Precise Point Positioning Using Combined GPS and GLONASS Observations

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Journal of Global Positioning Systems (2007)

Vol.6, No.1: 13-22

Precise Point Positioning Using Combined GPS and GLONASS

Observations

Changsheng Cai, Yang Gao

Department of Geomatic s Engineering, University of Calgary, AB, Canada

Abstract. Precise Point Positioning (PPP) is currently

based on the processing of only GPS observations. Its

positioning accuracy, availability and reliability are very

dependent on the number of visible satellites, which is

often insufficient in the environments such as urban

canyons, mountain and open-pit mines areas. Even in the

open area where sufficient GPS satellites are available,

the accuracy and reliability could still be affected by poor

satellite geometry. One possible way to increase the

satellite signal availability an d positioning reliability is to

integrate GPS and GLONASS observations. Since the

International GLONASS Experiment (IGEX-98) and the

follow-on GLONASS Service Pilot Project (IGLOS), the

GLONASS precise orbit and clock data have become

available. A combined GPS and GLONASS PPP could

therefore be implemented using GPS and GLONASS

precise orbits and clock data. In this research, the

positioning model of PPP using both GPS and

GLONASS observations is described. The performance

of the combined GPS and GLONASS PPP is assessed

using the IGS tracking network observation data and the

currently available precise GLONASS orbit and clock

data. The positioning accuracy and convergence time are

compared between GPS-only and combined

GPS/GLONASS processing. The results have indicated

an improvement on the position convergence time but

correlates to the satellite geometry improvement. The

results also indicate an improvement on the positioning

accuracy by integrating GLONASS observations.

Keywords. GPS, GLONASS, Precise Point Positioning,

Precise orbit and Clock

1 Introduction

Current Precise Point Positioning (PPP) system

developed is based on only GPS observations. The

accuracy, availability and reliability of PPP positioning

results however are quite dependent on the number of

visible satellites. Under environments of urban canyons,

mountains and open-pit mines, for instance, the number

of visible GPS satellites is often insufficient for position

determination (Tsujii, 2000). Further, even in the open

area where sufficient GPS satellites are available, the PPP

accuracy and reliability may still insufficient due to poor

satellite geometry. One possible way to increase the

availability of satellites as well as the reliability of the

positioning results is to integrate GPS and GLONASS

observations. The benefit from such integration is

obvious particularly for applications in urban canyons,

mountain and open-pit mining environments.

Since the International GLONASS Experiment (IGEX-

98) and the follow-on GLONASS Service Pilot Project

(IGLOS) conducted in 1998 and 2000 respectively

(Weber, 2005), the precise GLONASS orbit and clock

data have become available over times. Currently, four

organizations can provide independent GLONASS

precise orbits consistent at 10-15 cm level but only two

centers provide post-mission GLONASS clock data

(Oleynik, 2006). This provides opportunities to use

GLONASS observations to improve precise point

positioning accuracy and reliability currently based on

only GPS observations. Although GLONASS achieved

its Full Operational Capability (FOC) in January 1996

when 24 GLONASS satellites were available for

positioning and timing, its constellation had dropped to

several satellites by the year of 2001 due to a decrease in

GLONASS budget (Zinoviev, 2005). As of Nov. 19,

2007, there are 18 GLONASS satellites in orbit but only

9 of them are operational satellites. However, the Russian

government has approved a long-term plan to reconstitu te

a GLONASS constellation of 24 satellites. 18 satellites

are expected to be operational by 2008, and a full

operational capability with 24 satellites will be achieved

14 Journal of Global Positioning Systems

by 2010-2011. By that time, the number of GLONASS

satellites will not be a problem any more.

In this paper, we will investigate the integration of GPS

and GLONASS observations for improved accuracy and

reliability of positioning results using PPP. The quality

and characteristics of currently available precise

GLONASS orbit and clock products are first described.

The positioning model for combined processing of GPS

and GLONASS observations is then presented. IGS

tracking network observation data and available precise

GLONASS orbit and clock data are used to assess the

performance of combined GPS and GLONASS precise

point positionin g. Comparisons are also conducted on the

numerical results between GPS only and combined

GPS/GLON A S S p r o c e s s ing.

