL) • Immediate Applications in UTC: - Validate the BIPM calibrator with uB ~ 100 ps by the fibre - Verify the accurate GNSS and TWSTFT techniques • New challenges: - the theoretical issues - the practical issues: data processing, format, programs …

II. Optical Fibre

Tsoft.lnk

Reciprocity DAB = DBA in Two-Way time transfer

1.Two-Way Satellite Time Transfer; 2. Two-Way Optical-fibre Time Transfer

Z. Jiang, H. Konaté and W. LewandowskiTime Department, Bureau International des Poids et Mesures (BIPM)

Pavillon de Breteuil F-92312, SÈVRES CEDEX, Francezjiang@bipm.org

-Early history-1960-1997 [1,3]

As a promising accurate time transfer technique , TWSTT started its experiments earlier than GNSS. -1960 Echo-I: One-way time transfer, not ideal due to the unknown propagation delays; -1962, Telstar: The first transatlantic two-way clock comparisons between USNO-NPL (US-UK)-1965 Relay II: The first transpacific two-way clock comparisons between USNO-RRL (US-Japan).The type A (uA) and B (uB) uncertainties were near 10~100 ns and 100~1000 ns respectively1970s ATS-1, 1983 Intelsat-V: uA reached 0.2 ns level.

RELAY 196xCommunication Satellite

TELSTAR 1962 1st Active Communication Satellite

ECHO-I first PassiveCommunication Satellite

NASA images

Report to the 5th TW WG in 1997 (hand-writing transparent slides probably from PTB) [2]

Conclusion: TWSTFT Earth-stations can be monitored to be stable to a: *TDEV of 100 ps for τ=24h-1200h (1d to 50d)* Freq. transfer at 2.10-15 at τ =1 day

Hardware technology developments reduced the costs and dimensions of the devices. UTC Labs could equip more automated and operational timing transfer system. Since 1987, NIST, USNO, NRC, TUG, NPL, NMi, VSL DTAG, PTB and OCA etc., began to perform routine experiments of three measurements per week using commercial Ku-band satellites.

The declaration of 11th CCDS/CCTF (1989) encouraged the use of TWSTT and suggested to create at BIPM an ad-hoc working group on TWSTT.

In 1992, several commercial satellite systems were available, and modems adapted for the technique were commercialized.

In March 1993, the BIPM ad-hoc Working Group was transformed to the permanent CCDS/CCTF Working Group on TWSTFT. Then the TDEV of hundred ps was attainable.

ITU (Internat. Telecommunication Union), approved in 1995 the standard data format for TWSTFT.

The first TWSTT link officially introduced in the UTC was TUG-PTB in Circular T 139 published on the 17 August 1999. GPS therefore finished its solo role for UTC time link for more than a decade. In 2000, there were four TW time links: TUG-PTB, VSL-PTB, NPL-PTB and USNO-NPL.

The international UTC time link network in 2000 TW links in/between Asia, Europe and America in 2005

The high accurate Europe-Asia transcontinental link between NICT and PTB

was established in 2005

Establishmentof the devices

at PTB in 2005

at NICT

Mobile TW calibrators

The UTC links must be calibrated. Several calibration campaigns were organized in and between Europe, America and Asia.

Calibration Period Laboratories/campaigns

May-June 2003 IT-PTB-IT

July 2004 PTB-OP-NPL-VSL-PTB

Oct.-Nov. 2005 PTB-SP-VSL-NPL-OP-IT-PTB

May-June 2006 TUG-PTB-CH-TUG

Sept.-Oct. 2008 PTB-NPL-OP-IT-VSL-CH-TUG

2012-2013 PTB,OP,SP,CH,AOS …

2 . 0 2 . 44 . 7 5 . 5 5 . 5 5 . 5

8 . 2

561 10 561 15 56 120 5 6125 5 6130 5 6135-40

-30

-20

-10

0

10

20

30

40

AOS-PTB 1207303 points, s=0.779 nsT

W-o

nly

/ AO

S-P

TB

/ ns

2 . 0 2 . 44 . 7 5 . 5 5 . 5 5 . 5

8 . 2

561 10 561 15 56 120 5 6125 5 6130 5 6135-40

-30

-20

-10

0

10

20

30

40

AOS-PTB 1207303 points, s=0.779 nsT

W-o

nly

/ AO

S-P

TB

/ ns

1 . 5 2 . 3 4 . 1 5 . 0 5 . 2 5 . 47 . 7

56 10 5 561 1 0 5 61 15 56 1 20 56 1 25 56 13 0 5 61 35 5 61 40-4 0

-3 0

-2 0

-1 0

0

1 0

2 0

3 0

4 0

AOS-PTB 1207468 points, s=0.042 ns No longer diurnal and gaps

TW

+P

PP

/ AO

S-P

TB

/ ns

1 . 5 2 . 3 4 . 1 5 . 0 5 . 2 5 . 47 . 7

56 10 5 561 1 0 5 61 15 56 1 20 56 1 25 56 13 0 5 61 35 5 61 40-4 0

-3 0

-2 0

-1 0

0

1 0

2 0

3 0

4 0

AOS-PTB 1207468 points, s=0.042 ns No longer diurnal and gaps

TW

+P

PP

/ AO

S-P

TB

/ ns

Diurnal and Data missing

30

40

60

100

170 220

340

TW+PPP

s x /

ps

600 590 610

340

250300

550

h/2 h d/8 d/4 d/2 day 3d week

3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8-10.6

-10.4

-10.2

-10.0

-9.8

-9.6

-9.4

-9.2

-9.0

TW

Averaging time

TDevAOS-PTB1207

30

40

60

100

170 220

340

TW+PPP 30

40

60

100

170 220

340

TW+PPP

s x /

ps

600 590 610

340

250300

550

h/2 h d/8 d/4 d/2 day 3d week

3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8-10.6

-10.4

-10.2

-10.0

-9.8

-9.6

-9.4

-9.2

-9.0

TW

Averaging time

TDevAOS-PTB1207

s x /

ps

600 590 610

340

250300

550

h/2 h d/8 d/4 d/2 day 3d week

3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8-10.6

-10.4

-10.2

-10.0

-9.8

-9.6

-9.4

-9.2

-9.0

TW

Averaging time

TDevAOS-PTB1207

The diurnals and the data missing are well repaired in the combined link. The Tdev is greatly improved

