**The International GLONASS Experiment**

**(IGEX-98): Organization, Preliminary Results and Future Plans**

James A. Slater, *National Imagery and Mapping Agency, USA*

Pascal Willis, *Institut Géographique National, France*

Gerhard Beutler, *Astronomical Institute, University of Berne,
Switzerland*

Werner Gurtner, *Astronomical Institute, University of Berne,
Switzerland*

Wlodzimierz Lewandowski, *Bureau International des Poids et Mesures,
France*

Carey Noll*, NASA Goddard Space Flight Center, USA*

Robert Weber, *University of Technology Vienna, Austria*

Ruth E. Neilan, *Jet Propulsion Laboratory, USA*

Guenter Hein, *Institute of Geodesy and Navigation, University FAF
Munich , Germany*

**BIOGRAPHY**

*James A. Slater* has been actively involved in developmental work,
program management and policy issues related to satellite geodesy, GPS
and WGS 84 since 1980 at the (U.S.) National Imagery and Mapping Agency,
and co-chairs the ION GLONASS-GPS Interoperability Working Group.

*Pascal Willis* has worked in the field of geodesy at the Institut
Géographique National since 1983. After a Doctoral dissertation on
geodetic applications of GPS, he has been doing research work on other
satellite systems such as DORIS (for which he is the Coordinator for the
International Earth Rotation Service). He chairs the International
Association of Geodesy Sub-commission on Microwave Satellite Systems and
the IGEX-98 Steering Committee.

*Gerhard Beutler* is Director of the Center for Orbit Determination in
Europe (CODE) at the University of Berne, Switzerland and former Chair
of the International GPS Service Governing Board. He has a Ph.D. in
astronomy from the University of Berne.

*Werner Gurtner* received his Ph.D. from the Swiss Federal Institute of
Technology working on an astro-geodetic geoid of Switzerland. He
currently is on the staff of the Astronomical Institute, University of
Berne where he is the manager of the Zimmerwald Geodynamical
Observatory.

*Wlodzimierz Lewandowski* has a Doctorate in geographical science from
the Institut Géographique National. He is a physicist in the Time
Section of the Bureau International des Poids et Mesures in Sèvres,
where he helps evaluate and disseminate International Atomic Time (TAI)
and Coordinated Universal Time (UTC). He is studying the use of space
techniques, especially GPS and GLONASS, for ultra-accurate time
transfer, and currently chairs the Timing Subcommittee of the Civil GPS
Service Interface Committee.

*Carey Noll* is the manager of NASA\'s Crustal Dynamics Data Information
System (CDDIS) at Goddard Space Flight Center in Greenbelt, Maryland.
She is the Data Flow Coordinator for IGEX-98.

## Robert Weber is in the Department of Theoretical Geodesy, at the University of Technology, Vienna, Austria. He has a Ph.D. from the same institution, and is currently using GPS data to study Earth rotation. He is the Analysis Center Coordinator for IGEX-98 and chairs an IAG study group on regional permanent arrays. 

*Ruth E. Neilan* is Director of the IGS Central Bureau at the Jet
Propulsion Laboratory in Pasadena, California. She has a M.S. degree in
Civil and Environmental Engineering from the University of
Wisconsin-Madison and has been involved in the coordination and
establishment of the GPS Global Network since 1987.

*Guenter Hein* is Full Professor and Director of the Institute of
Geodesy and Navigation of the University FAF Munich. He has been working
with GPS since 1983 and with GLONASS since 1990 in several research
projects.

**ABSTRACT**

When considering future global navigation satellite systems, the
interoperability and seamless use of several constellations is an
important aspect. The International GLONASS Experiment (IGEX-98) was
organized to address these interoperability issues between GLONASS and
GPS. From October 1998 to April 1999, approximately 60 stations in over
25 countries collected GLONASS tracking data in the first global
campaign organized to investigate GLONASS. The experiment was made
possible through the voluntary cooperation of dozens of organizations
that tracked the satellites, supplied equipment, archived and
distributed data, and produced the results described here. Observations
were made continuously at each station, converted to RINEX format, and
transmitted to one of two data centers in France and the United States.
These data have been used by numerous organizations to determine precise
GLONASS orbits, examine the relationship between the PZ-90 and WGS 84
terrestrial reference frames, compute station positions and GLONASS-GPS
time offsets, perform time transfer, evaluate receiver performance, and
develop and test new software. The paper summarizes the preliminary
results of the campaign and notes the impacts of the experiment on the
ability to use GLONASS for scientific and practical applications. Due to
the success of the campaign, plans are underway for continuing the
global data collection and analysis.

### INTRODUCTION

The International GLONASS Experiment (IGEX-98) was conducted between
October 19, 1998 and April 19, 1999. This experiment was the first
internationally-coordinated tracking and analysis campaign dedicated to
the Russian GLONASS satellites. During the campaign, GLONASS receivers
were operated at 61 sites in 26 countries (or territories). In addition,
the International Laser Ranging Service helped coordinate an
unprecedented amount of laser tracking of the GLONASS satellites, each
of which is equipped with laser retroreflectors. Nine of the satellites
were chosen for concentrated tracking. Thirty satellite laser ranging
(SLR) observatories in 15 countries participated.

IGEX-98 was co-sponsored by the International Association of Geodesy
(Commission VIII, International Coordination of Space Techniques for
Geodesy and Geodynamics), the International GPS Service (IGS), the
Institute of Navigation (ION), and the International Earth Rotation
Service. A Steering Committee with members from these organizations,
chaired by Dr. Pascal Willis of the Institut Géographique National,
organized and managed the experiment. \[Willis et al., 1998\] By taking
advantage of the international community's extensive experience with GPS
campaigns and the infrastructure that already existed within the IGS,
the Steering Committee was able to organize the experiment relatively
quickly. Requirements for station monumentation, receivers,
documentation, satellite tracking protocols, data formats, data
transmission, data archiving, and an IGEX e-mail message system were
developed and put in place. \[Slater et al., 1998; Willis et al., 1998;
Willis et al., 1999\]

Receiver manufacturers cooperated by producing enough dual-frequency
combined GLONASS and GPS receivers to support the participating
organizations. A number of receivers were lent to Russian organizations
to help support Russian participation. Global data centers at the NASA
Goddard Space Flight Center in the U.S. and the Institut Géographique
National (IGN) in France archived all of the GLONASS receiver tracking
data.

A number of organizations modified their GPS and SLR software to handle
GLONASS data. Precise post-processed orbits were produced at the
sub-meter level for all the operational GLONASS satellites from the
GLONASS receiver data and, where possible, independently from the SLR
data. A weighted combination of the individual precise orbits was also
produced by an Analysis Center Coordinator at the University of
Technology, Vienna.

