Although in accuracy parameters GLONASS is correlated with GPS, using GLONASS signals for high-precision clock synchronization was, up to the recent time, of limited utility due to the lack of specialized time receivers. In order to improve this situation, in late 1992 the Russian Institute of Radionavigation and Time (RMT) began to develop a GLONASS time receiver using as a basis the airborne ASN-16 receiver. This paper presents results of estimating user clock synchronization accuracy via GLONASS signals using ASN-16 receiver in the direct synchronization and common-view modes

Bogdanov, Pyotr P.

Gevorkyan, Arvid G.

Gouzhva, Yuri G.

Ovchinnikov, Vitaly V.

English

NASA Technical Reports Server

USING GLONASS SIGNAL FOR CLOCK
SYNCHRONIZATION N94-30680
Prof. Yuri G.Gouzhva, Prof. Arvid G.Gevorkyan,
Dr. Pyotr P.Bogdanov, Dr. Vitaly V. Ovchinnikov
Russian Institute of Radionavigation and Time
2, Rastrelli square, St-Petersburg, 193124, Russia
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Abstract
Although in accuracy parameters GLONASS is correlated with GPS, using GLONASS signals
for high-precision clock synchronization was up to the recent time of limited utility due to the
lack of specialized time receivers. In order to improve this situation, in late 1992 the Russian
Institute of Radionavigation and Time (RIRT) began to develop a GLONASS time receiver
using as a basis the airborne ASN-16 receiver. This paper presents results of estimating user
clock synchronization accuracy via GLONASS signals using ASN-16 receiver in the direct
synchronization and common-view modes.
INTRODUCTION
At present the use of satellite navifation system signals is one of lhe most high-precision ways to clock
synchronization on global scale. Varioas aspects of time synchronization via GPS signals are discussed in
diverse references, for example [I, 2 ]. Over the last 2 - 3 years, several experiments of lime synchronization
via GLONASS signals were also held [3, 4 ]. But using GLONASS signals for clock synchronization was
limited for a long time because the lack of commercial time receivers. Therefore RIRT carried out a number
of experiments to estimate the possibility of using the commercial airborne ASN- 16 receiver as the GLO NASS
time receiver. Results of these experiments and its analysis are presented in this paper. This results
demonstrate that the use of ASN-16 gives the clock synchronization accuracy about 20 ns for one day in a
direct synchronization mode and about 10 ns for one day in a common-veiw mode. To provide the automat-
ization of the measuremets and their processing the interface between ASN-16 and PC was held.
BRIEF ANALYSIS OF GLONASS
The full-deployed Russian GLONASS should consist of 24 satellites placed witnin three orbital planes by 8
satellites each plane [5 ]. The orbital planes are separated by 120 °. Salellite orbits are the circular ones with
eccentricity less than 0,01, revolution period of I 1 h 45 min, altitude of 19100 krn and inclination of 64.8 °.
Now lhe space segment consists of 12 - 15 heallhy satellites disposed in first and third orbital planes. The
satellites are equipped with a cesium lime/frequency standard with daily frequency instability no more than
5"I0 "la [5 ]. This provides accuracy of satellite time synchronization relative to System Common Time (SCT)
about 15 ns (o) with uploading time and frequency corrections (TFC) to the satellite twice a day.
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https://ntrs.nasa.gov/search.jsp?R=19940026175 2020-06-16T12:22:55+00:00Z
SCTis generatedonthebasisof mainsynchronizer( MS) time.Hydrogenfrequencystandardswithdaily
frequencyinstabilitynomorethan 5"10 -14 are used as a part of MS. The difference in SCT and UTC (SU)
should be less then 1 ms. The accuracy of SCT corrections uploaded to the satellite once a day should be less
than 35 ns (o).
When UTC (SU) is corrected by an interger number of seconds, the time of all system components including
