Abstract Within the regional EUREF Permanent

Network (EPN) all positioning is purely based on GPS.

This paper investigates, using the Bernese GNSS

analysis software, the influence of adding GLONASS

observations to the EPN processing using fixed orbits

from the International GNSS Service (IGS) as well as

from the CODE analysis centre. The GPS-only coor-

dinates and GPS + GLONASS coordinates will be

compared and the change in their repeatabilities will

be investigated. The influence of the used orbits will

also be outlined. The results show that a combined

GPS + GLONASS data analysis can be set up without

major efforts and that it will not degrade the positions

obtained within the EPN.

Keywords Glonass Æ EPN Æ Positioning

Introduction

The EUREF Permanent Network (EPN) consists of

190 permanent GPS stations from which 29 are also

tracking GLONASS satellites. The primary purpose of

the EPN is to maintain the European Terrestrial Ref-

erence System (ETRS89) and EUREF does this by

making available the tracking data of the EPN stations

and by generating weekly coordinate estimates for all

of them (Adam et al. 2002; Bruyninx 2004). Up to now,

all coordinate estimates have been based on only GPS

data and no GLONASS data is used. However with

• the growing number of commercially available

GPS + GLONASS receivers;

• the recent revitalization of GLONASS (with a

constellation of 18 satellites expected in 2007) and

• the availability of short latency combined IGS or-

bits for GLONASS and consistent GPS + GLON-

ASS CODE orbits

it has become worthwhile to assess the advantages

and disadvantages of adding GLONASS data to the

routine data analysis of the EPN network. For this

investigation, two networks of GNSS stations belong-

ing to the EPN have been analyzed: a first network

consisting of only GPS/GLONASS stations and a sec-

ond mixed network of GPS and GPS/GLONASS sta-

tions. For both networks, the station coordinates have

been estimated using GPS only observations and

GPS + GLONASS observations.

Presently, with a constellation of 29 active GPS

satellites and 13 GLONASS satellites, no significant

improvement can be expected from adding GLONASS

observations to GPS: the number of observations is

increased by a factor of 1.4, resulting in an expected

reduction of the formal errors byffiffiffiffiffiffiffiffiffi

1:45p

¼ 1:2: This

assumes that the number of parameters is not changed

by the introduction of the GLONASS data. As we will

see later, this is not the case.

In addition, the exercise to process observations

from a mixed GPS and GPS/GLONASS network in

which one of the two GNSS has an incomplete con-

stellation is a worthwhile exercise when preparing for

the future GALILEO system.

C. Bruyninx (&)Royal Observatory of Belgium, Av. Circulaire 3,1180 Brussels, Belgiume-mail: C.Bruyninx@oma.be

GPS Solut (2007) 11:97–106

DOI 10.1007/s10291-006-0041-9

123

ORIGINAL ARTICLE

Comparing GPS-only with GPS + GLONASS positioningin a regional permanent GNSS network

Carine Bruyninx

Received: 23 June 2006 / Accepted: 22 August 2006 / Published online: 19 September 2006� Springer-Verlag 2006

Set-up of the data processing

Networks analyzed

Two networks have been analyzed in order to assess

the influence of adding GLONASS data to the GPS-

only analysis. The first network consists of stations all

equipped with combined GPS + GLONASS equip-

ment. It contains the 25 GPS/GLONASS stations in-

cluded in the EPN on January 2006. As can be seen in

Fig. 1 (left side) most of the GPS/GLONASS stations

are located in central and northern Europe. The GPS/

GLONASS equipment used is given in Table 1.

The second network corresponds to the network

who’s GPS data are routinely analyzed by the Royal

Observatory of Belgium (ROB) and used to contribute

to the weekly coordinate solution of the EPN. The

network is a mixture of stations, mostly located in and

around Belgium, equipped with GPS-only and

GPS + GLONASS receivers; it is shown on the right

side of Fig. 1. We are using this network in order to

investigate whether the GPS-only ROB solution could

be replaced by a GPS + GLONASS solution without

altering the station coordinate time series and the

ROB contribution to the combined EPN solution.