The precise orbits generated during the campaign facilitated other
applications including time transfer, datum transformations between the
Russian PZ-90 reference system and WGS 84 (or ITRF96), and geodetic
positioning. This paper describes the motivation for the experiment, its
operational aspects, and the results that have been produced thus far.
Maintenance of a "permanent" global tracking network, from a subset of
the IGEX-98 stations, and continued production of precise GLONASS orbits
are discussed as possible follow-on activities by the international
community. All of the results discussed here were reported at the
IGEX-98 Workshop held on September 13-14, 1999 in Nashville, Tennessee.
\[Slater et al., 1999\]

**MOTIVATION**

'Why GLONASS' and 'why now' are the two questions that come to mind
regarding the motivation behind this GLONASS campaign. The fact that
GLONASS existed was the first and most obvious consideration. There were
over a dozen operational satellites in orbit with characteristics very
similar to those of GPS. \[GLONASS, 1995; Langley, 1997\] This
immediately provided a way to experiment with a realization of a Global
Navigation Satellite System (GNSS). Both the navigation and the
scientific communities had an interest in exploring the possible
benefits of a 36 or more satellite constellation, and therefore had an
interest in resolving any interoperability problems that existed.
\[Misra and Slater, 1998; Senus and Misra, 1996; Slater and Misra, 1997;
Zarraoa et al., 1998\] One immediate application of combined GPS and
GLONASS receivers is real-time navigation of space platforms.
\[Klyushnikov et al., 1999\] Another is atmospheric monitoring and
research.

The availability of commercial receiver technology was no longer an
impediment. Several manufacturers were marketing combined GPS-GLONASS
receivers for positioning and timing applications, and dual-frequency
geodetic-quality receivers could be obtained at a reasonable cost.
Furthermore, data and hardware standards that had been established for
GPS applications could be readily adapted for GLONASS.

One of the attractions of GLONASS to the civil (non-military) user is
direct access to the P-code. Unlike GPS, GLONASS signals are
unencumbered by Selective Availability and anti-spoofing corruptions.
The GLONASS P-code has a resolution five times better than the GPS
C/A-code (the only code directly available to the civil user). This is
particularly useful to the timing community. \[Azoubib and Lewandowski,
1998\]

Differences between GLONASS's PZ-90 reference frame and GPS's WGS 84
reference frame remained a problem. Attempts had been made to relate
PZ-90 to WGS 84 (and the International Terrestrial Reference Frame
(ITRF)) on a local and regional basis, but no global solution existed.
\[Bazlov et al., 1999; Misra et al., 1996; Rossbach et al., 1996\] In
addition, GLONASS employed UTC(SU) as its time reference, while GPS used
UTC(USNO); therefore, time synchronization was still an issue.

Exploitation of GLONASS for many applications requires a global tracking
network and availability of precise orbits for the satellites. Neither
of these had been attempted before. However, because of the operational
capabilities of the IGS, the infrastructure was in place to deploy
receivers, and collect, archive and analyze the data. \[Beutler et al.,
1999\] The benefits of this expanded constellation to the scientific
community are essentially more observations per unit time and a higher
density of observations. This means more robustness, independence and
redundancy of measurements, and better temporal and spatial resolution.
Because of the similarity between GPS and GLONASS, it was relatively
easy to assimilate the GLONASS data into existing GPS processing
schemes. Furthermore, SLR observations of GLONASS satellites can be used
for independent quality control and validation of other results. Note
that most applications are considering GLONASS as an augmentation to GPS
and not as a stand-alone system. Thus, the number of GLONASS satellites
is not a limiting factor in most of this work.

Lastly, GLONASS had an uncertain future. Many of the satellites are well
beyond their design lives and could fail at any time. Three replacement
satellites were successfully launched halfway into the campaign,
maintaining a constellation of about 14 satellites for operational use.
Postponing the experiment may have resulted in a reduced constellation
to work with.

###  OBJECTIVES 

The IGEX Steering Committee established a list of objectives for the
experiment. \[Willis et al., 1999\] The foundation for much of the work
that would follow was the establishment of a global tracking network.
GLONASS tracking stations were required to be collocated with GPS
stations (with well-defined ITRF96 coordinates), monumented and
documented. The network's task was the collection of a continuous,
globally-distributed data set over a long time period.

In parallel with this, SLR stations around the world were requested to
track a subset of the GLONASS satellites to provide an independent data
set for orbit evaluation. These activities then supported the remaining
objectives: (a) precise orbit determination, (b) receiver evaluation,
(c) software development, (d) geodetic positioning, (e) reference frame
comparisons, and (f) time and time transfer applications. It was also
hoped that this work would be a stimulus for other applications and
developments.

#### NETWORK OPERATIONS

The global network of GLONASS and SLR stations that supported the
IGEX-98 experiment is shown in figure 1. GLONASS receivers were deployed
at 61 sites in 26 countries (or territories). Several of the sites had
more than one GLONASS receiver operating. Every station was required to
have a dual-frequency GPS receiver, in part to ensure that good ITRF
coordinates were determined for the station location. The stations that
operated GLONASS receivers during the tracking campaign are listed in
table 1. Receivers from Ashtech, MAN, Javad Positioning Systems and 3S
Navigation were the predominant receivers used. Two older Trimble 4000
SGL single-frequency receivers that had been produced in limited
quantities for Russia were operated, and one prototype receiver
developed for the European Space Agency by the Institute of Satellite
Navigation at the University of Leeds was tested as well. A total of 48
dual-frequency and 20 single-frequency GLONASS receivers formed the
global tracking network. (See table 2.)

In addition, the International Laser Ranging Service (ILRS) helped
organize the SLR stations worldwide to track nine GLONASS satellites,
three in each of the three orbit planes, during the campaign. (Only
three GLONASS satellites were being observed on a regular basis prior to
the campaign.) The satellites chosen are listed below by slot and plane
number (SLR satellite number is in parentheses):

Plane 1: Slots 3 (68), 4 (70), 6 (69)

Plane 2: Slots 9 (79), 12 (65), 16 (66)

Plane 3: Slots 17 (62), 20 (71), 22 (72)