SCT are corrected too. However, there exists a constant offset of 3 h between SCT and UTC (SU) due to
GLONASS monitoring specific features. Full deployement of GLONASS is scheduled on the end of 1995 [6 ].
Besides flying tests of modernization satellite named GLONASS-M will start in 1994. The main charac-
teristics of this satellite will be an increased satellite life ( span up to 5 years) and use of a new spaceborne
time/frequency standard with daily frequency instability no more than 1"10-13. This enables to realize a
synchronization of satellite times with an error not worse than 10 ns (a) with uploading TFC to the satellite
once a day. No special measures to degradate the GLONASS signal are planned.
CLOCK SYNCHRONIZATION VIA GLONASS SIGNALS
There are four main methods of clock synchronization via satellite navigation systems signals: direct
synchronization, common-view, clock transportation and very-long base interferometry (VLBI) Techniques
Over Short Baselines [1 ]. But the methods of the most ulility are the first two ones, which priciples are shown
in Fig. 1.
The direct synchronization method is the most simple. It provides the global coverage and requires no other
data than those received from satellite navigation message. The time difference between the user clock and
the SCT or UTC (SU) is given by the relationship ( 1 ):
a T = S - (D / C - r ion - rtrop - rrec) + A Tsar + a Tsct
where S
- measured pseudorange between the satellite and the user, i.e. the difference between
two identical codes, one received by the receiver, whereas the other one generated by
the receiver; each of these codes is synchronized by its own clock;
D - range from the satellite to the user;
C - speed of light;
rion - propagation delay due to the ionosphere;
rtrop - propagation delay due to the troposphere;
Trec - receiver delay;
a Tsar - difference between satellite clock and SCT;
A Tsct - difference between SCT and UTC.
(i)
The range from the satellite to the user is computed on the basis of broadcasted ephemerides Xi, Yi, Zi and
the known coordinates of the receiver antenna XA, YA, ZA. The difference between satellite clock and SCT is
determined on the basis of TFC ri, yi. The difference between SCT and UTC is directly contained in the
navigation message as a SCT correction rs.
Since GLONASS navigation messages doesn't include model's parameters for consideration for the
ionosphere delay, the ionospheric correction is computed by the user on the basis of parameters which are
autonomously stored in the receiver. The troposperic delay is determined in a similar way.. The receiver delay
is calculated by means of its periodical calibration. It follows from the relationship ( 1 ) , that the clock
synchronization accuracy while using direct technique is defined by the following components:
534
- the error of pseudorange measuring;
- the instability of receiver delay;
- the error of ionospheric and tropospheric influence consideration;
- the uncertainty of the antenna coordinates;
- the error of the satellite ephemerides;
- the error of the satellite clock;
- the error of SCT correction.
The last three components are diclated by GLONASS's characteristics. That's why the clock synchronization
accuracy via GLONASS signals in a direct synchronization mode doesn't exceed tens of nanoseconds.
The common-view method is a technique of a mutual clock synchronization. It presupposed simultaneous
measurements at the points of clock location via one of the satellite's signals and further exchange of the
results of the measurements. In this case, mutual difference of clock times is determined from the relationship
(2):
ATA-B = ATA - AT8 (2)
where zxTA.B - the result of determination of each clock's offset relative to SCT or UTC (SU) in conformity
with ( 1 ).
It is obvious that, due to the elimination of several components, stipulated by GLONASS characteristics,
which are common for A and B clock, its mutual synchronization accuracy increases. Therefore characteristics
of the receivers which determine the potential clock synchronization accuracy are of great importance.
