2017.06.25

Minutes of the 25th Meeting of the CCTF Working Group on TWSTFT

18-19 May, 2017

Host: National Time Service Center (NTSC), Chinese Academy of Sciences Conference Site: Elegant Oriental Hotel, Lintong, Xi’an, Shaanxi

（陕西省西安市临潼区凤凰大道 6 号，雅致东方酒店）

Chairman: Victor ZHANG (NIST) Executive Secretary: Zhiheng JIANG (BIPM)

Reporter/: Calvin LIN (TL) Local organizer: Wenjun WU (NTSC)

Edited by Zhiheng JIANG

Contenu Opening Ceremony .................................................................................................................................................................... 3

Agenda: ................................................................................................................................................................................. 3 Session I 8h00-11h30 and 13h00-17h30, 18 May ..................................................................................................................... 4

Welcome speech by Dr. Shougang ZHANG, Director of NTSC ........................................................................................... 4 Opening speech by Felicitas E Arias ..................................................................................................................................... 4 Designation of reporter: Calvin ............................................................................................................................................ 4 1.1 Laboratory reports ........................................................................................................................................................... 4

AOS (Jerzy NAWROCKI), .............................................................................................................................................. 4 BIRMM (Shengkang ZHANG) ........................................................................................................................................ 4 METAS/CH (Christian SCHLUNEGGER, presented by Victor ZHANG) ...................................................................... 5 GUM (Albin CZUBLA, presented by Jerzy NAWROCKI) ............................................................................................. 5 INRIM/IT (Ilaria SESIA/Tung Thanh THAI, authors absent, not presented) ................................................................... 5 NICT (Miho FUJIEDA) ................................................................................................................................................... 5 NIM (Zhiqiang YANG) .................................................................................................................................................... 5 NIST (Victor ZHANG)..................................................................................................................................................... 6 NTSC (Wenjun WU/Hong ZHANG) ............................................................................................................................... 6 OP (Joseph ACHKAR) ..................................................................................................................................................... 6 PTB (Dirk PIESTER) ....................................................................................................................................................... 7 ROA (Hector ESTEBAN) ................................................................................................................................................ 7 SP (Carsten RIECK) ......................................................................................................................................................... 7 TL (Yi-Jiun HUANG) ...................................................................................................................................................... 7 USNO (Angela MCKINLEY/Russell BUMGARNER, not presented) ............................................................................ 8 VNIIFTRI/SU (Andrey NAUMOV) ................................................................................................................................. 8 VSL (Erik DIERIKX, presented by Zhiheng JIANG) ...................................................................................................... 8

1.2 Network status ................................................................................................................................................................. 9 Status of the Europe-Europe and Europe-USA links (Victor ZHANG/Dirk PIESTER) ................................................... 9 Status of the Eu-Asia link (Andrey NAUMOV/Dirk PIESTER) ...................................................................................... 9 Status of the Asia-Asia and Asia-US links (Miho FUJIEDA) .......................................................................................... 9

1.3 Calibration and long-term variation in UTC time links ................................................................................................... 9 Europe TWSTFT and GNSS calibrations (Hecter Esteban) ............................................................................................. 9 Mobile calibration station and calibrations (Wolfgang Schaefer) ..................................................................................... 9 Improving the GNSS time transfer by an ensemble of the GPS receivers (Calvin S.Y. LIN) .......................................... 9 The draft GNSS recommendation to CCTF (Victor ZHANG/Zhiheng JIANG) ............................................................. 10

1.4 Discussion ..................................................................................................................................................................... 10 The development of the TWSTFT CP using NICT digital modem (Miho FUJIEDA) ................................................... 10 News and developments on TWSTFT equipment (Wolfgang SCHAEFER) .................................................................. 10

1.5 TWSTFT SDR installation and operation ...................................................................................................................... 10 Update on the TWSTFT SDR software (Yi-Jiun HUANG) ........................................................................................... 10 Reports and discussions of SDR TWSTFT operation by the TW Labs .......................................................................... 11 AOS, IT, KRISS, NICT, NIM, NIST, NTSC, ................................................................................................................ 11

Session II 8h30-11h30 and 13h00-14h30, 19 May ....................................................................................................... 11 2.1 The pilot study on the TWSTFT SDR for UTC computation (8h30-9h30) ................................................................... 11

SDR for UTC – the BIPM analysis (Zhiheng JIANG) ................................................................................................... 11 Approval of the TWSTFT SDR for UTC time links – Recommendation to CCTF (Victor ZHANG) ........................... 12

2.2 Other issues (10h00-11h00) ........................................................................................................................................... 12 TWOTFT in Europe (Jerzy NAWROCKI) ..................................................................................................................... 12 TWOTFT in NTSC（Tao LIU） ................................................................................................................................... 12 TWSTFT application: The Technology of Orbit Determination by Transfer Tracking (ODTT) (Xuhai YANG/Fen CAO) ........................................................................................................................................................................................ 12 CAPS time and frequency transfer (Wenfang JING/Shuo LIU) ..................................................................................... 12 On the TWSTFT 26thAnnual Working Group Meeting 2018 (Albin CZUBLA/Jerzy NAWROCKI) ............................ 13

2.3 Laboratory tour (13h00–14h30)..................................................................................................................................... 13 Road Map – Implementation of the SDR in UTC ................................................................................................................ 13 Attendees of the 25th Meeting of CCTF Working Group on TWSTFT ................................................................................... 14 

OpeningCeremony

Agenda:Session I    18 May    (8:00‐11:50 ,  13:00‐17:30) 

