8
ORIGINAL ARTICLE Monitoring of UTC(k)’s using PPP and IGS real-time products Pascale Defraigne Wim Aerts Eric Pottiaux Received: 17 January 2014 / Accepted: 12 April 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract The time transfer technique based on Precise Point Positioning (PPP) has proved to be a very effective technique allowing the comparison of atomic clocks with a precision of a hundred picoseconds, and with latency of 2 days. Using satellite orbit and clock information from the IGS real-time products, it is now possible to compute very precise time transfer solutions based on PPP in near real-time, i.e., with a latency down to some minutes. We present PPP-based time transfer results obtained in near real time with the new version of the GNSS data processing software Atomium, and the satellite products mentioned above. We additionally analyze the results that can be obtained with low latency using the NRCan Ultra-Rapid products (EMU) available with a delay of 90–150 min. From the statistics of the results, it is concluded that the real-time IGS products allow detection of a clock jump larger than 1.5 ns after some minutes; the detection threshold falls down to 0.8 ns when using the EMU products about 2 h later. It is also shown that in near real time, a frequency change larger than 2e-14 can be detected when looking at the last 24 h data or larger than 2e-13 when looking at the last 2 h. As demonstrated, the quality of the monitoring depends on the distance between the two stations, so that distances shorter than about 3,000 km should be preferably used for a near real-time comparison of atomic clocks based on PPP. Keywords UTC(k) Á PPP Á Time transfer Introduction The International Atomic Time (TAI) is computed by the International Bureau of Weights and Measurements (BIPM) from an ensemble time scale algorithm combining about 400 atomic clocks distributed in about 60 laborato- ries worldwide. The Universal Time Coordinated (UTC) is then obtained after correcting TAI by the number of leap seconds introduced since its origin in 1972. Since UTC is being computed a posteriori, it is not available in real time. Each time laboratory, therefore, maintains one realization of UTC, named UTC(k) where k is the acronym of the laboratory. UTC(k) is a clock, or a time scale, adjusted to produce the best possible extrapolation of UTC and serves then as an accurate time source available in real time. The monitoring of these realizations of UTC is classically done through the comparison with the true UTC, when this one is available. Presently, a rapid version of UTC is provided by the BIPM on a weekly period (Arias et al. 2012), giving a maximum latency of 10 days to get the comparison UTC(k)-UTCr. The official UTC is, however, computed on a monthly basis which imposes up to 45 days for the val- idation of UTC(k) via its comparison with the true UTC. While UTC(k) is continuously compared with the lab- oratory clocks, an external monitoring with parallel real- izations of UTC is a powerful tool for the time laboratories to increase the reliability of their time scale. The time transfer technique based on Precise Point Positioning (PPP) has proved to be a very effective technique allowing the comparison of atomic clocks with a precision at the level of a hundred picoseconds, and with latency of 2 days (Orgi- azzi et al. 2005; Defraigne et al. 2008). PPP time transfer is presently operationally used for many time links included in the computation of the TAI (Petit et al. 2011). PPP (Zumberge et al. 1997; Kouba and Heroux 2001) is based P. Defraigne (&) Á W. Aerts Á E. Pottiaux Royal Observatory of Belgium, Avenue Circulaire, 3, 1180 Brussels, Belgium e-mail: [email protected] W. Aerts e-mail: [email protected] E. Pottiaux e-mail: [email protected] 123 GPS Solut DOI 10.1007/s10291-014-0377-5

Monitoring of UTC(k)’s using PPP and IGS real-time products

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ORIGINAL ARTICLE

Monitoring of UTC(k)’s using PPP and IGS real-time products

Pascale Defraigne • Wim Aerts • Eric Pottiaux

Received: 17 January 2014 / Accepted: 12 April 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract The time transfer technique based on Precise Point

