Each site has a high-precision dual-frequency P-code 
  
  
least one order of magnitude better than the broadcast 
  
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996 
TAB! 
receiver which records measurements at 30-sec intervals. ephemeris, even when anti-spoofing is in effect. PRO 
IGS Operational Data Centers (Table 1) are in direct — 
contact with the tracking sites, and are responsible for IGS Rapid Orbits/Clocks are available about ten days. GPS : 
fetching raw receiver data, formatting it in a common IGS Final Orbits are available after about one month. The GPS « 
standard [4], and forwarding the data to Regional or two differ from each other mainly in the orientation of 
Global Data Centers. their reference frame. Lately, due to the improvement of Earth 
data quality and distribution, the differences have been Earth 
Reformatted global tracking data from several Operational negligibly small for all practical purposes. For this reason Statio 
Data Centers are collected at Regional Data Centers. A as of July 1, 1996 the IGS Final Orbit series is planned to 
local archive of the data received is maintained, and the be discontinued and the Rapid Orbits will become the — E 
data are transmitted to the Global Data Centers. Regional Final. At the same time daily IGS (Preliminary) 
Data Centers thus serve to reduce traffic on electronic Orbits/Clocks of somewhat lower accuracy will be made 13 
networks. available to the user community. 
will c 
TABLE 1. IGS DATA CENTERS TABLE 2. IGS ANALYSIS CENTERS windc 
equiv 
Operational & Regional Astronomical Institute-University of Berne Switzerland graph 
National Oceanic and Atmospheric US European Space Agency Germany Ar 
Administration GeoForschungsZentrum Germany 
Institut für Angewandte Geodäsie Germany Jet Propulsion Laboratory US The 
Natural Resources, Canada Canada National Oceanic and Atmospheric US Calt 
Norwegian Mapping Authority Norway Administration Supp 
Jet Propulsion Laboratory US Natural Resources Canada Canada * IG 
Nae se ca iiec tear Scripps Institution of Oceanography US am 
Global * TC 
NASA/Goddard Space Flight Center US fr 
2 ; : ; Estimates of earth orientation and station coordinates 
A NAR Rene from the ACs are coordinated with the IERS [6]. Through B 
Scripps Institution of Oceanography US the IGS, GPS-derived station locations are contributing p 
more and more to the ITRF. As mentioned above, a major sp 
goal of the IGS is to include in the ITRF the coordinates 
; of a number of sites that comprise dense regional GPS Thes 
The Global Data Centers serve as interfaces to the networks. The computations of coordinates for these à ve 
n Analysis Centers and the Cent UR communs. Em regional stations will be handled by a number of Regional 
| purpose 1s to Re fee are (ve, dm provi e om ue Network Associate Analysis Centers (RNAACS) currently Fors 
li access to tracking data. In addition, Global Data Centers being appointed in several countries around the world. ae 
| provide access to Analysis Center products. B y 
n ; ; ; igsc 
i Table 3 summarizes the approximate accuracies of IGS ATP 
There are currently seven IGS Analysis Centers (ACs) orbits, clocks, earth orientation, and station coordinates d UT i 
(Table 2) — three in the US, one in Canada, three in ([6], [7], [8]). the mi 
Europe — that routinely analyze some subset of the data snm PTS | 
where 
from the IGS global network. ACs compute precise GPS CENTRAL BUREAU INFORMATION SYSTEM 
ephemerides and earth orientation parameters. Daily 
'esult iodi ; i : ; i BENE 
results are posted periodically, typically once each week TheiCentrab Bureau InformationSvstem éCBISMprovides 
s public access to products of the IGS, and also provides a The I 
The IGS Analysis Coordinator "xt Natural Resources means of electronic messaging among IGS participants. ; S e 
Canada — is responsible for producing the official IGS The System runs automanital! and de 
orbits/clock corrections based on the combination of y y its dat: 
orbits from the separate ACs [5]. Because of consistency : 
checks, the combined orbit is largely free of some In CBIS isa entem the Dems y orld Wo Ar e. Highc 
systematic errors that occasionally remain in results from a rat pu wi usen ren s sais m m netwoi 
individual ACs. Agreement among ACs in GPS satellite Tepe or, example, ON; MOsl: LIMX Systems, the brc 
ephemerides is generally at the level of 10 to 30 cm. It is comma precis 
estimated that the absolute accuracy of the IGS orbits is at eventu