The spatial techniques currently used in accurate time transfer are based on GPS, TWSTFT, and GLONASS. The International Bureau of Weights and Measures (BIPM) is mandated for the generation of Coordinated Universal Time (UTC) which is published monthly in the BIPM
GLONASS (from GLObal NAvigation Satellite System, GLN for short) is a radio-based satellite navigation system operated by the Russian Space Forces with the aim of providing real-time, all-weather, three-dimensional positioning, velocity measuring, and timing with a worldwide coverage. The completely deployed GLN constellation is composed of 24 satellites in three orbital planes of which the ascending nodes are 120° apart. Eight satellites are equally distributed in each plane. The first satellite was launched on 12 October 1982, and the constellation was completed in 1995, although until recent years it has not always been well maintained.
With respect to present and future techniques for accurate time transfers, GLN is comparable to other global navigation satellite systems (GNSSs): the United States’ Global Positioning System (GPS), the upcoming Chinese Compass navigation system, and the Galileo positioning system of the European Union.
To guarantee the accuracy and robustness of UTC generation, a multitechnique strategy for UTC time transfer is indispensable. Over the last two decades much effort has been devoted to introducing GLN in UTC. However, earlier GLN studies [
The situation has greatly improved in recent years. As of 2008, there were 15 GLN timing receivers operating at UTC laboratories (see Table
Status in November. 2011 (
In this study, we investigate the receivers available at present in the UTC time transfer. The data in the numerical tests were collected mainly using the 3S Navigation and the AOS TTS GPS/GLN receivers. The conclusion obtained in this study is applicable in these two types of receivers. The numerical analysis was carried out using the BIPM UTC/TAI software package Tsoft, with the usual monthly procedure. When the study was initialed, there were no TTS4 receiver data in the UTC databank. We have a couple of TTS4 receiver data recently and start to study them. As for Septentrio receivers, there is no software currently available to convert the receiver measurements to the CCTF CGGTS format used in UTC code time transfer. TTS-4 and Septentrio receivers are not investigated in this study.
In the following section we describe the technical features for the use of GLN in UTC, and then in Section
In an earlier publication [
GLN distributes three codes that can be used for time transfer: L1C, L1P, and L2P. The L1C code is authorized for civil applications in GLONASS ICD [
Histograms of the monthly mean value (Mean) of the CV clock differences and the standard deviations of the smoothing residuals (±
The IGS analysis centres did not supply precise corrections for GLN satellite clocks (the IGS analysis center CODE recently announced the availability of the GLN clock product that we need to validate before using for UTC computation). hence the All in View (AV) technique [
The present study is therefore concentrated on L1C code CV time transfer and its application in UTC. In the following discussions, because the short-term measurement noise of the L1C time link is about 0.7 to 1.5 ns, as given in Tables
Bias of the GLN PRN 11 L1C link (+) relative to the GPS PPP link (●) for the data set UTC 1009. Here
GLN Frequency L1C biases in order of the nominal frequencies corresponding to Table
GLN L1C time links OP-PTB and Time deviations for UTC 1009 and 1109.
GLN L1C time link OP-PTB 1009 where all the frequency biases have been corrected
Comparison of the time deviations
Comparison of the Time deviations
GLN L1C time links SU-PTB and Time deviations for UTC 1009 and 1109.
The GLN L1C UTC time link SU-PTB 1109 after correction for the frequency biases
Comparison of time deviations of the time link SU-PTB 1009 with and without correction for frequency biases
Comparison of time deviations of the time link SU-PTB 1109 with and without correction for frequency biases (here 1109 is one year after 1009 in Figure
Consistency of UTC links between GPS C/A and GLN L1C (10-month comparison corresponding to Table
Before GLN can be used in UTC, the following points need to be clarified: use of precise orbit and ionosphere corrections, biases due to the multiple GLN PRN and/or frequencies, short- and long-term stabilities, calibration and its long-term variation.
The first point has been fully discussed in earlier studies, such as [
In the following sections, we discuss the three remaining points, based on test CGGTTS L1C data from UTC 1005 to UTC 1110 (May 2010 to October 2011), assuming that all the raw measurements have been corrected for precise orbits and atmosphere delays.
