contributions of the international gps service for geodynamics (IGS) to global geodynamics, astronomy and atmosphere science: review and outlook

Acta Geod. Geoph. Hung., Vol. 35(1), pp. 3-24 (2000)

CONTRIBUTIONS OF THE INTERNATIONAL GPS SERVICE FOR GEODYNAMICS (IGS) TO GLOBAL

GEODYNAMICS, ASTRONOMY AND ATMOSPHERE SCIENCE: REVIEW AND OUTLOOK

G BEUTLERl

Keywords: CPS; International CPS Service; satellite orbits; station coordinates; terrestrial reference frames

Contents

- IGS objectives and mission

- Development of the IGS

- Structure and products of the IGS

- Fully exploiting the GPS/IGS for fundamental astronomy, geodynamics and atmosphere science.

The IGS is an International Scientific Service working under the auspices of the lAG (International Association of Geodesy). The IGS is closely cooperating with the IERS (International Earth Rotation Service) . The IGS is a FAGS service since 1996.

IGS objectives and mission

The IGS terms of reference contained, e.g., in the IGS colleague directory state:

- IGS collects, archives and distributes GPS observation data sets

- These data sets are used by the IGS to generate:

- high accuracy GPS satellite ephemerides,

- Earth rotation parameters,

- coordinates and velocities of the IGS tracking stations,

- GPS satellite and tracking station clock information,

- atmosphere information.

I Astronomical Institute, University of Berne, Switzerland

1217-8977/2000/$ 5.00 ©2000 Akademiai Kiad6, Budapest

4 G BEUTLER

The accuracies are sufficient to support current scientific objectives including the improvement and the realization of the accessibility to the ITRF, the International Terrestrial Reference Frame.

The IGS accomplishes its mission through the following components

- a network of tracking stations,

Data Centers, where Operational, Regional, and Global Data Centers are associated with stations, regions, and the global IGS network,

Analysis Centers generating daily global products without interruption,

Associated Analysis Centers generating unique products like ionosphere models, regional networks, etc. , possibly only for a limited amount of time per year

an Analysis Center Coordinator,

a Central Bureau, and

an International Governing Board, consisting of 15 members.

Date Aug 89-Feb 90

16 Mar 90

25 Apr 90

02 Sep 90

01 Feb 91 01 Apr 91 01 May 91 24 Jun 91 17 Aug 91

24 Oct 91

IGS development

Chronicle of IGS-events 1989- 1998 Event

lAG General Meeting in Edinburgh. Original ideas by I I Mueller, G Mader, W G Melbourne, B Minster and R E Neilan lAG Executive Committee Meeting in Paris decides to establish a Working Group to explore the feasibility of an IGS under lAG auspices with I I Mueller as chairman The Working Group is redesignated as the lAG Planning Committee for the IGS in Paris Planning Committee Meeting in Ottawa to prepare the Call for Participation (CFP) CFP mailed. Letters of intent due 1 April 1991 CFP attachments mailed to interested parties Proposals due Proposals evaluated and accepted in Columbus, Ohio Planning Committee reorganized and renamed IGS Campaign Oversight Committee (OSC) at 20st IUGG General Assembly in Vienna First IGS Campaign Oversight Committee Meeting in Greenbelt, MD. Preparation of the 1992 IGS Test Campaign, scheduled for 21 June - 23 September 1992 and for a two weeks intensive campaign called Epoch 92

Acta Geod. Geoph. Hung. 35, 2000

Date 17 Mar 92 04 May 92 21 May 92 21 Jun 92 01 Jul 92 27 Jul 92 23 Sep 92 15 Oct 92 01 Nov 92

