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A few comments may be permitted on the list of reports of the Directors of the national ephemerides. There is one omission and one addition as compared with former lists. Omitted is the Astronomisches Recheninstitut, Potsdam-Babelsberg, which coincident with the cessation of the Berliner Astronomisches jfahrbuch has become a division of the Babelsberg Observatory. As far as activity in the field of the Commission is concerned the Observatory is continuing to carry out calculations of special local interest, e.g. solar eclipse data for geophysicists. Added to the list is the Nautical Almanac Unit of the India Meteorological Department, Calcutta, which is publishing the Indian Ephemeris and Nautical Almanac. National ephemerides and exchange of dataThe identity (except for the title) of The American Ephemeris and Nautical Almanac and the (British) Nautical Almanac, newly entitled The Astronomical Ephemeris, as from the editions for the year i960, has to be considered as an important step towards unification of the national ephemerides. G. M. Clemence and D. H. Sadler (1) write: 'Many dearly-held, but essentially unimportant, standards and prejudices have had to be sacrificed on both sides; it is surprising how quickly these lose their former importance in the satisfaction of a comprehensive agreement. That same co-operation, goodwill and confidence exists between all the national ephemeris offices, and, although differences of language will introduce some further difficulties, there is no obstacle to complete unification that will not be overcome in course of time.' Although it seems still too early for the ultimate achievement of a single international astronomical ephemeris these words of Clemence and Sadler should have an encouraging effect.Reproducible material of the unified Ephemeris can be made available to the Offices of other B 1 https://www.cambridge.org/core/terms.

The problem of training symbol placement in data packets for channel tracking is considered, where the channel is time-varying Rayleigh flat fading. We use the minimum mean-square error (MMSE) estimator for channel tracking. A minmax approach is considered. We optimize the placement by minimizing the maximum MSE over a packet. It is shown that training symbols should be scattered throughout the packet with equal space to achieve the optimal performance.