![](media/image1.png){width="6.75in" height="8.868055555555555in"}

+--------+------------------+-------+-----+------+---+------------+----+
| #####  |                  |       |     |      |   |            |    |
| # **Ta |                  |       |     |      |   |            |    |
| ble 1. |                  |       |     |      |   |            |    |
|  IGEX- |                  |       |     |      |   |            |    |
| 98 GLO |                  |       |     |      |   |            |    |
| NASS R |                  |       |     |      |   |            |    |
| eceive |                  |       |     |      |   |            |    |
| r Stat |                  |       |     |      |   |            |    |
| ions a |                  |       |     |      |   |            |    |
| nd Dat |                  |       |     |      |   |            |    |
| a Hold |                  |       |     |      |   |            |    |
| ings** |                  |       |     |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| O      |                  |       |     |      |   |            |    |
| ctober |                  |       |     |      |   |            |    |
| 19,    |                  |       |     |      |   |            |    |
| 1998 - |                  |       |     |      |   |            |    |
| April  |                  |       |     |      |   |            |    |
| 1      |                  |       |     |      |   |            |    |
| 9,1999 |                  |       |     |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
|        |                  |       |     |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| C      | Station          | St    | La  | L    |   | Receiver   | N  |
| ountry | Location/Name    | ation | tit | ongi |   | Model      | o. |
|        |                  | ID    | ude | tude |   |            | Da |
|        |                  |       |     |      |   |            | ys |
+--------+------------------+-------+-----+------+---+------------+----+
| Anta   | McMurdo          | CRAR  | -7  | 166° |   | Javad      | 43 |
| rctica | Stn./Crary       |       | 7°5 | 40\' |   | Legacy GGD |    |
|        | Science Lab      |       | 1\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Arg    | Rio Grande       | RIOZ  | -5  | -67° |   | MAN        | 1  |
| entina |                  |       | 3°4 | 45\' |   | NR-R124    | 41 |
|        |                  |       | 7\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Aus    | Brisbane         | SUNM  | -2  | 153° |   | Javad      | 90 |
| tralia |                  |       | 7°2 | 02\' |   | Legacy GGD |    |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Aus    | Canberra/        | STRR  | -3  | 149° |   | Ashtech    | 1  |
| tralia | Stromlo          |       | 5°1 | 01\' |   | Z18        | 23 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Aus    | Hobart           | HOBR  | -4  | 147° |   | Ashtech    | 3  |
| tralia |                  |       | 2°4 | 26\' |   | GG24       |    |
|        |                  |       | 8\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Aus    | Lin              | LINR  | -3  | 151° |   | 3S Nav     | 90 |
| tralia | dfield/Lindfield |       | 3°5 | 12\' |   | R100/30T   |    |
|        | NML2             |       | 0\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Aus    | Perth/ Bellevue  | BELR  | -3  | 116° |   | Ashtech    | 27 |
| tralia | 157              |       | 1°5 | 00\' |   | GG24       |    |
|        |                  |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Aus    | Yaragadee/ AU053 | YARR  | -2  | 115° |   | Ashtech    | 1  |
| tralia |                  |       | 9°0 | 21\' |   | Z18        | 37 |
|        |                  |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| A      | Graz             | GRAB  | 4   | 15°  |   | Ashtech    | 1  |
| ustria | /Graz-Lustbuehel |       | 7°0 | 30\' |   | Z18        | 15 |
|        |                  |       | 6\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| A      | Innsbruck        | IBK1  | 4   | 11°  |   | Ashtech    | 1  |
| ustria |                  |       | 7°1 | 21\' |   | GG24       | 20 |
|        |                  |       | 6\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| A      | Mattersburg      | MTBG  | 4   | 16°  |   | Ashtech    | 1  |
| ustria |                  |       | 7°4 | 24\' |   | GG24       | 37 |
|        |                  |       | 4\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| B      | Brussels         | BRUG  | 5   | 4°   |   | 3S Nav     | 1  |
| elgium |                  |       | 0°4 | 22\' |   | R100/30T   | 54 |
|        |                  |       | 8\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Chile  | Santiago         | SANG  | -3  | -70° |   | 3S Nav     | 1  |
|        |                  |       | 3°0 | 40\' |   | R100/40    | 56 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| China  | Hong Kong        | HKPU  | 2   | 114° |   | Ashtech    | 4  |
|        |                  |       | 2°1 | 11\' |   | GG24       |    |
|        |                  |       | 8\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| France | B                | BRSG  | 4   | -4°  |   | Martech    | 1  |
|        | rest/Brest-EPHOM |       | 8°2 | 30\' |   | Mira 24    | 56 |
|        |                  |       | 4\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| France | B                | BRST  | 4   | -4°  |   | Martech    | 1  |
|        | rest/Brest-EPHOM |       | 8°2 | 30\' |   | Mira 24    | 56 |
|        |                  |       | 4\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| France | Grasse/ Caussols | GRAC  | 4   | 6°   |   | Martech    | 1  |
|        |                  |       | 3°4 | 55\' |   | Mira 24    | 19 |
|        |                  |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| France | Sèvres/BIPM      | BIPD  | 4   | 2°   |   | 3S Nav     | 1  |
|        |                  |       | 8°5 | 13\' |   | R100/30T   | 44 |
|        |                  |       | 0\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| France | Vernon           | LRBA  | 4   | 1°   |   | Ashtech    | 1  |
|        |                  |       | 9°0 | 30\' |   | Z18        | 38 |
|        |                  |       | 6\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Gabon  | N\'Koltang       | NKLG  | 0°1 | 9°   |   | Ashtech    | 37 |
|        |                  |       | 3\' | 24\' |   | Z18        |    |
+--------+------------------+-------+-----+------+---+------------+----+
| G      | Neubiberg        | BLVA  | 4   | 11°  |   | 3S Nav     | 10 |
| ermany |                  |       | 8°0 | 35\' |   | R101+R100  |    |
|        |                  |       | 8\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| G      | Neustrelitz/ DLR | NTZ1  | 5   | 13°  |   | 3S Nav     | 1  |
| ermany | DFD Neustrelitz  |       | 3°2 | 04\' |   | R101       | 83 |
|        |                  |       | 0\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| G      | Oberpfaffenhofen | DLRA  | 4   | 11°  |   | 3S Nav     | 1  |
| ermany |                  |       | 8°0 | 17\' |   | R100/40T   | 32 |
|        |                  |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| G      | Ko               | WTZG  | 4   | 12°  |   | 3S Nav     | 1  |
| ermany | etzting/Wettzell |       | 9°0 | 53\' |   | R101+R100  | 23 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| G      | Ko               | WTZZ  | 4   | 12°  |   | Ashtech    | 68 |
| ermany | etzting/Wettzell |       | 9°0 | 53\' |   | Z18        |    |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Gre    | Thule Airbase    | THU2  | 7   | -68° |   | Ashtech    | 1  |
| enland |                  |       | 6°3 | 50\' |   | Z18        | 12 |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| I      | Reykjavik        | REYZ  | 6   | -21° |   | Ashtech    | 85 |
| celand |                  |       | 4°0 | 57\' |   | Z18        |    |
|        |                  |       | 8\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| India  | New Delhi        | NPLI  | 2   | 77°  |   | 3S Nav     | 2  |
|        |                  |       | 8°3 | 10\' |   | GNSS-300T  |    |
|        |                  |       | 8\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Japan  | Mitaka/ Mitaka A | MTKA  | 3   | 139° |   | Ashtech    | 1  |
|        | Site             |       | 5°4 | 34\' |   | Z18        | 27 |