Data processing

When processing a regional network, such as the two

networks considered, precise a priori orbit information

in needed. The IGS makes separately combined orbits

available for GPS and GLONASS. The GLONASS

orbits are based on the computations from four anal-

ysis centres: BKG, CODE, ESA/ESOC, and IAC

(Weber and Fragner 2000; Gendt 2006). As we can see

in Fig. 2, recently the latency of the combined IGS

GLONASS orbits has been significantly reduced. Since

the analysis centres BKG and ESA use the final IGS

GPS orbits as input for the computation for their

GLONASS orbits, the combined IGS GLONASS or-

bits are mostly available with a delay of a few days with

respect to the final IGS GPS orbits. Following the

Fig. 1 Left GPS/GLONASSstations belonging to the EPNon 1 January 2006; rightmixed GPS and GPS/GLONASS network (GPS/GLONASS stations areindicated with station namelabel)

Table 1 GPS/GLONASS equipment used within the EPN(January 2006)

Station Receiver Antenna + radome

BISK ASHTECH Z18 ASH701946.2 NONEBOGI JPS E_GGD ASH701945C_M SNOWCAGZ JPS E_GGD JPSREGANT_DD_E NONECOMO TPS E_GGD TPSCR3_GGD CONEGOPE ASHTECH Z18 ASH701946.3 SNOWHELG JPS LEGACY ASH700936D_M SNOWHERT ASHTECH Z18 ASH701946.2 NONEHOE2 JPS LEGACY TPSCR3_GGD CONEJOZ2 ASHTECH Z18 ASH701941.B SNOWKIR0 JPS E_GGD AOAD/M_T OSODMAR6 JPS LEGACY AOAD/M_T OSODMARJ ASHTECH Z18 ASH701946.2 NONEONSA JPS E_GGD AOAD/M_B OSODPOUS TPS GB-1000 TPSCR3_GGD CONESASS JPS LEGACY TPSCR3_GGD CONESKE0 JPS E_GGD AOAD/M_T OSODSNEC ASHTECH Z18 ASH701946.2 NONESOFI TPS E_GGD AOAD/M_T NONESPT0 JPS LEGACY AOAD/M_T OSODVACO ASHTECH Z18 ASH701946.2 NONEVENE ASHTECH Z18 ASH701941.B NONEVILO JPS LEGACY AOAD/M_T OSODVISO JPS E_GGD AOAD/M_T OSODWARN JPS LEGACY TPSCR3_GGD CONEWTZR TPS E_GGD AOAD/M_T NONE

98 GPS Solut (2007) 11:97–106

123

Products web-page at the IGS Central Bureau, the

accuracy of the final IGS orbits is less than 5 cm for

GPS and less than 15 cm for GLONASS.

The CODE analysis centre is presently the only

analysis centre generating fully consistent GPS

+ GLONASS orbits, meaning that both orbits result

from the same simultaneous processing of GPS and

GLONASS observations. Figure 3 shows that the

GLONASS orbits from CODE agree with the IGS

orbits at the 5 cm level. Following work from Urschl,

based on an SLR validation, the CODE orbits have an

accuracy of 2.5 cm for GPS and 5 cm for GLONASS

(Urschl, in press). The IGS and CODE orbits are ex-

pressed in the IGb00 reference frame, which is the IGS

realization of the ITRF2000.

Neither IGS nor CODE make available GLONASS

satellite clock information. As we will show later, this

will cause no problem for the data analysis.

The GPS and GPS + GLONASS data analysis has

been done using the Bernese 5.0 software (Beutler et al.

2006) following the scheme displayed in Fig. 4. In a first

step, the ionospheric free P3 GPS code is analyzed to

compute the receiver clocks. This is done using only the

GPS data as no GLONASS satellite clocks are avail-

able. Then, baselines are formed between stations with

a maximum number of common dual frequency

GPS + GLONASS carrier phase observations. In the

first test only GPS observations are considered, later

both GPS and GLONASS observations are used. In

both cases, identically the same baselines are formed.

After the creation of the independent baselines, phase

double differences are created in order to perform

Fig. 2 Latency of precise IGS orbits and CODE orbits

Fig. 3 Agreement of CODEorbits with IGS GLONASSorbits (source http://www.gfz-potsdam.de). The SLR orbitsfrom MCC (Mission ControlCentre Moscow) do notcontribute to the IGScombined GLONASS orbits,but it is only shown ascomparison

Fig. 4 GNSS data processing

GPS Solut (2007) 11:97–106 99

123

baseline-by-baseline GPS ambiguity resolution using

the QIF (Quasi Ionospheric Free) method (Beutler

et al. 2006) supported by the CODE ionospheric model.

