GNSS and Time Metrology: Report from the CCTF Working ...

GNSS and Time Metrology:

Report from the CCTF Working

Group on GNSS

Pascale Defraigne, Chair, Royal Observatory of Belgium

2

Current use of GNSS for UTC

3

To date: all UTC(k) labs are connected via GPS, and 87% of the links are entirely based on GPS

GLONASS was used for some links during more than ten years

The accurate receiver calibration for GLONASS signals is still an issue. The GLONASS common view is used as back up for some links.

Galileo : data analysis shows a better performances than GPS in terms of code noise. - Calibration available since June 2020- Use in UTC – in test phase

BeiDou : transition from BDS-2 to BDS-3 (different signals and frequencies)- only a few receivers get the BeiDou 3 signals- Absolute calibration for BDS-3 just started

Current use of GNSS for UTC

Time transfer techniques in UTC

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Topics

• Calibration of GNSS equipment

• Time transfer with new GNSS

• Update on Circular T, Section 4

(UTC-Broadcast_UTCGNSS), to include all GNSS, with uncertainties

• Task Group on traceability to UTC from GNSS Measurements

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Hardware delay calibration

Latching pointd1

d4d3d2

6

Calibration scheme

Absolutely

calibrated

BIPM reference

G1 reference

G1 referenceG1 reference

G1 reference

G2G2

G2

G2 G2G2

7

Calibration trip

G1 reference

G2 G2

Traveling station

Traveling station

Traveling station

Uncertainties: 2.5 ns (or better)

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Calibration for GNSS hardware delays

G1 calibration trips:Since 1001-2018 all of them include Galileo

Excellent stability on GPS:

Ensemble Nr DIDP1 DIDP2 DIDC1 DIDP3

APMP (2018 – 2016) Ave 9 -0.1 -0.1 -0.2 -0.1

EURAMET (2018 – 2016) Ave 9 0.1 0.0 0.1 0.2

SIM (2018 – 2016) Ave 5 0.0 0.2 0.2 -0.2

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Stability of G2 calibration results

0

1

2

3

4

5

6C

S21

20

16

-20

18

DL0

4 2

01

6-2

01

9

DL0

5 2

01

6-2

01

9

ES0

3 2

01

2-2

01

6

ES0

4 2

01

2-2

01

6

ES0

3 2

016

-20

20

ES0

4 2

01

6-2

02

0

IF2

0 2

01

7-2

02

0

IT1

Z 2

01

6-2

01

6

IT2

_ 2

01

6-2

01

6

IT__

20

16

-20

16

IT0

8 2

01

9-2

02

1

IT0

9 2

01

9-2

02

1

IT1

0 2

01

9-2

02

1

IT1

1 2

01

9-2

02

1

IT1

2 2

01

9-2

02

1

MI0

4 2

01

5-2

01

9

NP

L1 2

01

7-2

01

9

RIT

1 2

01

6-2

01

9

SP0

2 2

01

6-2

01

9

SP0

5 2

01

6-2

01

9

SP0

1 2

01

6-2

01

9

ZA0

8 2

016

-20

18

BR

UX

20

17

-20

20

OR

4Z

20

17

-20

20

NTP

1 2

01

6-2

01

8

NTP

2 2

01

6-2

01

8

NTP

3 2

01

6-2

01

8

difference between successive G2 calibrations (P3)

uB = 2.5 ns

(ns)

10

G2 calibration status – June 2021

2020-2021

2019

2018

20172016

More than 5.5 yr

Age of last calibration

77

16

67

31 labs

labs

labs

labs

labs labs

Improved information distributed to time laboratories. (July 2021)

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Progress in absolute calibration

GPS Intercomparisons

Galileo

Recent absolute calibrations agree within (peak-to-peak)4 ns for GPS 2.5 ns for Galileo

To date : absolute calibration by

ESA – CNES – JPL - VNIIFTRI

GNSS signal simulator

GNSS receiver

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Time transfer with new GNSS

Several studies carried out.The conclusions converge:• BeiDou-3 (B1C&B2a) provides similar

performances as GPS• Galileo code measurements are less

noisy, and provide short-term stability typically better than GPS P3, but not always at t ~ 0.25 day

→ Introduction of Galileo links in the computation of UTC, as backup to GPSPPP

(~20 links in 12/2020 )

