CHARTER FOR IGS ANALYSIS CENTERS AND ASSOCIATE ANALYSIS CENTERS Definition of Analysis Center and Associate Analysis Center activities as adopted by the IGS Governing Board, May 1998 (NOW SUPERSEDED BY MAR 2005 VERSION) Background The IGS analysis products are formed from a combination of the results submitted by the IGS Analysis Centers (ACs). To the extent that the effect of data and mismodeling among different Analysis Centers are independent, properly weighted combinations of results can be superior. In this way, the IGS products benefits in precision, accuracy, stability, reliability, and robustness compared to the results of any individual AC. It is thus advantageous to encourage participation by a new AC whenever its results would improve the quality and/or robustness of the combination. The ACs, which produce the IGS Core Products, are particularly important because their results are essential for the formation and maintenance of a highly accurate and consistent IGS Reference System. While it might be advantageous for individual ACs to contribute only some components of the Core Products, this would undermine the consistency and quality of the IGS combined products. Since combination strategies normally assume that the weighted average result is preferred, it is important for the IGS and the ACs to be aware of potential common mode errors which can degrade the accuracy of the IGS combined products compared to those of an individual AC. For this reason, the IGS must continuously encourage efforts to calibrate and test the absolute performance of its products. Core Products The IGS core products consist of the weekly Final products, which are: * GPS satellite ephemerides and clock values, tabulated at 15-min. intervals for each day (in sp3 format) * Earth orientation parameters (polar motion, polar motion rate, and length-of-day, estimated at daily intervals in SINEX format) * station coordinate and velocity values for the global tracking network from a combination of weekly data sets (SINEX format) determined for each GPS week of data and delivered to the IGS within a specified deadline (currently 10 days after the end of that week). The Final products from each AC must be fully self-consistent. The detailed specifications for the Final products are established by the Analysis Center Coordinator, in consultation with the ACs. The Final products, when combined with proper weighting, allow for the definition and maintenance of the IGS Reference Frame [Kouba et al., 1998]. In addition, for more timely applications the IGS provides the daily Rapid products, which are: * GPS satellite ephemerides and clock values, tabulated at 15-min. intervals for each day (in sp3 format) * Earth orientation parameters (polar motion, polar motion rate, and length-of-day, estimated at daily intervals) determined for each day of data and delivered to the IGS within a specified interval after the end of that day. The Rapid products from each AC must apply the current IGS Reference Frame. The detailed specifications for the Rapid products are established by the Analysis Center Coordinator, in consultation with the ACs. Related to these, the IGS also produces daily Prediction products, which are: * GPS satellite ephemerides and clock values, tabulated at 15-min. intervals for each day (in sp3 format) extrapolated for each day and delivered to the IGS within a specified interval before the start of that day. The Prediction products from each AC must apply the current IGS Reference Frame. The detailed specifications for the Prediction products are established by the Analysis Center Coordinator, in consultation with the participating ACs. Other Products In addition to the Core and related Products listed above, the IGS encourages contributions of other analysis products of interest to various users such as the current global tropospheric delay and future ionospheric delay products, station clock solutions, etc. Analysis Centers (ACs) The IGS Analysis Centers are those groups, recognized by the Governing Board, which commit to deliver to the IGS the Core Products listed above promptly and reliably. (Satellite clock estimates are not strictly required of those ACs which process differenced observables, but they are nevertheless encouraged.) Commitment to deliver Rapid and Prediction products is not required but is strongly urged. It is the Core Products of the ACs which are combined by the IGS Analysis Center Coordinator to form the IGS Reference Frame accessed by the user community. All other IGS products must be maintained consistent with this frame and together in fact facilitate the realization of the IGS Reference System. Associate Analysis Centers (AACs) Associate Analysis Centers are those groups, recognized by the Governing Board, which produce specialized products or portions of the Core Products. AAC functions will be designated as the scope of the IGS evolves and new products emerge. Currently, the following types of AACs are recognized: * Rapid Service Associate Analysis Centers (RSAACs) RSAACs contribute Rapid and/or Prediction products to the IGS in compliance with the procedures established for the ACs. * Regional Network Associate Analysis Centers (RNAACs) RNAACs contribute station coordinate and velocity values for regional sub-networks of tracking stations from a combination of weekly data sets (in SINEX format). The RNAAC products must adopt the current IGS Reference frame, including appropriate GPS ephemerides. The RNAAC results are combined by the Global Network Associate Analysis Centers (GNAACs) together with similar results from the ACs for the global tracking network to form the IGS Polyhedron. * Global Network Associate Analysis Centers (GNAACs) GNAACs combine the station coordinate and velocity covariance information contributed by the ACs and RNAACs (in SINEX format) to form global combinations for each week of data. A specified GNAAC combines the global combinations to form the official IGS combined network result, an essential element of the IGS Reference System. Future AACs are anticipated to contribute products in a number of areas, such as: ionospheric delay, specialized clock estimates, etc. New IGS Analysis Center/Associated Analysis Centers An aspiring AC/AAC must demonstrate a long term commitments as well as satisfactory performance of product delivery and solution quality. In particular the solution precision and reliability must be commensurate to the quality of the current AC/AAC solution products. The AC Coordinator will, by mutual agreement with AC/AAC candidates, evaluate and test the quality and timeliness of the prospective AC/AAC products. This evaluation should be based on a long test period of at least several months. An AC/AAC officially attains the IGS AC/AAC status only when accepted by the IGS Governing Board, upon recommendation of the AC Coordinator. Reference: Kouba, J., J. Ray and M.M. Watkins, IGS Reference Frame Realization, Proceedings of the 1998 Analysis Workshop held at ESAOC Darmstadt, Germany, February 9-11, 1998.