Positioning (PPP) has proved to be a very effective technique

allowing the comparison of atomic clocks with a precision of a

hundred picoseconds, and with latency of 2 days. Using

satellite orbit and clock information from the IGS real-time

products, it is now possible to compute very precise time

transfer solutions based on PPP in near real-time, i.e., with a

latency down to some minutes. We present PPP-based time

transfer results obtained in near real time with the new version

of the GNSS data processing software Atomium, and the

satellite products mentioned above. We additionally analyze

the results that can be obtained with low latency using the

NRCan Ultra-Rapid products (EMU) available with a delay of

90–150 min. From the statistics of the results, it is concluded

that the real-time IGS products allow detection of a clock jump

larger than 1.5 ns after some minutes; the detection threshold

falls down to 0.8 ns when using the EMU products about 2 h

later. It is also shown that in near real time, a frequency change

larger than 2e-14 can be detected when looking at the last 24 h

data or larger than 2e-13 when looking at the last 2 h. As

demonstrated, the quality of the monitoring depends on the

distance between the two stations, so that distances shorter than

about 3,000 km should be preferably used for a near real-time

comparison of atomic clocks based on PPP.

Keywords UTC(k) � PPP � Time transfer

Introduction

The International Atomic Time (TAI) is computed by the

International Bureau of Weights and Measurements

(BIPM) from an ensemble time scale algorithm combining

about 400 atomic clocks distributed in about 60 laborato-

ries worldwide. The Universal Time Coordinated (UTC) is

then obtained after correcting TAI by the number of leap

seconds introduced since its origin in 1972. Since UTC is

being computed a posteriori, it is not available in real time.

Each time laboratory, therefore, maintains one realization

of UTC, named UTC(k) where k is the acronym of the

laboratory. UTC(k) is a clock, or a time scale, adjusted to

produce the best possible extrapolation of UTC and serves

then as an accurate time source available in real time. The

monitoring of these realizations of UTC is classically done

through the comparison with the true UTC, when this one

is available. Presently, a rapid version of UTC is provided

by the BIPM on a weekly period (Arias et al. 2012), giving

a maximum latency of 10 days to get the comparison

UTC(k)-UTCr. The official UTC is, however, computed on

a monthly basis which imposes up to 45 days for the val-

idation of UTC(k) via its comparison with the true UTC.

While UTC(k) is continuously compared with the lab-

oratory clocks, an external monitoring with parallel real-

izations of UTC is a powerful tool for the time laboratories

to increase the reliability of their time scale. The time

transfer technique based on Precise Point Positioning (PPP)

has proved to be a very effective technique allowing the

comparison of atomic clocks with a precision at the level of

a hundred picoseconds, and with latency of 2 days (Orgi-

azzi et al. 2005; Defraigne et al. 2008). PPP time transfer is

presently operationally used for many time links included

in the computation of the TAI (Petit et al. 2011). PPP

(Zumberge et al. 1997; Kouba and Heroux 2001) is based

P. Defraigne (&) � W. Aerts � E. Pottiaux

Royal Observatory of Belgium, Avenue Circulaire, 3,

1180 Brussels, Belgium

e-mail: [email protected]

W. Aerts

e-mail: [email protected]

E. Pottiaux

e-mail: [email protected]

123

GPS Solut

DOI 10.1007/s10291-014-0377-5

on a consistent modeling and analysis of GPS, and possibly

GLONASS (Defraigne and Baire 2011), dual-frequency

code and carrier phase measurements, using external pro-

ducts for the satellite orbits and clocks, in order to deter-

mine the station position, its clock synchronization error

and the troposphere zenith delay at each observation epoch.

The differences between the clock solutions obtained for

two remote stations provide, therefore, the synchronization

offset between the two station clocks and its evolution over

time. This technique is widely recognized for its high

resolution, with a classical sampling of one point each 30 s,

its low statistical uncertainty of 0.1 ns, when ignoring the

uncertainty of the instrumental hardware delays calibration,

and its ability to reach a stability of 10-15 or better at an

averaging time of 1 day, thanks to the very low noise level

of the carrier phases (Larson et al. 2000). The systematic

uncertainty for PPP time transfer is equivalent to that of the

code-only time transfer, as the code data are the only

observables providing access to the timing information, the

carrier phases being ambiguous. Clock solution accuracy

is, therefore, limited to some nanoseconds, due to the

colored nature of the code measurements (Defraigne and

Bruyninx 2007) and the performances of instrumental

calibrations, i.e., 1 ns on each frequency for absolute cal-

ibration (Proia et al. 2011) and 1.5 ns for carefully done

individual long-distance relative calibration (Esteban et al.