2 GLONASS Precise Orbit and Clock Products

GLONASS has been on the way to its modernization. In

2003, the first GLONASS-M satellite was launched,

where “M” stands for Modified. On December 25, 2006,

three GLONASS-M satellites (GLONASS 715,

GLONASS 716 and GLONASS 717) were launched. All

the three satellites are placed on orbit II. The GLONASS-

M is a modernized version of the GLONASS spacecraft

which supports a number of new features, such as the

satellite design-lifetime increased to 7 years, a second

civil modulation on L2 signal, and improved clock

stability. The third generation GLONASS satellite

“GLONASS-K” is expected to launch in 2008. The

satellite service life is further increased to 10-12 years

and a third civil signal frequency and Synthetic Aperture

Radar function will be added (Sergey, 2007). The

GLONASS-K represents a radical change in GLONASS

spacecraft design, adopting a non pressured and modular

spacecraft bus design (Kaplan, 2006).

The International GLONASS Experiment (IGEX-98) is

the first global GLONASS observation and analysis

campaign for geodetic and geodynamics applications,

conducted from October 19, 1998 to April 19, 1999 and

organized jointly by the International GNSS Service

(IGS), the International Association of Geodesy (IAG)

and the International Earth Rotation Service (IERS). The

main objectives of the experiment were to collect a

globally-distributed GLONASS dataset by using dual-

frequency GLONASS receivers and determine the precise

GLONASS satellite orbit. IGEX-98 has a global network

consisting of 52 stations with 19 dual-frequency and 13

single-frequency receivers. For the IGEX-98 campaign,

an infrastructure comparable to that of the IGS was

established (Habrich, 1999). IGEX-98 includes the

production of precise orbits for all the operational

GLONASS satellites (Weber, 2005).

The International GLONASS Service Pilot Project

(IGLOS) is a follow-on project of IGEX-98 that began in

2000 with the major purpose to integrate the GLONASS

satellite system into the operation of IGS. The IGLOS

Pilot Project has a global network consisting of about 50

tracking stations with dual-frequency GLONASS

receivers. The GLONASS data are collected continuously

and archived in RINEX format at the IGS Global Data

Centers (Weber, 2005). The GPS and GLONASS

observations are processed simultaneously and therefore

the precise orbit products for both systems are given in

one unique reference frame (Weber, 2002).

Currently four IGS analysis centers are routinely

providing GLONASS precise orbit products. They are

CODE (University Berne, Switzerland), IAC

(Information - Analytical Center), ESA/ESOC (European

Space Operations Center, Germany) and BKG

(Bundesamt für Kartog raphie und Ge odäsie, Germ any).

CODE can generate the final GLONASS orbit as well as

the rapid and predicted rapid orbit products (Weber,

2005; Schaer, 2004). The CODE orbits are expressed in

the IGb00 reference frame, which is the IGS realization

of the ITRF2000 (Bruyninx, 2007). IAC is a department

at MCC (Russian Mission Control Center) which is

routinely monitoring the GLONASS performance.

Starting from 2004, IAC started to conduct routine orbit

and clock determination based on IGS tracking network

data. Since 2005 IAC has become one of the four IGS

analysis centers who are routinely providing GLONASS

post-mission orbit and clock data including

(Oleynik,2006):

a) the final orbit and clock data with a delay of 5 days;

b) the rapid orbit and clock data with a delay of 1 day.

ESA/ESOC began to process and analyze GNSS data for

precise orbit determination in 1991, first using its

GPSOBS/BAHN software to compute the precise GPS

orbits and clock parameters and then aligning its

GLONASS solution to the ITRF2000 reference frame

using the GPS orbits and tight constrains on the

coordinate s of 7 ob serving sta t i ons ( R omero, 2004 ).

BKG has processed and analyzed the combined

GPS/GLONASS observations from a network of global

tracking stations since the beginning of the IGEX-98.