Unlike GPS, the satellites of GLN is divided into groups according to the frequencies used. Early studies based on certain physical considerations and on first-generation GLN receivers, for example, 3S Navigation, addressed the question of so-called frequency biases disturbing the CV time transfer [
We estimate PRN and/or frequency biases, based on the most acceptable hypothesis, for example, [
As of mid-2010, the GLN system comprises 20 satellites operating the L1C code. Table
Operational GLN PRNs recorded using TTS-2 and TTS-3 time receivers (
PRN | Fr. code |
|
Receiver |
---|---|---|---|
GLN 11 | 0 | 904 | TTS-3 |
GLN 15 | 0 | 915 | TTS-3 |
| |||
GLN 01 | 1 | 900 | TTS-3/TTS-2 |
GLN 05 | 1 | 906 | TTS-3/TTS-2 |
| |||
GLN 20 | 2 | 901 | TTS-3/TTS-2 |
| |||
GLN 24 | 2 | 912 | TTS-3/TTS-2 |
| |||
GLN 13 | −2 | 944 | TTS-3 |
| |||
GLN 19 | 3 | 921 | TTS-3/TTS-2 |
GLN 23 | 3 | 903 | TTS-3 |
| |||
GLN 18 | −3 | 920 | TTS-3 |
GLN 22 | −3 | 892 | TTS-3 |
| |||
GLN 17 | 4 | 895 | TTS-3/TTS-2 |
GLN 21 | 4 | 905 | TTS-3/TTS-2 |
| |||
GLN 02 | −4 | 893 | TTS-3 |
| |||
GLN 03 | 5 | 251 | TTS-3/TTS-2 |
GLN 07 | 5 | 885 | TTS-3/TTS-2 |
| |||
GLN 04 | 6 | 851 | TTS-3/TTS-2 |
GLN 08 | 6 | 902 | TTS-3/TTS2 |
| |||
GLN 10 | −7 | 915 | TTS-3 |
GLN 14 | −7 | 912 | TTS-3 |
(a) GLN PRN/Fr L1C biases relative to GPS PPP for the link OP-PTB 1009. (b) GLN Frequency L1C biases in increasing order of the nominal frequencies.
PRN | Fr |
|
|
|
|
---|---|---|---|---|---|
11 | 0 | 1602.0 | 750 | −7.204 | 0.963 |
15 | 0 | 744 | −7.083 | 0.920 | |
01 | 1 | 1602.5625 | 727 | −6.882 | 0.995 |
05 | 1 | 733 | −6.518 | 0.896 | |
20 | 2 | 1603.125 | 740 | −7.071 | 0.968 |
24 | 2 | 745 | −7.175 | 1.016 | |
13 | −2 | 1600.8750 | 757 | −6.793 | 0.975 |
19 | 3 | 1603.6875 | 754 | −5.559 | 0.928 |
23 | 3 | 730 | −5.851 | 1.077 | |
18 | −3 | 1600.3125 | 759 | −5.724 | 0.906 |
22 | −3 | 732 | −5.800 | 1.034 | |
17 | 4 | 1604.25 | 745 | −8.287 | 1.031 |
21 | 4 | 736 | −8.277 | 1.070 | |
02 | −4 | 1599.7500 | 751 | −5.328 | 0.983 |
03 | 5 | 1604.8125 | 220 | −6.206 | 1.081 |
07 | 5 | 722 | −6.120 | 1.017 | |
04 | 6 | 1605.375 | 710 | −6.727 | 0.931 |
08 | 6 | 720 | −6.907 | 1.069 | |
10 | −7 | 1598.0625 | 753 | −4.706 | 0.888 |
14 | −7 | 748 | −4.823 | 0.966 |
Fr | Fr′/MHz |
|
|
|
---|---|---|---|---|
−7 | 1598.0625 | 753 | −4.76 | 0.65 |
−4 | 1599.7500 | 751 | −5.33 | 0.98 |
−3 | 1600.3125 | 759 | −5.76 | 0.69 |
−2 | 1600.8750 | 757 | −6.79 | 0.98 |
0 | 1602.0 | 750 | −7.14 | 0.66 |
1 | 1602.5625 | 727 | −6.70 | 0.67 |
2 | 1603.125 | 740 | −7.12 | 0.70 |
3 | 1603.6875 | 754 | −5.70 | 0.70 |
4 | 1604.25 | 745 | −8.28 | 0.74 |
5 | 1604.8125 | 220 | −6.16 | 0.74 |
6 | 1605.375 | 710 | −6.81 | 0.71 |
Gains in the standard deviation of the smoothing residuals for the GLN L1C baseline OP-PTB after correction for the frequency biases calculated for the period 1009 (comparison over 18 months).