2010). Usually, IGS Rapid or Final products are used to

remove the satellite clock errors from the measurements

and to model the satellite positions. These products are,

however, available after a minimum of 17 h. IGS is also

providing Ultra-Rapid products but the clock products

available in real time are in that case based on predictions

over several hours, producing satellite clock uncertainties

of 3 ns, which is too large for a comparison of ground

clocks at the nanosecond level. We, therefore, propose here

to use two sets of estimated satellite products available

with a low latency to provide PPP solutions with the same

low latency. The first one is the IGS real-time products

(Caissy and Agrotis 2011), available with a latency of 30 s,

allowing a clock monitoring in near real-time, i.e., down to

some minutes after the last observation. The second one is

the NRCan Ultra-Rapid products (EMU) generated hourly

with a delay of 90 min after the last observation (Mireault

et al. 2008), and already proposed by Cerretto et al. (2012)

for clock monitoring with a latency from 2 to 3 h. This

capability can be considered useful for the National

Metrology Institutes, being provided with an additional tool

for comparisons of their local realization of UTC or for any

time scale for which the real-time monitoring is crucial.

We start with a presentation of the satellite orbit and

clock products used, followed by a description of our

analysis scheme based on the PPP software tool Atomium

(Defraigne et al. 2008). In particular, the modifications

introduced in the GNSS data processing to be real-time

product compliant, are detailed. The next two sections

present a quantification of the quality of the results

obtained from the analysis conducted using the EMU or

IGS real-time products for the satellite clocks and orbits, in

order to determine at which level it can serve as near real

time or low latency monitoring of atomic clocks connected

to a GNSS receiver.

Near real-time satellite orbit and clock products

As mentioned in previous section, two sets of satellite

clocks and orbits available with a very short latency have

been considered: the first one is the IGS real-time clocks

and orbits which can be obtained from the RTCM streams

delivered in real time by the NTRIP Broadcaster http://

www.igs-ip.net. These products are the output of an IGS

Real-time Working Group established in 2001 with the

goal to design and implement real-time IGS infrastructure

and processes (Caissy and Agrotis 2011). The second set of

products is the EMU Ultra-Rapid products delivered by the

NRCan Analysis Center of the IGS (Mireault et al. 2008),

in the same format as the IGS products, i.e., SP3 and

RINEX clock format.

Real-time IGS products

The IGS presently manages a global real-time GNSS

tracking network and generates combined real-time ana-

lysis products. These products are available with a sam-

pling rate of 5 s and a latency of about 30 s. As for the

classical post-processed IGS products, the different IGS

real-time Analysis Centers (ACs) provide real-time clocks

and orbits. A combination is then operated by ESOC who

assumes the role of IGS real-time Analysis Center Coor-

dinator, producing the combined products IGC01 we used

here. These products are provided as corrections to the

broadcast orbits and clocks, which we converted in a first

step into classical IGS standard files of post-processed

orbits and clocks (sp3 and clk formats). Note that the orbits

streamed in the message IGC01 are very close to the Ultra-

Rapid combined orbits IGU. We tested the use of either the

IGC01 or the IGU orbits and found that the impact on the

results, few picosecond only, was negligible.

The quality of the real-time satellite clocks was esti-

mated to have a 2-sigma uncertainty of 200–400 ps (Hadas

and Bosy 2014) with an rms two times larger. The refer-

ence for the IGS real-time satellite clocks is not a contin-

uous clock or time scale, so that the clock solution of one

particular station is not continuous. For this reason, only a

difference between the clock solutions of two stations can

be used for clock monitoring. This is illustrated in Fig. 1

GPS Solut

123

where the clock solutions of two IGS stations equipped

with a H-maser (BRUX and PTBB) have been depicted.

We retrieve indeed the same jumps at the same epochs,

which are jumps in the reference of the IGS real-time

clocks. When looking at the differences between the two

stations (Fig. 1, bottom part), these jumps cancel and the

solution directly represents the clock offset evolution

between the two clocks that are compared.