|        |                  |       | 1\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Japan  | Tsukuba/ Tsukuba | TSKA  | 3   | 140° |   | Ashtech    | 1  |
|        | A                |       | 6°0 | 05\' |   | Z18        | 36 |
|        |                  |       | 6\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Kyrg   | Bishkek          | BISZ  | 4   | 74°  |   | MAN        | 1  |
| yzstan |                  |       | 2°5 | 36\' |   | NR-R124    | 47 |
|        |                  |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| La     | La Reunion       | REUN  | -2  | 55°  |   | Ashtech    | 64 |
| R      |                  |       | 1°1 | 34\' |   | Z18        |    |
| eunion |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Nethe  | Delft            | DLFT  | 5   | 4°   |   | Ashtech    | 1  |
| rlands |                  |       | 1°5 | 23\' |   | GG24       | 20 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Nethe  | Delft            | DLFT  | 5   | 4°   |   | Javad      | 53 |
| rlands |                  |       | 1°5 | 23\' |   | Legacy GGD |    |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Nethe  | Delft/ VSL-Delft | VSLD  | 5   | 4°   |   | 3S Nav     | 1  |
| rlands |                  |       | 2°0 | 23\' |   | R100/40T   | 65 |
|        |                  |       | 0\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Poland | Borowiec         | BORG  | 5   | 17°  |   | 3S Nav     | 1  |
|        |                  |       | 2°1 | 04\' |   | R100/30T   | 67 |
|        |                  |       | 7\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Ekaterinburg     | EKAT  | 5   | 59°  |   | Javad      | 8  |
|        |                  |       | 7°0 | 33\' |   | Legacy     |    |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Irkutsk          | IRKG  | 5   | 104° |   | Trimble    | 1  |
|        |                  |       | 2°1 | 19\' |   | 4000SGL    | 69 |
|        |                  |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Irkutsk          | IRKZ  | 5   | 104° |   | Ashtech    | 1  |
|        |                  |       | 2°1 | 19\' |   | Z18        | 74 |
|        |                  |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Khabarovsk       | KHAB  | 4   | 135° |   | Ashtech    | 1  |
|        |                  |       | 8°3 | 02\' |   | Z18        | 50 |
|        |                  |       | 1\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Magadan          | MAGD  | 5   | 150° |   | Javad      | 54 |
|        |                  |       | 9°3 | 48\' |   | Legacy     |    |
|        |                  |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Mendeleevo       | MDVG  | 5   | 37°  |   | Trimble    | 1  |
|        |                  |       | 6°0 | 13\' |   | 4000SGL    | 18 |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Mendeleevo       | MDVZ  | 5   | 37°  |   | Ashtech    | 1  |
|        |                  |       | 6°0 | 13\' |   | Z18        | 81 |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Petropavl        | PKST  | 5   | 158° |   | Javad      | 28 |
|        | ovsk-Kamchatskiy |       | 3°0 | 38\' |   | Legacy     |    |
|        |                  |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Svetloe          | SVT3  | 6   | 29°  |   | Javad      | 34 |
|        |                  |       | 0°3 | 47\' |   | Legacy     |    |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Yakutsk          | YAKT  | 6   | 129° |   | Javad      | 52 |
|        |                  |       | 2°0 | 41\' |   | Legacy     |    |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Russia | Zvenigorod       | ZWEG  | 5   | 36°  |   | Ashtech    | 79 |
|        |                  |       | 5°4 | 46\' |   | GG24       |    |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| South  | Pretoria         | CSIR  | -2  | 28°  |   | 3S Nav     | 1  |
| Africa |                  |       | 5°4 | 17\' |   | R100/30T   | 65 |
|        |                  |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| South  | Sutherland       | SUTG  | -3  | 20°  |   | MAN        | 1  |
| Africa |                  |       | 2°2 | 49\' |   | NR-R124    | 36 |
|        |                  |       | 4\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Sweden | Kiruna           | KR0G  | 6   | 21°  |   | Ashtech    | 1  |
|        |                  |       | 7°5 | 04\' |   | Z18        | 70 |
|        |                  |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Sweden | Maartsbo         | MR6G  | 6   | 17°  |   | Ashtech    | 1  |
|        |                  |       | 0°3 | 16\' |   | GG24       | 77 |
|        |                  |       | 6\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Sweden | Onsala           | OS0G  | 5   | 11°  |   | Ashtech    | 1  |
|        |                  |       | 7°2 | 56\' |   | Z18        | 63 |
|        |                  |       | 4\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Sweden | Visby            | VS0G  | 5   | 18°  |   | Ashtech    | 1  |
|        |                  |       | 7°3 | 22\' |   | GG24       | 69 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Switz  | Zimmerwald/      | ZIMJ  | 4   | 7°   |   | Javad      | 48 |
| erland | ZimmerwaldGPS87E |       | 6°5 | 28\' |   | Legacy     |    |
|        |                  |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Switz  | Zimme            | ZIMZ  | 4   | 7°   |   | Ashtech    | 1  |
| erland | rwald/Zimmerwald |       | 6°5 | 28\' |   | Z18        | 72 |
|        | GPS97            |       | 3\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| Taiwan | Taiwan           | NCKU  | 2   | 120° |   | Ashtech    | 1  |
|        |                  |       | 3°0 | 13\' |   | GG24(2)    | 17 |
|        |                  |       | 0\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| UK     | Herstmonceux     | HERP  | 5   | 0°   |   | 3S Nav     | 1  |
|        |                  |       | 0°5 | 20\' |   | R100/40    | 51 |
|        |                  |       | 2\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| UK     | Leeds            | LDS1  | 5   | -1°  |   | ESA/ISN    | 1  |
|        |                  |       | 3°4 | 33\' |   | GNSS       | 81 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| UK     | Leeds            | LDS3  | 5   | -1°  |   | Ashtech    | 1  |
|        |                  |       | 3°4 | 33\' |   | GG24       | 80 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| UK     | Teddington       | NPLC  | 5   | -0°  |   | 3S Nav     | 1  |
|        |                  |       | 1°2 | 20\' |   | R100/40T   | 31 |
|        |                  |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| UK     | Gr.              | GTY2  | 5   | 1°   |   | Ashtech    | 74 |
|        | Yarmouth/Great   |       | 2°3 | 44\' |   | GG24       |    |
|        | Yarmouth 2       |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| USA    | Gainesville      | GATR  | 2   | -82° |   | Javad      | 1  |
|        |                  |       | 9°2 | 21\' |   | Legacy GGD | 20 |
|        |                  |       | 9\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| USA    | Greenbelt        | GODZ  | 3   | -76° |   | Ashtech    | 1  |
|        |                  |       | 9°0 | 50\' |   | Z18        | 70 |
|        |                  |       | 1\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| USA    | Irvine           | 3SNA  | 3   | -    |   | 3S Nav     | 1  |
|        |                  |       | 3°4 | 117° |   | R100/40T   | 39 |
|        |                  |       | 1\' | 52\' |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| USA    | Lexington/       | SL1X  | 4   | -71° |   | Ashtech    | 1  |
|        | MIT/LL South Lab |       | 2°2 | 16\' |   | Z18        | 72 |
|        |                  |       | 8\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| USA    | Ft.              | MDOA  | 3   | -    |   | Javad      | 1  |
|        | Davis/McDonald   |       | 0°4 | 104° |   | Legacy     | 38 |
|        | Observatory      |       | 1\' | 01\' |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+
| USA    | Washington, DC   | USNX  | 3   | -77° |   | 3S Nav     | 1  |
|        |                  |       | 8°5 | 04\' |   | R100/30T   | 66 |
|        |                  |       | 5\' |      |   |            |    |
+--------+------------------+-------+-----+------+---+------------+----+