Similar to ESA/ESOC, BKG first computes GPS orbits,

clock estimation and earth orientation parameters and

then utilizes the Bernese software to produce precise

GLONASS orbits and station coordinates on a daily basis

using double-differenced phase observations (Habrich,

2004). It provides GLONASS precise orbits, receiver-

specific estimates of the system time difference between

GPS and GLONASS, and the station coordinates (SINEX

files).

Cai et al.: Precise Point Positioning Using Combined GPS and GLONASS Observations 15

The independent GLONASS orbits from the above four

organizations are consistent at the 10-15cm level and

have been combined to generate the IGS final GLONASS

orbits using a procedure similar to IGS final GPS orbit

(Weber, 2005).

As to precise satellite clock data, currently only two data

analysis centers, namely IAC and ESA/ESOC, provide

post-mission GLONASS clock data. The direct

comparison of precise colock data from different centers

however can hardly be conducted due to different

reference time scales used and different inter-frequency

biases applied to the GLONASS code measurements. The

agreement between the IAC and ESOC post-mission

GLONASS clock values is considered at the level of

1.5ns (Oleynik, 2006).

3 Combined G PS /G L ONASS Data Processing for PPP

In the following, the positioning model for a combined

GPS and GLONASS PPP system is described along with

mathematical equations.

Based on a dual-frequency GPS/GLONASS receiver, the

pseudorange and carrier phase observables on L1 and L2

between a receiver and a satellite can b e described by the

following observation equations:

gPmult

gPion

trop

orb

dddcdTcdtP

+++−+=

(1)

mult

ion

trop

orb

dddcdTcdt

ΦΦ

+++

−++−+=Φ

ελ

(2)

rPmult

rPion

trop

orb

dddcdTcdtP

+++−+=

(3)

mult

ion

trop

orb

dddcdTcdt

ΦΦ

+++

−++−+=Φ

ελ

(4)

where the superscript

and r refer a GPS and a

GLONASS satellite respectively, and

P is the measured pseudorange on i

L (m);

Φ is the measured carrier phase on i

L (m);

is the true geometric range (m);

c is the speed of light (m/s);

dt is the receiver clock error (s);

dT is the satellite clock error (s);

orb

d is the satellite orbit error (m);

trop

d is the tropospheric delay (m);

Lion

d/ is the ionosph eric delay on i

L (m);

is the wavelength on i

L (m/cycle);

N is the integer phase ambiguity on i

L (cycle);

Pmult

d/ is th e multipath effect in the measured

pseudorange on i

L(m);

mult

dΦ/ is the multipath effect in the measured carrier

phase on i

L(m);

is the measurement noise (m);

A system time difference unknown parameter should be

introduced for mixed GPS/GLONASS observation

processing (Habrich, 1999). A receiver clock error can be

described as

sys

ttdt

−

(5)

where sys

t denotes either GPS system time GPS

t for GPS

observations or GLONASS system time GLONASS

t for

GLONASS observations. Since the receiver clock error is

related to a system time, the combined GPS and

GLONASS processing includes two receiver clock offset

unknown parameters, one for the receiver clock offset

with respect to the GPS time and one for the receiver

clock offset with respect to the GLONASS time. We can

also describe the GLONASS receiver clock offset as

follows:

sys

GLONASSGPSGPS

GLONASS

dtdt

tttt

ttdt

−+−=

−=

(6)

which is a function of the GPS receiver clock offset and a

system time difference between GPS and GLONASS.

Applying equation (6) into equations (3) and (4) results in

the following pseudorange and carrier phase observation

equations:

rPmult

rPion

trop

orb

sys

dddcdTcdtcdtP

+++−++=

(7)

mult

ion

trop

orb

sys

dddcdTcdtcdt

ΦΦ

+++

−++−++=Φ

ελ

(8)

Before GPS and GLONASS observations are used for

position determination, the GPS and GLONASS precise

orbit and clock data should be first applied to correct

satellite orbit errors and satellite clock offsets. The

ionospheric refraction bias can be eliminated by

constructing an ionosphere-free combination of phase or

pseudorange observable from the L1 and L2 frequencies.

