Acra Astronautica Vol. 38, Nos 4-8, Copyright 8 !996 btemational Astronautical Federation. Published by E P,
p. 437-443, 1996 sewer Snence. Ltd
Printed in Great Britain
PII: SOO94-5765(96)00016-l 009~5765/96 $15.00 + 0.00
ORBITAL AND OTHER PRODUCTS OF THE INTERNATIONAL GPS
SERVICE FOR GEODYNAMICS (IGS)
J.M. Dow, T. Martfn-MIX. J. Feltens, C. Garcia-Marthez, M. Bayona Perez**
EM/European Space Operations Centre Darmstadt. Germany
EDS-mbp at ESOC;**GMV at ESOC
The paper reviews the current status of the routine prod- ucts of the International GPS Service for Geodynamics (IGS) from the point of view of one cf the many partic- ipating institutions. Stie recent developments are highlighted. These include monitoring of the earths ionosphere.
The International GPS Geodynamics Service (IGS) be- came operational on 1 January 1994. following an ex- tended period of testing which began with the three ma& IGS-92 Campaign in the swllmerof 1992,ad continued through a Pilot Service from November 1992. A large number of Agencies and Institutions are involved in the various aspects of this operation: track- ing the GPS satellites; retrieving and formatting data; providing archiving and retrieval facilities for users: analysing the data: assessing the results; and overseeing the functioning of the service. In addition to determin- ing the orbits of the complete GPS constellation (with accuracy currently at the levei of 10 cm). the IGS sup- plies mutineiy to the International Earth Rotation Serv- ice (IERS) solutions for earth aientatim and global station coordinate solutions. which am essential for the maintenance of the IERS terrestrial reference frame (ITRF). Some cenhes am striving to provide orbital and regional position determinations with yet-y ahat &lays in order to facilitate. rapid analysis of earthquake move- ments Ionospheric products. which in turn provide im- proved orbit determinatim capability for other applicationa. am becaning available.
ESA ia participating in IGS with its GPS Tracking and Data Analysis Facility. High quality receivers and asso- ciated communications equipment are installed at glo- bally distributed ESA ground stations Data from curmntly about 50 stations from the IGS network are re- trieved daily from a global data cenue. and are processed to compute orbits for the complete GPS constellation. station coordinates. earth a&nation parameters. and clock corrections The paper reports on the status of IGS
Copyright 8 1995 by the International Astm- nautical Federation. All rights reserved.
products of particular interest for crbital dynamics appli- cations, with the aim of providing an idea of the current status of the IGS from the point of view of one of the many institutions involved.
The objectives of the IGS are to support. thrargh GPS products. geodetic and geophysical research. and also a range of operational activities performed by govemmen- tal and selected commercial organisations. The main products are:
*high accuracy GPS satellite ephemerides; *earth rotation parameters; *coordinates and velocities of trlrcking stations; l GPS satellite and ground receiver clock corrections: *ionospheric information.
Applications currently beiig supported by these prod- ucts include: l realization of globsl accessibility to and improve- ment of the IERS Terrestrial Reference Frame lTRF. which is today the standard earth-fixed refer- ence frame for precise positioning and geodetic research (maintained by the Intematirmal Earth Rotation Service IERS); l mcoitoring of earth rotation (polar motion and varia-
tions in url); l scientific orbit determination. notably the case of TOPEX/Poseidon; l maritoring of deformations of the solid and liquid earth; *atmospheric investigations, includJng tropospheric and ionospheric monitoring.
Data from more than 80 stations (mid-EM) which are permanently tracking the GPS satellites flow daily
through a number of Regional and Operational Data Cenms into one of the three Global Data Centms (locat- ed respectively at CDDIS in INASA/GSFC. Greenbelt; IGN. Paris; and Scripps Iminm of Oceanography SIO. San Diego). Them are seven Analysis Centres. located at the University of Bern: Natural Resources of Canada NRCan. Ottawa; ESA/ESOC: GFZ. Potsdam; JPL. Pa- sadena; NOAA. Silver Spring: and SIO. San Diego. The Analysis Centres (AC) retrieve data from one of the Glo- bal Data Centres. and return to the latter their solutions
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for the GPS satellite orbits. earth orientation (polar mo- tion and UT1 variations). and satellite clock conections. The. orbital solutions are combined into an official IGS solution. which is made available to interested users within about 10 days from the date of data collection. The orbit combination is the responsibility of the Analy- sis Centre Coordinator. a function being carried out by the MCan Analysis Ceatre. The IGS earth pole solu- tions are now an essential component in the earth orien- tation parameters generated and distributed by the WS. due to the high temporal resoludon and consistent accu- racy and availability of the GPS solutions (equivalent to about I cm at the earths surface. with 24 hour resolu- tion). The routine coordination of the IGS. including in- terfaces with external bodies such as the IERS, is perfotmed by a Central Bureau located at JPL. while the overall policy is determined by a Governing Board, in which the active elements of the IGS are represented.