Ultra-rapid NRCan products

The second set of products is the EMU Ultra-Rapid pro-

ducts delivered by the NRCan Analysis Center of the IGS

(Mireault et al. 2008) in the same format as the IGS pro-

ducts, i.e., SP3 and RINEX clock format. A new set of two

files (sp3 and clk) is delivered each hour. Each clk file

contains satellite clocks for the last 24 h, and each sp3 file

contains the satellite positions estimated in the last 24 h,

and predicted for the following 24 h. The EMU products

are available around 90 min after the last epoch of the clk

file, so that the latency for the products of any epoch is

between 90 and 150 min. As post-processed products, they

are based on a higher degree of internal consistency

checking like, e.g., outlier rejection, than the real-time IGS

products described in previous section. The 2-sigma

uncertainties on these clock products are estimated to

170 ps (Cerretto et al. 2012). The advantage of using the

EMU products rather than the combined IGU products is

that they are available more rapidly, i.e., 90 min instead of

3 h, they are updated, each hour rather than every 6 h for

the IGU, and with a higher sampling rate, 30 s instead of

900 s.

There is, therefore, a major difference between the IGS

real-time products for which a given epoch is only

computed and provided once, in real time, and the EMU

products which are computed for a moving window of

24 h. For each new hourly computation performed with the

EMU, a new set of orbits and clocks is, therefore, used. The

reference for the satellite clocks is continuous inside one

24 h EMU data set, but is changing from one set to the next

one. Figure 2 illustrates the impact of these changes on the

station clock solutions obtained with PPP. Consequently, as

for the real-time IGS products, the clock solution obtained

for one particular station with the EMU cannot be used

alone for the monitoring of the clock stability. Only the

differences between two station clocks will, therefore, be

presented in the following.

PPP software modifications

All the PPP results presented have been obtained using the

PPP software Atomium, developed at the Royal Observa-

tory of Belgium (Defraigne et al. 2008). Only GPS data are

used in this study; the addition of GLONASS observations

does not significantly improve the PPP-based time transfer

solutions as shown in Defraigne and Baire (2011). Ato-

mium is based on a least-squares analysis over a given data

batch, providing one station position for the data batch, one

clock solution at each observation epoch and tropospheric

zenithal delays with a chosen sampling rate. In the present

case, the troposphere was determined as one estimation

each 20 min, with a relative constraint of 1.5 mm between

two successive values. A linear interpolation of the tro-

pospheric zenith delay is used between the epochs where a

value is estimated.

Some preliminary results of near real-time monitoring of

atomic clocks using Atomium and real-time IGS products

Fig. 1 PPP clock solutions for BRUX and PTBB obtained using

Atomium and the IGS real-time products for one 24 h data batch.

Bottom time transfer solution obtained as the differences of the two

PPP solutions

Fig. 2 PPP clock solutions for BRUX and PTBB obtained using

Atomium and the EMU products for five successive 24 h data batch

(top), and time transfer solution obtained as the differences of the two

PPP solutions (bottom)

GPS Solut

123

have been presented in Defraigne et al. (2012). These

results contained an important number of outliers, which is

not acceptable for a real-time monitoring. For this reason, a

new cleaning procedure has been implemented in Atomi-

um. In its original version, a pre-inversion cleaning based

on the Hatch-Melbourne-Wubbena (Hatch 1982) combi-

nation was applied on the raw data in order to detect out-

liers and cycle slips, and a second cleaning was then

operated for the rejection of smaller outliers based on the

residuals after a first inversion. However, due to some

erroneous satellite clock products in the real-time data

introduced after the first cleaning, the first inversion was

contaminated, affecting all the residuals and making the

whole procedure unsuccessful. In order to overcome that

problem, an additional cleaning was introduced after cor-

recting the code measurements for the satellite distance, the

satellite clock, and the dry atmosphere, just before the first

inversion. This allows removing new outliers caused by

problems in the real time or EMU satellite orbits and

clocks. It must be noted that the IGS real-time products

have no accuracy codes and no information concerning the

quality of the individual values, so that a software-based

cleaning is really necessary to reject observations of sat-

ellites with erroneous clock products. Furthermore, as real-

time products have epochs without any satellite clock,

mainly due to short interruptions of the streaming, an

additional software modification was introduced to keep

the same ambiguities rather than to determine new ones

when a data gap is due to the absence of satellite clock for

one epoch; this avoids the presence of frequent jumps

associated with new ambiguity determinations in the

solutions.