After the application of precise orbit and clock

16 Journal of Global Positioning Systems

corrections, the ionosphere-free code and phase

combinations can be written as follows:

trop

dcdt

ffPfPfP

ερ

+++=

−⋅−⋅= )/()( 22

1 (9)

trop

Ndcdt

ffff

++++=

−Φ⋅−Φ⋅=Φ

ερ

)/()( 22

1 (10)

tropsys

dcdtcdt

ffPfPfP

ερ

++++=

−⋅−⋅= )/()(2

1 (11)

tropsys

Ndcdtcdt

ffff

+++++=

−Φ⋅−Φ⋅=Φ

ερ

)/()( 2

1 (12)

where

P is the ionosphere-free code combination (m);

Φ is the ionosphere-free phase combination (m);

f is the frequency of i

L (Hz);

N is the combined ambiguity term (m);

contains measurement noise, multipath as well as

other residual errors.

The unknown parameters of the positioning model based

on the above observation equations include three position

coordinates, a receiver clock offset, a system time

difference, a zenith wet tropospheric delay, and

ambiguity parameters equal to the number of observed

GPS and GLONASS satellites. The dry tropospheric

delay error is first corrected using the Hopfield

tropospheric model and the remained zenith wet

tropospheric delay (ZWD) including the residual dry

delay is to be estimated as an unknown parameter. The

Niell Mapping Functions have been used for hydrostatic

and wet mapping functions. The positions, clock offset,

system time difference and ZWD are modeled as a

random walk process while the ambiguity parameters are

modeled as constants and are to be estimated using a

Kalman filter.

The basic procedure of PPP processing based on

combined GPS and GLONASS observations is

demonstrated in Fig. 1.

4 Numerical Results and Analysis

To assess the obtainable positioning accuracy based on

combined GPS and GLONASS observations, numerical

computations are conducted and the obtained results are

presented in this section. At first, the PPP processing

results including the positioning error, the receiver clock

offset, the zenith wet tropospheric delay and the system

time difference are given. Then comparisons are

conducted to assess the positioning accuracy and the

convergence time. Slow position convergence time is

currently an obstacle for PPP applications and the

additional observations from GLONASS are expected to

reduce the required convergence time.

Fig. 1 PPP processing for combined GPS and GLONASS

Data Descriptions

GPS/GLONASS observation datasets, collected on April

26th, 2007 at the IGS station HERT, GOPE and YARR,

were downloaded from the IGS website. Each station was

installed with an ASHTECH Z18 dual-frequency

GPS/GLONASS receiver. Data sampling rate was 30s.

The mixed GPS/GLONASS precise satellite orbit and 5-

minute clock data generated by IAC data analysis center

were downloaded from the IAC website. A total of 12

GLONASS satellites were operational on that day.

Positioni ng Results

Twelve hours of observations acquired at the station

HERT from EPN (EUREF Permanent Network) Local

Analysis Centers were first used. The elevation mask was

set to 10 degrees. The GPS only an d the GPS/GLONASS

observations were processed respectively. Fig. 2 shows

the position errors over the period. It can be clearly

observed that the positioning errors for GPS only and

combined GPS/GLONASS processing are at a quite

similar level.

Table 1 shows the mean, RMS, and standard deviation

(one-sigma) of the converged positio n errors based on the

results from local time 3:00 to 12:00. The differences in

the mean, RMS and STD values for all three coordinate

components are less than 1.5 cm.

GPS/GLONASS

RINEX file Precise orbit and

clock data

PPP processing

3-D coordinates

Receiver clock offset

System time difference

Zenith wet tropospheric delay

Cai et al.: Precise Point Positioning Using Combined GPS and GLONASS Observations 17

-1

E a st Error (m)

GPS Only

GPS/GLONASS

-1

Nort h Error (m )

0:00 2:00 4:00 6:00 8:0010:00 12:00

-1

Up Error (m)

GPS Time (HH:MM)

Fig. 2 GPS only vs. GPS/GLONASS positionin g e rrors

Tab. 1 Statistics of Position Results (m)