Combination of TW and GPS

The symmetry in the two-way transfer when the signals go through atmosphere or optical fibre

DAB

DBA

LabA /TALabB/TB

Net

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TT

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Y axis unit – nsX axis unit – UTCOn MJD 56454

The real-time clock comparison between UTC(AOS) and UTC(PL) through a fibre link,

www.optime.org.pl/node/47

First Operational Optical Fibre Time Link 420 km between UTC Laboratories AOS-PL

Combined uncertainty 112 ps [4]

Table 7.2 The first 25 lines of the proposed TW%TFT data file T%PTB56.150 in unit ns for delay and ps for statistical terms * tfptb56.150* FORMAT 01 * LAB PTB * REV DATE 2011-08-03 * ES PTB01 LA: N 52 17 49.787 LO: E 10 27 37.966 HT: 143.41 m * REF-FRAME ITRF * LINK 14 fibre: Dark Channel Length: 420.00 Km Amplifiers: 6 * OPTICAL-TX: 1552.1500 nm RX: 1552.1550 nm * MODEM: Dedicated hardware SIGNAL: 1 PPS on square wave * Link Stabilization: YES * LINK 16 fibre: AAA Network Length: 72.00 Km Amplifiers: 0 * OPTICAL-TX: 1542.1000 nm RX: 1542.1500 nm * MODEM: SATRE 037 SIGNAL: PRN, 20 Mcps * Link Stabilization: NO * CAL xxx TYPE: CAL xxx BRIDGED MJD: 55769 EST. UNCERT.: x.xxx ns* CAL 214 TYPE: CAL 141 BRIDGED MJD: 55769 EST. UNCERT.: 5.000 ns* CAL 213 TYPE: CAL 142 BRIDGED MJD: 55769 EST. UNCERT.: 1.300 ns* LOC-MON NO * COMMENTS unit in 0.1 ps*--- data body proposition (I)* EARTH-STAT LI MJD STTIME NTL TW DRMS SMP ATL REFDELAY RSIG CI S CALR ESDVAR ESIG TMP HUM PRES* LOC REM hhmmss s 0.1ps 0.1ps s 0.1ps 0.1ps 0.1ps 0.1ps 0.1ps C % mbar PTB01 TIM01 14 56150 000400 119 265739347023X 1226X 120 119 0000000040870X 0020X 999 9 999999999 1035000X 2800X 12 98 1013 PTB01 PTB01 14 56150 000700 119 266718670995X 2491X 120 119 0000000040870X 0020X 999 9 999999999 1035000X 2800X 12 98 1013 PTB01 OCA01 14 56150 001000 119 264311268059X 1497X 120 119 0000000040870X 0020X 999 9 999999999 1035000X 2800X 12 98 1013 PTB01 IT02 14 56150 001300 119 264702466195X 1937X 120 119 0000000040870X 0020X 213 1 479209X 1035000X 2800X 12 98 1013 PTB01 ROA01 14 56150 001600 119 260338922342X 2520X 120 119 0000000040870X 0020X 217 1 298673X 1035000X 2800X 12 98 1013

It is suggested adapting the ITU TWSTFT data format for TWOTFT [4]. Hence all the data exchanges, processing, calibrations, computations and the related methodology can be kept with only slight modifications. This will save huge time and man powers and speed up its applications.

TL 25 km fibre experiment at TL [4]

Acknowledgement and Reference

We thank all TW labs for their contributions to UTC and to this paper

1. http://tycho.usno.navy.mil/twstt_hi.html, history of TWSTFT2. http://www.bipm.org/wg/AllowedDocuments.jsp, documents of CCTF WG on TWSTFT back to 19893. Z. Jiang, H. Konaté and W. Lewandowski 2013 TM220, Review and preview of Two-Way Time and Frequency Transfer for UTC generation -history, present and future4. Śliwczyński Ł., Krehlik P., Czubla A., Buczek Ł., Lipiński M.: Dissemination of time and RF frequency via stabilized fibre optic link over the distance of 420 km, Metrologia, vol. 50, pp.

133-145, 2013

Summary TWOTFT, with a combined uncertainty of an order of hundred ps level, would provoke fundamental changes in the UTC construction. E.g. TWOTT requires a few instant to calibrate a satellite time link with 100 ps uncertainty. Another example, the configuration of the international UTC time transfer network. Since more than three decades, the UTC generation has been using the space-techniques. A new era of the ground based techniques is returning …

I. Use of Carrier phase and DPN The current TWSTFT observable is only the code. It is affected by measurement noise and diurnals hence limits in the accuracy. Further improvements should come from other observables, such as the DPN (dual pseudo-random noise) and Carrier Phase.

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The DPN allows doubly reducing the measurement uncertainty, in particular that from the diurnals. Because the resolution of the carrier phase is 100 to 1000 times more precise than that of the code, TWSTFT carrier phase transfer may reach a stabilities of 0.1 ps in time and 10-12@1s or 10-16@1-day in frequency.

Principle of TWTT

The third author at the CCTF TW WG 6th meeting San Fernando, 29 - 30 October 1998