An IGS Information System has been set up to provide a central reference data base describing for example the configuration of the IGS sites and the Analysis Ceatre processing models. as well as information cm the availa- ble products and how to access them. Connection to the IGS Lnformation Service is by anonymous ftp at Internet node igscb.jpl.nasa.gov (directory /igscb). or through the World Wide Web at http://igscb.jpl.aasa.gov/.
In order to develop support capability for future ESA projects involving GPS. ESOC is developing a GPS Tracking and Data Analysis Facility (GPS-TDAF). A pi- lot version of this is beiig used in the Operational Data Ceatre and Analysis Centre activities being carried out for IGS. The emphasis is on the use of on-board GPS i-e- ceive-rs for orbit determination. including applications which may have very stringent accuracy requirements.
The objectives of the ESA GPS-TDAF can be summa- rised as follows:
*establishment of a network of high precision geo- detic GPS receivers on ESA ground sites. including communications to ESOC:
*development of software for fast. automatic transfer of data from the stations followed by decornpres- sioa. quality control. reformatting. and further rout- ing to global and other external data ceatres:
*advanced software for processing GPS data. solving for the orbits of the GPS satellites. station positions and motions, and earth orientation parameters; *me degradation of the accuracy of some types of GPS measurement by Selective Availabilitv (SA) and Anti-Spoofing (A/S) should be minim&d or even eliminated:
*monitoring of offsets and variations in GPS satellite clocks and ground receiver clocks;
*active participation in the IGS; *ionospheric mapping in near-real time for ES.4 track-
ing stations. providing support to all ESA satellites being tracked from these sites, and eliminating the need for complex and inaccurate models of ioao-
spheric propagation corrections in orbit determina- tion: *monitoring of the ionosphere. satellite clocks and satellite and signal health in near-real time will pro- vide the potential of providing measurement correc- tions of essential interest to users of GPS signals for navigation purposes (vehicles on or near the earths surface):
*orbit determination for earth satellites equipped with on-board GPS receivers. Navigation for future sci- ence and earth observation missions and In-orbit Infrastructure is envisaged. The TOPEX/Poseidoa spacecraft. launched in August 1992. has provided a first test data set with high precision measurements..
The central part of the GPS-TDAF. involving the simul- taneous computation of multiple orbits and many asscci- ated geophysical. geodetic. satellite- and tracking system-specific parameters. has developed out of a more general software facility for handling orbit determination for a wide range of earth satellite application@.
So far. high quality GPS receivers have been installed in five ESA stations: Maspalomas (Gran Canaria. Spain) in June 1992; Kourou (French Guyana) in August 1992: Kinma (Sweden) in July 1993; Perth (Western Australia) in August 1993: and Villafranca, near Madrid (Spain) in November 1994. A further installation at Malindi. Kenya is being prepared to begin operations in October 1995. making it the second permanent GPS station on the Afri- can continent. and the third oa the African plate. (Harte- beesthoek in South Ahica and Maspalomas are the others.) A survey of these and other stations was made in 1991 by the TH Darmstadt and University of Bern in connection with the GIG91 Campaign.