GPS Only GPS / GLONASS

East 0.045 0.057

North 0.012 0.016

MEAN Up 0.012 0.001

East 0.016 0.014

North 0.006 0.006

STD Up 0.020 0.020

East 0.048 0.058

North 0.014 0.017

RMS Up 0.024 0.020

In addition to position determination , PPP can also outpu t

receiver clock offset solution which has the potential to

support precise time transfer applications. The estimated

receiver clock offsets in both GPS only and

GPS/GLONASS processing are presented in Fig. 3. The

red curve stands for the results from GPS only processing,

which is completely overlapped by the green curve from

the GPS/GLONASS processing results. Since the clock

offset difference, which has a RMS value of 0.01 ns, is

very small, the addition of GLONASS observations did

not have a significant impact on the estimation of the

receiver clock.

Presented in Fig. 4 is the estimated zenith wet

tropospheric delay. As can be seen, the ZWD difference

between the GPS only processing and the combined

GPS/GLONASS processing after the position

convergence is not significant with a RMS value of 2 mm.

The estimated system time difference is presented in Fig.

5. The system time difference varies in a range of about 4

ns over the twelve hours, which partially reflects the

accuracy of the GLONASS system time scale. The

greater variation before the GPS time 2:00 is due to the

position convergence process. The obtained system time

difference from the combined GPS/GLONASS data

processing in PPP includes not only the time difference

between the GPS and GLONASS system times but also

the receiver hardware delay differences between GPS and

GLONASS. Since they can not be separated from each

other, the obtained value is the combined system time

difference and receiver’s inter-system hardware delay. As

a result, the estimated system time difference should be

considered as only an approximation to the true system

time difference and is quite dependent on the receiver

used.

0:00 2:004:006:00 8:0010:0012:00

100

120

Receiver clock offset (ns)

GPS Time (HH:MM)

GPS Only

GPS/GLONASS

Fig. 3 GPS only vs. GPS/GLONASS receiver clock offset estimates

0:00 2:004:00 6:00 8:00 10:00 12 :00

0.1

0.2

0.3

Zeni th wet tropospheric delay (m )

GPS Time (HH: MM)

GPS Only

GPS/GLONASS

Fig. 4 GPS only vs. GPS/GL O N A SS zenith we t t ropospheric delay

estimates

18 Journal of Global Positioning Systems

0:00 2:00 4:00 6:00 8:0010:00 12:00

740

741

742

743

744

745

746

747

748

S y st em t i m e dif ferenc e (ns )

GPS Time (HH:MM)

Fig. 5 Estimated system time differences

Positioning Accurac y and Converge nce Analysi s

Four processing sessions, each with three-hour data from

three IGS stations, namely HERT, GOPE and YARR,

were included in the data analysis. The elevation masks

were set to 10 degrees. For each session, in addition to

the position errors, the PDOP value and the number of

used satellites were also calculated. The computation of

the PDOP values in GPS/GLONASS processing is based

on the design matrix corresponding to the unknowns of

the three position coordinates, the receiver clock offset

and the system time difference. This design matrix has

one more column compared to the design matrix used for

PDOP computation in the GPS only processing. The

processing results are presented in Figs. 6-17.

Fig. 6 shows the positioning results between 0:00 and

3:00 at HERT station. No significant PDOP improvement

is found before the position solutions converge and as a

result, no significant convergence improvement is found.

Presented in Fig. 7 are the processing results from the

GPS time 3:00 to 6:00. In this session, two GLONASS

satellites were utilized on average. Although in the

beginning the PDOP value has only a slight improvement

by adding GLONASS observations, th e conv ergence time

has been reduced significantly in the east and up

directions.

In Fig.e 8, although PDOP has a significant improvement

from the local time 6:42 to 7:02, no significant

convergence improvement is found. This is because such

a geometry improvement with more visible satellites was

present after the position solutions have already

converged. Looking at the results in Fig. 9, the PDOP

improvement occurred at the first half an hour and during

the convergence process. As a result, it has reduced

significantly the position conv ergence time for horizontal

coordinate components.