Period yymm |
|
|
Gain |
---|---|---|---|
1005 | 1.199 | 1.073 | 11% |
1008 | 1.199 | 1.110 | 7% |
|
|
|
|
1109 | 1.180 | 1.134 | 4% |
Gains in standard deviation of the smoothing residuals after correction for the frequency biases calculated for the OP-PTB UTC 1009.
Baseline | Distance/km |
|
|
Gain |
---|---|---|---|---|
AOS-PTB | 500 | 1.721 | 1.381 | 20% |
NIS-PTB | 3000 | 1.338 | 1.276 | 5% |
|
|
|
|
|
SG-PTB | 6300 | 2.500 | 2.557 | −2% |
UME-PTB | 1900 | 1.398 | 1.403 | 0 |
GLN PRN/Fr L1C biases computed with SU-PTB 1009 versus GPS C/A (a constant of −200 ns is subtracted from the
PRN | Fr. |
|
|
|
---|---|---|---|---|
11 | 0 | 700 | −8.699 | 0.639 |
15 | 0 | 697 | −8.331 | 0.649 |
01 | 1 | 683 | −8.809 | 0.743 |
05 | 1 | 693 | −8.507 | 0.684 |
20 | 2 | 691 | −8.673 | 0.691 |
24 | 2 | 696 | −8.578 | 0.760 |
13 | −2 | 711 | −8.857 | 0.711 |
19 | 3 | 701 | −8.833 | 0.627 |
23 | 3 | 685 | −8.700 | 0.751 |
18 | −3 | 703 | −9.988 | 0.690 |
22 | −3 | 684 | −9.730 | 0.698 |
17 | 4 | 703 | −10.495 | 0.756 |
21 | 4 | 678 | −10.365 | 0.701 |
02 | −4 | 692 | −8.927 | 0.674 |
03 | 5 | 200 | −8.871 | 0.613 |
07 | 5 | 663 | −8.811 | 0.686 |
04 | 6 | 663 | −9.895 | 0.673 |
08 | 6 | 689 | −9.880 | 0.735 |
10 | −7 | 708 | −9.329 | 0.744 |
14 | −7 | 713 | −9.078 | 0.763 |
Gains in standard deviation of the smoothing residuals before and after corrections for the frequency biases calculated for the SU-PTB link for the period 1009.
Period yymm |
|
|
Gain |
---|---|---|---|
1005 | 1.252 | 1.254 | 0% |
1008 | 1.068 | 1.043 | 2% |
|
|
|
|
1109 | 1.150 | 1.177 | −2% |
Comparing GLN PRN biases computed using OP-PTB and SU-PTB 1009.
PRN | Fr |
|
|
|
|
Mean1 | Mean2 |
---|---|---|---|---|---|---|---|
11 | 0 | 0 | 0.963 | 0 | 0.639 | 0.0 | 0.0 |
15 | 0 | 0 | 0.920 | 0 | 0.649 | ||
01 | 1 | 0.322 | 0.995 | −0.110 | 0.743 | 0.4 | −0.1 |
05 | 1 | 0.565 | 0.896 | −0.176 | 0.684 | ||
20 | 2 | 0.133 | 0.968 | 0.026 | 0.691 | 0.0 | −0.1 |
24 | 2 | −0.092 | 1.016 | −0.247 | 0.760 | ||
13 | −2 | 0.411 | 0.975 | −0.158 | 0.711 | 0.4 | −0.2 |
19 | 3 | 1.524 | 0.928 | −0.502 | 0.627 | 1.4 | −0.3 |
23 | 3 | 1.353 | 1.077 | −0.001 | 0.751 | ||
18 | −3 | 1.359 | 0.906 | −1.657 | 0.690 | 1.4 | −1.3 |
22 | −3 | 1.404 | 1.034 | −1.031 | 0.698 | ||
17 | 4 | −1.204 | 1.031 | −2.164 | 0.756 | −1.1 | −1.9 |
21 | 4 | −1.073 | 1.070 | −1.666 | 0.701 | ||
02 | −4 | 1.755 | 0.983 | −0.596 | 0.674 | 1.8 | −0.6 |
03 | 5 | 0.998 | 1.081 | −0.172 | 0.613 | 1.0 | −0.3 |
07 | 5 | 0.963 | 1.017 | −0.480 | 0.686 | ||
04 | 6 | 0.477 | 0.931 | −1.196 | 0.673 | 0.3 | −1.4 |
08 | 6 | 0.176 | 1.069 | −1.549 | 0.735 | ||
10 | −7 | 2.498 | 0.888 | −0.630 | 0.744 | 2.4 | −0.7 |
14 | −7 | 2.260 | 0.966 | −0.747 | 0.763 |
Calibration consistency of GPS C/A versus GLN L1C links over 10 months (values given in the table are the mean of the differences between GPS and GLN links and its standard deviation).