USE OF ASN-16 RECEIVER FOR CLOCK SYNCHRONIZATION
In order to estimate the possibility of using the commercial airborne GLONASS ASN-16 receiver as a time
receiver, a number experiments was carried out in RIRT. The measurements were held simultaneously using
two ASN-16 receivers on the basis of RIRT's secondary time/frequency standard (STFS). The uncertainty
of the antenna coordinates was approximately 5 m.
The ASN- 16 unit, designed by RIRT, is one-channel one-frequency navigation user equipment. While solving
the time task it provides lhe operation via one chosen satellite with output of l-Hz and the result of
determination of the time difference between this signal and UTC (SU). That's why for clock
synchronization via GLONASS signals using of ASN-16 an additional counter of a 1-Hz signal difference in
ASN-16 output and user's clock is requied. In order to eliminate this defect, one of ASN-16 reciever was
reworked to provide a time task solving relative to the external signal of 1-Hz. As a result of this, the ASN-16
reciever takes part of a time receiver. The experiment configuration is shown in Fig.2.
The procedures of experiments were as follows. The 15-minute sessions of measurements were held every
half of hour during the day. The data registrations took place every 15 seconds. Then average value AT and
root-mean-square value 6 of the offset between STFS and UTC (SU) were estimated for each session, for
every day and over all the interval of the measurements (the drift of the offset between STFS and UTC was
eliminated). Simultaneously, the data received had been used for estimation of a mutual clock lime difference
measurements accuracy using ASN-16 as an average value aTra and rms value 6m for each session, for every
day and over all the interval of the measurements. The results for each session of measurements are given in
Table I, for every day and whole intervals - in Table 2.
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The average value of time synchronization accuracy via GLONASS signals in the direct synchrenization mode
was 6,8 ns for session, 20.6 ns for one day and 23.3 ns for 5-day intervals for the serial ASN-16 receiver and
7.1 ns; 20.8 ns and 24.4 ns respectively for the reworked ASN-16 receiver. Practically absolute coincidence of
the results proved that accuracy characteristics of ASN-16 had not become worse due to the reworking the
receiver.
The systematic error of mutual synchronization of the clock with the two ASN-16 receivers was close to zero
for a day, and the random error was 6.1 ns for a session, 10.3 ns for a day and I0.0 ns for the whole interval.
An approximate coincidence between the random error of the mutual synchronization of the clock for a session
and that of the clock synchronization in direct synchronization mode can be explained by the fact that within
one session the influence of the parts, stipulated by receivers characteristics, prevail upon the parts, stipulated
by GLONASS characteristics. So there were no essential synchronization accuracy increase. However, for the
averaging over a day, the clock synchronization accuracy in a common-view mode is twice better then for
direct synchronization mode.
PROSPECTS FOR ASN-16 RECEIVER USE
In order to provide a wider use of ASN- 16 as GLONASS time receiver the task of its interfacing with a PC was
solved. This provides fully automatic process of measuring and data processing. Module of data exchange
between ASN-16 and PC interface is a board installed in PC. With this interface module the setting of
ASN-16 operation modes and the reception of the necessary measuring and processing data is provided. The
rate of the data output from ASN-16 is 1 second with a period of change of 2 seconds.
The developed software for PC realizes as follows:
= the GLONASS state indication;
- measurements programs generating and realizing;
- data reception and registration;
- real-time time/navigation tasks solving;
- statistical processing and analysis of data;
