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PRECISE TIME TRANSFER USING GPS CARRIER PHASE-BASED TECHNIQUES

Jan Johansson and Kenneth Jaldehag

SP Swedish National Testing and Research Institute, Box 857, S-501 15, Borås, Sweden

ABSTRACT

We have used the carrier phase signal from the

satellites in the Global Positioning System (GPS) in an

attempt to perform precise time and frequency transfer.

The method is commonly used in the geodetic

community to obtain mm-level precision over global

distances. When estimating the station position other

parameters must be solved for. Such parameters include,

e.g., the atmospheric propagation delay as well as

determination of clocks in the GPS-receivers. In this

study we have focused on the evaluation of these clock

solutions for two Swedish GPS stations with particular

interest for the timing community. One station is

collocated with the Swedish national laboratory for time

and frequency and equipped with several cesium

frequency standards. The other station is located at the

Onsala Space Observatory where all receivers are

connected to a hydrogen maser.

1. INTRODUCTION

The GPS carrier phase data have been in extensive use

in the geodetic and geophysical community for more

than a decade. It is possible to reach mm-level precision

even over global distances using GPS carrier phase- based relative positioning. Simultaneously, several ”by-

products” have come available because of the modeling

and the estimation technique used. For example, it is

necessary to calculate or estimate the signal propagation

path delay in the atmosphere in order to obtain accurate

coordinates of a station. Products such as the amount of

water vapor in the troposphere and the total electron

content in the ionosphere are standard deliverables from

several GPS data analysis centers.

Another parameter, which has to be solved for in the

estimation process, is the receiver clock. If the clock of

the GPS receiver is based on the signal from an atomic

frequency standard the parameter may be preciselydetermined. We might actually be able to study the

relative performance of an external frequency standard

connected to a high-quality GPS receiver.

Recent developments in atomic frequency standards

such as the cesium fountain and frequency standards

based on linear ion traps calls for new methods in time

and frequency transfer techniques. GPS carrier phase-

based time transfer is considered to have the potential

required [1]. In order to take full advantage of this

technique, the delays of various parts of the receiving

equipment must be stabilized and measured (includes

e.g., cables, receiver, and antennas). Especially, serioushardware delay instabilities may result from temperature

variations in the vicinity of the receiver system. For timelaboratories it is essential to minimize this effect.

2. GPS NETWORKS AND PRODUCTS

2.1 The Swedish permanent GPS network.

The Swedish permanent GPS network SWEPOS® has

been in continuos operation since August 1993. The

network consists of 21 stations (see figure 1). The

average station separation is 200 km covering a region

from latitude 55° to 69° north [2]. The communication

between the SWEPOS® station and the network control

center, hosted at the National Land Survey of Sweden, is

managed via 64 kbit/s TCP/IP lines. With this real time

data flow, SWEPOS® is a multipurpose network with

applications stretching from navigation to geophysics.

Figure 1: The Swedish network of continuously

operational GPS stations, SWEPOS®.

Data from the permanent GPS stations are analyzed

daily within several different projects such as studies of

present-day glacial isostatic adjustment and sea-level

rise, monitoring of water vapor in Earth’ troposphere,

and for addressing reference frame issues

(IGS/EUREF). In order accomplish this, the design of

each site and the choice of hardware are importantissues [2].

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2.2 The International GPS Service (IGS).

The IGS was founded to improve geodetic

applications of GPS by providing an infrastructure for a

global network of high quality and continuously

operational GPS stations, data collection and archiving,

and by making available products based on post- processing of GPS data. The basic products include

precise determination of satellite orbit parameters and

clocks. In high-precision carrier phase-based relative

positioning we need to obtain accurate information

about the satellite orbital parameters and the satellite

clocks. This can be obtained from the IGS with a lag-

time of 24 hours up to 11 days depending on the level of

accuracy needed. One may also determine these

parameters simultaneously with other parameters but

this requires a fairly big network of GPS tracking

stations.

3. EXPERIMENTAL SETUP

The two SWEPOS® stations, Borås and Onsala, are of

special interest to the time keeping community since

they contribute to the realization of the atomic time

scale and to the global reference frame. Data from

several receivers hosted at these two stations have been

used in order to investigate different aspects of precise

frequency transfer. In this section we will briefly

describe the hardware at each site and the strategy used

for GPS data processing.

3.1 The GPS station at SP in Borås

The station in Borås, located at the SP Swedish

National Testing and Research Institute, is a member of

SWEPOS® and collocated with the national laboratory

for time and frequency. Three cesium frequency

standards are available at SP and one is used for the

realization of UTC(SP). Data from two HP5071A high-

performance and one Oscilloquartz 3200 cesium

standards are contributing to TAI. The entire time and

frequency laboratory is hosted in a temperature-

controlled environment where the temperature is

continuously monitored and known to be 24 ± 0.5 °C.

