Position paper for the IGS Workshop

Multi–GNSS Processing

Rolf Dach1 Stefan Schaer2 Tim Springer3

Gerhard Beutler1 Felix Perosanz4 Urs Hugentobler5

1 Astronomical Institute of the University of BernSidlerstrasse 5; CH–3012 Bern; Switzerland

2 Swiss Federal Office of Topography swisstopoSeftigenstrasse 264; CH–3084 Wabern; Switzerland

3 European Space Operations CentreRobert Bosch Strasse 5; DE–64293 Darmstadt; Germany

4 Centre National d’Etudes Spatiales18, avenue Edouard Belin; FR–31401 Toulouse Cedex 09; France

5 Technische Universitat Munchen; Forschungseinrichtung SatellitengeodasieArcisstrasse 21; DE–80333 Munich; Germany

Miami Beach, U.S.A.; June 02–06, 2008

1 Introduction

When the IGS was renamed from “International GPS Ser-vice” to “International GNSS Service” the IGS GoverningBoard and the IGS community expressed their expectationto extend activities from the well–established GPS to otheractive and to planned systems. We expect the EuropeanGalileo system (and possibly the Chinese Compass) to be-come active in future.

Independent of these future systems the RussianGLONASS is a second active GNSS. We observe a contin-uously increasing number of stations in the IGS networktracking both, GPS and GLONASS satellites (see Sec-tion 2). Currently, the following analysis centers provideorbit products for the GLONASS satellites:

• BKG: Bundesamt fur Kartographie und Geodasie,Frankfurt am Main, Germany

• CODE: Center for Orbit Determination in Europe,AIUB, Bern, Switzerland

• ESOC: European Space Operations Center, ESA,Darmstadt, Germany

• IAC: Information–Analytical Center, Russia

BKG and IAC solve for GLONASS satellite orbits, intro-ducing the information for the GPS satellites from the IGSsolution as known. Since May 2003 CODE is providingorbits for GPS and GLONASS based on a rigorously com-bined analysis of the data of both GNSS. In January 2008ESOC adopted this strategy, as well. So far, GLONASS

satellite clock corrections are available only from the ESOCand IAC analysis.

In the two–step procedure the full consistency betweensystems is not guaranteed — even if the same softwareis used to establish the GPS and GLONASS satellite or-bits, because the orbits of the satellites from the two GNSSemerge from different procedures. Currently, the results areprovided in separate files for each GNSS to the user com-munity. Because there is only one analysis centers submit-ting results for the rapid and ultra–rapid products, it is onlypossible to provide combined GLONASS orbits for the finalproduct series. Currently, satellite clock corrections maynot combined because of the lack of contributing centers.

This position paper first documents the development ofthe combined GPS/GLONASS receivers in the IGS network(Section 2). The quality and accuracy of the GLONASSsatellite orbits is discussed in Section 3.

In Section 4 we compare the global parameters from so-lutions of a GPS–only and a combined GPS/GLONASSprocessing using one year of data. After comparing the orbitcharacteristics and their impact on positioning in Section5,we study the benefit of adding GLONASS measurements tothe GPS observations for positioning in Section 6.

In Section 7 we review the existing file formats concern-ing their capability of covering all aspects of current andfuture multi–GNSS constellations.

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2 The GNSS Subnetwork of the IGS

Figure 1 shows the number of stations in the IGS networkproviding GLONASS measurements. The number of IGSsites providing GPS and GLONASS data has significantlyincreased and the distribution improved since Spring 2003.

At the end of the IGEX campaign the IGS terminated thegeneration of combined GLONASS satellite orbits, becauseonly two analysis centers continued to submit solutions.In May 2003 CODE started its activities related to multi–GNSS processing by providing GLONASS satellite orbitsas the third analysis center. This third contribution allowedit to the IGS to re–initiate the combination of GLONASSorbits.