A validation of this new version of Atomium is pre-

sented in Fig. 3 which illustrates the differences between

the PPP clock solution computed with Atomium, and the

IGS Final combined solution for five IGS stations over the

6 month period from October 1, 2012 to March 31, 2013.

The figure confirms the announced precision of PPP-based

time transfer: the standard deviations of these differences

extend from 62 to 88 ps. Some specific multi-day corre-

lations can be seen in the differences shown in Fig. 3.

These can be explained by differences in the data pre-

processing which, associated with some multipath or near-

field effects can induce some diurnal effect as the data are

analyzed in daily data batches.

PPP using real-time and EMU products

A set of IGS stations equipped with a Hydrogen Maser was

selected in order to investigate the quality of the clock

comparisons based on a PPP computation using the IGS

real-time products and EMU products. Most of these sta-

tions are co-located with a time laboratory providing a

local realization of UTC. They are illustrated in Fig. 4.

We then used BRUX (time laboratory ORB, Brussels,

Belgium), PTBB (time laboratory PTB, Braunschweig,

Germany) and USN3 (time laboratory USNO, Washington

Fig. 3 Differences between the PPP clock solution, computed with

Atomium using the IGS final products, and the IGS final combined

clock solution

Fig. 4 Distribution of stations

used in the present study

GPS Solut

123

DC) as core stations, from which we formed all the base-

lines of different lengths. A period of 6 months has been

chosen for the analysis, covering October 1, 2012 to March

31, 2013.

Figure 5 presents the PPP time transfer solutions

obtained using Atomium and IGS real-time products, the

EMU and IGS final products for the 6 month period cited

above, applied to one long baseline (TWTF-USN3,

10,700 km) and one continental baseline (BRUX-PTBB,

450 km). Figure 6 presents the differences between the

PPP time transfer solutions obtained in short latency and

those obtained with the final IGS products, the standard

deviations of these differences are also indicated on the

Figure.

A first observation of Fig. 6 is that the magnitude of the

differences is clearly dependent on the baseline length.

This can be easily explained by the spatial correlation

between satellite position and clock errors: these errors

have a similar impact on the PPP clock solutions for nearby

stations and hence cancel when taking the difference of the

solutions for time transfer. For remote stations, the impact

on the clock solution is different since the satellite visi-

bility is different, and this impact does not cancel when

taking the difference between the two PPP solutions. It

must, therefore, be recommended to use short baselines for

making real-time monitoring of time scale using real-time

products. This thesis was confirmed by the study of all

baselines formed from the three reference stations USN3,

BRUX and PTBB. A second observation of Fig. 6 is that as

expected, the solutions obtained with the IGS real-time

products are characterized by a noise level significantly

larger than those obtained with the EMU. The precision of

the solutions is indeed increasing with the latency of the

satellite products used. In the following, we quantify the

possible clock problem detection that can be obtained

either after some seconds to minutes using the IGS real-

time problem or after about 2 h using the EMU products.

Figures 7, 8 and 9 present the average difference, the

standard deviation and the maximum of the differences

between the PPP time transfer solutions, obtained using

Atomium and either the IGS real-time products or the

EMU, and the IGS final products for the same 6 month

period and for all the time links presented in Fig. 4. In

Fig. 7, we observe that some bias can exist between solu-

tions obtained using the real-time products and those

obtained with the final IGS products; however, this bias is

not systematic but depends on the link, and its magnitude is

increasing with the distance between the stations. The

estimated bias for all the time links investigated is lower

than 150 ps, and it is always significantly smaller than the

standard deviation of the differences shown in Fig. 8. We

Fig. 5 PPP time transfer solutions obtained using Atomium and the

IGS real-time products (red), the EMU (green) and the IGS final

products (black); the curves have been vertically shifted by 2 ns in

order to improve the visibility

Fig. 6 Differences between the time transfer solutions obtained with

the IGS real-time products (red) or the EMU (green) with respect to

the solution obtained with the IGS final products for the same time

links as in Fig. 5

Fig. 7 Average of the differences, i.e., bias between the Atomium

solution obtained with final IGS products and the Atomium solution

based on IGS real-time products (red), or EMU (green)

GPS Solut

123

can, therefore, conclude that no systematic bias exists in

the solution obtained with real-time or EMU products.