8:00-8:15 Welcome speech by Dr. Shougang ZHANG, Director of NTSC 8:15-8:30 Opening presentation “The evolution of the UTC time transfer techniques” by Dr. Felicitas E

ARIAS on behalf of CCTF, Director of Time Department BIPM. 8:30-10:00 Laboratory reports by:

n° Participants 1 AOS (Jerzy Nawrocki) 2 BIRMM (Shengkang ZHANG) 3 CH (Christian Schlunegger, presented by Victor Zhang) 4 GUM (Albin Czubla, presented by Jerzy Nawrocki) 6 NICT (Miho Fujieda) 7 NIM (Zhiqiang YANG) 8 NIST (Victor ZHANG) 9 NTSC (Wenjun WU/Hong ZHANG) 10 OP (Joseph ACHKAR) 11 PTB (Dirk PIESTER) 12 ROA (Hector Esteban) 13 SP (Carsten Rieck) 14 SU (Andrey Naumov) 15 TL (Yi-Jiun Huang) 16 USNO (Angela MCKINLEY/Russell BUMGARNER) 17 VSL (Erik Dierikx, Jerzy Nawrocki by Victor Zhang)

10:30-11:50 Network Status 1. Status of the Europe-Europe and Europe-USA links (Victor ZHANG/Dirk PIESTER) 2. Status of the Europe-Asia link (Andrey NAUMOV/Dirk PIESTER) 3. Status of the Asia-Asia and Asia-US links (Miho FUJIEDA)

13:00-14:00 Calibration and long-term variation in UTC time links 1. Europe TWSTFT and GNSS calibrations (Hecter ESTEBAN) 2. Europe TWSTFT calibration in 2017 and 2018 (Wolfgang SCHAEFER) 3. Improving the GNSS time transfer by an ensemble of the GPS receivers (Calvin S.Y. LIN) 4. The draft GNSS recommendation to CCTF (Victor ZHANG/Zhiheng JIANG)

14:00-15:00 Discussion 1. The development of the TWSTFT CP using NICT digital modem (Miho FIJIEDA) 2. News and developments on TWSTFT equipment (Wolfgang SCHAEFER) 3. Others

15:30-17:50 TWSTFT SDR installation and operation 1. Update on the TWSTFT SDR software (Yi-Jiun HUANG) 2. Reports and discussions of SDR TWSTFT operation by (AOS, IT, KRISS, NICT, NIM, NIST, NTSC,

OP, PTB and VNIIFTRI etc.) Session II    19 May    (8:30‐11:30 , 13:00‐14:30)  8:30-9:30 Pilot study on the TWSTFT SDR for UTC computation

1. SDR for UTC – the BIPM analysis (Zhiheng JIANG) 2. On the Recommendations to CCTF 2017 on TWSTFT (Victor ZHANG)

10:00-11:30 Other Issues 1. TWOTFT in Europe (Jerzy NAWROCKI) 2. TWOTFT in NTSC (Tao LIU) 3. TWSTFT Application：the technique of orbit tracking and determination by transfer (Xuhai YANG) 4. High Precision Time Comparison Technique Based on CAPS (Wenfang Jing and Shuo LIU) 5. On the TWSTFT 26th Annual Working Group Meeting 2018 (Jerzy NAWROCKI)

13:00-14:30 Visit NTSC 1. Museum of Time Science 2. Time keeping laboratory 3. Quantum frequency standards division

SessionI 8h00‐11h30and13h00‐17h30,18May

WelcomespeechbyDr.ShougangZHANG,DirectorofNTSC

Dr. ZHANG opens the meeting with a welcome address to all participants.

OpeningspeechbyFelicitasEArias

Direction of Time Department BIPM on behalf of CCTF

Title: On the evolution of time transfer techniques used for UTC -- since I joined the BIPM

Dr. Arias concisely and forcibly introduced the formation, generation, and improvements of UTC links and UTC, her speech includes people, review and outlook:

- From 1999 to 2017, the UTC clocks time comparison started via GPSCV links. After 2001, TWSTFT (2001) then TWOTT links increased the redundancy of UTC computation.

- The improvements of time transfer techniques such as GNSS all in view, GPSPPP, and TWGPPP let uA of UTC links achieve sub-ns level.

- Uncertainty study and the development of calibration improved uB of UTC links. - Dr. Arias commends those people who erected the milestones, Dr. G. de Jong, Dr. Dieter

Kirchner, Dr. J. Azoubib, Dr. W. Lewandowski, Mr. M. Imae, Dr S. Leschiutta, Dr W.J. Klepczynski, and Dr. Dirk Piester; furthermore Dr. Arias appreciates the contribution of Chinese researchers, especially mentions Mr. Victor Zhang and Dr. Zhiheng Jiang.

- Institutionalization and frequent GNSS and TWSTFT calibration campaigns improved the stability and accuracy of UTC links. The uncertainty of UTC-UTC(k) now achieve 1-2 ns.

- New studies such as the development of SDR receiver and optical fibre time transfer keep improving the performance of UTC links and UTC. The total uncertainty of UTC-UTC(k) may achieve sub-ns in near future.

Designationofreporter:CalvinMr. Calvin Lin accepted the duty as the reporter and to prepare the minutes of the meeting.

Approval of the agenda: the proposed agenda was accepted without changes.