When processing GPS and GLONASS observations,

the double difference GLONASS ambiguities are esti-

mated but no attempt is made to fix them to their

integer value. Since double differences are used, there

is no need for a priori information on the satellite

clocks, which are eliminated by the double differencing.

After fixing the GPS ambiguities to their integer values,

the ionospheric free L3 double differences are formed

and the station coordinates are estimated together with

the troposphere. The troposphere is modelled as piece-

wise linear functions using the dry-Niell a priori

mapping function, and estimating each hour the tro-

posphere using the wet-Niell mapping function.

The GPS-only analysis is preformed using IGS final

orbits/clocks while the GPS + GLONASS analysis is

done using on one hand the final IGS GPS orbits/

clocks and GLONASS orbits (which we merged at the

SP3 level) and on the other hand the fully consistent

GNSS orbits from CODE (Ineichen et al. 2001).

For the first network, the data were analysed from 1

October 2005 to 28 February 2006. The second net-

work was analysed form 5 February until 1 April 2006.

For each day a set of coordinates was determined. The

datum of the coordinates was defined by applying three

translation conditions (minimal constraint) to the EPN

realization of the ITRF2000.

Results

GPS/GLONASS network

When processing the network of 25 GPS/GLONASS

stations, the additional GLONASS satellites increase

the number of observations by 47%. The associated

maximal reduction of the formal errors has a factor of

1.2. However, in our case, the introduction of the

GLONASS data also increases the number of param-

eters to be estimated considerably (by 47%). These

additional parameters are the GLONASS ambiguities.

Consequently no significant improvement in terms of

formal errors can be expected from adding GLONASS

data to GPS.

During the period that we analysed, several of the

GPS/GLONASS stations provided data of degraded

quality. The most striking example is the station SNEC

(Snezka, Czech Republic) whose coordinates wan-

dered away (see Fig. 5), especially in the height com-

ponent, because of a receiver malfunctioning. The

SNEC data have therefore been eliminated starting

from GPS week 1349 at the first symptoms of the

receiver error.

Starting 1 January 2006 the data from the ASH-

TECH Z-18 receivers at JOZ2 (Jozefoslaw, Poland)

and GOPE (Ondrejov, Czech Republic) became

unusable. After the midnight epoch these two receivers

started tracking all GLONASS satellites with a 1-s

delay causing the pseudoranges to be increased by

300,000 km. As a leap second was introduced at this

date, a link to this event was suspected. The other

ASHTECH Z-18 receivers in the EPN behaved nor-

mally. The problem at JOZ2 and GOPE was narrowed

down to the TEQC (GNSS Translating, Editing, and

Quality Checking) software (Estey and Meertens 1999)

used to convert the native data to the RINEX format.

The problem was solved by updating TEQC to its

latest version from 15 December 2005.

The data from the station SOFI (Sofia, Bulgaria)

had to be discarded from the processing because of a

lack of reliable data caused by a malfunction of the

station PC.

As can be seen in Fig. 6, in addition to the problems

mentioned above, the three Italian GPS/GLONASS

stations (CAGZ, COMO and VENE) are missing in

almost 20% of the final solutions. The data of these

stations are regularly missing at all the Data Centers

(without correlation between the missing days from the

Fig. 5 Estimated coordinatesfor the station SNEC

100 GPS Solut (2007) 11:97–106

123

different stations). Note that in Fig. 6 as well as the

following graphs, the stations are ordered according to

increasing latitude.

Following the scheme displayed in Fig. 4, daily

coordinates have been estimated for the remaining

stations in the network: first using only GPS data (and

final IGS orbits/clocks), and secondly using GPS as

well as GLONASS data. The last processing was done

once using IGS orbits and once using CODE orbits.

Figure 7 shows no significant differences in the

repeatabilities of the station coordinates depending on

the observations and orbits used. An exception is the

station SNEC with a significant degradation of the Up-

RMS caused by the introduction of GLONASS data.