-1

East (m)

-1

North ( m )

-1

Up (m)

SVs

0:00 0:30 1:001:30 2:00 2:30 3:00

PDOP

GPS Time (HH:MM)

GPS only

GPS/GLO

Fig. 6 Processing results at HERT (Session 1)

-1

East (m)

-1

North (m )

-1

Up (m)

SVs

3:00 3:30 4:00 4:30 5:00 5:30 6:00

PDOP

GPS Time (HH:MM)

GPS only

GPS/GLO

Fig. 7 Processing results at HERT (Session 2)

-1

East (m)

-1

Nort h (m )

-1

Up (m )

SVs

6:00 6:30 7:00 7:30 8:00 8:309:00

PDOP

GPS Time (HH:MM)

GPS only

GPS/G LO

Fig. 8 Processing results at HERT (Session 3)

Cai et al.: Precise Point Positioning Using Combined GPS and GLONASS Observations 19

-1

East (m)

-1

North (m )

-1

Up (m )

SVs

9:00 9:30 10:00 10:30 11:00 11:3012:00

PDOP

GPS Time (HH:MM)

GPS o nly

GPS/GLO

Fig. 9 Processing results at HERT (Session 4)

Table 2 shows the RMS statistics of the positioning errors

at HERT station using the position results obtained from

the last one and a half hours of observations from each

session. A significant accuracy improvement is found in

Session 2 where the improvements in the east and up

components reach 4cm and 3cm respectively.

Tab. 2 RMS Statistics of Positi o ni ng Results at HERT (m)

GPS Only GPS / GLONASS

East 0.101 0.093

North 0.031 0.034

Session 1 Up 0.082 0.092

East 0.129 0.087

North 0.019 0.018

Session 2 Up 0.060 0.029

East 0.063 0.085

North 0.024 0.012

Session 3 Up 0.083 0.091

East 0.037 0.035

North 0.012 0.011

Session 4 Up 0.013 0.013

Figs. 10-13 show the processing results at GOPE station.

No convergence improvement is found in Fig. 10 while a

slight improvement in the east component can be seen

from Fig. 11. Look at Fig. 12, the convergence in the

combined PPP processing appears more stable and

smooth between 7:00 and 7:40 when compared to the

GPS-only processing results. Fig. 13 indicates a slight

improvement in the beginning of the convergence process.

-1

East (m)

-1

North (m )

-1

Up (m )

SVs

0:00 0:30 1:00 1:302:002:30 3:00

PDOP

GPS Time (HH:MM )

GPS only

GPS/GLO

Fig. 10 Processing resul ts at G OP E (Session 1)

-1

East (m)

-1

Nort h (m )

-1

Up (m )

SVs

3:00 3:30 4:00 4:30 5:00 5:30 6:00

PDOP

GPS Time (HH:MM)

GPS only

GPS/GLO

Fig. 11 Processing resul ts at G OP E (Session 2)

-1

East (m)

-1

Nort h (m )

-1

Up (m )

SVs

6:00 6:30 7:00 7:30 8:00 8:30 9:00

PDOP

GPS Time (HH:MM)

GPS only

GPS/GLO

Fig. 12 Processing resul ts at G OP E (Session 3)

20 Journal of Global Positioning Systems

-1

East (m)

-1

Nort h (m)

-1

Up (m)

SVs

9:00 9:30 10:00 10:30 11:00 11:30 12:00

PDOP

GPS Time (HH:MM)

GPS o n ly

GPS/GLO

Fig. 13 Processing resul ts at G OP E (Session 4)

Presented in Table 3 is the RMS statistics of positioning

results at GOPE station. The maximum accuracy

improvement is 3cm which can be seen in the east

component of Session 3 while the accuracy degradation

of 2cm is also found in the up component of Session 1.

Tab. 3 RMS Statistics of Positioning Results at GOPE (m )

GPS

Only GPS /

GLONASS

East 0.008 0.008

North 0.010 0.018

Session 1 Up 0.030 0.051

East 0.147 0.128

North 0.019 0.017

Session 2 Up 0.044 0.031

East 0.098 0.069

North 0.018 0.014

Session 3 Up 0.084 0.059

East 0.045 0.040

North 0.010 0.008

Session 4 Up 0.099 0.098

The processing results at YARR station are presented in

Figs. 14-17. A significant convergence improvement has

been found in the east direction in Fig. 14 where

observations from an average of four GLONASS

satellites are utilized in the combined processing during

the period of 0:00 to 1:30. No convergence improvement

is found in the other figures by adding the GLONASS

observations due to limited number of visible GLONASS

satellites. This indicates a correlation between position

convergence improvement and satellite geometry

improvement. Table 4 shows the RMS statistics results of

the poisoning errors at YARR station. The maximum

accuracy improvement of 13cm occurs in the east

direction of Session 1.