24 Mar 93

27 May 93

09 Aug 93 12 Oct 93 18 Oct 93

08 Dec 93 01 Jan 94

21 Mar 94 25 Mar 94 30 Nov 94

06 Dec 94 15 May 95 06 Jul 95 12 Dec 95 19 Mar 96 30 Jun 96

16 Oct 96 12 Mar 97 05 Sep 97 11 Dec 97 12 Dec 97 09 Feb 98

INTERNATIONAL GPS SERVICE

Chronicle of IGS-events 1989- 1998 (contd.) Event

Second IGS OSC Meeting at OSU, Columbus, Ohio Communication tests IGS e-mailbox established at University of Berne Start of 1929 IGS Test Campaign First results about 2 weeks after beginning of campaing Start of Epoch-92 (2 weeks duration) Official end of campaign; data collection and processing continue Third IGS OSC Meeting at GSFC in Greenbelt, MD Start of IGS PILOT Service. Regular weekly orbit comparisons performed by IGS Analysis Center Coordinator 1993 IGS Workshop and fourth IGS OSC Meeting at University of Berne Fifth IGS OSC Meeting at AGU spring meeting in Baltimore, MD lAG Symposium in Beijing. Official lAG approval of the IGS Analysis Center Workshop in Ottawa Network Operations Workshop and first IGS Governing Board Meeting in Silver Spring, MD GB Business Meeting in San Francisco during AGU meeting Start of official IGS. Production of combined IGS Orbits, Central Bureau Information System (CBIS) installed Combined IERS/IGS Workshop in Paris (one week) Second IGS Governing Board Meeting in Paris IGS Workshop densification of the ITRF through regional GPS analyses in Pasadena Third IGS Governing Board Meeting in San Francisco IGS Workshop Special Topics and New Directions in Potsdam Fifth IGS Governing Board Meeting in Boulder Fifth IGS Governing Board Meeting in San Francisco IGS Analysis Center Workshop in Silver Spring Major Review of Processing (use of ITRF94, subdaily ERP terms, start of activities for new AACs, compliance to IERS Conventions 96) Sixth IGS Governing Board Meeting in Paris IGS Analysis Center Workshop in Pasadena Sevent IGS Governing Board Meeting in Rio de Janeiro Eighth IGS Governing Board Meeting in San Francisco IGS retreat 1997 in San Francisco IGS Analysis Center Workshop in Darmstadt

5


6 G BEUTLER

GPS TRACKING NETWORK

International GPS Service for Geodynamics

Fig. 1. CPS tracking network. International CPS service for geodynamics

Present structure of the IGS

Today the IGS consists of

- a Global Network of about 80 stations (Fig. 1),

- three global Data Centers

- CDmS (Crustal Dynamics Data Information System at Goddard Space Flight Center, USA),

- IGN (Institut Geographique National, France),

- SIO (Scripps Institution of Oceanography),

- seven Analysis Centers, namely CODE, EMR, ESA, JPL, GFZ, NGS and SIO,

- the Analysis Center Coordinator Jan Kouba, Natural Resources, Canada,

- the Central Bureau with Ruth Neilan as director,

- the International IGS Governing Board with Gerhard Beutler as chairman.

The IGS Network

Since 1992 the IGS Network went through a considerable development. Important empty areas like those in South America and in Russia could be filled with permanent receivers. At present Africa is considered as the most critical IGS area.


INTERNATIONAL GPS SERVICE 7

Table I. Some IGS institutions

Abbreviation Institution Function Participation

CODE at Astronomical Institute, Uni Berne A since 21 June 1992 Collaboration of AIUB, IfAG, IGN and Swiss Federal Office of Topography

NRCan Natural Resources, Canada (former EMR) A since September 1992 ESOC European Space Agency, Germany A since 21 June 1992 GFZ Geoforschungszentrum, Germany A since 21 June 1992 JPL Jet Propulsion Laboratory, USA A since 21 June 1992 NOAA Nat. Oceanic and Atmosph. Adm. USA A since March 1993 SIO Scripps Institution of Oceanography, USA A since 21 June 1992 UTX University of Texas at Austin, USA A 21 June to 23 Sep 1992

CDDIS Goddard Space Flight Center, USA D since 21 June 1992 IGN Institut Geographique National, France D since August 1992 SIO Scripps Institution of Oceanography, USA D since 21 June 1992

OSU Ohio State University C 21 June 92 - Dec 1993 NRCan Natural Resources, Canada (former EMR) C since January 1994

JPL Jet Propulsion Laboratory, USA B since 21 June 1992

A: Analysis Center, D: Global Data Center, C: Coordinator, B: Central Bureau

Present structure of the IGS: The Central Bureau Information System (CBIS)

The Central Bureau Information System (CIBS) was installed with the start of the official service on January 1, 1994.

The CBIS contains

information concerning the availability of IGS observational data,

information concerning IGS data and analysis centers,

information concerning IGS stations,

information concerning status of the GPS,

the official IGS orbits and pole coordinates,

IGS-mail, IGS-reports.