**Table 2. Numbers and Types of GLONASS**

**Receivers Deployed**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

3S Navigation R100 (various models) 12

3S Navigation R101 1

3S Navigation R101/R100 2

3S Navigation GNSS-300T 1

Ashtech GG24 (including Martech MIRA 24)\* 15

Ashtech Z-18 20

ESA/ISN GNSS 1

Javad Positioning Systems Legacy 11

MAN NR-R124\* 3

Trimble 4000 SGL\* 2

##### TOTAL 68

\*Single frequency.

Some of the other GLONASS satellites were also tracked. The 30 SLR
stations that participated are shown in table 3.

Operational data centers transmitted data from one or more stations to
regional or Global Data Centers. Operational centers that collected data
from multiple stations included Deutsche Forschungsanstalt fuer Luft-und
Raumfahrt (DLR), European Space Agency (ESA), GeoForschungsZentrum
(GFZ), and National Cheng Kung University (NCKU). Regional centers were
located at the Australian Surveying and Land Information Group (AUSLIG)
and Bundesamt fuer Kartographie und Geodaesie (BKG). The two Global Data
Centers were at NASA Goddard Space Flight Center's Crustal Dynamics Data
Information System (CDDIS) and the IGN. The goal was for data to be
transmitted once a day to the Global Data Centers within 48 hours after
collection at the

+------+--------+--------------+------+------+--------+-----+------+
| #### |        |              |      |      |        |     |      |
| ## * |        |              |      |      |        |     |      |
| *Tab |        |              |      |      |        |     |      |
| le 3 |        |              |      |      |        |     |      |
| . IG |        |              |      |      |        |     |      |
| EX-9 |        |              |      |      |        |     |      |
| 8 Sa |        |              |      |      |        |     |      |
| tell |        |              |      |      |        |     |      |
| ite  |        |              |      |      |        |     |      |
| Lase |        |              |      |      |        |     |      |
| r Ra |        |              |      |      |        |     |      |
| ngin |        |              |      |      |        |     |      |
| g Ob |        |              |      |      |        |     |      |
| serv |        |              |      |      |        |     |      |
| ator |        |              |      |      |        |     |      |
| ies  |        |              |      |      |        |     |      |
| and  |        |              |      |      |        |     |      |
| Data |        |              |      |      |        |     |      |
|  Hol |        |              |      |      |        |     |      |
| ding |        |              |      |      |        |     |      |
| s**  |        |              |      |      |        |     |      |
+------+--------+--------------+------+------+--------+-----+------+
| Oct  |        |              |      |      |        |     |      |
| ober |        |              |      |      |        |     |      |
| 19,  |        |              |      |      |        |     |      |
| 19   |        |              |      |      |        |     |      |
| 98 - |        |              |      |      |        |     |      |
| A    |        |              |      |      |        |     |      |
| pril |        |              |      |      |        |     |      |
| 19,  |        |              |      |      |        |     |      |
| 1999 |        |              |      |      |        |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      |        |              |      |      |        |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      |        | Station      | Sta  |      |        | No. |      |
|      |        | Location/    | tion |      |        | of  |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | C      | Name         | ID   | Lati | Lon    | Pas |      |
|      | ountry |              |      | tude | gitude | ses |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Aus    | Mount        | 7849 | -35° | 14     | 535 |      |
|      | tralia | Stromlo      |      | 19\' | 9°01\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Aus    | Orroral      | 7843 | -35° | 14     | 78  |      |
|      | tralia |              |      | 38\' | 8°56\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Aus    | Yaragadee    | 7090 | -29° | 11     | 1,  |      |
|      | tralia |              |      | 03\' | 5°21\' | 073 |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | A      | Graz         | 7839 | 47°  | 1      | 601 |      |
|      | ustria |              |      | 04\' | 5°30\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | China  | Beijing      | 7249 | 39°  | 11     | 39  |      |
|      |        |              |      | 36\' | 5°54\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | China  | Changchun    | 7237 | 43°  | 12     | 386 |      |
|      |        |              |      | 50\' | 5°20\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | China  | Kunming      | 7820 | 25°  | 10     | 35  |      |
|      |        |              |      | 02\' | 2°48\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | China  | Shanghai     | 7837 | 31°  | 12     | 169 |      |
|      |        |              |      | 06\' | 1°12\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | China  | Wuhan        | 7236 | 30°  | 11     | 26  |      |
|      |        |              |      | 35\' | 4°19\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | F      | Metsahovi    | 7806 | 60°  | 2      | 1   |      |
|      | inland |              |      | 13\' | 4°24\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | France | Grasse       | 7835 | 43°  | 6°55\' | 1   |      |
|      |        |              |      | 45\' |        |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | France | Grasse (LLR) | 7845 | 43°  | 6°55\' | 608 |      |
|      |        |              |      | 45\' |        |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | G      | Potsdam      | 7836 | 52°  | 1      | 101 |      |
|      | ermany |              |      | 17\' | 7°05\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | G      | Wettzell     | 8834 | 49°  | 1      | 417 |      |
|      | ermany |              |      | 09\' | 2°53\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Japan  | Kashima      | 7335 | 35°  | 14     | 4   |      |
|      |        |              |      | 57\' | 0°40\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Japan  | Koganei      | 7328 | 35°  | 13     | 17  |      |
|      |        |              |      | 26\' | 9°17\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Japan  | Miura        | 7337 | 35°  | 13     | 2   |      |
|      |        |              |      | 05\' | 9°22\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Japan  | Simosato     | 7838 | 33°  | 13     | 3   |      |
|      |        |              |      | 34\' | 5°56\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Japan  | Tateyama     | 7339 | 34°  | 13     | 19  |      |
|      |        |              |      | 35\' | 9°30\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Poland | Borowiec     | 7811 | 52°  | 1      | 26  |      |
|      |        |              |      | 17\' | 7°05\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Russia | Komsomo      | 1868 | 50°  | 13     | 33  |      |
|      |        | lsk-Na-Amure |      | 52\' | 6°59\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Switz  | Zimmerwald   | 7810 | 46°  | 6°55\' | 230 |      |
|      | erland |              |      | 53\' |        |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Tahiti | Papeete      | 7124 | -17° | -14    | 42  |      |
|      |        |              |      | 35\' | 9°36\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | U      | Simeiz       | 1873 | 44°  | 3      | 2   |      |
|      | kraine |              |      | 16\' | 3°36\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | UK     | Herstmonceux | 7840 | 50°  | 0°20\' | 413 |      |
|      |        |              |      | 52\' |        |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | USA    | Greenbelt    | 7105 | 39°  | -7     | 360 |      |
|      |        |              |      | 01\' | 6°50\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | USA    | Haleakala    | 7210 | 20°  | -15    | 225 |      |
|      |        |              |      | 43\' | 6°15\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | USA    | McDonald     | 7080 | 30°  | -10    | 245 |      |
|      |        |              |      | 41\' | 4°01\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | USA    | Monument     | 7110 | 32°  | -11    | 896 |      |
|      |        | Peak         |      | 54\' | 6°25\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+
|      | Uzbe   | Maidanak     | 1864 | 38°  | 6      | 52  |      |
|      | kistan |              |      | 41\' | 6°56\' |     |      |
+------+--------+--------------+------+------+--------+-----+------+