1.1Laboratoryreports

AOS(JerzyNAWROCKI),Time and frequency activities at AOS, including equipment, time scale, time transfer, PFS and optical clock development:

- AOS maintains 2 5071a cesium clocks, 2 hydrogen masers, and 1 cesium fountain clock AOSF1 (constructed at NPL). The second cesium fountain will be set up around September 2017.

- TA(PL) is generated by 16 cesium clocks and 4 hydrogen masers in 7 labs, consists of 3 fibre optical links and 4 GNSS time comparison links. New realization of UTC(AOS) introducing the AOSF1 is examining and will be implemented in future.

- Time transfer system includes 4 regular GNSS stations, 4 fibre optical links, and one TW station, the new installed SDR receiver was complete and started operation).

BIRMM(ShengkangZHANG)Briefly introduces the tasks of BIRMM and reports the status of time scale, TWSTFT stations and new development all digital modem:

- The clock ensemble of BIRMM consists of 3 HM and 2 CS clocks. Further development of space borne Rb and Hm clocks for BeiDou system is proceeding.

- BIRMM uses GNSS time comparison as UTC link. BIRMM has four TWSTFT earth stations, one with 1.8 m antenna is for domestic TW link and the other one with 1.8 m antenna is ready for UTC link. 2 stations with 1.2 m antennae are used for experiment and test.

- BIRMM keeps working on the development of the all-digital, 6 channels TWSTFT modem. The preliminary results showed the 1 and 2.5 MChip/s DCD result are 0.26 and 0.13 ns (1 σ).

- Furthermore, investigations are carried out on different modulation schemes such as dual pseudo-random noise codes (DPN) and flexible binary offset carrier (FBOC) modulation.

METAS/CH(ChristianSCHLUNEGGER,presentedbyVictorZHANG)Reports the jump event of UTC(CH), furthermore report the installation and preliminary results of their SDR receiver:

- There is about 7 ns time jump of UTC(CH) on MJD 57848, due to the master reference source HM made a large frequency step. METAS switched to backup Master Clock on MJD 57849.

- The un-calibrated SDR system is finished installation and kept operating. Respect to SATRE modem, the results of SDR receiver is obviously more stable, but not for CH-NIST SDR link. METAS thanks TL team for the help of SDR.

- Together with other laboratories, METAS is interested in performing a new calibration of TWSTFT link.

GUM(AlbinCZUBLA,presentedbyJerzyNAWROCKI)Introduces the new TWSTFT station of GUM and its link:

- A new TW station located at National Institute of Telecommunications (NIT) link to Asia labs via AM-22 satellite is ready to perform tests and start continuous operation.

- A 28 km dark fibre link transmits the UTC(PL) (generated by GUM) to NIT.

INRIM/IT(IlariaSESIA/TungThanhTHAI,authorsabsent,notpresented)Reports the current configuration of INRIM, including their 2 TWSTFT stations, calibration, and the new installation of SDR receiver: - INRIM operates two TWSTFT stations, IT02 and IT01, equipped with SATRE modems. The

nominal station IT02 will be switch to IT01 in the end of 2017 due to the failure of some Rx module of IT02. Both stations were calibrated in 2016 Galileo Calibration Campaign.

- SDR receiver installed with OP support and running since 29/10/2016 (MJD 57690).

NICT(MihoFUJIEDA)She introduces the organization change of NICT time department, and report the status of Japan Standard Time (JST) and current developing time and frequency transfer system. She also reports the current status of Asia-North American TWSTFT link.

- The time keeping and time transfer department of NICT now belongs to Space-Time measurement group. The group leader was changed from Dr. Yoko Honado into Dr. Tetsuya Ido.

- The complete JST clock ensemble includes the clocks in NICT headquarter, Kobe sub-station, Fukushima LF station and Kyushu LF station. All distributed clocks are linked via GPSTT and TWSTFT.

- NICT keep developing the In+ and 87Sr optical clock standard. A real time physical clock steered by a 87Sr lattice clock has continuously run over 5 months.

- NICT will start G2 calibration campaigns (3-4 G2 labs, including KRISS and NMIJ) in 2017 fiscal year.

- Operation of Hawaii relay station (KPGO) stopped in September 2016 due to the end of QZSS project.

- NIST already tested the satellite ABS-2A at 04/21 2017, the C/No is low but still possible to operate TWSTFT. Dr. Fujieda request PTB and SU to increase of Tx power (minima 3 dB) for next test.

- NICT-KRISS TWCP test starts from Dec 2016 and is coexistent with SATRE TWSTFT. The disagreement with respect to PPP is about 3.810-16 and -0.4310-16 with respect to IPPP.

- The developing of a FPGA-base, cost effective all digital modem is almost done, NICT will organize a long base line test with TL and KRISS. The test will start around early 2018.

NIM(ZhiqiangYANG)He introduces the organization and facilities of time and frequency division of NIM, also reports the domestic and international TWSTFT results especially for the new installed SDR system. At final Dr. Yang mentions the next working plan of NIM.

- NIM is active in the development of a Cs fountain clock and an optical clock. Current operated Cs fountain NIM5 with an uncertainty of 210-15, and next generation of fountain clock will be better than 210-15.

- The time keep laboratory of NIM is equipped with: 6 Symmetricom H-masers, 4 Vremya H-masers and 7 cesium clocks.

- For time transfer NIM is equipped with TWSTFT and GPS. (NIM is a G1 laboratory for GPS receiver calibrations.)

- There are two TWSTFT stations at NIM: NIM01 is used for time links via AM22 and NIM02 is used for domestic links via China Sat-10. NIM02 has a temperature controlled box on the rooftop to maintain the transceiver at an ambient with 25 °C.