Perhaps as a consequence of these additional efforts thenumber of GLONASS tracking stations grew in the IGS net-work from about 20 to 30 till the end of the year 2003.This number of stations remained stable for quite a longtime. With the availability of a new generation of com-bined GPS/GLONASS receivers, produced by several well–known GPS receiver manufacturers in 2006/2007, the num-ber of GLONASS tracking stations in the IGS network in-creased steadily and continues to increase today. Orbitsof GLONASS satellites may now be determined from thedata of more than 50 tracking stations in the IGS network.GLONASS tracking data from about 35 IGS stations maybe used for the ultra–rapid solution (the number is limitedby data latency). Note that a good global distribution of ob-serving sites is at least as important for orbit determinationas their number.

In Summer 2003 the global coverage of IGS stationstracking GLONASS satellites was very inhomogeneous.Most of the 20 stations with GLONASS tracking capabil-ity were located in Europe (see Figure 2(a)). The 30 stationsnetwork available in the 2003–2006 time frame is in essencethat shown in Figure 2(b). Figure 2(c) shows the current sit-uation (early 2008). The relation between GPS–only (blackdots) and combined GPS/GLONASS (grey stars) receiversis now balanced in all regions — except for the Ameri-can continent, where GPS–only receivers still dominate. In

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Figure 1: Number of sites in the IGS network providingGLONASS data, which were used for orbit determination inthe CODE rapid (grey line) and final (black line) solutions.

(a) GNSS subnetwork of the IGS in July 2003 (day of year 182)

(b) GNSS subnetwork of the IGS in March 2005 (day of year 075)

(c) GNSS subnetwork of the IGS in April 2008 (day of year 110)

Figure 2: Geographical distribution of multi–system GNSSreceivers (grey stars) and GPS-only receivers (black dots)used for the CODE final processing.

summary we may state that today orbit determination forthe GLONASS satellites may be based on a truly globaltracking network of geodetic–type receivers. This signifi-cant improvement is due to the efforts of many IGS stationmanagers and their institutions.

The number of active GLONASS satellites also grewconsiderably since 2003. Unfortunately, a large number ofreceivers was unable to track satellites flagged as “unus-able”, which considerably reduced the number of receiverstracking these satellites. In 2007 GLONASS moved the fre-quency range of the system to a new frequency band (an-nounced as a system update already in 2002). The frequen-cies of the 24 GLONASS satellites of the nominal constel-lation are no longer computed by the frequency numbers1 to 12, but by−7 to +6. When the first satellites withfrequency numbers≤ 0 became active, several firmwareupgrades were necessary to enable the receivers to providedata from these satellites.

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3 GLONASS Orbit Determination

The number of GNSS satellites contained in the CODE1

final solution is shown in Figure 3. The light–grey curveshows the number of available GPS satellites, which isquite stable around 30 since the year 2004. The numberof GLONASS satellites tracked by a sufficient number ofsites of the IGS network to allow for precise orbit determi-nation is represented by the dark–grey curve in Figure 3.Currently (April 2008), 16 GLONASS satellites are active.As two more triple–launches are announced for this yearwe can expect 22 active GLONASS satellites by the end ofthis year (or, more realistically, 20 satellites assuming thattwo satellites may be decommissioned during this year). Inthat case GLONASS will nearly have completed its nominalconstellation of 24 satellites.

Figure 3 shows that the number of active GLONASSsatellites varies much more than the number of active GPSsatellites, because of two reasons: (1) during the mainte-nance phase a GPS satellite is flagged as unhealthy, butit continues to emit signals. GLONASS satellites, how-ever, do not transmit signals for about 1 up to 3 days atirregular intervals. The duration and frequency of theseevents are comparable to the duration and frequency ofmaintenance periods for GPS satellites. These GLONASSevents are usually not announced by the system operators.Whereas GPS maintenance periods are often associatedwith repositioning events, no repositioning events were de-tected for GLONASS up today. It is thus possible to predictGLONASS orbits over long time intervals for the purposeof re–initialization of the orbit determination process, whenthe satellite is again tracked by the receivers in the IGS net-work — even if broadcast information is not yet available.(2) If an orbital plane is partially eclipsed, the GLONASSsatellites were often switched off for a few weeks. Whenthe satellite start to broadcast again after such a long time,a new initialization of the orbit determination process is re-quired.

System–specific outages are compiled in Figure 4: Each

1CODE is chosen as example because the center provides orbitsevenif the number of tracking stations for a satellite is very limited.