operational stations. Daily files containing RINEX-formatted \[Gurtner,
1994\] GPS and GLONASS observations collected at a 30 second sampling
rate and navigation messages were archived at the global centers for the
entire six-month campaign. CDDIS also archived the SLR observations
during IGEX. Analysis Centers downloaded the station data from the
Global Data Centers for orbit determination and other applications. In
turn, the Analysis Centers uploaded computed GLONASS orbits and clock
information, Earth rotation parameters, and station coordinates to the
Global Data Centers. The orbits are in SP3 format \[Remondi, 1989\] and
the station coordinates are in SINEX format \[Blewitt et al., 1995\].

As might have been expected, a number of operational problems occurred
during the tracking campaign. Some manufacturers' firmware had problems
in a continuous operational environment and had to be fixed by the
manufacturers during the campaign. Several stations experienced repeated
failures. Some file formatting and naming convention problems were
discovered. Site logs describing station location, equipment and ties to
other geodetic control on the site were received for most stations;
however, some were never submitted or are incomplete.

About 20 different RINEX converters and versions were used in the
campaign. Several problems were found including incorrect time tagging
of data and incorrect field sizes for observations. All of these
problems were fixed by the manufacturers, when they were identified
during the network operations.

Information exchange among the participants was greatly helped by an
Internet mail system "IGEX-mail" through which 300 messages were
distributed during the campaign. Web sites at ION
(http://[www.ion.org/workgroup.html](http://www.ion.org/workgroup.html))
and at IGN (http://lareg.ensg.ign.fr/IGEX) posted IGEX information
throughout the campaign.

#### PRECISE ORBIT AND CLOCK DETERMINATION

One of the major accomplishments of the IGEX was production of precise
ephemerides for all the operational GLONASS satellites for every day of
the campaign. Seven organizations independently generated precise
GLONASS orbits from the GLONASS receiver tracking data. Four
organizations used the SLR observations to compute GLONASS orbits. In
addition, "refined" orbits were generated at the University of
Technology, Vienna through a weighted combination of a subset of the
precise orbits that were being routinely produced. Table 4 lists the
organizations that computed orbits and, when known, the software used.

Six of these groups -- BKG, CODE, ESA, GFZ, JPL and MCC -- were able to
deliver, more or less regularly, precise

**Table 4. Organizations Generating Precise**

**GLONASS Orbits**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Organization Data Type Software

BKG Phase Bernese

CODE Phase Bernese

ESA/ESOC^1^ Phase/Code BAHN

GFZ Phase EPOS.P

NASA Jet Propulsion Lab Phase/Code GIPSY/OASIS

U. of Olsztyn, Poland Phase/Code TOP

U. of Texas CSR^1^ Phase GIPSY/OASIS

AUSLIG SLR MICROCOSM

NERC^1^ SLR SATAN

Russian MCC/GEO-ZUP^1^ SLR

U. of Texas CSR^1^ SLR UTOPIA

U. of Technology, Vienna Combined

^1^ESA/ESOC = European Space Agency/European Space Operations Center;
CSR = Center for Space Research; NERC = United Kingdom SLR facility; MCC
= Mission Control Center.

orbits and station coordinate solutions. Of these, the MCC solution is
the only one based solely on laser ranging measurements. Although these
Analysis Centers used different data types (undifferenced vs.
double-differenced observations), different parameterizations of the
force field, and different fit spans (3-8 days), all of them were able
to estimate from the start GLONASS orbits well below the meter level.
This was one of the goals of the experiment. For example, ESA used 72
hours of carrier phase and pseudorange data from 32 IGEX GLONASS
receivers and 14 GPS stations from the IGS network for orbit
computation. Twenty-four-hour orbit overlaps with the preceding 72-hour
spans were used to evaluate orbit precision. ESA used the GPS orbits and
satellite clocks produced for IGS as constraints in its GLONASS orbit
determination processing. Similarly, GFZ used 72 hours of carrier phase
data from both dual-frequency and single-frequency GLONASS receivers
with constrained GPS orbits from IGS to compute GLONASS orbits. Six-hour
orbit overlaps with the previous fit were used to judge orbit quality.
JPL also employed a 72-hour fit span to derive a solution, which was
compared to the solution from the previous span using a 6-hour overlap.
Table 5 shows sample orbit overlap statistics from ESA, GFZ and JPL as
daily RMS values averaged over 13 satellites for which each of these
three organizations computed orbits. These numbers are a good indication
of the repeatability of the orbits from day to day.

**Table 5. Sample Repeatability of GLONASS Orbits**

**for GPS Week 1000 (7-13 March 1999)**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Average Overlap RMS (cm) Over All Satellites^1^

7 Mar 8 Mar 9 Mar 10 Mar 11 Mar 12 Mar 13 Mar

ESA 27 21 21 19 20 20 18

GFZ 66 71 110 99 80 150 \--^2^

[JPL 31 17 12 11 10 11 \--^2^]{.underline}

^1^Computed for 13 GLONASS satellites: Slot nos. 1, 3, 4, 6, 7, 8, 9,

11, 13, 16, 17, 20, 22.

^2^No overlap data.