The CBIS is available through internet (ftp: igscb.jpl.nasa.gov, dir: /igscb, world wide web, (http:j /igscb.jpl.nasa.gov) and e-mail}.


8 G BEUTLER

Table II

Perturbation

Two-body term of Earth's gravity field Oblateness of Earth Lunar gravitational attraciton Solar gravitational attraction Other terms of Earth's grav. field Radiation pressure (direct) y-bias Fixed body tides

Acceleration m/s2

0.59 5.10- 5

5.10-6

2.10-6

3.10-7

9.10-8

5.10- 10

1.10-9

Orbit error after one day

(m)

00

10000 3000 800 200 200

2 0.3

IGS products and quality: satellite orbits

Forces (accelerations) acting on GPS satellites. Radiation pressure (direct, ybias and others) have to be modeled in the orbit determination process. Interesting new RPR models were presented at the 1998 Darmstadt IGS Workshop.

IGS makes available through its Global Data Centers the orbits of the seven IGS Analysis Centers. In addition each week the official IGS orbit is produced:

- A weight (corresponding to the standard deviation of the transformation of the individual solution to the unweighted mean orbit of the seven centers) is assigned to each analysis center.

- The official IGS orbit consists of the satellite positions computed as the weighted mean of the positions as computed by the IGS analysis centers.

- The mean errors and the transformation parameters of the individual solutions with respect to the IGS orbit are made available every week for each day of the (preceeding) week in an IGS-Report.

Motivation

Bauersime (1983) computed the error ~x in a component of a baseline of length I as a function of an orbit error of size ~X:

1 I (km) ~x = d · ~X::::3 25000 (km) . ~X (1)

d is the mean distance between the survey area and the satellite system.



IGS Final omit 0.3 ,----------,---------,,---------,----------,----------,

0.25

0.05

cc:: emr -.. "' gfz -

sio -

o L-________ _L ________ ~ __________ ~ ________ ~ ________ ~

700 750 800 850 900 950 GPS-Week

9

Fig. 2. IGS final orbit quality from November 1993 - February 98 as computed by the IGS Analysis Center Coordinator. Most IGS analysis centers are now consistent to better than 10 cm

rms per satellite coordinate for the IGS final orbits

lOS Il a ~id Orhit (1..1

...: mr -• t'i~

O. l~

.~ "5 :E 0.2 E 0 V

~ 0 ~ 0. 15 'Q.

~ c

5 Vl lI. t ~

'" :=

0.05

oL-__ ~ ____ _L ____ L-__ ~ ____ _L ____ L_ __ ~ ____ _L ____ ~ __ ~

5MOO 50450 50500 50550 50(.00 50650 50700 50750 50S00 50850 50900 GPS·W""k

Fig. 3. IGS rapid orbit quality from December 96 - February 98 as computed by the IGS Analysis Center Coordinator. Consistency level 10- 15 cm


10 G BEUTLER

IGS Rapid Orbit 0.3 .----,----r--,-----,----,---,----,-----,---,----,

.2 ~

0.15

:E 0.2 E 8

~ ~ 0.15

.ii

V) 0.1 :;:

~

o.os

.).

gr, ..... _ ..

o ~-~-~--~-~-~--~-~-~--~-~ 5().100 50450 50500 50550 50600 50MO 50700 50750 50800 50850 50900

GPS-Wc<:~

Fig. 4. IGS rapid orbit quality from December 96 - February 98 as computed by the IGS Analysis Center Coordinator. Only GFZ and CODE rapid orbits shown

5045() 50500 50550 5()(,oo 50650 5070() 50750 50800 50850 50900 GPS-Week

Fig. 5. IGS predicted orbit quality from December 96 - February 98 as computed by the IGS Analysis Center Coordinator. Prediction quality over second day of predictions



rGS Predkl~d Orhil

1.8

1.(>

5 ' '::;

" :E 1.4 ~

i; v :e 1.2

0

" ~ ;:; ;;; ~ "-c II.X

g '" U.6 ~ ?:

U.4

i II : 'j i' i I I' [II

I

!Ii

I: I II'

1,1 I! , !llllill , I !

i I i

I 1

,

II I'