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Figure 3: Number of satellites included in the CODE finalorbit product since 2004.

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Figure 4: Days for which orbits of the individualGLONASS satellites are provided by CODE since July2003 are indicated by black squares. If the orbit determi-nation was not very reliable because of the lack of track-ing data a grey square is used instead (in most cases thesatellites were flagged as unusable). Dark–grey bars indi-cate intervals when the PRN slot was occupied by a newGLONASS–M satellite. Light–grey bars indicate eclipsingperiods for the satellites in a particular orbital plane.

day since May 2003 is marked by a black square, if theCODE final solution contained an accurate GLONASSsatellite orbit. Grey squares mark days, where orbit deter-mination was of poor quality due to a limited number ofreceivers tracking the satellite. Blank squares mark dayswhen no orbit determination was possible, because of miss-ing data (e.g., due to inactive satellites). Many gaps (blanksquares) occur during the eclipsing phases marked by greybars. Satellites R05, R18, or R21 illustrate the behavior.

In 2003 the first GLONASS–M satellite — member ofa new generation of GLONASS satellites — has beenlaunched (R06 was running in a testing mode for severalmonths in 2004). The replacement of an old–style by amodernized GLONASS satellite is indicated by dark bars inFigure 4. The current constellation mainly consists of mod-ernized GLONASS satellites, because many of the oldersatellites have been decommissioned by now. R01 and R08are the only active GLONASS satellites of the old genera-tion. The new satellites continue operating during eclipsephases, which is a big advantage for orbit determination.Also, the lifetime of the new generation of GLONASSsatellites seems to be longer than that of the old ones (everyfew years the old generation GLONASS satellites neededto be replaced), which is another factor contributing to thegreater variability in the GLONASS satellites constellationdisplayed in Figure 3.

Let us make the attempt to asses the precision of theGLONASS (and GPS) orbits provided by CODE. For thispurpose we use the ephemerides of the final orbit seriesof three consecutive days. The positions, at a 15 minutesspacing, are used as pseudo–observations in an orbit deter-mination process, where only six initial osculating elementsand nine empirical parameters (three constant and six once–per revolution parameters in D−, Y−, and X−directions,

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Figure 5: Median of the RMS for the fit of a three–dayarc for the GPS (light–grey) and GLONASS (black) satel-lites obtained in the combined GPS/GLONASS processingat CODE since 2003.

[Beutler et al., 1994]) were set up. The RMS error of onesatellite coordinate (referred to simply as RMS hereafter)isused as a precision indicator.

We do not want to include problems of marginally ob-served satellites and therefore display the median of theRMS over all GLONASS satellites for each day (black dotsin Figure 5). For reference the corresponding values for theGPS satellites are given in light–grey. There is a clear cor-relation of the RMS with the number of stations trackingGLONASS satellites (see Figure 1): For a long time in-terval the median of the RMS for the GLONASS satelliteswas of the order of 8 to 10 cm. With the significantly in-creased number of GLONASS tracking stations in the IGSnetwork this value was recently reduced to about 5 to 6 cm.Note that the median of the RMS error is much larger thanthe corresponding value for the GPS satellites. This factmainly reflects the smaller number of tracking stations andthe less optimal global distribution (as compared to GPS). Itis, however, remarkable that long time series of GLONASS

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Figure 6: Consistency of the GLONASS orbits inthe combination procedure (http://www.ngs.noaa.gov/igsacc/WWW/). (The MCC solution is based onSLR data for only three GLONASS satellites.)

ephemerides with sub–decimeter precision are now avail-able. This precision is sufficient for many purposes of “ev-eryday surveys”.

Because of the limited number of contributing analysiscenters the consistency of the GLONASS satellite orbitsfrom the combination (displayed in Figure 6) is not a verymeaningful information to assess the quality of the orbits.Nevertheless, an improvement of the consistency is evident— in particular after week 1400. Unfortunately, we can-not decide whether the network densification or the mod-elling refinements implemented into the processing causedthis improvement. It is worth mentioning that since the be-ginning of 2008 — when ESOC started contributing withits new software package — a consistency level of 5 cmbetween the solutions of the four analysis centers has beenreached.