In general, over the entire campaign, solutions were consistent at the
20-30 cm level. At the University of Technology, Vienna, the precise
orbits submitted by the six Analysis Centers noted above were combined
into one final solution. This was done by applying small reference frame
corrections and then calculating a weighted mean for each satellite from
the independent solutions of the six Analysis Centers. The weights are
based on the results of a 7-day fit that is performed by the Analysis
Coordinator, to the daily orbit solutions for each satellite for each
Analysis Center. At this time, the broadcast satellite clock values are
being used with the orbits until better values can be derived.

The accuracy of the orbits computed from the GLONASS receiver data can
be evaluated by using the SLR observations. The NERC SLR facility in the
U.K. computed 7-day orbits from the SLR measurements, which gave
post-fit residual RMS values of about 6-10 cm. These orbits were then
compared to orbits derived by CODE from the GLONASS receiver data. The
SLR and receiver-derived orbits compared at the one-meter level in the
along-track and cross-track directions, and at about the 20-cm level
radially. The along-track and cross-track differences may be due to
systematic biases between the two systems. AUSLIG found RMS differences
between SLR orbits and CODE's orbits in the 8-9 cm range radially and 50
cm range in the along-track direction. Similar results were obtained by
the University of Texas CSR. A direct comparison done by CODE between
its microwave GLONASS orbits and the raw SLR range data revealed an
average range bias of 4 cm and an RMS difference of 16 cm.

#### RELATIONSHIPS BETWEEN REFERENCE FRAMES 

Two methods are generally used to quantitatively relate one reference
frame to another. One requires knowledge of a set of ground (antenna)
positions in both reference frames. Thus, for example, the PZ-90
coordinates of the ground positions could be mathematically related to
the WGS 84 coordinates of the same positions. The resulting
transformation parameters can then be used to convert other point
positions from one coordinate system to the other when needed. The
second method relies on comparisons of satellite orbits which are
defined independently in two different reference frames, and which then
can be mathematically related to one another. A 7-parameter
transformation is usually employed to derive three translation and three
rotation parameters, and one scale parameter. PZ-90 coordinates are not
available for most places outside the Former Soviet Union. Therefore,
most attempts to compute transformations have been based on the orbit
approach. BKG, JPL and MCC all used the orbit approach. BKG computed
daily values for transformation parameters using its precise GLONASS
orbits and the broadcast orbits. A rotation about the z-axis of 0.3--0.4
arcseconds was found to be the most significant parameter. JPL obtained
the same result after smoothing the broadcast orbits and then comparing
them to JPL's precise ITRF96 orbits. The MCC computed precise ITRF94
orbits based on SLR data, which they then compared to the broadcast
orbits. They noted that the transformation parameters seem to vary
periodically and attribute this to errors in the Earth Orientation
values used to generate GLONASS broadcast orbits. A 0.3-0.4 arcsecond
z-axis rotation and a 1.1 m z-translation are the dominant parameters in
their proposed transformation. The station positioning method was used
by IGN to derive results similar to the MCC's. IfEN GmbH employed an
alternative method for computing transformation parameters, that
involved incorporating the transformation parameters directly in the
range observation equations. Using the known ITRF station positions and
the broadcast PZ-90 satellite positions with the pseudorange
observations, the transformation parameters were obtained by solving
these range equations. The z-axis rotation was again the most
significant parameter. Note that the accuracy attainable with any of
these methods is limited by the accuracy of the broadcast PZ-90
satellite positions.

#### TIME TRANSFER APPLICATIONS

IGEX-98 was well received by the international time metrology community,
as the results of this experiment are contributing greatly to research
on the best use of the GLONASS constellation for international time
transfer. The international time scales -- International Atomic Time
(TAI) and Coordinated Universal Time (UTC) -- are based on data from
some 220 atomic clocks located in about 50 time laboratories around the
world. Today, the sole method of comparing these clocks to provide data
for TAI is the GPS C/A-code common-view technique. \[Lewandowski et al.,
1996\] The performance of this technique barely covers present needs,
and will not be sufficient in the near future to compare the new
generation of clocks. Thus, the time metrology community is engaged in a
number of studies of new time transfer techniques, among which is the
use of GLONASS or combined GPS and GLONASS systems. A gain of a factor
of 4 for time transfer stability was observed with a GPS+GLONASS
multi-channel link compared to a one-channel GPS link.

In order to take full advantage of GLONASS capabilities for
intercontinental time transfer, however, the use of post-processed
precise ephemerides is necessary, as it is for GPS applications. An
uncertainty of about 2 ns or better is achievable for intercontinental
GLONASS time transfer when using IGEX precise ephemerides, and only
about 10 ns when using broadcast ephemerides. An international effort is
currently under way to calibrate GLONASS timing receivers around the
world. When this is completed, the timing community will be able to
apply these techniques in earnest.

### FUTURE DIRECTIONS

Since April 20, 1999, the IGEX activities have continued on a "best
effort" basis. A global network of about 20 combined GLONASS/GPS
dual-frequency receivers is still tracking GLONASS satellites on a
routine basis. The ILRS network continues to track three GLONASS
satellites routinely. These data are being forwarded to the Global Data
Centers, and several of the Analysis Centers continue to produce precise
orbits that are publicly available from the Data Centers. These orbits
are in the same reference frame as the GPS orbits produced by the IGS.

Although the future of GLONASS is uncertain, it is clear that the
technology, methodology and organizations are in place to collect and
process GLONASS data. GLONASS data, when combined with GPS data, are a
valuable resource for satellite geodesy, definition of the terrestrial
reference system, time transfer, navigation, and atmospheric research.
Participants in the IGEX-98 Workshop in Nashville voted overwhelmingly
in favor of continuing global, internationally-coordinated tracking and
orbit determination for GLONASS for a four-year period ending in 2003.
This new pilot service should be organized under the auspices of the
IGS. If successful, the GLONASS pilot service should eventually be
merged with GPS into one dual service within the IGS. A new Call for
Participation will be issued in January 2000.

#### 

#### CONCLUSIONS

IGEX-98 was a great success. The results of the project exceeded
expectations. All of the major problems that previously stood in the way
of exploiting GLONASS were overcome. Receivers, a global tracking
network, data exchange formats, software, precise orbits and datum
transformations were produced for this experiment. More than 60
receivers located in 26 countries and 30 SLR observatories in 15
countries contributed data during the 6-month campaign. Eleven different
organizations computed precise orbits for GLONASS satellites from these
data and achieved accuracies of 20-50 cm consistently. The SLR data were
crucial in verifying the microwave receiver-derived orbits. More work
needs to be done to improve modeling of solar radiation pressure and
other non-gravitational forces, as well as to explain small systematic
biases between SLR- and receiver-based orbits. A number of independent
comparisons of PZ-90 and ITRF orbits and station coordinates confirmed a
difference of about 0.3-0.4 arcseconds in rotation about the z-axis and
a possible 1.0 meter translation in the z-direction.