- TWSTFT links via AM22 started from February 2016. Results are shown from the links NIM-PTB, NIM-NTSC and NTSC-PTB. The TWSTFT links from NIM via satellite AM22 have been calibrated between 6 and 13 July 2016 by means of the BIPM calibrators. The calibration loop has not yet been completed.

- NIM setup and operated SDR receiver since Feb 2017 with the help of Yi-Jun Huang of TL. The ITU format files and 1 s data files are uploaded into BIPM regularly.

- In the future NIM will test the new satellite of Europe-Asia link and rebuild the China Sat-10 link.

- Comment by Victor Zhang (or Dirk Piester?): reminds NIM to check the 0.5 second time tag shift inside SATRE modem which possible cause the huge diurnal in NIM01-SU01 TWSTFT results.

NIST(VictorZHANG)

- NIST has one operational TWSTFT station with a 3.7 m antenna, used for transatlantic links. A second station with 2.4 m antenna is being set up on the PML building and will be used to link to USNO.

- NIST has 4 SATRE modems: one as primary, the others are used as back-ups and experiments. - SDR system has been in operation since September, 2016. It is configured to simultaneously

receiver AOS, CH, IT, OP, PTB, ROA, SP and VSL signals. Links have established between NIST and AOS, CH, IT, OP. The short-term (averaging time from 2 to 4 hours) noise is decreased in SDR TWSTFT links, but no significant diurnal reduction from SDR TWSTFT.

- On April 28, 2016, the calibration results (TM256) of the NIST to Europe links have been implemented. This calibration was performed with the BIPM travelling station in 2014-2015.

- An independent verification has been performed on June 6, 2016 by means of the USNO mobile station (SUV) calibration of UTC(NIST)-UTC(USNO). The results of this verification confirm the results of BIPM calibration.

NTSC(WenjunWU/HongZHANG)Wenjun Wu reports the current status of TWSTFT equipment and activities, including the calibration of PTB-NTSC TWSTFT link and the results of SDR receiver links.

- NTSC has four TWSTFT stations: NTSC01 is used for domestic links on satellite ZHONGXING10, in the Ku-band with the SATRE modem. NTSC02 is used for Europe and Asia link via satellite AM22, in the Ku-band with the SATRE modem and the SDR. NTSC03 is a C-band system. NTSC04 is a Ku-band system with a SATRE modem and is still under construction.

- The link between NTSC and PTB via AM22 has been active since February 2016. - From 19 to 31 July 2016 BIPM travelling GPS calibrators visited NTSC to calibrate the TW

links NTSC-PTB and NTSC-NIM. - From 19 to 29 July 2016 Y. J. Huang visited NTSC to install the SDR system. SDR TW links

are already setup to connect to SU, PTB, and NIM. Until MJD 57832, the short-term noise decreased PTB-NTSC and SU-NTSC SDR links but no significant diurnal reduction.

- Comment by Victor Zhang/Dirk Piester: Reminds NTSC to check the 0.5 second time tag shift inside SATRE modem which possible cause the huge diurnal in NIM01-SU01 TWSTFT results.

OP(JosephACHKAR)He reports the status and activities of GNSSTT and TWSTFT at OP. He also reports the results of SDR receiver, ABS-2A satellite line up test, calibration campaign, and the study of long term discrepancy between GNSSTT and TWSTFT:

- OP owns 5 geodesic receivers and 4 travelling receivers. 3 geodesic receivers were calibrated during 2016 BIPM G1 campaign.

- The SDR system is installed and regular operated with collaboration of TL in the frame of the WG/BIPM pilot project.

- The line-up test of ABS-2A at 75 °E (candidate link to Asia) is finished, the result is not good due to the elevation is too low.

- The P3AV-TWSTFT DCD of OP-PTB has a slope about 6 ns/2200 days. The Refdelay change caused a time jump in UTC(OP) of 4.2 ns.

- OP re-derived the TWSTFT formula, ITU-R TF1153.4, and concluded that the introduced parameter REFDELAY(lab) should be defined without ambiguity and with respect to the ITU-R TF1153.4.

- OP (Paris) and LNE-LTFB (Besançon) want to participate in 2017 calibration campaign, and may add the second station OP02 in the calibration campaign schedule (if possible).

PTB(DirkPIESTER)The updates in the time and frequency activities and TWSTFT at PTB.

- The TWSTFT station PTB01 was replaced by PTB05. PTB05 station has 2.4 m antenna and the temperature stabilized outdoor unit.

- Calibration transfer from PTB01 to PTB05 by adjustment of ESDVAR(PTB05) from common clock data PTB01-PTB05. All CALR values and CIs to remote links remain the same. The calibration adjustment is already applied for all links. (Comment by Victor Zhang: the performance of PTB05 is better than PTB01 due to its temperature control.)

- PTB03 is still used for Asia links and PTB04 is a spare station which used for the tests of ABS-2A satellite link.

- The calibration of USNO-PTB TWSTFT link is planned at June 2017. - PTB will contribute to AM-22 transponder lease for June. Further operation beyond June is

unsure. - PTB thanks to TL guys (Y.J. Huang) for the help of SDR installation.