4 Impact of GLONASS on the GlobalProducts

ESOC has twice processed the data from the year 2007: afirst time as a GPS–only solution and a second time as acombined GPS/GLONASS solution. In Figure 7 comparesthe following orbits using the RMS and the median of thedifferences:

1. IGS combined orbits versus CODE final orbits

2. IGS combined orbits versus ESOC submitted final or-bits generated by the old software package)

3. IGS combined orbits versus the orbits from a GPS–only solution generated with the new software packageat ESOC

4. IGS combined orbits versus the orbits from a com-bined GPS/GLONASS solution generated with thenew software package at ESOC

5. GPS–only versus combined GPS/GLONASS solution,both generated with the new ESOC software package

All relevant parameters were estimated in both ESOC solu-tions: station coordinates, troposphere delays and gradients,Earth rotation parameters, orbit parameters, and ambiguityparameters that were not resolved to integer values.

The last bar shows that differences between the two so-lutions (with and without GLONASS) are of the order of5 mm (violet bar). On the other hand, comparing both solu-tions to the combined IGS orbits (green and cyan bars) noadvantage for one or the other solution can be detected. Thefigure confirms the (expected) improvement of the ESOCsolution due to the transition to the new software packagefor the orbit products (blue and cyan bars).

The same kind of comparisons is provided for the Earthrotation parameters in Figure 8. The last bar (violet) indi-cates some differences between the GPS–only and the com-bined GPS/GLONASS solutions. As in the case of the or-

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Figure 8: Comparison of the Earth rotation parametersfrom a GPS–only and a combined GPS/GLONASS solution(computed from all days of year 2007).

bits the comparison to the combined IGS ERP–solution re-veals no significant differences (green and cyan bars) be-tween the two solutions with and without the GLONASSmeasurements. The same conclusion can be drawn for thestation coordinates from the ESOC study.

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Figure 9: RMS of the coordinate differences obtained indaily GPS–only and combined GPS/GLONASS solutionsin a global network during the year 2007, respectively.

A similar study was carried out at the CODE analysiscenter. The results in general confirm the findings of theESOC. Figure 9 shows the daily RMS values in the North,East, and Up components of the coordinate differences be-tween two solutions with and without using GLONASS.About 150 sites are included in the global analysis. Theprocessing strategy corresponds closely to that used by theCODE analysis center. Figure 9 has a simple message: Theglobal reference frame is only marginally affected whenadding GLONASS measurements to the processing.

5 GPS and GLONASS Orbit Char-acteristics

The sub–satellite track of one particular GPS satellite re-peats every day. It is therefore possible to show all sub–satellite tracks for the entire GPS constellation using oneday as an example. As long as the satellites are not movedto a different position within the orbital plane, the sameground tracks result for each day. Figure 10(a) showsthe ground tracks for all GPS satellites during ten days inFebruary 2008. The GPS–specific ground tracks show thata particular satellite follows the same azimuth–elevationpaths (at maximum two visibility intervals per day) for oneand the same site. This implies in particular that the ob-servation scenarios of particular GPS satellites are — fora given latitude — longitude–dependent. As the IGS net-work is not really global and homogeneous, this fact im-plies that different GPS satellites are most likely not ob-served with the same “intensity” and with the same quality.Figure 11(a) shows an example for the Zimmerwald site,located at a Northern latitude of about45◦. Note that theground track actually corresponds to10 days, underliningthat the particular GPS satellite follows the same track dayafter day. A GPS track culminating almost at90◦ elevationresults in this case. A site at the same latitude as Zimmer-wald, but separated in longitude by±90◦ would see twotracks of the same GPS satellite per day, both culminatingat lower elevations, one in the East and one in the West.