Figure 8 clearly confirms the increase in the magnitude of

the differences with increasing distance between the sta-

tions. Figure 8 furthermore reproduces the standard devi-

ations of the differences between the Atomium and the IGS

final solution. It can be observed that with the EMU pro-

ducts, although producing differences increasing with the

distance, the standard deviations of these differences are

always smaller than the standard deviations of the differ-

ences between the Atomium solution based on the IGS

final products, and the IGS solution itself. The impact of

the choice of products in this case is therefore smaller than

the impact of the computation procedure. On the contrary,

the use of IGS real-time products has an impact larger than

the computation procedure for all time links larger than

3,000 km, as seen from the comparison of the standard

deviation of the differences with respect to the Atomium

solutions computed with the IGS final products. Finally,

looking at Fig. 9, we can determine the level at which a

clock jump can be detected and attributed unambiguously

to the clock and not to an artifact of the satellite clock

products used. For baselines shorter than 1,000 km, this

level is 0.5 ns when the EMU are used, i.e., with a latency

of 90–150 min, and 1 ns when the IGS real-time products

are used, i.e., with a latency of 5 min. While this level is

still valid for long baselines with the EMU products, it is

raised to 1.5 ns for baselines larger than 3,000 km with the

IGS real-time products.

In order to determine which level of frequency change

can be detected by the near real-time monitoring based on

IGS real-time/EMU products, we plotted in Fig. 10 the

frequency differences over 24 h and over 2 h between the

PPP solution computed with the IGS real-time/EMU pro-

ducts and the IGS Final combined solution. These fre-

quency differences are computed as the linear regression

coefficient over the 24 h solution differences (real-time/

EMU—Final) in the first case, and over each 2 h window

starting at an integer hour of the same differences in the

second case. We can observe that for a clock frequency

monitoring, looking at a 24 h solution, allows detection of

any frequency change larger than 1.8e-14; when using only

the last 2 h for the monitoring, only frequency changes

larger than 1.8e-13 can be detected. For smaller frequency

changes observed in the solution, no distinction can be

done between a clock frequency change and an artifact due

to the satellite orbit/clock errors.

Considering the results presented here above, two rec-

ommendations can be made for near real-time comparison

Fig. 8 Standard deviation of the differences between the Atomium

solution obtained with final IGS products and the Atomium solution

based on IGS real-time products (red), EMU (green) and the IGS final

combined solution (black)

Fig. 9 Maximum differences between the Atomium solution

obtained with final IGS products and the Atomium solution based

on IGS real-time products (red), EMU (green) and the IGS final

combined solution (black)

Fig. 10 Maximum frequency differences as a function of the baseline

length

GPS Solut

123

of atomic clocks based on PPP: the first one is to use

preferably intra-continental links, since long-distance links

are affected by a larger noise, especially when the IGS real-

time products are used. Furthermore, there is an advantage

of using the EMU products when they are available, i.e.,

with a latency of 90–150 min, as these refine the solution

obtained in near real-time with the IGS real-time products.

Note that the results obtained in this study concerning the

use of the EMU are in good agreement with a similar

experiment based on the NRCan PPP analysis software

(Cerretto et al. 2012).

Conclusions

Using real-time or Ultra-Rapid products for satellite orbits and

clocks, it is now possible to compute near real-time or low

latency PPP solutions and hence provide a near real-time or

low latency clock monitoring. We proposed an analysis of the

results obtained from the PPP analysis conducted using the IGS

real-time products for the satellite clocks and orbits, in order to

determine the level at which these PPP solutions can serve as

near real-time monitoring of the UTC(k)’s or any other clock

connected to a GNSS receiver. A comparison was, moreover,

made with the results obtained with the EMU products pro-

vided hourly by the NRCAN analysis center of the IGS.

A new version of Atomium was used for this study,

including a new data cleaning able to remove from the data

the satellites having bad orbits or clocks, avoiding the

presence of outliers in the clock solutions computed with

the IGS real-time products or any clock/orbit products of

lower quality than the IGS combined Final or Rapid pro-

ducts. Such outliers would indeed impede the method to be

used for near real-time monitoring of clocks.