ROA(HectorESTEBAN)Simply introduces the time and frequency equipment of ROA and reports the 2016 calibration campaign of Europe TWSTFT links:

- 2 new operative TW stations by summer 2017, backup each other - TW calibration campaign from 4/4 to 3/6, 5 UTC(k) sites PTB, ROA, INRIM (calibrate 2

station IT01 and IT02), OP, and SP participate the tour. - Instead of the site-based one calibration, the campaign used baseline-based TW calibration

method; the fixed reference station and mobile station are provided by TimeTech. - In general, the combined uncertainty for TW links is about 1 ns (involving IT01 and IT02),

Hector Esteban also reports the CALR variation since 2014.

SP(CarstenRIECK)He introduces the new organization of SP, and report the TWSTFT and TWOTT activities. Furthermore he reports the passive TW time dissemination system of SP.

- In the beginning of 2017, SP changed name into RISE, Research Institutes of Sweden. The RISE mission is also a NMI mission.

- SP has one TW station and no significant changes were introduced since the previous meeting. The hardware of SDR is ready and will be installed soon.

- Carsten Rieck reports the analysis of UTC(PTB)-UTC(SP)-UTC(ROA) TWSTFT closures, using Kalmar filter to estimate the diurnal and time scale difference. Another Kalman filter is applied for near real time processing of TW data using RT streams from PTB, NIST, ROA and SP. The results are published on: http://igsrt.sp.se/TWSTFT

- A White Rabbit time transfer experiment setup of a P2P WR-LEN pair over 3 km is on-going. Its long term stability can achieve 10-16 or better.

- For the IGS SPT0 station, the antenna was replaced because its internal LNA showed significant temperature dependence. This affects all SP GPS receiver contributing to the TAI computation.

- The study of passive TWSTFT is still on-going. With improving the orbit maneuvers of TW satellite, the time difference of the passive station with respect to the active stations can be determined. SP will start a passive TW experiment cover Europe and North America, and inquire the participation labs. (Comment by Victor: how about sending an email (proposal) to EU/US colleagues for discussion (the joint program of passive TW))

TL(Yi‐JiunHUANG)Introduces lab staffs of TL, time scale and time transfer equipment. Furthermore reports their recent studies of GNSSTT and TWSTFT and the future works of TL.

- For the realization of UTC(TL), the laboratory is equipped with 11 cesium clocks (5071a) and 4 active H-masers.

- TL has four TWSTFT station: the TL03 is the main Ku-band station for Asia–Pacific links with KRISS and NICT via satellite EutelSat 172A (SATRE and SDR). The other stations TL01, TL02, and TL04 are idle and used as backup stations.

- TL will not join ABS-2A due to the poor footprint coverage and can’t pass the line up test. Furthermore, the KPGO station was stopped on 31 august 2016, so that the link to North America is also no-longer active. TL will work for finding a direct or relay link to Europe labs in the next year.

- The studies of long-term instability between GNSSTT and TWSTFT are kept on-going. The development of ensemble GNSS system and the investigation of propagation time correction on earth's orbital motion for TWSTFT may be the alternative way to figure out the long term behaviour of time transfer.

- Furthermore, TL is investigating the diurnals effects that may be caused by outdoor devices in the earth stations.

- TL will keep supporting the diffusion of SDR under the BIPM pilot study group (PSG) by upgrading the SDR receiver software and (if necessary) visiting laboratories to help them to install the SDR equipment and software.

USNO(AngelaMCKINLEY/RussellBUMGARNER,notpresented)USNO person cannot give a formal report without the permission of USNO. The following is extracted from personal communication and agreed by the authors:

- Angela and Rusell are the contact persons hereafter on TWSTFT issuers at USNO. - USNO continues participating in the Europe-USA TWSTFT and the calibration of the

PTB/USNO link once every year. - USNO calibrates UTC(NIST)-UTC(USNO) once or twice every year since 2009. - USNO will install the SDR system and participate in the SDR TWSTFT soon.

VNIIFTRI/SU(AndreyNAUMOV)Andrey Naumov introduces the new equipments and the TWSTFT stations of VNIFTRI and Khabarovsk. Furthermore reports the absolute calibration campaign from VNIIFTRI to PTB.

- VNIIFTRI installed 4 New Active H-masers and two 8-Channels Phase Comparator. 4 Rb fountains were setup and will join the clock ensemble. A 87Sr lattice clock is operating and the uncertainty budget is less then 1E-16.

- New station in far east branch of VNIIFTRI, KM01, was completely setup and start joined to AM-22 links from March 2017, ITU files available in: ftp://ftp.vniiftri.ru/Atomic_Time/Km/links/TWSTFT/

- TWSTFT station SU01 joined to the Europe-Asia AM-22 link in December 2016. Links with PTB, NIM, NTSC, KM since March 2017. SDR receiver installed to the SU01 station and links to PTB, NTSC, and NIM from February 2017.

- TWSTFT station SU03 is idle due to its Tx part failed. Another station SU02 is a position for parking mobile station.

- A GTR51 absolute calibration campaign from VNIIFTRI to PTB and back to VNIIFTRI had been carried out at April 24-28, 2017. The data is processing and will be prepared after that the official report to BIPM.

VSL(ErikDIERIKX,presentedbyZhihengJIANG)An overview is given of the staff and most relevant facilities for time and frequency. VSL Reports activities on TWSTFT, TWOTFT, and EGNSS robust timing, also calibration status and the instability of VSLTWSTFT links.

- VSL is involved in the EMPIR SIP project TIMEFUNC, exploring the use of White Rabbit time and frequency transfer for telecom applications. A Delft-Amsterdam white rabbit link was setup and continuously operating.