GLONASS ground tracks repeat after 8 sidereal days(which corresponds to a17 : 8 commensurability with Earthrotation). The ground tracks of all 16 GLONASS satellitesactive on the same days in 2008 are shown in Figure 10(b).The ground track of a particular satellite is shifted by45◦ inlongitude per day. As the satellites in one and the same or-bital plane are separated by45◦ in the full nominal constel-lation, the ground track generated by one particular satelliteon dayi is the same as the ground track of its two neigh-bors on daysi ± 1 . Therefore, one orbital plane of theGLONASS in essence generates one ground track, whereall ground tracks are much steeper than the GPS groundtracks as a consequence of the 8 sidereal day repeat cy-cle. From the scientific perspective it is unfortunate thatthe arguments of latitude of the satellites in the three orbitalplanes are defined in such a way that the satellites in the

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three orbital planes all generate one and the same groundtrack. This characteristic may be attractive for the systemoperators (it reduces the number of necessary control sta-tions) but it would be better from the scientific point of viewto have a less regular pattern.

Be this as it may: It is an important difference of theGLONASS w.r.t. the GPS constellation that, in the averageover 8 sidereal days, all sites at one and the same latitudeobserve each GLONASS satellite in essence in the sameway (shifted only by a time offset governed by the lon-gitude difference). Figure 11(b), which was generated inthe same way as Figure 11(a) covering the time interval of10 days, illustrates this behavior. One GLONASS satellite

(a) Ground tracks for the GPS satellites

(b) Ground tracks for the GLONASS satellites

Figure 10: Ground tracks of the GPS (top) and GLONASS(bottom) constellation for 10 days (day 60 to 69 of year2008) in February 2008.

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Figure 11: Sky plot for one GPS (left) and one GLONASS(right) satellite for Zimmerwald, Switzerland, covering10 days (day 60 to 69 of year 2008) in February 2008.

in essence fills the entire azimuth–elevation plot (except forthe hole in the North, caused by the satellites’ inclinationof65◦). Due to the special selection of the arguments of lat-itude in the three orbital planes, Figure 11(b) also charac-terizes the ground tracks of all GLONASS satellites. TheseGLONASS orbit characteristics lead, as a matter of fact, toan eight–hour repeat cycle in the satellite geometry for eachstation.

As each GLONASS satellite transmits its signal on anindividual frequency the impact of frequency–dependentef-fects such as multipath on station–specific parameters (suchas coordinates and troposphere) should be reduced for theconstellation (compared to GPS). For such issues we expecta basic period of four sidereal days (as opposed to one side-real day for the GPS), because GLONASS satellites sepa-rated by180◦ in the orbital plane use the same frequencies.

6 Benefit of the Combined GNSSProducts on Navigation and RapidPositioning

The global distribution of the PDOP for a GPS–onlyconstellation is shown in Figure 12 (left). The currentGLONASS constellation consists of only 16 active satel-lites (13 in November 2007). From these numbers we mayexpect an accuracy gain when using the combined sys-tem (and not only the GPS) for navigation and for posi-tioning based on short (few minutes) time spans of about√

31/16 ≈√

1.5 ≈ 1.22 in a least squares adjustment.Figure 12 (right) shows the gain according to the differentlatitudes: about 10% in equatorial regions, about 20% inmid–latitude regions and almost 30% in the polar regions.The much better performance in the polar regions is a con-sequence of the higher inclination of the GLONASS satel-lites (55◦ for GPS; 65◦ for GLONASS).

The PDOP value in essence gives the average of the meanerrors in the three orthogonal directions North, East, Up ofa position determination assuming code observations of theaccuracy of one meter (remember that smaller PDOP val-ues correspond to better satellite geometry . . . ). The samePDOP may be used for phase observations with resolvedambiguities, where the unit would be mm. The expectedaccuracy gain is not dramatic. With the full 24 satelliteconstellation the gain is expected to be

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(32 + 24)/32 ≈1.32. More important, but more difficult to illustrate, is thegain in robustness of the solution, e.g., the higher redun-dancy for the preprocessing in case of kinematic solutions.

This improvement in the PDOP by adding the GLONASSdata seems to be in contradiction to the conclusions of Sec-tion 4. The PDOP reflects the satellite geometry for thelocation of a receiver for a specific epoch, which is mainlyrelevant for a kinematic positioning. So we carried out thefollowing experiment: The European network solution, theCODE contribution to the EPN (European Permanent Net-work, [Bruyninx and Roosbeek, 2007]), was processed in

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Figure 12: The PDOP for a GPS–only constellation (left) and the improvement of the PDOP by adding the GLONASSconstellation (as it was available in November 2007).