Guang et al 2020 Metrologia 57 065023

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Revised section 4 for Circular T

GOAL :Provide a validation of tGNSS – “UTC” → quantify UTC – “UTC”

Current situation :

- Only GPS and GLONASS

[UTC-UTC(USNO)_GPS] = C0', [TAI-UTC(USNO)_GPS] = 37 s + C0'

[UTC-UTC(SU)_GLONASS]= C1', [TAI-UTC(SU)_GLONASS]= 37 s + C1’

- No specified uncertainty

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G1 laboratories - laboratories regularly calibrated

and monitored by the BIPM,- directly calibrated differentially against

an absolutely calibrated station- Geographically distributed over the world

Revise section 4 for Circular T

Naming Convention : “UTC” → Broadcast_UTCXXX (xxx= BDS GAL GLO GPS)

Pivot UTC(k)

UTC – Broadcast_UTCXXX

= [UTC(k) – Broadcast_UTCXXX ]GNSS– [UTC(k) – UTC]circular T

NIST NIMOPNICT

Uncertainties

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Uncertainties on UTC-Broadcast_UTCGNSS

1. GNSSTIME – Broadcast_UTCGNSS from different satellites

2. The uncertainties in the solution UTC(k)-GNSST due to multipath, and satellite clocks and orbits in the navigation messages.

3. Receiver calibration

4. Uncertainty on UTC-UTC(k) for the pivot UTC(k)

5. Differences between single-frequency and dual-frequency solutions.

GPST – Broadcast_UTCGPS

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Uncertainties on UTC-Broadcast_UTCGNSS

Final uncertainty Budget:BeiDou Galileo GLONASS GPS

Calibration 2.6 2.4 3.8 2.6

Broadcast value dispersion

3.0 0.5 1.7 1.3

Code noise and multipath

1.5 0.7 3.5 0.9

UTC-UTC(k) pivot 2.2 2.2 2.2 2.2

Total 4.8 ns 3.4 ns 5.9 ns 3.7 ns

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Traceability to UTC using GNSS measurements

Increasing use of GNSS for synchronization& increasing demand for traceability

Need for guidelines on - how the user can get UTC from GNSS (including equipment and calibration)- and how traceability can be obtained when using GNSS for synchronization to UTC (UTC from Signal in Space or UTC from UTC(k) )

Task force of the GNSS WG, with the help of the WG on MRA.

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Task Force on Traceability to UTC

using GNSS measurements

CCTF survey → some questions on the traceability to UTC using GNSS

Answers : from the UTC(k) laboratories + stakeholders (telecom, IT, Science,TF equipment manufacturers, standardization bodies, …)

Observations from the answers received ➢ The term „traceability“ is used with different connotation in the various user groups. often ignoring the definition of (metrological) traceability in the Vocabulaire International de Metrologie (VIM). ➢ Traceability and accuracy are sometimes not clearly distinguished.➢ GNSS signals are employed for getting a reference for frequency, for epoch,

for time-of-day, with quite different accuracy requirements.

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Task Force on Traceability to UTC

using GNSS measurements

Final Goals : - Produce unified guidelines on how to get traceability to a realization of

UTC through GNSS measurements

- Different options are on the table,- Work in progress

- Disseminate the information to the end user, via e. g. RMOs, ICG, GNSS providers, GNSS stakeholders

Pascale Defraigne, [email protected]

Thank You• Gérard Petit (BIPM)• Andreas Bauch (PTB) • Giancarlo Cerretto (INRIM) • Michael Coleman (U.S. NRL) • Jérôme Delporte (CNES) • Héctor Esteban (ROA) • Johann Furthner (DLR) • Marina Gertsvolf (NRC) • James Hanssen (USNO) • Ryuichi Ichikawa (NICT) • Artem Karaush (VNIIFTRI) • Paul Koppang (USNO) • Alexander Kuna (UFE) • Judah Levine (NIST) • Zhiqiang Yang (NIM) • Shinn-Yan Lin (TL) • Andrey Naumov (VNIIFTRI) • Jerzy Nawrocki (AOS) • Bijunath Patla (NIST) • Daniele Rovera (LNE-SYRTE) • Pierre Uhrich (LNE-SYRTE) • Pierre Waller (ESA) • Michael Wouters (NMIA) • Wenjun Wu (NTSC)