II Ii , I ~ "~I I

II f

" I III ,,, I I I ,

I 1." 1

O.~

n 50450 505()() 51)551) 506tXI 5()(>.~O 50700 50750 51)~OO 50H50 50<)00

GPS·Week

Fig. 6. Predicted orbit quality from December 96 - February 98 as computed by the CODE and GFZ Analysis Centers. Prediction quality over second day of predictions

Table III. Impact of orbit errors on the estimated station coordi-nates for different baseline lengths

Orbit error Baseline length Baseline error Baseline error m km ppm mm

25 1 1 25 10 10 25 100 100 25 1000 1000

2.5 10 .1 1 2.5 100 .1 10 2.5 1000 .1 100

.25 100 .oI 1

.25 1000 .01 10

.05 100 .002

.05 1000 .002 .5


12 G BEUTLER

Table IV. Orbit types available today

Orbit type Quality

Delay of availability m

Broadcast orbit 3.0 real time CODE predicted orbit .25 real time IGS predicted orbit .25 real time CODE rapid orbit .10 after 12 hours IGS rapid orbit .07 after 24 hours IGS final orbit .05 after 11 days

Orbit type Quality

A vail able at m

Broadcast orbit 3.0 broadcast message CODE predicted orbit .25 real time, IGS Data Centers IGS predicted orbit .25 real time, CODE CODE rapid orbit .10 CODE through FTP IGS rapid orbit .07 IGS Data Centers IGS final orbit .07 IGS Data Centers

IGS product changes on June 30, 1996

All IGS combined products (orbits, EOPs, station coordinates) are based on ITRF94, realized by the published positions and velocities of the stations TROM, MADR, KOSG, WETT, HART, FAIR, KOKB, YELL, ALGO, GOLD, SANT, TIDB, YARl.

The IGS preliminary orbit/clocks became the rapid orbit, produced daily within 24 h. The IGS rapid orbit became the IGS final orbit. The former IGS final orbit is discontinued. All IGS solutions employ the 1996 IERS diurnal and semi diurnal EOP model. The operational broadcast ephemerides are included in the IGS rapid orbit summaries for information (they have no weight in the combination). Polar motion rates are taken into account for the combination process.

IGS predicted orbits

Several IGS Analysis Centers started predicting GPS orbits based on the rapid orbits (available within 12 to 24 hours after the observations. The Analysis Coordinator started comparing and combining these predictions with GPS week 895. There is an official predicted IGS orbit available for every day 30 minutes before start of new day. The quality ofthe IGS predictions (for "normal" satellites) is close to what the quality of final IGS orbits was back in 1992! In general the IGP orbits are an order of magnitude better than the broadcast orbits. The predicted orbits are of importance for time-critical applications (for meteorological community).



IGS products and quality: station coordinates

So far, station coordinates and the associated estimated velocities were sent by the individual Analysis Centers on an annual basis to the !ERS. They contributed - together with the corresponding results from VLBI, SLR, DORIS, PRARE -to the realization of the current ITRF. The ITRF94 is the latest ITRF available. VLBI and SLR with about 120 sites each, GPS with about 70 sites, and DORIS with about 60 sites were the main contributors. The ITRF96 being prepared right now!

In view of the growing number of permanent regional GPS networks (Australia, Canada, Europe, Japan, USA) the systematic densification of the ITRF using GPS becomes more and more important.

IGS products and quality: densification of the ITRF

The IGS strategy was defined at the Workshop Densification of the IERS Terrestrial Reference Frame through Regional GPS Analyses in December 1994 at JPL in Pasadena. It was decided to proceed along the following lines:

- The densified IGS network shall consist of about 200 globally well distributed sites.

- The densification shall be realized in cooperation with other groups setting up permanent GPS networks.

Regional analyses are performed by (new) IGS Associate Analysis Centers. These make extensive use of the global products (orbits, ERPs, ITRF coordinates and velocities of sites well established in the Global IGS Network) . These AACs produce free network solutions on a weekly basis.

The AACs make available their solutions to specialized Associated Analysis Centers which compare and combine the individual solutions.

The pilot phase of the IGS Densification Project started in September 1995 and will last till the end of 1997.

The Associate Analysis Centers combining the solutions are

JPL,

MIT and

University of Newcastle.

Initially only the seven IGS Analysis Centers were making available their free network solutions on a weekly basis. Several Regional Analyses Centers contribute solution since 1996. A special format, the SINEX format was created to make the solutions and the associated covariance information available. The Pilot Phase should be terminated by the end of 1997. The Governing Board will decide on the future of the activities.