The inspection of the coordinate time series of

SNEC (Fig. 8) shows that the degradation of the RMS

is the caused by a few outliers in the GPS + GLON-

ASS solution of GPS week 1345.

We have also drawn the coordinates time series of the

nearby station POUS, which, as can be seen in Fig. 9,

shows an offset in its height-component when the GPS-

only results are compared to the GPS + GLONASS

estimates. However, in these coordinate time series, no

special events are noted.

Figure 9 shows that adding GLONASS data to the

GPS-only analysis changes the coordinates up to

2.5 mm in the horizontal components. However, as can

be seen from Fig. 10, these differences are mainly due

to differences in the reference frame. After a Helmert

transformation, the horizontal differences are below

1.5 mm, with a general RMS of 0.4 mm. In the up-

component, the coordinate differences between GPS

and GPS + GLONASS are mostly below 4 mm, but

reach for one station (POUS) up to 6 mm.

The 3D RMS of the coordinate differences is

1.8 mm, which is reduced to 1.4 mm by the Helmert

transformation. In all cases, the GPS + GLONASS-

based coordinates obtained using IGS or CODE orbits,

agree at the 1-mm level.

The origin of the difference between the GPS-only

and GPS + GLONASS estimates for the up-compo-

nents of POUS (6 mm) is unclear.

As a by-product of our analysis tropospheric Zenith

Total Delays (ZTD) are estimated each hour. As can

be seen in Fig. 11, GPS + GLONASS underestimates,

for all stations except POUS (!), the ZTDs compared

to GPS only. It is clear that the station POUS is

showing an atypical response to the introduction of

Fig. 6 Percentage ofobservation days included foreach station in the dataprocessing

0

1

2

3

]m

m[

RMS East

0

1

2

3

0

1

2

3

0

1

2

3

]m

m[

RMS North

GPS onlyGPS+GLONASS using IGS orbitsGPS+GLONASS using CODE orbits

ZGA

C

EN

EV

OM

OC

OCA

V

RZTW

EPO

G

SUOP

KSIB

JR

AM

CEN

S

TR

EH

2Z

OJ

IGO

B

NRAW

GLE

H

SS

AS

2EOH

AS

NO

0SI

V

0T

PS

6RAM

0LI

V

0EKS

0RI

K

egarevA

01234567

]m

m[

RMS Up

01234567

Fig. 7 Coordinaterepeatabilities obtained usingGPS-only, GPS + GLONASSwith IGS orbits,GPS + GLONASS withCODE orbits

GPS Solut (2007) 11:97–106 101

123

GLONASS. Figures 12 and 13 show examples of the

ZTD behavior for the stations VACO and JOZ2.

Results for mixed network

In the network of mixed GPS and GPS/GLONASS

receivers, the introduction of the GLONASS data

increases the amount of used observations by 14%. A

similar increase is also noted in the number of esti-

mated parameters. As we can see in Fig. 14, as ex-

pected, the repeatabilities of the estimated coordinates

are independent of the introduction of the GLONASS

data (GPS/GLONASS stations are: HELG, HERT,

HOE2, KARL, ONSA, SPT0, WARN, WTZR).

In addition, Fig. 15 shows that no significant changes

in the coordinates can be seen. We can therefore con-

clude that, for this specific network, GLONASS data

can be introduced in the data analysis without any

problems. However, to avoid influencing the site

velocities, the introduction of GLONASS should be

done simultaneously with the introduction of the abso-

lute phase centre variations and the switch to ITRF2005.

Conclusion

The goal of this study was to investigate the advanta-

ges/disadvantages of analyzing combined GPS/

Fig. 8 Coordinate time seriesfor SNEC (top) and POUS(bottom) obtained using aGPS-only and aGPS + GLONASS analysisusing IGS orbits

102 GPS Solut (2007) 11:97–106

123

+

-3-2-10123

]m

m[

East

-3-2-10123

-3-2-10123

-3-2-10123

]m

m[

North

GPS versus GPS+GLONASS using IGS orbitsGPS versus GPS+GLONASS using CODE orbitsGPS+GLONASS using IGS or CODE orbits