- VSL observed significant improvements in the VSL-PTB and VSL-CH links. The fluctuation in the DRMS values for VSL-PTB and VSL-CH had been significantly reduced after MJD 57883 and 57843. VSL raises a question why these 2 links improved so much.

- The GPS station VSLF and VSLG were calibrated by PTB travelling receiver at 2016. The TWSTFT station VSL01 is scheduled (to be confirmed) to be calibrated by TimeTech mobile TW station at 2018.

1.2Networkstatus(about 5-10 minutes/each, 10h30-11h50)

StatusoftheEurope‐EuropeandEurope‐USAlinks(VictorZHANG/DirkPIESTER)- The links use three Ku-band transponders on the T-11N satellite. The contract with RiteNet

Corp is signed by NIST and PTB. - The aim is to get a multi-year contract structure, starting with a 6 month base period from

November 27, 2016 till May 26, 2017, and then four optional periods for renewal in 2017-18, 2018-19, 2019-20 and 2020-21.

- Cost estimates have been requested for three bandwidth options: 4.9 MHz, 7.6 MHz and 11.2 MHz.

StatusoftheEu‐Asialink(AndreyNAUMOV/DirkPIESTER)- The orbit of AM-22 inclined since April 2017, its data quality became worse both in Satre and

SDR results. - The line-up tests of ABS-2A were performed at February and April, 2017. 1.0 Mcps

Participant labs are PTB, VNIIFTRI, NICT, NTSC, and TL. The results showed TW sessions is possible with PTB, VNIIFTRI (SU and KM), NTSC, NIM (not tested); NICT looks like possible too; KRISS need to check by transmission from KRISS; NPLI need to check by transmission from NPLI; TW sessions is not possible with TL and OP.

- Except ABS-2A and AM22, Yamal 401 at 90E + Yamal 402 at 55E can be another possible candidate for Europe-Asia TWSTFT links. VNIIFTRI and some Asian stations can be covered by both two satellites. The Intersputnik agreed to provide the line-up test for both satellites. The lease price is about $2000/1MHz/Month/satellite, Andrey will in charge of all stuff.

- Comment by Jerry: can we look for 2 transponders in one satellite cover all stations? - Comment by Joseph: use 2 satellites and each satellite use single transponder will be better for

ranging.

StatusoftheAsia‐AsiaandAsia‐USlinks(MihoFUJIEDA)- Main issues are contented in the status report of NICT by Miho Fujieda. - Operation of Hawaii relay station (KPGO) stopped in September 2016 due to the end of QZSS

project. And the lease of Eutelsat 172a will be continued.

1.3Calibrationandlong‐termvariationinUTCtimelinks(about 10-15 minutes/each, 13h00-14h00)

EuropeTWSTFTandGNSScalibrations(HecterEsteban)Hecter Esteban reports the latest Europe TWSTFT calibration campaign:

- TW calibration campaign from 4/4 to 3/6 2016, 5 UTC(k) sites PTB, ROA, INRIM (calibrate 2 station IT01 and IT02), OP, and SP participate the tour.

- Instead of the site-based one calibration, the campaign used baseline-based TW calibration method; the fixed reference station and mobile station are provided by TimeTech.

- In general, the combined uncertainty for TW links is about 1 ns (involving IT01 and IT02), Hector Esteban also reports the CALR variation since 2014.

Mobilecalibrationstationandcalibrations(WolfgangSchaefer)- The calibration mobile Station is now stable and fully operational, and will be full availability

during 2019 or longer, - The optical link stability has improved; PLL parameters have been optimised to be more

tolerant against disturbances. - CH and VSL have expressed preliminary interest to join future calibrations

ImprovingtheGNSStimetransferbyanensembleoftheGPSreceivers(CalvinS.Y.LIN)- Study is motivated from how to improve the long term performance of GPSTT. - Long term monitor the latch point migration of 4 GPS receivers of TL, Z12T, GTR50,

PolaRx4, and TTS-4, and compare their “REFGPS” from P3 all in view. There is about 0.5-1 ns/year discrepancy in each P3 CCD pair.

- The total delay variation of TTS-4 receiver vs. GTR50 receiver was smaller than vs. Z12T receiver and PolaRx4 receiver, Calvin Lin note the latch points inside the TTS-4 and GTR-50 receiver are more rigorously synced with 1 PPS input or the definition point of UTC(TL).

- A 4-receiver ensemble using equal weighting but eliminate outliers is under testing to average out the migration of latch point inside GPS receiver. For NICT-TL time comparison, the long term DCD of ensemble results with TW is better than the primary receiver link, NC01-TLT1.

ThedraftGNSSrecommendationtoCCTF(VictorZHANG/ZhihengJIANG)Report the study of “Task group of long-term comparisons of GPS and TWSTFT links” and the recommendation:

- The study of long term task group was published in 2017 PTTI “Long-term instability in UTC time links”.

- Recommendations (partial): Monitor the CCDs between the receiver systems and the TW-GNSS, either in-house

or with the BIPM’s web pages, and report any anomalies to the BIPM; Monitor and record the inside and outside temperature and humidity and provide the

data to the BIPM for its web site and database; Monitor the reference signals at the input to the time transfer equipment (GNSS

receivers and TW modems); - Reports cases of discrepancy between TW and GPSTT DCD, and comments that the

discrepancy of NIST-USNO calibrations may come from GPSTT. - Comment by Arias: how to report temperature and humidity data, what is the file form? - Comment by Arias: labs should keep all data; the store space in BIPM is very limited now.

