Last Updates of GLONASS Program and GLONASS Interoperability with GALILEO and GPS
Koshelyaevsky, N.B.
VNIIFTRI

According to the governmental Regulation No. 423 issued 14 July 2006 [1] many important changes have been introduced into the Federal Program "Global Navigation System" approved by governmental Regulation No. 587 issued 20 August 2001.

The first of the two goals was mentioned is further development and effective utilization of GLONASS for the sake of social and economy progress and ensuring of national security. And as a second goal was mentioned rendering of navigation signals to domestic and foreign users.

With regard to announced goals time support of GLONASS have to be considerably improved. According to GLONASS Interface Control Document [2] GLONASS system time is strongly linked to the National time scale of Russian Federation - UTC(SU). On other hand GLONASS itself is the most powerful and accurate mean of UTC(SU) dissemination through out Russia and the world. That's why one of requirements of GLONASS updates is to keep UTC UTC(SU) difference within 10 ns. Simultaneously GLONASS proposes to disseminate UTC(SU) time scale with uncertainty considerably less than 10 ns (1 sigma). Analysis [3] of this requirement based on step by step uncertainty spreading from GLONASS satellite clock, ephemeris uncertainty through time link to on land GLONASS SYNCHRONISER (MASTER CLOCK), it's ability to keep time, then time link to UTC(SU) leads us to long term stability requirements of H-masers ensemble and necessity of primary clocks with uncertainty type B be not more than 5*E-16.

Apart from expected considerable improvement of the National Time and Frequency Standard a set of state secondary laboratories will be reequipped with new time keeping/measuring instruments. Basing on all available clocks, including for example clocks of GLONASS SYNCHRONISER and other DoD facilities, composite time scale will be generated. All these laboratories will be well time linked and stability of resulted composite time scale is expected to be not more than 3*E-16 for 1 month.

Despite GNSS inter-operability problem looks to be purely technical one the legal component of it turned out to be very important. First of all due to the fact that GNSS at least in two countries are federal property, article of federal responsibility and have a significant military component. So first of all countries have to start a sort of intergovernmental negotiations. By the way such a negotiations have started between US and RF about two years ago and at the moment GPS/GLONASS Interoperability and Compatibility Working Group (WG-1) have met three times [4].

Regarding technical aspects of the problem our ideas are approximately following. To achieve cost effective solution first have to be solved problem of signal compatibility, FDMA and CDMA issues. On other hand a key problem to get navigation solution using signals of two or three different GNSS is transformation formulae for geodetic reference frames and system time scale difference. A geodetic reference frames are quite conservative, so one may mention their constant and transformation formulae in a proper interface documents. Contrary the system time scale difference requires continuously operating service. Taking into consideration military aspects of GNSS it is too difficult to imagine direct time links between proper GNSS control centers. Much more feasible looks time links between well-known internationally recognized centers, responsible at national level for proper GNSS. In Russia it may be VNIIFTRI, in USA - USNO. Time links between such a centers and a proper GNSS are proposed to be solved at na-tional level. The predicted system time scale difference has to be loaded to navigation message of proper GNSS by control center. More detailed proposals will be delivered in full presentation.

References

1. http://www.glonass-icb.rsa.ru/pls/htmldb/f?p=101:6:1319297756958002504::::P6_ID:364
2. Metrologia, 20, 1, 25 Comptes rendus de la 17e CGPM (1983), 1984, 97
3. Global Navigation Satellite System, GLONASS, Interface Control Document, edition No.5, Moscow, 2002.
4. V.P.Kostromin, N.B.Koshelyaevsky, Metrology Ensurance of Federal Task Program "GLOBAL NAVIGATION SYSTEM", All Russian scientific and technical conference "Metrology Insurance of Modern Military Technologies", Abstracts, Mitischi, 2006.
5. http://www.glonass-ianc.rsa.ru/i/glonass/joint_statement_eng.pdf