14 G BEUTLER

The following table shows (in abbreviated form) an extraction from the MIT report:

Transformation of Loose solutions to COMB

X-tran Y-tran Z-tran Scale rms (mm) (mm) (mm) (ppb) (mm)

EMR 1.8 111.5 - 108.2 - 1.920 4.3 JPL - 8.3 - 1.0 - 45.1 - 2.569 2.6 GFZ - 12.6 - 13.0 -61.9 - 2.701 4.6 COD -16.9 9.2 - 20.5 - 2.797 3.5 SIO -20.2 6.5 - 72.2 - 1.330 2.1 NGS 4.1 214.8 -148.0 -1.962 8.3 ESA 3.1 -45.0 -144.4 - 2.663 4.1

(Mailed from: Tom Herring <[email protected]»

The report reflects the results of GPS Week 898. The reports of all three centers (JPL MIT, Newcastle) indicate a consistency between the individual coordinate series of well below the cm for the horizontal, of about 1 cm for the vertical components.

Today the following institutions are contributing solutions as RNAACs on a weekly basis:

AUS: Australian Surveying and Land Information Group (AUSLIG) EUR: EUREF-Solution (Euref Subcommission of lAG) with many con

GIA: GSI: PGC: SIR:

tributors (following page) Geophysical Institute, University of Alaska, Fairbanks Geographical Survey Institute, Japan Natural Resources Canada, Pacific Geoscience Centre, Canada SIRGAS Solution prepared by the Deutsches Geodatisches Forschungsinstitut, Abt. I (DGFI/I).

EUR: EUREF-Solution Contributors:

ASI: Nuova Telespazio S.p.A., Space Geodesy Center BEK: International Commission for Global Geodesy of the Bavarian

Academy of Sciences (BEK) COE: European solution created at CODE (Center for Orbit Determi

GOP: IFG: LTP: NKG: OLG: ROB: WUT:

nation in Europe) Geodetic Observatory Pecny, Czech Republic Institute for Applied Geodesy in Germany (HAG) Bundesamt fUr Landestopographie (L+T), Switzerland Nordic Geodetic Commission (NKG) Observatory Lustbiihel Graz (OLG) Royal Observatory of Belgium (ROB) Warsaw University of Technology (WUT).



Velocities derived from EUREF weekly solutions (gpsweeks: 0834- 0898)

_ ' cm/yr Veloc ity

Fig. 7. Estimation of velocities of EUREF sites based on data from GPS week 834 (first week of year 1996) to GPS week 898 (end of March 1997)

IGS products and quality: densification of the ITRF EUREF solution

IGS products and quality: Earth orientation

An uninterrupted series of Earth orientation parameters is available starting with 21 June 1992. A combined IGS pole is available since 28 May 1995! With the increasing number stations and with growing experience the quality of EOPestimates grew from about 1- 2 mas in 1992 to 0.1-0.2 mas in 1998.

LOD (or UT1-UTC-drift values) are available with a one-day resolution from IGS Analysis Centers. LOD values corresponding to combined series are available as well!

Exploiting the GPS/IGS for fundamental astronomy

At the CODE processing center the attempt is made to exploit the IGS network for the following purposes:

- Not only the UT1-UTC-drift but also the drifts in the nutation in longitude .iw sin f and in obliquity .if are estimated. (If you are solving for length-ofday there is no reason not to solve for these nutation drifts!)

- The polar wobble components x and y and UT1-UTC-drift are estimated with a high time resolution (2 hours). It is thus possible to extract ocean


16 G BEUTLER

Velocities derived from EUREF weekly solutions (9Psweeks: 0834- 0898)

_ i cm/yr Veloc'ly

-----_._._._-----

Fig. 8. Estimation of velocities of EUREF sites based on data from GPS week 834 (first week of year 1996) to GPS week 898 (end of March 1997). Mean velocity for Europe subtracted

tide models from these series. More precisely: models for tide-driven oceanic angular momenta.

So far Markus Rothacher (AWB), Robert Weber (TV Vienna), Jan Hefty (TV Bratislava), Tim Springer (AWB) and Tom Herring (MIT) were analysing these series. All IGS Analysis Centers have to model tropospheric refraction in order to obtain good results. Different techniques are used. Some Analysis Centers use stochastic models (based on Kalman filters), others just solve for station- and timespecific troposphere parameters.