The GPS station in Borås is hosted in the samelaboratory and all the receivers are consequently in the

same temperature-controlled environment. Similarly as

all SWEPOS® sites the Borås station is equipped with

two Ashtech Z12 receivers both connected to same

antenna through a power splitting device. These two

receivers are both hooked up to TCP/IP and delivers

real-time data to the SWEPOS® operational center. In

addition, two TurboRogue SNR-8000 geodetic receivers

and three timing receivers also receive GPS signals from

the same GPS antenna. The four ”geodetic” GPS-

receivers in Borås all utilizes external 5 MHz from the

cesium frequency standard from which UTC(SP) is

based.

Figure 2: The SWEPOS® station at SP in Borås.

The GPS station in Borås is unique in the sense that,

in addition to the GPS receivers, also the antenna cable

is in a temperature controlled environment. The antennacable is placed in a water pipe where the temperature of

the water is kept at 7 ± 1 °C. Thus, we do not expect any

significant fluctuation in the electrical delay of the cable

due to temperature variations. The three-meter high

concrete pillar, on top of which the GPS antenna is

mounted, is shown in figure 2. The entire pillar is

temperature-controlled by means of electrical heating

and cooling water.

As at all other SWEPOS® sites the Borås station has

a Dorne-Margolin T choke-ring antenna. A hemispheric

radome is used to protect the antenna. The GPS antenna

is not included in the temperature-controlled

environment and may contribute to the error budget. Atemperature sensor was therefore installed next to the

GPS antenna in order to model this effect.

3.2 The IGS station at Onsala Space Observatory

The Onsala station, located at the radio astronomy

facility Onsala Space Observatory, is a long-standing

member of the global GPS-satellite tracking network,

IGS and also a member of SWEPOS®. The station is

equipped with two TurboRogue SNR-8000, three

Ashtech Z12 GPS receiver, and one Ashtech Z18,

tracking both GPS and GLONASS satellites. All the

receivers are connected to the same Dorne-Margolin B

GPS antenna via a power splitting device. The antenna

is mounted on top a one meter concrete pillar and

covered by a hemispheric radome (see figure 3).

The Onsala GPS-receivers operates using an external

hydrogen-maser frequency standard which is equivalent

to the frequency standard used in the radio-astronomy

and geodetic observations with the Very Long Baseline

Interferometry (VLBI) technique. The GPS antenna,

antenna cable, and GPS receivers are not in a

temperature controlled environment. However, the

temperature are carefully logged every half hour near

the antenna, the antenna cable, and inside the cabinhosting the GPS-receivers.

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Figure 3: The GPS antenna and pillar system at the

Onsala station in front of the radome-enclosed 20 meter

radio telescope used for e.g., in geodesy VLBI.

3.3 The GPS data analysis

The analysis of GPS data basically adapted

traditional methods used in space geodesy and remote

sensing applications. Even though data from about 50

stations have been processed, this study includes only

the results obtained from the TurboRogue receivers at

Onsala and Borås. The entire data set was analyzed

using the GIPSY software [3], based on a Kalman filter

approach. Station coordinates were constrained to their a

priori value within a mm. The signal propagation path

delay in the troposphere is modeled as a random-walk

process with stochastic updates of one zenith delay parameter and two gradient parameters per stations

every five minutes. We used dual-frequency

measurements, which eliminate most of the delay caused

by the ionosphere. The receiver clock parameters were

updated every five minutes using a white-noise process

with very loose (1 s) a priori sigma. We have used the

most accurate products produced by the IGS community

as described above. The satellite orbits from the IGS

were held fixed. Finally, we should mention the

inclusion of accurate modeling of Earth tides and ocean

loading.

4. RESULTS

4.1 Zero baseline test

In order to eliminate all error sources and

demonstrate the capability of the method, we have used

a zero baseline test. Two GPS receivers were connected

to the same antenna and also utilizing external 5 MHz

from the same cesium frequency standard. Basically, all

other error sources were eliminated except those

associated with instrumental biases within the two GPS

receiver themselves, data registration, and data analysis

methods. Under these ideal conditions the GPS carrier phase-based technique are capable of frequency transfer

at the level of a few parts in 1016

after less than 12 h

averaging time. Figure 4 shows the Allan deviation

based on the zero-baseline test over 3 days. Apparently,

the slope of the Allan deviation is close to –1 and may

be restricted by the measurement phase noise.

Figure 4: The Allan deviation as a function of

averaging time between two receivers sharing the same

antenna, antenna cable, and external cesium standard.