ZGAC

EN

EV

OMOC

OCAV

RZTW

EPOG

SU

OP

KSIB

JR

AM

CENS

TR

EH

2ZOJ

IGOB

NRA

W

GLEH

SS

AS

2EO

H

AS

NO

0SI

V

0T

PS

6RA

M

0LIV

0E

KS

0RIK

-6-4-202468

]m

m[

Up

-6-4-202468

Fig. 9 Coordinate differencesbetween GPS-onlycoordinates and coordinatesobtained usingGPS + GLONASS data,computed respectively withIGS orbits and with CODEorbits

-3-2-10123

-3-2-10123

]m

m[

East

-3-2-10123

-3-2-10123

]m

m[

North

GPS versus GPS+GLONASS using IGS orbitsGPS versus GPS+GLONASS using CODE orbitsGPS+GLONASS using IGS or CODE orbits

ZGAC

ENEV

OMOC

OCAV

RZTW

EPOG

SUOP

KSIB

JRAM

CENS

TREH

2ZOJ

IGOB

NRAW

GLEH

SSAS

2EOH

ASNO

0SIV

0TPS

6RAM

0LIV

0EKS

0RIK

-6-4-202468

]m

m[

Up

-6-4-202468

Fig. 10 Residuals of7-parameter Helmerttransformation between GPS-only coordinates andcoordinates obtained usingGPS + GLONASS data,computed respectively withIGS orbits and with CODEorbits

-2,5-2

-1,5-1

-0,50

0,51

1,5

ZGAC

ENEV

OMOC

OCAV

RZTW

EPOG

SUOP

KSIB

JRAM

CENS

TREH

2ZOJ

IGOB

NRAW

GLEH

SSAS

2EOH

ASNO

0SIV

0TPS

6RAM

0LIV

0EKS

0RIK

]m

m[D

TZ

Fig. 11 Mean bias betweenZTDs from GPS andGPS + GLONASS

GPS Solut (2007) 11:97–106 103

123

2005.8 2005.9 2006 2006.1

Epoch

2050

2100

2150

2200

2250

2300

2350]

DT

Zm

m[

GPSGPS + GLONASS

2050

2100

2150

2200

2250

2300

2350

2005.94 2005.941 2005.942

Epoch

2160

2200

2240

2280

]D

TZ

mm[

Fig. 12 Comparison of ZTDs based on GPS-only and on GPS + GLONASS observations

2005.8 2005.9 2006

Epoch

2250

2300

2350

2400

2450

2500]

DT

Z m

m[

GPSGPS + GLONASS

2250

2300

2350

2400

2450

2500

2005.8 2005.9 2006

Epoch

-10

-5

0

5

10

]D

TZ

mm[

-10

-5

0

5

10

Fig. 13 Top ZTDs from GPSand GPS + GLONASS;bottomZTD(GPS + GLONASS) –ZTD(GPS)

0123456

0123456

]m

m[

RMS North

GPS onlyGPS+GLONASS, with IGS orbits

0

1

2

3

]m

m[

RMS East

0

1

2

3

MMIZ

LRAK

AZTW

RZTW

UDE

R

POLK

KS

UE

SURB

SREH

TREH

GSO

K

BBT

P

STOP

TR

SW

UBOH

ERAD

KR

OB

NRAW

GLEH

2EOH

PR

OM

DIM

S

PDUB

DLUS

AS

NO

EVNI

0TPS

SLS

O

eg

arevA

0

1

2

3

4

5

]m

m[

RMS Up

0

1

2

3

4

5

Fig. 14 Coordinaterepeatabilities obtained usingGPS-only andGPS + GLONASSobservations

104 GPS Solut (2007) 11:97–106

123

GLONASS data in a regional network of GPS and

GPS/GLONASS receivers.

For all tests, we used the Bernese 5.0 data analysis

software, which allows to process GPS-only or

GPS + GLONASS observations using identically the

same processing strategy (except for the ambiguity

resolution).

We have compared the GPS-only and GPS +

GLONASS coordinates obtained in the two networks:

• a regional network consisting of 25 GPS/GLON-

ASS stations (all GPS/GLONASS included in the

EPN at January 2006)

• a typical regional network of mixed GPS and GPS/

GLONASS stations (20 GPS and 8 GPS/GLON-

ASS stations).