Gendt and Beutler (1996) showed that the consistency of estimates stemming from different IGSS Analysis Centers is relatively high. It is thus posible to extract on a routine basis the precipitable water content for the entire IGS network with a high temporal resolution (two hours or finer) - provided high accuracy barometers are deployed in the IGS network. The issue was decided at the 1998 AC Workshop in Darmstadt to come up with an official IGS troposphere product (troposphere zenith delays).

The ionosphere is a dispersive medium:

E dr[(v) = ±a . 2.

v (2)

The difference between the L1 and L2 GPS observations may thus be used for ionosphere modeling.



-0.3

-0.2

'iii" -0.1 "0

~ U)

I 0.0 !

~ r::: 0.1 'E § I

X 0.2

0.3

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

Y -Coordinate [Arc- Seconds)

Fig. 9. Polar motion (19 July 1993 - 13 February 1998), as estimated by CODE

The motivation to produce ionosphere models is manyfold:

local or regional models may be used to remove (reduce) biases in single frequency surveys,

regional or global models may be used to calibrate altimeter measurements (ERS-1 is an example),

there are pure research projects (ionosphere maps, extraction of geomagnetic indices).

CODE, ESA, JPL plus University of New Brunswick (UNB) and DLR Neustrelitz started producing ionosphere models. The example (Fig. 19) shows a single layer model produced by CODE, where the density within the shell is computed as series of harmonical functions. A common format, IONEX, was accepted.

New IGS products: antenna calibration tables

Zenith-dependent phase center variations exist for each antenna type, small variations even exist between antennas of the same type. Although the DorneMargolin antenna is dominant in the IGS network, there is a considerable number of Trimble-, Ashtech-antennas in the network.

It must be considered a constructive achievement that it was decided to produce tables of phase center variations relative to the Dorne-Margolin antenna (Rothacher


18

0.15

0.1

0.05

0

·0.05

·0.1

G BEUTLER

chi3(code) and chi3(aam)

code aam

.0.15 '--_.l..-_..l.-_...J...._--L.._ ....J...._--'-_--L_---I __ L..-~

49000 49200 49400 49600 49800 50000 50200 50400 50600 50800 51000

MID Fig. 10. Axial angular momentum calculated from CODE UT1·UTC·drift estimates and from

IERS·AAM·Subbureau values (NCAR)

3

2

o

·1

·2

·3

chil (code) and chil (aam)

tilde aalll

·4 ~-~--L--~---~--~-~--L--~--L-~

49000 49200 49400 49600 49800 50000 50200 50400 50600 50800 51000

MJl)

Fig. 11. First equatorial angular momentum calculated from CODE x·, and y. pole estimates and from IERS·AAM·Subbureau values (NCAR). Smoothing applied over three days



chi2(code) and chi2(aam) 6.---.---.----,---,---.--~,_--._--._--._--~

chde aam

4

2

o

-2

-4

-6

-8

_IOL---~---L--~----~--~---L--~----~--~--~

49000 49200 49400 49600 49800 50000 50200 50400 50600 50800 51000

MJ()

19

Fig. 12. First equatorial angular momentum calculated from CODE x-, and y- pole estimates and from IERS-AAM-Subbureau values (NCAR). Smoothing applied over three days

chi3(code) and chi3(llllm) 0.1 ,----,----,----,,_---,----,----,-----,----,

code --aam

0.05

-0.05

-0.1

-0.15 L--_-1... __ .l....-_--L. __ .1...-_---L. __ ...I..-_---L_----J

50450 50500 50550 50600 50650 50700 50750 50800 50850

MJD

Fig. 13. Axial angular momentum calculated from CODE UT1-UTC-drift estimates and from IERS-AAM-Subbureau values (NCAR). Results for year 1997. AAM-series smoothed over one day


20 G BEUTLER

.12 L--_ -'-----_ ---'----_----'---_ -----L-_ ------'-_ ------' __ -'----_-'--------'