It would not have been possible to organize and successfully execute
this experiment in such a short period of time without the GPS
infrastructure that has been developed during the last 20 years. The IGS
provided a backbone for the entire campaign, through its operational
tracking network, protocols for data communication and formats, and its
analysis centers and data centers. This combined with the collective
expertise of the GPS and GLONASS communities enabled us to meet the
objectives that we originally set forth.

Most users view GLONASS as an augmentation to GPS and not a stand-alone
system at this time. As such, the two systems form a 36-satellite
constellation. Since receivers, orbit determination software and a
tracking network now exist for GLONASS and GPS in a common reference
frame, it should be possible to take advantage of the combined satellite
systems for many different applications. The timing community is already
using the precise orbits to enhance intercontinental time transfer. The
global tracking network was called upon twice during the campaign to
provide information for special purposes -- once to help the Russians
pinpoint the time that one of the satellites stopped transmitting while
it was out of view of Russian ground stations, and another time to
provide high sampling rate data for ionospheric studies during the solar
eclipse in August. The experience gained here will lay the groundwork
for using any future global navigation satellites, such as the proposed
European Galileo system, in combination with GPS. Due to continued
interest in using GLONASS by many of the IGEX participants, plans are
under way to continue the IGEX effort as a pilot operational service
within the IGS on a test basis beginning next year.

#### ACKNOWLEDGMENTS

The success of this experiment was due to the work of many dedicated
individuals and organizations around the world that voluntarily
collaborated on this project. We are indebted to Dr. Pratap Misra, Chair
of the ION GLONASS-GPS Interoperability Working Group, and ION for
providing a forum for discussion of GLONASS and for directly assisting
with Russian participation in these activities. We appreciate the
efforts of receiver manufacturers to make sure receivers were available
to all those who wanted to participate in the campaign. The authors also
wish to thank David Hansen of the National Imagery and Mapping Agency
who created and maintained the ION IGEX web site during the entire
experiment.

**REFERENCES**

Azoubib, J. and W. Lewandowski, A Test of GLONASS P-Code Time Transfer,
Proc. ION GPS-98, Nashville, September 15-18, 1998, pp.1729-1736 (1998).

Bazlov, Y. A., V. F. Galazin, B. L. Kaplan, V. G. Maksimov and V. P.
Rogozin, GLONASS to GPS, A New Coordinate Transformation, *GPS World*,
10, 1, pp. 54-58 (1999).

Beutler, G., M. Rothacher, S. Schaer, T. Springer, J. Kouba, and R. E.
Neilan, The International GPS Service (IGS), an interdisciplinary
service in support of Earth sciences, 32nd COSPAR Scientific Assembly,
Nagoya, Japan, July 1998, *Adv. Space Res.*, 23, 4, pp. 631-653 (1999).

Blewitt, G., Y. Bock and J. Kouba, Constructing the IGS Polyhedron by
Distributed Processing, Proc. IGS Workshop on the Densification of the
IERS Terrestrial Reference Frame Through Regional GPS Networks, J. F.
Zumberge and R. Liu, Eds., JPL, Pasadena, Nov. 30 -- Dec. 2, 1994, pp.
21-37 (1995).

Gurtner, W., RINEX: The Receiver-Independent Exchange Format, *GPS
World*, 5, 7, pp. 48-52 (1994).

GLONASS Interface Control Document, International Civil Aviation
Organization, (ICAO), GNSSP/2-WP/66, Montreal, Canada, 14 November 1995
(1995).

Klyushnikov, S., S. Filatchenkov, N. Mikhailov, S. Pospelov, and M.
Vasilyev, "Soyuz"-"Mir" Orbital Flight GPS/GLONASS Experiment: First
Results, presented at 6^th^ St. Petersburg Int. Conf. on Integrated
Navigation Systems, St. Petersburg, Russia, May 24-26, 1999 (1999).

Langley, R.B., GLONASS: Review and Update, *GPS World*, 8, 7, pp. 46-51
(1997).

Lewandowski, W., J. Danaher and W. J. Klepczynski, Experiment Using
GPS/GLONASS, Common-View Time Transfer Between Europe and North America,
Proc. ION GPS-96, Kansas City, September 17-20, 1996, pp. 271-277
(1996).

Misra, P., R. Abbot and E. Gaposchkin, Integrated Use of GPS and
GLONASS: Transformation Between WGS 84 and PZ-90, Proc. ION GPS-96,
Kansas City, September, 17-20, 1996, pp. 307-314 (1996)

Misra, P. and J. Slater, A Report on the Third Meeting of the
GLONASS-GPS Interoperability Working Group, Proc. ION GPS-98, Nashville,
September 15-18, 1998, pp.2103-2106 (1998).

Remondi, B. W., Extending the National Geodetic Survey Standard Orbit
Formats, NOAA Tech. Report NOS 133 NGS 46 (1989).

Rossbach, U., H. Habrich, and N. Zarraoa, Transformation Parameters
Between, PZ-90 and WGS-84, Proc. ION GPS-96, Kansas City, September
17-20, 1996, pp. 279-285 (1996).

Senus, W. and P. Misra, A Report on the First Meeting of the GLONASS-GPS
Interoperability Working Group, Proc. ION GPS-96, Kansas City, September
17-20, 1996, pp. 1929-1930 (1996).

Slater, J. and P. Misra, A Report on the Second Meeting of the
GLONASS-GPS Interoperability Working Group, Proc. ION GPS-97, Kansas
City, September 16-19, 1997, pp.1893-1894 (1997).

Slater, J., P. Willis, W. Gurtner, C. Noll, G. Beutler, G. Hein, R. E.
Neilan, The International GLONASS Experiment (IGEX-98), Proc. ION
GPS-98, Nashville, September 15-18, 1998, pp.1637-1644 (1998).

Slater, J. A., C. Noll and K. Gowey, Eds., Proc. International GLONASS
Experiment 1998 (IGEX-98) Workshop, Nashville, Sept. 13-14, 1999, in
preparation (1999).

Willis, P., G. Beutler, W. Gurtner, G. Hein, R. Neilan and J. Slater,
International Call for Participation to the IGEX-98 campaign, *J. of
Geodesy*, 72, 5, p. 313 (1998).

Willis, P., G. Beutler, W. Gurtner, G. Hein, R. E. Neilan, C. Noll, and
J. Slater, IGEX: International GLONASS Experiment - Scientific
Objectives and Preparation, 32nd COSPAR Scientific Assembly, Nagoya,
Japan, July 1998, *Adv. in Space Res*., 23, 4, pp. 659-663 (1999).

Zarraoa N., W. Mai, E. Sardon, and A. Jungstand, Preliminary evaluation
of the Russian GLONASS system as a potential geodetic tool, *J. of
Geodesy*, 72, 6, pp. 356-363 (1998).