4.2 Zero-baseline with different frequency standards

In order to test the capability of frequency transfer

the two TurboRogue receivers in Borås were connected

to different cesium standards (see fig. 5). However, both

receivers were sharing the same GPS antenna. Using a

Time Interval Counter (TIC) the 1 PPS from each GPS

receiver and cesium standard can be monitored. Thus,

we may compare the difference between the cesium 1

and the cesium 4 obtained from the readings from theTIC and from the GPS data analysis, respectively.

Cesium1

Cesium4

FDA

Switch

TR2

TR1

Digitalclock

µ-phasestepper

TIC

5 MHz

CS1

5 MHz CS4

1-pps

1-pps

5 MHz 5 MHz

5 MHz

1-pps

1-pps

UTC(SP)

Figure 5: The setup used in the zero-baseline frequency

transfer between cesium 1 and 4. The dashed ”5 MHz-

line” corresponds to the setup used in section 4.1.

The difference between these two methods over a

period of almost three weeks in March 1999 is plotted in

figure 6. The GPS solutions were obtained daily and

based on daily IGS orbits. Jumps of up to 150 ps in the

time series are clearly evident from day-to-day and

related to the GPS solutions. The entire period have a

standard deviation of 120 ps. However, if we just look

over a one-day periods (one GPS solution) we obtain 60

ps. The IGS orbits have an uncertainty of about 5 cm.

Since a jump of 100 ps is roughly equivalent to 3 cm,

orbit errors are significant. The corresponding Allan

deviation is shown in fig. 7. We have also included a

line describing white frequency noise reflecting thecesium standards.

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4.3 A 60 km baseline with different frequency standards

Using the same time period as above, frequency-transfer

between the stations in Borås and Onsala have been

performed. The two stations are separated by

approximately 60 km. In figure 7 the time difference

between the two receivers is shown. The frequencyoffset has been removed before plotting. In the same

figure we have included both the indoor and outdoor

temperature at the Onsala GPS station. A correlation

between, especially, the temperature in the GPS cabin

and the time difference is evident. Similar hardware

temperature dependence has been detected by other

investigations.

In figure 8, the Allan deviation is plotted. The results

closely follow the –0.5 slope and one may conclude that

this is the performance level of the cesium frequency

standard. Variations, most likely caused by changes in

the temperature at Onsala, are visible in fig. 8.

-0,5

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0,4

0,5

69 71 73 75 77 79 81 83 85 87

Day of Year 1999

T I C - G P S [ n s ]

Figure 6: The difference between the time intervalcounter and the GPS estimates.

Figure 6: The Allan deviation plotted as a function of

averaging time for a zero-baseline experiment where the

two GPS receivers were setup according to fig. 5.

5. DISCUSSION AND CONCLUSIONS

The capability of using GPS carrier phase-based

techniques in time and frequency transfer applications

has been demonstrated. In a zero-baseline test we have

obtained an Allan deviation of about 4 x 10-16

when

averaged over less than 12 hours. Furthermore, a GPS

zero-baseline was used to compare two cesium

frequency standards hosted at the same site and

simultaneously we also compared these two cesium

standards with a hydrogen maser over a 60 km baseline.

The Allan deviation plots show that the performance

level of cesium frequency standards is reached.

However, there is a correlation between temperature

variation in the vicinity of the GPS hardware at Onsalaand the results obtained in the frequency comparison.

The Onsala IGS stations will now be upgraded. The

receivers, the antenna cable, and the antenna will be

temperature controlled similar to the Borås site. Finally,

we intend for Borås to become an IGS station within the

framework of the new IGS/BIPM pilot project.

-6

-4

-2

0

2

4

6

8

10

78 79 80 81 82 83 84 85 86 87Day of Year 1999

B o r å s - O

n s a l a

[ n s ]

-40

-30

-20

-10

0

10

20

30

T e m p e r

a t u r e [ C ]

GPS

Onsala outdoor temperature

Onsala indoor temperature

Figure 7: The time difference between Borås and

Onsala GPS receivers over 9 days (lower curve). Also

plotted are the outdoor (mid curve) and the indoor (top

curve) temperature at the Onsala station.

Figure 8: Frequency stability obtained from a 60 km

baseline between Onsala and Borås.

6. REFERENCES

[1] G. Petit, C. Thomas, Z. Jiang, P. Uhrich, and F. Taris. Use

of GPS Ashtech Z12T Receivers for Accurate Time and

Frequency Comparisons. IEEE International Frequency

Control Symposium, Pasadena, Ca, May 27-29, 1998.

[2] H.-G. Scherneck, J.M.Johansson, J.X.Mitrovica, and

J.L.Davis. The BIFROST Project: GPS determined 3-D

displacement rates in Fennoscandia from 800 days of

continuous observations in the SWEPOS network.

Tectonophysics, 294, pp. 305-321, 1998.

[3] F.H.Webb and J.F. Zumberge. An Introduction to

GIPSY/OASIS-II. Jet Propulsion Laboratory, 1993.

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