J Geod (2009) 83:327–334 DOI 10.1007/s00190-008-0240-y ORIGINAL ARTICLE Use of IGS products in TAI applications G. Petit · E. F. Arias Received: 11 March 2008 / Accepted: 4 June 2008 © Springer-Verlag 2008 Abstract The Bureau International des Poids et Mesures (BIPM) is in charge of producing International Atomic Time TAI. In this aim, it uses clock data from more than 60 lab- oratories spread worldwide. For two decades, GPS has been an essential tool to link these clocks, and products from the International GNSS Service (IGS) have been used to improve the quality of these time links since its creation in the early 1990s. This paper reviews the various interactions between the IGS and time activities at the BIPM, and shows that TAI has greatly benefited from IGS products so that their avail- ability is now an essential need for the quality of TAI links. On the other hand, IGS has also benefited from introducing time laboratories equipped with highly stable clocks in its network of stations. In the future, similar products will be needed for an ensemble of satellite systems, starting with GLONASS and GALILEO. It will be a major challenge to the IGS to obtain a consistent set of products, particu- larly for what concerns satellite clocks and inter-system bias values. Keywords Time tranfer · GNSS · TAI 1 Introduction The international timescales TAI and Coordinated Univer- sal Time UTC are calculated at the Bureau International des Poids et Mesures. They are the result of the worldwide cooperation of more than 60 national metrology laborato- ries and astronomical observatories that operate commer- cial atomic clocks. In addition, about ten of them develop and maintain primary frequency standards that provide TAI G. Petit (B ) · E. F. Arias BIPM, Pavillon de Breteuil, 92312 Sèvres, France e-mail: gpetit@bipm.org accuracy. The algorithm used for the calculation of TAI has been designed to guarantee the reliability, the long-term sta- bility, the frequency accuracy and the accessibility of the scale. Nevertheless, the quality of TAI rests critically on the methods of clock comparison which may bring significant instability mostly at short averaging time (5–10 days). Local representations of UTC, named UTC(k ) are maintained in laboratories “k ” contributing clock data to the BIPM. The BIPM organizes the international network of time links to compare local realizations of UTC in contributing laborato- ries and uses them in the formation of TAI. The network of time links presently used by the BIPM is non-redundant and relies on the observation of GPS satellites and on two-way satellite time and frequency transfer (TWSTFT). The International GNSS Service (Dow et al. 2005) is an voluntary federation of institutions which operate, on a vol- untary basis, a network of several hundred permanent GPS and GLONASS stations, many analysis centres, and data cen- tres that distribute its products. GPS and GLONASS obser- vation data sets coming from GNSS stations are analysed and combined to form the IGS products which support many Earth science applications. Some of these products have been used in the process of calculation of TAI since the IGS incep- tion; e.g. maps of the ionosphere total electronic content (TEC) have allowed obtaining high-quality time transfer using existing single-frequency receivers; post-processed precise satellite ephemerides from the IGS have been used for cor- recting satellite positions. The cooperation between the IGS and the BIPM has there- fore contributed to significant improvement in time trans- fer. An example of this successful association is the pilot project conducted jointly by the BIPM and the IGS between 1998 and 2002, focusing on the feasibility of accurate time and frequency comparisons using GPS phase and code mea- surements (Ray and Senior 2003). As a conclusion of some 123