YYMM | AOS-PTB/ns | SU-PTB/ns | UME-PTB/ns |
---|---|---|---|
1002 | −0.6 ± 1.6 | −0.2 ± 1.4 | 0.0 ± 1.4 |
1001 | −1.4 ± 1.6 | −0.3 ± 1.6 | −0.4 ± 1.4 |
0912 | −1.0 ± 1.5 | −0.2 ± 1.6 | −0.4 ± 1.4 |
0911 | −0.7 ± 1.6 | −0.4 ± 1.6 | |
0910 | −0.9 ± 1.4 | −0.3 ± 1.6 | −0.4 ± 1.3 |
0909 | −0.4 ± 1.6 | −0.4 ± 1.6 | −0.0 ± 1.4 |
0908 | −0.4 ± 1.6 | −0.4 ± 1.6 | −0.6 ± 1.4 |
0907 | −0.3 ± 1.6 | −0.7 ± 1.6 | −0.6 ± 1.4 |
0906 | −0.2 ± 1.6 | −0.3 ± 1.6 | −0.7 ± 1.4 |
0905 | −0.0 ± 1.6 | −0.0 ± 1.6 | −0.0 ± 1.4 |
| |||
Mean | −0.6 | −0.3 | −0.3 |
|
0.4 | 0.2 | 0.3 |
UTC laboratories operating two or three time and frequency transfer facilities as of 2008 [
Lab | GPS | GLN | TW |
---|---|---|---|
AOS |
|
|
|
AUS |
|
|
|
CH |
|
|
|
IT |
|
|
|
KRIS |
|
|
|
LDS |
|
|
|
MIKE |
|
|
|
NICT |
|
|
|
NIM |
|
|
|
NIS |
|
|
|
NIST |
|
|
|
NMIJ |
|
|
|
NPL |
|
|
|
NPLI |
|
|
|
NTSC |
|
|
|
OP |
|
|
|
PTB |
|
|
|
ROA |
|
|
|
SG |
|
|
|
SP |
|
|
|
SU |
|
|
|
TL |
|
|
|
UME |
|
|
|
USNO |
|
|
|
VSL |
|
|
|
ZA |
|
|
Our main interest is the influence of the so-called frequency biases on the CV time links. According to previous studies, we assume first that the frequency biases exist and are physically caused by the GLN frequencies, significantly receiver dependent, and are constant. The frequency biases should therefore be universal and could be corrected for in the UTC time transfer. We focus our analysis on the SU-PTB and OP-PTB baselines because both are UTC links, and for the latter we also have GPS PPP and TW links, which are more precise and provide good references for the evaluation of the GLN links. All three laboratories are equipped with TTS-3 receivers. To study the physical cause(s) of the frequency biases, we proceeded as follows: we first split the raw data file containing all the PRNs into subfiles for each PRN and then compute the one-PRN links; we then compare the one-PRN links to the GPS PPP link to compute the frequency biases and use them to calibrate the raw link data; we study if there are gains by comparing the time deviations and the differences versus GPS PPP and TW; finally, we apply the “frequency biases” obtained from a month of a baseline to “calibrate” the raw data from other months and other baselines to see if the biases are “universal” (independent of receivers, months, and locations).
Figure
Table
In Tables
We also estimated the so-called frequency biases using other references such as P3 and TW, and the results are almost the same as those listed in Table
It is expected that application of the frequency bias corrections to the raw GLN measurements should lead to a significant reduction in noise level and improvement in the short-term stability of the link. In Figure
Figure
The standard deviation of the smoothing residuals is also an index of the gains. If the frequency biases are constant for that baseline, they should be applicable to the raw data of other periods. We used the frequency bias corrections listed in Table
Because the same type of the receiver TTS3 is used (hence the hardware delay for same frequency is similar if not equal) we may further assume that the frequency corrections obtained from OP-PTB can be used for other receivers at AOS, NIS, SU, UME, and SG. We may expect a global gain of about 9%. Table
We can use the same method to study the GLN UTC link SU-PTB. Because neither GPS PPP nor TW data exist for this baseline, we have to use GPS C/A as the reference to compute the so-called frequency biases.