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Figure 13: Allan deviations of the kinematic positions (at3-minute intervals over60 days) of the combinedGPS/GLONASS receiver in Zimmerwald (ZIM2) using only GPS measurements (grey line) and observations from bothGNSS (black line), respectively.

daily batches, for a two month interval. The orbits and thecoordinates of the reference stations were introduced fromthe official CODE contribution to the IGS (final solution)and to the EPN. The coordinates of the other sites and thetroposphere parameters were adjusted in the experiment.The combined GPS/GLONASS receiver at the Zimmer-wald observatory (ZIM2) was considered as “mildly kine-matic”, i.e., coordinates were determined at 3 minute inter-vals, where the ambiguities were introduced as known fromthe standard network processing. Only the GPS observa-tions were used for all stations in the solution in the first partof the experiment, all observations (GPS and GLONASS)were used in the second part. It would have been attrac-tive to generate a third solution using only the GLONASSmeasurements. In view of the limited number of simulta-

neously visible GLONASS satellites, such a solution wouldhave made little sense.

Allan deviations (see [Allan, 1987]) were generated withthe two sets of three minutes solutions for the Zimmerwaldstation (all in all 30’240 data points within 63 days). TheAllan deviations referring to a spacing ofτ between datapoints are given by

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The black line in Figure 13 refers to the combined pro-cessing of GPS and GLONASS measurements, whereas the

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grey line is obtained from the GPS–only solution. Forshort time intervals (up to a few minutes) the Allan devi-ation is dominated by the noise of the carrier phase (seealso [Dach et al., 2007]). In this domain the additionalGLONASS measurements help according to the

√n–law

to reduce the noise of the kinematic positions by 20 to 30%.For longer intervals — let us say half an hour or longer— the improvement becomes very small. For intervals ofone hour and longer the difference between both curves iseven smaller, but the black one (GPS/GLONASS solution)remains slightly below the grey line (GPS–only solution).

This behavior might — at least partially — be explainedby the higher variability in the satellite geometry of the in-complete GLONASS constellation.

7 Reviewing file formats concerningmulti–GNSS

In this section we address the issue whetherthe currently used file formats cover the needsfor the current (GPS/GLONASS) and future(GPS/GLONASS/Galileo/Compass) multi–GNSS con-stellations. We also summarize the status of the currentdevelopments:

RINEX The new RINEX 3 has been defined and covers allneeds for the multi–GNSS processing in future. Un-fortunately it is not really in use within the IGS.

SINEX The current format covers all needs of a routinelymulti–GNSS analysis.

Troposphere SINEX No adaption for a multi–GNSS pro-cessing is necessary.

clock RINEX Station clocks must be separable for dif-ferent GNSS because some receivers show morethan only an offset between the systems (see[Schaer, 2007]).

SP3c In the current format there are not enough positionsfor the satellites from all expected GNSS. A corre-sponding format extension is under discussion.

ERP The current format meets all needs of a multi–GNSSprocessing.

DCB Today we have for GPS only P1−C1, P2−C2,P1−P2 code biases and the 1/4–cycle–phase shift fora very limited number of receivers. For GLONASSone GPS–GLONASS receiver clock bias as well asGLONASS inter–frequency code biases need to beconsidered ([Dach et al., 2006]).Many more biases have to be expected in future, be-cause there will be three GPS and GLONASS and fiveGalileo frequencies, respectively, in future. This as-pect should be addressed and solved in the workinggroup on “biases and calibrations”.

8 Summary: Perspective of Multi–GNSS in the IGS

The IGS has promised to become a GNSS service by chang-ing its name two years ago. In practice it is, however, stilla GPS–only service today — with a marginal extension toGLONASS. On the other hand, there are commercial com-panies providing real multi–GNSS products on at least acomparable quality level as the IGS product lines. In thissection we describe the current status and the expected de-velopment within the IGS in the near future. We also de-scribe what would be necessary to develop the IGS into afull GNSS service.