1.4Discussion(about 10-15 minutes/each, 14h00-15h00)

ThedevelopmentoftheTWSTFTCPusingNICTdigitalmodem(MihoFUJIEDA)Miho Fujieda reports the status of the developing of NICT all digital modem:

- The modem is re-programmable FPGA-based with high sampling rate (250 MHz Tx, 100 MHz Rx) and can perform code and carrier phase TWSTFT.

- Main system is designed by NICT and manufactured by SPECTRA Corp. - Easy-to-use and no need of high-end PC and band pass filter. - The performance evaluation showed a good result and NICT will organize a long base line test

with TL and KRISS. The test will start around early 2018.

NewsanddevelopmentsonTWSTFTequipment(WolfgangSCHAEFER)- Timetech determined to support the SATRE 2-way operations in the next 5 years and beyond

with hardware and calibrations. - Current digital modem uses 50 MHz bandwidth and can be worked down to C/N 60 dBHz. It is

capable of performing dual PN codes and code and integer carrier phase TWSTFT. However, it can’t be improved due to the chosen design approach.

- Next digital SATRE modem is under development together with colleagues at Stuttgart University. The modem uses high bandwidth, and will be an enhanced version of current digital modem. It is not open source, but is ready for commercialisation. Availability is in 2018 (tentative) to have a successor, once the current SATRE is no longer available.

1.5TWSTFTSDRinstallationandoperation(about 5-10 minutes/each, 15h30-17h50)

UpdateontheTWSTFTSDRsoftware(Yi‐JiunHUANG)He reports the update of SDR software version 2017.2 and the current status of Europe-Europe, Europe-Asia, and Europe-North America SDR links. Furthermore he give the suggestion to each SDR user lab.

- SDR software v2017.2 was announced, should be no more time jump in principle. New software will set/check time stamp using NTP and provide an executive program “obs2itu” to convert raw data into ITU files.

- Also report the status and analysis of current SDR stations and links, from triangle closure sum, the results of SDR results are more precise than SATREs’.

- Yi-Jiun suggest to measure 1 PPS out or use longer/shorter PPS-in cable to avoid the for time jump when restart SDR. And give suggestions lab by lab for individual SDR setup.

ReportsanddiscussionsofSDRTWSTFToperationbytheTWLabsAOS, IT, KRISS, NICT, NIM, NIST, NTSC, OP, PTB and VNIIFTRI etc.

- Question by Dirk Piester: how many parallel Rx can measure at the same time? Answer by Yi-Jiun Huang: the simultaneous measurement channel number is limited by the ability of GPU card, for Nvidia K20 is 6 channels.

- Comment by Dirk Piester: PTB consider to buy another SDR for Europe-Asia link or simultaneous receive the signal from all Europe Labs.

- Comment by Andrey: the PPS out may be necessary for calibration. - Comment by Victor: 10 or 5 ns jump in SDR raw measurement still exist when NIST use SDR

v2017.2. Response by Yi-Jiun: to use longer/shorter PPS-in cable may avoid it.

SessionII 8h30‐11h30and13h00‐14h30,19May

2.1ThepilotstudyontheTWSTFTSDRforUTCcomputation(8h30‐9h30)

SDRforUTC–theBIPManalysis(ZhihengJIANG)He reports the status and results of SDR joint pilot project. Since the most of SDR results are better than Satres’, the Working Group of TWSTFT recommends using SDR in UTC.

- There are 12 TWSTFT labs already installed SDR system; more labs (USNO, SP, NPL, ROA) are ready to joint.

- All TWSTFT short term results of SDR links of Europe-Europe, Europe-North American, Europe-Asia links are better than Satre links, especially for short-base line.

- 400 days KRIS-TL DCD also showed SDR’s long term ability is conformed with Satre. - The gain factor, DCD with PPP, and triangle closures also demonstrate the great improvement

of SDR in most of links - In TM268, Zhiheng examined the 3 steps calibration test to align the SDR link in order to trace

the UTC calibration, that The CALR of the SDR links can be implemented in the TWSTFT ITU data files.

- he raises some open points for SDR Why the gains in SDR vs. Satre depend on the regions, satellite coverage and baseline

lengths? Why there is no significant diurnal reduction in the trans-Atlantic SDR links? The calibration issues; Time steps when restarting a SDR receiver; Definition of reference-delay;

- he proposes the schedule of implement of SDR May 2017: WGTWSTFT approve the recommendation of using the SDR in UTC; June 2017: the recommendation approved by the CCTF; July-Sept. 2017: complete the open issues of SDR and test the computations and

calibrations of SDR, then finish the BIPM Technical Memorandum Oct.-Des. 2017: Use SDR links as the backup links in the BIPM monthly Circular T

computation, Publish on the BIPM web; Jan. 2018: use in BIPM Circular T; June 2018: final report of the pilot study to the 26th TW WG meeting in Poland

- Comment by Arias: the final report should be approved by CCTF before using SDR links as the official links. The following step may be followed: 1. Test SDR for CirT backup link, 2. Test SDR for CirT official link, 3. Final report for using SDR to CirT in next WG meeting.

- Comment by Joseph: for the triangle calibration, the total calibration should be the combine uncertainty of 2 links; Dirk: we should recalibrate the indirect link, better not using the combine uncertainty. Calvin: some common noise may be cancelled out in the indirect calculation; to recalibrate the indirect link is a better way.

Approval of the TWSTFT SDR for UTC time links – Recommendation to CCTF (VictorZHANG)

He reads out the recommendation and asks for comments; the main articles are approved except some minor typos.