Table
Figure
The standard deviations of the smoothing residuals for the months 1005, 1008, 1009, and 1109 are listed in Table
According to Tables
The previous results do not fully support the previous studies summarized in the beginning of Section
Let us use the exclusion method to examine a seemed impossible possibility.
If the biases are physically caused by the GLN signal frequencies alone, they should be constant with time, isotropically equivalent, and independent of receivers and baselines. As we now have two sets of frequency biases, obtained from the baselines OP-PTB and SU-PTB (Tables
This numerical evaluation based on two CV links does not prove the existence of the impact of the biases which are bigger than the measurement noise and depend on the GLN frequencies. Again, we cannot exclude the effects of other frequency-dependent factor(s) including the impact of the temperature variations. Considering the gain in applying the frequency bias corrections is not significant and the complexity of the computation is, it has been decided [
A time link technique can be used in UTC only when it is calibrated, and its short- and long-term stabilities are proven. In the following study we use GPS as reference.
Table
As the short- and long-term stabilities of GPS are well proven and GPS and GLN are completely independent systems, this close consistency between the data sets demonstrates that the GLN time transfer technique is as stable as GPS in both the short and long terms. The same conclusion holds for the long-term variations in their calibrations (cf. [
Since January 2011, a combination of the GLN L1C and GPS C/A code time links has been used for SU-PTB and UME-PTB in UTC time transfer [
The UTC time transfer strategy until the end of 2010 was the so-called
As discussed previously, (cf. Table
In the numerical tests, we use the more precise TW and GPS PPP links as references to estimate the gains. Both are available for the baseline OP-PTB. In March 2010, the measurement uncertainty
Table
Comparison of the standard deviations of the clock differences for the GPS-only, GLN-only, and GLN&GPS links for the baseline OP-PTB 1005 (MJD 55313 to 55346).
Compared to | GPS-only |
GLN-only |
(GLN&GPS) |
Gains ( |
---|---|---|---|---|
TW | 1.240 | 1.369 | 1.215 | 6.5% |
PPP | 1.182 | 1.285 | 1.149 | 7% |
Figures
Comparison of the time deviations between the GPS-only, GLN-only, and GLN&GPS links for the baselines SU-PTB (Figure
The baseline SU-PTB for the time links of UTC1102
The baseline INPL-PTB for the time links of UTC1110
Figure
The GLN&GPS time transfer baseline OP-PTB.
Time link OP-PTB of the combination (GLN&GPS) for UTC 1009
Comparison of the Time Deviations for the GPS-only, GLN-only, and combined link GLN&GPS for OP-PTB 1005
To compare the long-term stabilities, we look at the GPS-only, GLN-only, and GLN&GPS data over a 15-month period (1007–1109: MJD 55378–55834) for the UTC baseline OP-PTB. Figure
15-month long-term comparison of the TDev of GLN/L1C, GPS C/A, and GLN&GPS over the baseline OP-PTB between 1007 and 1109. The GLN&GPS is the most stable one and is used as the official UTC link. The TDev of the three links converges up to 1 day.
The combination thus leads to an improvement in the short-term stability for averaging times of up to 1 day. Since January 2011 combined solutions have therefore been applied in UTC generation. We gave some examples of the links based on a combination of two fully independent techniques to be used in UTC time transfer [
The possible use of P3-code clearly merits further investigation. Other open issues are the use of the carrier phase, the calibrations, and the raw data recording. We briefly outline our considerations for the coming future studies at the BIPM.
Given the success of GPS PPP [
One difficulty with the PPP is the ambiguity of the carrier-phase information. In addition, PPP relies on the Earth geocentric reference and related quantities, such as the geocentric coordinates of the satellites in space and of the antenna centres of the receivers on the ground, and the processing is complex.
The result of a time link is the
Study of the GLN RCD option is an ongoing activity at the BIPM. One way to generate the difference in rates between two clocks is to differentiate the PPP data [
Table
Comparison of the TW, GLN-only, GPS PPP, and GLN©RCD time links for OP-PTB 1005.