-1

East (m)

-1

North (m )

-1

Up (m )

SVs

0:00 0:301:001:30 2:002:303:00

PDOP

GPS Time (HH:MM)

GPS on ly

GPS/GLO

Fig. 14 Processing results at YA RR ( Se ss io n 1 )

-1

East (m)

-1

North (m )

-1

Up (m )

SVs

3:00 3:30 4:00 4:30 5:00 5:30 6:00

PDOP

GPS Time

(

HH:MM

)

GPS only

GPS/GLO

Fig. 15 Processing results at YA RR ( Se ss io n 2 )

-1

East (m)

-1

Nort h (m )

-1

Up (m )

SVs

6:00 6:30 7:007:30 8:00 8:309:00

PDOP

GPS Time (HH:MM)

GPS only

GPS/GLO

Fig. 16 Processing results at YA RR ( Se ss io n 3 )

Cai et al.: Precise Point Positioning Using Combined GPS and GLONASS Observations 21

-1

East (m)

-1

North (m)

-1

Up (m )

SVs

9:009:3010:00 10:30 11:00 11:30 12:00

PDOP

GPS Time (HH:MM)

GPS only

GPS/GLO

Fig. 17 Processing results at YA RR ( Se ss io n 4 )

Tab. 4 RMS Statistics of Positi o ni ng Results at YARR (m)

GPS

Only GPS /

GLONASS

East 0.209 0.074

North 0.011 0.009

Session 1 Up 0.112 0.086

East 0.063 0.064

North 0.016 0.016

Session 2 Up 0.078 0.080

East 0.021 0.025

North 0.021 0.020

Session 3 Up 0.100 0.075

East 0.017 0.018

North 0.005 0.005

Session 4 Up 0.047 0.050

In order to compare the positioning accuracy between

using GPS-only observations and combined

GPS/GLONASS observations, the positioning accuracy

derived from three-dimensional coordinate component

errors is presented in Fig. 18. As can be seen, the

improvement of the positioning accuracy is obvious for

most of the position results, and the maximum

improvement reaches 12cm.

0.1

0.2

0.3

Error (m)

GPS only

GPS/GLONASS

HERT GOPE YARR

Fig. 18 Positioning accuracy comparison

5 CONCLUSI ONS

A positioning model based on combined GPS and

GLONASS observations has been proposed in this paper

for precise point positioning. In order to assess the

positioning accuracy and convergence time improvement

of the combined GPS and GLONASS data processing, a

12-hour and four 3-hour sessions of datasets have been

used in the data analysis. Comparisons have been

conducted between GPS only and combined

GPS/GLONASS processing. Based on the results, current

GLONASS constellation has not caused a significant

impact on the positioning results including position

coordinates, receiver clock offset and zenith wet

tropospheric delay since only two or three GLONASS

satellites were observed most of time at any specific time.

More significant improvements are expected when with

more GLONASS satellites available in space. The

research results further indicate that even with limited

number of GLONASS satellites the improvement of the

position convergence time is dependent on the

improvement level of the satellite geometry for position

determination. The results also indicate that the

positioning accuracy can be improved by additional

GLONASS observations in most cases. Further

investigation will be conducted to assess the combined

GPS/GLONASS precise point positioning in a kinematic

mode.

ACKNOWLEDGMENTS

The financial supports from NSERC and GEOIDE are

greatly appreciated. The contribution of data from the

International GNSS Service (IGS) and Information-

Analytical Center (IAC) is also appreciated.

Based on a paper presented at The In stitute of Navigation

International Technical Meeting, Fort Worth, Texas,

September 2007.

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