2.2Otherissues(10h00‐11h00)

TWOTFTinEurope(JerzyNAWROCKI)Introduces the optical time transfer network in Poland, German, UK, France, Scandinavian, Italy, and Europe.

- TWOTFT network of Poland includes the labs of AOS, GUM, IL, CBR, and LT (Lithuania), the results of AOS-GUM TWOTFT are already regularly reported to BIPM.

- EUROMET optical time transfer network including: PTB, 2CMI, IEN, NPL, OP, SP, TUBITAK, AGH, CNRS, PSNC, Chalmers

- Other Europe optical fibre time transfer links including: PTB-DTAG (two links), NPL-London City (for trade exchange) TP at Prague – BEV at Vienna LPL-OP SP-STUPI-Mikes (urban SONET frame) IEN-Florence

TWOTFTinNTSC（TaoLIU）Tao introduces the optical time transfer network in NTSC and Jiangsu province; furthermore introduce the future network plan of China.

- NTSC optical time transfer links using lab made time transfer equipments include: NTSC-HTC, 56 km DWDM fibre with Bi-EDFA repeater. NTSC-Pucheng BPL, 128 km fibre.

- 817 km Jiangsu Province BC&TV optical network is used for syncing 11 relay broadcast station

- Tao announces a proposed china optical time transfer network from XAO, NTSC, NIM, WIPM, SOIM, and SHAO

TWSTFTapplication:TheTechnologyofOrbitDeterminationbyTransferTracking(ODTT)(XuhaiYANG/FenCAO)

Xuhai and Fen Cao report the development of ODTT (Orbit Determination by Transfer Tracking) technology.

- ODTT is a technology based on TWSTFT, used for precisely tracking GEO satellite and determining its orbit.

- ODTT co-observe the IGS station to get the correction information of troposphere delay, the instruments delay are calibrated by moving TWSTFT station

- There are total 6 ODTT station in China, orbit precision for IGSO satellite is about 0.86 m - Question by Carson: how often is the measurement period? Answer by Xuhai Yang: 1 second

CAPStimeandfrequencytransfer(WenfangJING/ShuoLIU)Introduce the China area positioning system and its applications especially for time and frequency transfer”

- CAPS (China Area Positioning System) is an independent navigation technology verification system, servicing for BeiDou navigation system, including the space, ground and user segments.

- CAPS uses specific technology to implement the navigation and time and frequency transfer. Including common view, TWSTFT, and TWCP.

- Long base line ground stations are being built by NTSC, and need to check the diurnal effect of TWSTFT and the undulation effect of TWCP, would like to ask for more cooperation.

On the TWSTFT 26thAnnual Working Group Meeting 2018 (Albin CZUBLA/JerzyNAWROCKI)

- GUM is interested in hosting the 26th annual working group meeting and presented an invitation.

- The 25th annual working group meeting gratefully accepted the invitation by GUM. The next meeting will be held at GUM, in Warsaw, Poland around next June. The dates of the meeting and agenda will be announced soon.

2.3Laboratorytour(13h00–14h30)1) Time keeping laboratory 2) Quantum frequency standards division 3) Museum of time science

RoadMap–ImplementationoftheSDRinUTC

Proposed by Joseph Achkar and provoked by Victor Zhang, a small meeting was held after the WG meeting to discuss the road map of how to implement the SDR in UTC. Participants: Dirk Piester, Joseph Achkar, Calvin Lin, Yi-Jiun Huan, Victor Zhang and Zhiheng Jiang It was decided to create a small Task Group:

Members: the persons presented this meeting Goal: Implementation of the SDR in UTC List of the opening, mainly the technical-practical, problems in SDR application Solutions of these problems proposed to all SDR labs The numerical experiences performed by the Labs and the BIPM Schedule of the above works Prepare a final report of the pilot project on SDR and presented to the WT WG 2018 Meeting List of future studies Zhiheng should prepare a draft to initiate the task group (TM269) and circular it to the

members

Attendeesofthe25thMeetingofCCTFWorkingGrouponTWSTFT

The attendees of the 25th WGTWSTFT meeting Number Name Institution Email

1 Victor ZHANG NIST victor.zhang@nist.gov

2 Zhiheng JIANG BIPM zjiang@bipm.org

3 Joseph ACHKAR OP joseph.achkar@obspm.fr

4 Dirk PIESTER PTB dirk.piester@ptb.de

5 Shinn-Yan (Calvin) LIN TL sylin@cht.com.tw

6 Carsten RIECK SP carsten.rieck@sp.se

7 Andrey NAUMOV VNIIFTRI(SU) naumov@vniiftri.ru

8 Yi-Jiun HUANG TL dongua@cht.com.tw

9 Wenjun WU NTSC wuwj@ntsc.ac.cn

10 Zhiqiang YANG NIM yangzq@nim.ac.cn

11 Shengkang ZHANG

BIRM zhangsk@126.com

12 Angela

MCKINLEY USNO angela.mckinley@navy.mil

13 Russell

BUMGARNER USNO russell.bumgarner@usno.navy.mil

14 Hector Esteban ROA hesteban@roa.es

15 Elisa Felicitas Arias BIPM farias@bipm.org

16 Yury Smirnov VNIIFTRI(SU) yfsmirnov@vniiftri.ru

17 Miho Fujieda NICT miho@nict.go.jp

18 Jerzy Nawrocki AOS nawrocki@cbk.poznan.pl

19 Wlofgang Schäfer TimeTech Wolfgang.Schaefer@timetech.de