We compared the GPS + GLONASS coordinates

obtained from the GPS/GLONASS network using on

one hand the IGS orbits and on the other hand the

CODE orbits. The CODE orbits are consistent GNSS

orbits, while the IGS computes separately its combined

GPS and its GLONASS orbits. The GPS-only coordi-

nates were computed using the IGS final orbits. A first

conclusion is that the GPS + GLONASS-based coor-

dinates obtained using either IGS or CODE orbits

agree in all three components at the 1-mm level after

applying a 7-parameter Helmert transformation.

From the two networks processed, we can see that

adding GLONASS data to the GPS data does not

significantly change the repeatabilities of any of the

station coordinates. For some stations, the repeatabil-

ities are slightly better using GPS-only, for others, the

repeatabilities improve when adding GLONASS.

In the GPS/GLONASS network, the differences

between the GPS-only coordinates and the GPS

+ GLONASS coordinates show that adding GLON-

ASS data can change the coordinates at the level of

1–2 mm in the horizontal components and between 2

and 6 mm for the vertical component. For the hori-

zontal components, the coordinate differences are

mainly caused by reference frame differences between

the two regional networks. For the vertical component,

one of the stations in the network shows an offset of

almost 6 mm when GPS-only coordinates are com-

pared to GPS + GLONASS coordinates. The cause of

this difference is not clear presently and will be subject

of further study.

In the mixed network, which corresponds to the

reality, all coordinate differences are below the 1 mm

level.

Acknowledgment The author wishes to thank DominiqueMesmaker for his help with the figures.

References

Adam J, Augath W, Boucher C, Bruyninx C, Caporali A, GublerE, Gurtner W, Habrich H, Harsson B, Hornik H, Ihde J,Kenyeres A, van der Marel H, Seeger H, Simek J, Stangl G,Torres J, Weber G (2002) Status of the European ReferenceFrame—EUREF. In: IAG Symposia 125:42–46

Beutler G, Bock H, Brockmann E, Dach R, Fridez P, GurtnerW, Habrich H, Hugentobler U, Ineichen D, Jaeggi A, Me-indl M, Mervart L, Rothacher M, Schaer S, Schmid R,Springer T, Steigenberger P, Svehla D, Thaller D, Urschl C,Weber R (2006) Bernese GPS software version 5.0 (draft)eds Urs Hugentobler, R. Dach, P. Fridez, M. Meindl, Univ.Bern, 464 pp

-1

-0.5

0

0.5

1

-1

-0.5

0

0.5

1

]m

m[

NorthGPS versus GPS+GLONASS

-1

-0.5

0

0.5

1

]m

m[

East

-1

-0.5

0

0.5

1

MMIZ

LRAK

AZTW

RZTW

UDE

R

POLK

KSU

E

SUR

B

SREH

TREH

GSO

K

BBT

P

STOP

TR

SW

UBO

H

ERAD

KROB

NRAW

GLE

H

2EO

H

PRO

M

DIMS

PDUB

DLU

S

ASNO

EVNI

0TP

S

SLSO

-2

-1

0

1

2

]m

m[Up

-2

-1

0

1

2

Fig. 15 Coordinatedifferences obtained usingGPS-only andGPS + GLONASSobservations

GPS Solut (2007) 11:97–106 105

123

Bruyninx C (2004) The EUREF Permanent Network a multi-disciplinary network serving surveyors as well as scientists.GeoInformatics 7:32–35

Estey LH, Meertens C (1999) TEQC: the multi-purpose toolkitfor GPS–GLONASS data. GPS Solut 3(1):42–49

Gendt G (2006) IGSREPORT-14104: Wk 1383 IGLOS Final Orbits.http://www.igscb.jpl.nasa.gov/mail/igsreport/2006/msg00751.html

Ineichen D, Springer T, Beutler G (2001) Combined processingof the IGS and IGEX network. J Geod 75:575–586

Urschl C, Beutler G, Gurtner W, Hugentobler U, Schaer S(in press) GPS/GLONASS orbit determination based oncombined microwave and SLR data analysis. In: Proceed-ings of IAG, IAPSO, IABO Joint Conference, DynamicPlanet 2005, 22–26 August 2005, Cairns, Australia

Weber R, Fragner E (2000) IGEX Analysis. In: IGS 1999Technical Reports, pp 273–278

106 GPS Solut (2007) 11:97–106

123