Using the IGS real-time products available with a

latency of about 30 s, it was observed that the magnitude of

the differences with respect to the time transfer solutions

obtained with the IGS Final products are significantly

correlated with the distance between the stations. From

about 100 ps for a 500 km baseline, it grows up to about

300 ps for the inter-continental baselines. This results from

satellite position and clock errors which have a similar

impact on the PPP clock solutions for nearby stations and

hence cancel when making the difference between the

solutions for time transfer, while they do not cancel for

more remote stations. Detection of clock problems using

IGS real-time products is, therefore, preferable using short

baselines. However, the results presented show that near

real-time PPP based on these real-time products allows

detection in a few minutes of any clock jump larger than

1.5 ns, and any frequency change larger than 2e-14 when

looking at the last 24 h data, or larger than 2e-13 when

looking at the last 2 h.

These results were compared with those obtained using

the EMU products available with a 90–150 min latency. It

was observed that these products provide a solution of

better quality than the IGS real-time products. The mag-

nitude of the differences with respect to the time transfer

solution obtained with the IGS Final products is always

lower than the differences between the Atomium and the

IGS final solutions, which means that the impact of the

products (EMU or IGS Final) is lower than the impact of

the analysis strategy (Atomium PPP or IGS combined). It

was also shown that using the EMU products, it is possible

to detect any clock jump larger than 0.8 ns slightly more

than 90–150 min after the event, and a frequency change

larger than 1.2e-14 when looking at the last 24 h data, or

larger than 1.6e-13 when looking at the last 2 h available,

i.e., ending 90 min before the current epoch. This capa-

bility is the same as the IGS Rapid of Final products, but

with longer latency.

A low latency clock comparison for a given set of time

links is presently running at the Royal Observatory of

Belgium; the results are stored in a database, and a web

page (http://clock.oma.be) displays the associated plots

updated for each new computation. The clock comparisons

are provided with a latency of 12 min after the end of the

hour using IGS real-time products. After 90 min, the

solutions in the database and the associated plots on the

web pages are replaced by new solutions computed with

the EMU products produced by the NRCan IGS analysis

center. One day later, the solutions in the database are

replaced by those computed with the IGS Rapid products,

and 2 weeks later they are replaced by the solutions com-

puted with the Final IGS orbits.

From the statistics obtained from this analysis, it is

concluded that the solutions proposed on the webpage

allows, within an hour, detection of a clock jump larger

than 1.5 ns after 12 min, or 0.8 ns after 90 min, and a

frequency change larger than 2e-14 when looking at the

last 24 h data, or larger than 2e-13 when looking at the last

2 h.

Acknowledgments The authors thank the IGS community, and

especially the IGS Real-Time Working Group members for the real-

time clocks and orbits and the NRCan analysis centers for the hourly

EMU products which allow us to process these near real-time clock

comparisons.

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Dr. Pascale Defraigne received

her doctoral degree in 1995

from the Universite Catholique

de Louvain. She is presently

responsible for the time and

frequency laboratory at the

Royal Observatory of Belgium.

She chairs the Working Group

of the Consultative Committee

for Time and Frequency on

GNSS time transfer.

Dr. ir. Wim Aerts (GNSS

research group, Royal Observatory

of Belgium since 2009) obtained a

PhD degree in electrical engineer-

ing from the KatholiekeUniversi-

teit Leuven (KULeuven), Belgium

in 2009. His research interests

include improving accuracy of

timing and positioning with GNSS.

From 2009 onwards he is guest

professor at the Katholieke Uni-

versiteitLeuven (FIIWKU Leuven

@ KHLim) with an engineering

course on analog and digital

telecommunications.

Dr. Eric Pottiaux (GNSS

research group, Royal Observa-

tory of Belgium since 1998)

received his Ph.D. degree from

the Universite Catholique de

Louvain (UCL), Belgium, in

2010. He is member of the

geodetic expert team within the

EUMETNET EIG GNSS water

VApour Program (E-GVAP),

member of the IGS troposphere

working group and co-chairs

working group 2 of the COST

Action ES1206 GNSS4SWEC.

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