49000 49200 49400 49600 49800 50000 50200 50400 50600 50800

MJD

Fig. 14. First equatorial angular momentum calculated from CODE x-, and y- pole estimates and from IERS-AAM-Subbureau values (NCAR) . Results for 1997. Smoothing applied over one day

chi2(gps) ulld chi2(aam) 25 r--.--.---r--'--.--.--.--'--~

20

15

10

5

O L--~-~--L-~-~--L--~-~-~

49000 49200 49400 49600 49800 50000 50200 50400 50600 50800

MJD

Fig. 15. Second equatorial angular momentum calculated from CODE x-, and y- pole estimates and from IERS-AAM-Subbureau values (NCAR). Results for 1997. Smoothing applied

over one day



Total Zcni rh Delay (cstimarcd whirh GPS) 2.15

D

2.25

E ::J Ul 2.2 Cl Ul P-O

2.15

2. 1

2.05 4.96 4.98 5 5.02 5'<)4 5.06 5.08

MJD[d) x \04

Fig. 16. GPS estimates from CODE (6 hours binning) for tropospheric zenith delays for Zimmerwald, October 1994 to November 1997

Total Zeni lh Delay (using model of Sn:tslamoincn) 2.3

2.25

Ii I 2.2

i ;::

-~ Ul <: I

~~ <: 2. 15

111 ~II Ul

~ 2. 1

2.05 4.96 4.98 5 5.02 5.04 5.06 5.08

MJD[d] 1l)04

Fig. 17. Tropospheric zenith delays for Zimmerwald using surface met measurements for October 1994 to November 1997


22 G BEUTLER

Total Zenith Delay (estimated with GPS) TSKB 2.7

2.65 ~

2.6 -

2.55

E 2.5 ::r Ul a Vl 2.45 " 0.. 0

2.4 "

2.35 ~

2.3

2.25 · 4.96 4.98 5 5.02 5.04 5.06 5,08 5,1

MJDI"l xl04

Fig. 18. GPS estimates from CODE (6 hours binning) for tropospheric zenith delays for Tsukuba (Japan), October 1994 to February 1998. Observe that the annual signal is much more pronounced

than for Zimmerwald!

and Mader 1996) and to use them within the IGS starting with GPS week 860. More information is available at the CBIS, directory igscb/station/general.

News from the 1998 Darmstadt Workshop and Governing Board meetings

A call of participation will be issued in February 1998 for the IGEX-98. IGEX-98, the International GLONASS Experiment by the end of 1998, is a three months GLONASS test campaign. CSTG is coordinating this effort (chair Pascal Willis).

The IGS adopts the ITRF96. The IGS realization of ITRF96 will consist of about 50 GPS stations. This should lead to much more consistent EOPs (rapid products in particular) .

The IGS/BIPM Time transfer experiment is well underway. The proposals answering this call for participation are due in March 1998.

The recommendations from the 1997 IGS retreat are discussed and viewed positively by the Governing Board. Formal approval will take place in May 1998 in Boston.

It was decided to establich and maintain a Global IGS Ionosphere model.



Vertical Total Electron Content in TECU

00 2

-60

--------------~-------_OOL---~-----L----~~==±==--=6===~~~-L--~

-180 -135 -90 -45 0 45 90 135 180 Sun-fixed longitude in degrees

Fig. 19. Global ionosphere model for day 073 of Year 1996

Summary and conclusions

The IGS Orbits allow it to perform regional GPS analyses of highest accuracy without further orbit refinement. The results refer to the best available realization of the ITRF.

The IGS pole positions x and yare important contributions to the IERS ERPseries (Bulletins A and B), today.

The IGS significantly contributes to the realization and densification of the ITRF.

IGS solution series containing estimates of the time derivatives of nutation may significantly contribute to the establishment of nutation terms with periods < 40 days.

A re-processing starting from raw observations of the years 1992- 1998 would lead to much more consistent ERP- and other series.

Important characteristics of the IGS:

The IGS actually is an international and a multi-agency service.

Although the IGS is a service with the goal to facilitate research, a considerable amount of research is taking place within the IGS. In this respect it is essential that there is a high redundancy in all IGS components (network, data centers, analysis centers).


24 G BEUTLER

The IGS products (even those of the Analysis Centers) always were freely available to the scientific community. This led, e.g., to the detection of biases in the official IERS ERP-series.

- The IGS always was and is open to cooperate with other groups operating permanent networks.

IGS standards for data formats and GPS station monumentation are observed by many other network operators today.


Documents

contributions of the international gps service for geodynamics (IGS) to global geodynamics, astronomy and atmosphere science: review and outlook