Network: An increasing number of multi–GNSS receiversis expected in the IGS network. Because not only thereceiver but also the antenna needs to be changed inorder to guarantee optimum performance, the stationmanagers are asked to consider options that help toretain the stability of the reference frame (e.g., byproviding data from the old and new receiver in par-allel as far as possible — according to the generalrecommendations of the IGS).Galileo: the exchange of the equipment within the net-work may take place more rapidly than the transitionfrom the GPS–only to the combined GPS/GLONASSnetwork observed today. This may enhance the prob-lem of the stability of the reference frame.

To become an International GNSS Service:Replace-ments of receivers in the IGS network should be asso-ciated with the transition to modern multi–GNSS sta-tions. New IGS–sites are only accepted if they providethe observations from all active GNSS.

Processing: Since the beginning of 2008 ESOC providesfully consistent GPS/GLONASS products from arigorously combined processing comparable to theapproach CODE follows since May 2003. There areplans for including GLONASS into the processing atthe CNES–CLS analysis center. Unfortunately thereare no activities in view by the established analysiscenters of the IGS to contribute to GLONASS orbitsneither in a separate nor in a fully combined mode. Onthe other hand, nearly all analysis centers announcedan interest in processing of Galileo data.

To become an International GNSS Service:All (or atleast a substantial number of the) analysis centers haveto provide combined products from all active GNSS.The ACs included into the combination should providecombined GPS/GLONASS products after a develop-ment phase of at maximum two years.

Combination: As we have now two ACs providing fullycombined and consistent orbits we do not need a GPS–only plus an experimental, independent GLONASS–

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only combination, but a combined GPS/GLONASS or-bit combination procedure (with an alternative GPS–only combination or — better — an extraction of or-bits).To become an International GNSS Service:a rigor-ous combined analysis of the satellites from all activeGNSS is required. A corresponding update of the com-bination software should not only consider GLONASSas the second active GNSS today but it should be opento all future GNSS.

Validation: The two GPS satellites carrying SLR reflec-tors are very old and their decommissioning is ex-pected in the near future. An independent valida-tion using the SLR technique of the GNSS orbits de-rived from the microwave signals will only be possiblewith non–GPS satellites (in near future GLONASS,later on also Galileo or possibly Compass). Studies(e.g., [Urschl et al., 2007]) have demonstrated the im-portance of this validation process.

References

[Allan, 1987] Allan, D. (1987). Time and Frequency(Time-Domain) Characterization, Estimation, and Pre-diction of Precision Clocks and Oscillators. InIEEETransactions of Ultrasonics, Ferroelectrics, and Fre-quency Control, volume UFFC–34(6).

[Beutler et al., 1994] Beutler, G., Brockmann, E., Gurtner,W., Hugentobler, U., Mervart, L., and Rothacher, M.(1994). Extended Orbit Modeling Techniques at theCODE Processing Center of the International GPS Ser-vice for Geodynamics (IGS): Theory and Initial Results.Manuscripta Geodaetica, 19:367–386.

[Bruyninx and Roosbeek, 2007] Bruyninx, C. and Roos-beek, F. (2007). EPN Status and New Developments.In Torres, J. and Hornik, H., editors,Subcommission forthe European Reference Frame (EUREF).

[Dach et al., 2007] Dach, R., Beutler, G., and Gudmund-son, G. (2007). Analysis of GPS data from an AntarcticIce Stream. InIUGG General Assembly, Perugia, July2–13, 2007, IAG Symposia. in press.

[Dach et al., 2006] Dach, R., Schaer, S., Hugentobler, U.,and Schildknecht, T. (2006). Combined Multi–SystemGNSS Analysis for Time and Frequency Transfer. InProceedings of the20th European Frequency and TimeForum EFTF 06, Braunschweig, Germany.

[Schaer, 2007] Schaer, S. (2007). Inclusion of GLONASSfor EPN Analysis at CODE/swisstopo. In Torres, J. andHornik, H., editors,Subcommission for the EuropeanReference Frame (EUREF).

[Urschl et al., 2007] Urschl, C., Beutler, G., Gurtner, W.,Hugentobler, U., and Schaer, S. (2007). Contribution

of SLR tracking data to GNSS orbit determination.Ad-vances in Space Research, 39(10):1515–1523.

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