Link differences |
|
Mean/ns |
|
---|---|---|---|
TW-(GLN-only) | 324 | 3.873 | 1.346 |
TW-GLN RCD | 324 | 3.858 | 0.497 |
GPSPPP-GLN RCD | 2870 | −0.921 | 0.100 |
Comparison between the TW (+) and GLN©RCD (●) time links for the baseline OP-PTB.
Figure
Time deviations of the GLN-only, GPS PPP, and the combined GLN©RCD links for OP-PTB 1005.
The total uncertainty in (UTC-UTC(
The CCTF GGTTS data format was designed in the early 1980s when GPS was introduced into time transfer using the receivers available at the time. The format has since been updated to accept GLN data as well but its basic specifications remain unchanged, and it is still used to facilitate the computation of UTC/TAI. However, some conventions defined in the GGTTS are now outdated due to the ever-progressing technology in GNSS receiver manufacturing and the introduction of new time-transfer techniques.
For example, one of the major outdated points in the GGTTS convention is that for a tracking arc of 16 minutes of data collection only 13 minutes of them are recorded and 3minutes of data are wasted. In addition, the time tagging with a fixed interval of 780s and a lag of about 4 minutes every day is impractical for most users. The data are round off at 0.1 ns and only code data without CP information are recorded. The BIPM therefore envisages a reform of the raw data collection conventions and an update of the GGTTS format [
To guarantee the precision, the accuracy, and the robustness of UTC generation, the multitechnique strategy for UTC time transfer is indispensable. Efforts towards introducing GLN to complement GPS and TW in the generation of UTC began in the early 1990s, and in November 2009 the first two GLN time links were introduced into the UTC worldwide time link network.
In this paper we present the technical features of GLN time transfer as important for UTC production: a study of the so-called frequency biases, the short- and long-term stabilities, the calibration process, and the advantages of combining GLN and GPS. We also describe various ongoing projects at the BIPM, particularly concerning the use of carrier-phase data.
The present study is focused on the application of GLN L1C code in the generation of UTC, which yields a short-term stability of 1 ns to 1.5 ns. The calibration uncertainty is 5 ns, and the long-term stability is about the same as for GPS. The combination of the GLN L1C and GPS C/A codes makes sense in reducing the short-term stability and particularly in increasing the accuracy and the robustness in the UTC links.
The cause of the so-called frequency biases remains unclear for the authors. Although correction for estimated frequency biases leads to some slight gains for certain baselines, these gains are not seen ubiquitously, and, pending further research, it has been decided not to apply such corrections for GLN links used in the computation of UTC.
Coordinated Universal Time
International Bureau of Weights and Measures
GLONASS (GLObal Navigation Satellite System) [
Global Positioning System
Global Navigation Satellite Systems
International GNSS Service
TWSTFT (Two-Way Satellite Time and Frequency Transfers)
PseudoRandom Noise code signal. Each GPS satellite transmits a unique code sequence (Code Division Multiple Access) and may be identified according to its PRN number. All GLN satellites transmit the same PRN signals using different frequencies (Frequency Division Multiple Access). In the UTC/TAI data format (CGGTTS), PRN is the nominal number of a GLN satellite
Frequency or frequency code
Bias in time delay of a GLN PRN
Bias in time delay of a GLN frequency
Code-based common view time transfer
Code-based and/or
Time transfer (CV and/or AV) using the linear combination of L1 and L2 measurements to achieve ionosphere-free code measurements
Time transfer using carrier-phase precise point positioning technique [
Time transfer combining GPS C/A and GLN L1C codes
In percentage to indicate the improvement in time transfer quality. The gain in
Carrier phase
Clock difference
Rate of CD
Year and month (an UTC computation month), for example, 0910 for 2009 October and 1005 for 2010 May.
Astrogeodynamical Observatory, Borowiec (Poland)
National Metrology Institute of South Africa (NMISA, South Africa)
National Physical Laboratory, Jerusalem (Israel)
National Institute for Standards, Cairo (Egypt)
Observatoire de Paris (France)
Physikalisch-Technische Bundesanstalt, Braunschweig and Berlin (Germany)
Agency for Science Technology and Research (
Institute for Physical-Technical and Radiotechnical Measurements, Rostekhregulirovaniye of Russia (VNIIFTRI), Moscow, (Russian Federation)
Ulusal Metroloji Enstitüsü/National Metrology Institute, Gebze-Kocaeli (Turkey)
Dutch Metrology Institute, Delft (Netherlands).
The authors are grateful to the UTC contributing laboratories for the data used in this study and the reviewers for their constructive scientific suggestions.