- ASN-16 operation monitoring;
- data exchange with other users.
The creation of such a measurement system on the basis of ASN-16 receiver and PC will make possible more
careful estimation of GLONASS characteristics as the time transfer system and to begin using GLONASS
signals for high-precision clock synchronization.
CONCLUSION
The results of the experiments held in RIRT have shown that already at present GLONASS provides time
synchronization accuracy about 10 - 20 ns. While using ASN-16 receiver, clock synchronization accuracy of
20 ns for one day in a direct synchronization mode and about 10 ns for a day in a common-view mode is
provided.
The further increasing of time synchronization accuracy via GLONASS signals with ASN-16 may be realized
using direct pseudorange measurements and solving a time task with PC.
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ACKNOWLEDGEMENTS
Theauthorsarepleased to express their gratitude to their colleagues in RIRT for their participation in the
experiments and data analysis.
REFERENCES
1. D.W.Allan and M.Weiss. Accurate time and frequency transfer during common-view of a GPS satellite,
Proc. 34th Ann.Syrup. on Frequency Control, pp.334-346, May 1980.
2. W.Lewandowski, C.Thomas. GPS Time transfer, Proc. of the IEEE, Special Issue on Time and Frequency,
pp.991-1000, July 1991.
3. P.Daly, G.T.Cherenkov, N.B.Koshelyaevsky, S.B.Pushkin. Satellite lime transfer between UTC (USNO)
and UTC (US) using Navstar GPS and GLONASS, Proc. 4th Institute of Navigation Technical Meeting,
pp.199-206, September 1991.
4. P.Daly, N.B. Koshelyaevsky, W.Lewandowski, G.Pelit and C.Thomas. Comparison of GLONASS and
GPS time transfers between two West European time laboratories and VNIIFTRI, Proc. 23rd PTTI meeting,
pp.341-350, 1991.
5. V.Kazantsev, V.Karnaukhov. GLONASS Satellite Radionavigation System, First International
Symposium "Real Time Differential Applications of the Global Positioning System", 1991.
6. Y.G.Gouzhva, A.G.Gevorkyan, A.B.Bassevich, P.P.Bogdanov. High-precision Time and Frequency
Dissemination with GLONASS, GPS World, pp.40-49, July 1992.
537
Table1. TheestimationsoflimesynchronizationaccuracyviaGLONASSsignalsforsessions
Time ASN-16/ 1 ASN-16 / 2 common-view
h, min AT, ns $, ns AT, ns _, ns ATm, ns 3m, ns
8.00 2522.0 3.8 2539.5 10.1 -17.5 I0
8.30 2520.0 0.0 2512.0 0.0 8.0 0.0
9.00 2470.0 0.0 2465.0 7.5 5.0 7.5
9.30 2456.0 8.7 2464.0 4.4 -8.0 I0.0
10.00 2520.0 9.9 2524.5 4.3 -4.5 10.3
10.30 2510.5 2.2 2507.0 4.9 3.5 5.0
11.00 2524.0 6.4 2528.5 4.6 -4.5 7.5
11.30 2455.5 5.8 2455.0 7.1 0.5 10.2
12.00 2471.5 3.7 2470.0 3.6 1.5 4.5
12.30 2491.0 3.0 2488.0 4.4 3.0 5.4
13.00 2490.0 5.6 2489.5 4.4 0.5 5.5
13.30 2470.5 2.2 2488.5 4.3 -18.5 4.6
14.00 2470.5 2.2 2468.5 4.6 2.0 4.8
14.30 2511.0 3.1 2506.5 5.1 4.5 5.0
15.00 2511.0 3.1 2508.5 4.4 2.5 4.8
15.30 2532.0 3.9 2534.5 7.4 -2.5 8.0
16.00 2532.5 4.0 2514.0 4.0 18.5 6.3
16.30 2491.0 3.0 2490.0 3.6 1.0 4.4
17.00 2572.0 3.5 2571.0 2.2 1.0 4.1
17.30 2492.0 4.1 2491.5 0.0 0.5 4.1
Table 2. The estimations of time synchronizalion accuracy via GLONASS signals for every day
and the whole interval
Dale ASN-16 / 1 ASN-16 / 2 common-view
h,min AT, ns ,_, ns _T,ns 3,ns aTm,ns $m,ns
21.12 2511.0 18.2 2511.5 17.5 -0.5 9.8
22.12 2591.0 22.1 2591.5 16.4 -0.5 9.3
23.12 2688.0 21.5 2685.5 26.8 2.5 10.9
24.12 2778.0 20.9 2780.0 22.4 -2.0 11.4
25.12 2863.0 19.5 2862.5 20.8 0.5 10.2
21-25.12 2686.0 23.3 2662.5 24.3 0.0 I0.0
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X,Y,Z
'IS
A
w
A(X A ,YA ,ZA)
a) The direct-synchronization technique
._A (XA, "_YA ,ZA) B(XB ,YB,ZB)_
b) Common-view method
Figure I. The direct-synchronization technique
and common-view method via GLONASS signals
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,// /
ASN - 16 1Hz ASN - 16
1 _ COUNTER 2
A ,_STFS - UTC
A "ESTFS - UTC
Figure 2. The experiment configuration
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Using GLONASS signal for clock synchronization

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