The GPS-TDAF system confiiation is shown in Figure I . It is structured in three levels: a hardware/canmunica- tions layer; a software layer; and an operations layer. At the core of the system is the Data Analysis software. which performs the numerical computations. The Soft- ware In.frastructum supports this with utilities for data re- ttieval. conversion. storage, archiving . . . . . and deals with the execution and monitoring of the automatic (or if te- quired. manual) operations. Overall control of the data acquisition is maintained by a Unix based workstation lo- cated at ESOC. rum&g an automatic script and a com- munication program. The basic approach to communicadoa with the station is with the Xmodem pro- tocol through high performance modems over the public telephone network A ma-e reliable appoach. which is adopted when it is available, is to share the use of an ex- isting. high capacity leased line to the station The trans- fer is then initiated with the Zmodem protocol. and the standard X.25 protocol is used between the packet assem- bler/disassembler (PAD) installed at the ESOC Commu- nications Centre and that at the station. In this case. it is more convenient to commtmicate with a PC oa the sta- tion. rather than with the receiver directly (lack of flow conaol from the receiver). Typical data rates am 6-8 kb/ s with both methods. In the curreat daily downloads, about 0.5 MBytes of data per station are retrieved (30 s sampling).
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GPS IDAF I
GPS TDAF Coufiruration LweQ
ESOC Routine IGS Activities
Typically about 50 stations are now processed daily, the data being relieved from CDDIS through the Internet. The RINEX observation files are decompressed and are input into the GPSOBS program, which identifies cycle slips. generates double-differenced phase observations (corrected for ionospheric effects). and computes initial estimates of receiver clock biases. The resulting meas- urement file is input into BAHN. our orbit and geodetic parameter estimation program. which makes a least squares adjustment for all the GPS orbits (including radi- ation pressure parameters). the positions of a number of ground antennas. tropospheric model parameters. phase ambiguities. receiver clock biases and earth orientation parameters (MPs). A subsequent run of GPSOBS then allows the computation of precise (ns level) satellite clock biases. by post-processing code and carrier phase observations.
Daily solutions for orbits. EOPs and satellite clocks are sent to CDDIS (and from there to the other two IGS Glo- bal Data Centres). along with a weekly summary file de- suibiig the sohnion. The orbital and clock solutions generated by the seven IGS Analysis Centres are com- bined to form an IGS solutions. This takes advantage of the different algorithms. software, and data sets used by the various analysis centres. and successfully eliminates poor solutions. The rms discrepancies with respect tq the IGS solution has been around 10-20 cm per satellite co- ordinate since me beginning of 1994, even with the ad- vent of A/S at the end of January 1994. and less than 15 cm during 1995. Satellite clock solutions are stable at the level of a few ns. and the frequency dither due to SA can be weti reconstructed. The polar motion parameters, which are forwarded weekly to the International Earth Rotation Service IERS as solution ERP(JZSOC)94 P 01. are determined with 1 day resolution and have accuracy
Figure 1: ESA GPS-TDAF System Configuration
of about 0.3-0.4 masec (I-1.3 cm) when compared with combined solution of the IERS using VLEtI and laser ranging in addition. A fde giving a full description of the current processing assumptions at each Analysis Cenue can be retrieved from the IGS Central Bureau.
GPS satellite clock information is currently only availa- ble through IGS with 15 minute sampling. which is not sufficient to be able to correct for the effects of Selective Availability (SA). Our clock solutions have recently been successfully applied with I minute sampling to sup- port a time transfer experiment carried out by the Univer- sity Qf Waiest. Tie transfer at the level of 4 ns was achieved. using a standard. single-frequency receiver. Such clock solutions will be used in evaluating future in- orbit GPS tracking experiments. for example in co~ec- tion with the flight experiments planned within the ATV Rendezvous Predevelopment (AI&P) programme.
Table I shows the accuracies currently being achieved
Table I: Accuracies of IGS Products (mid-l!%)
Individual AC orbit series (per satellite coordinate)
I Combined IGS orbit (per satel- I IOcm lite coordinate) I Polar motion (xp. yp)
Excess length of day (LOD)
Horizontal position coordinates
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for the main operational products of the IGS. There has been a very significant improvement in all of these sine the initiation of operational activities in 1992: this can be attributed to a large extent to the continuous feedback be- tween the various Analysis Cenees and the Analysis Centre Coordinator. and to the variety of software and computational approaches available to them.
Terrestrial Referer~ce Frarue and Den&cation
&I IGS Workshop was held at Pasadena in November 1994 on the topic Densitication of the IERS Terrestrial Reference Frame through Regional GPS Networks*. The motivation for developing this topic within the IGS is to take advantage of the increasing number of regional GPS networks being now being deployed in various parts of the world. The relatively low cost of GPS hardware and operations makes this technique ideal for densifica- tion of the terrestrial reference frame, in pticular for studies of regional deformation (earthquakes. volca- nos....). Tke main outcome of the...