Dear Colleague, The last time the definition of the SI second was changed was in 1967 when Caesium atom hyperfine frequency transition was chosen as the reference value. Since then first the primary thermal beam caesium clocks and then caesium fountains have been providing the SI second realization values to steer the International Atomic Timescale. The current fountain uncertainties are at the low 1E-16 level. The frequency metrology research in recent years has surpassed the microwave clocks, and optical frequency standards based on trapped ions and atoms now provide frequency measurements with evaluated uncertainties at the level 1E-18. The frequency comparisons have also been advancing both through improved techniques in Global Navigation Satellite Systems signal analysis and the Two Way satellite Time and Frequency transfer as well as with fibre based frequency links and portable optical clocks. Frequency combs used to compare frequencies between clock lasers of different optical clocks and also between optical frequencies and microwave 133Cs frequency of 9.192631770 GHz are now routine laboratory equipment and are commercially readily available. These factors now provide a solid foundation to prepare for the redefinition of SI second based on the optical frequency standard. The exact approach is currently under discussion and it may be selecting one atomic candidate or building an ensemble. The specific requirements and the conditions for the redefinition to take place are also being finalized. The Consultative Committee for Time and Frequency (CCTF) has started preliminary work on this subject, to investigate possible scenarios and their possible impact. It is within this framework that we contact you an expert in the field of Telecommunications, in order to make sure that the needs and wishes of your community are integrated into the general debate that will take place and may, eventually, lead to the redefinition of the second. With the new definition, the frequency measurement accuracy will surpass that of Caesium-based timescale by at least one order of magnitude and will provide an adequate tool for current and future research into fundamental physics as well as technology exploitation. The redefinition is not expected to affect any of the existing commercial frequency standard uncertainties such as those of caesium beam clocks or hydrogen masers, because their uncertainties are generally orders of magnitude higher than the expected accuracy of the new definition. Same is true for caesium fountains. The computation and dissemination of UTC will also not change, but UTC will become more stable and accurate based on the improved optical realizations of the SI second. We would like to ask your expert opinion on whether the redefinition may have any impact be it negative or positive on your field of study and operation. The issues we would like you to consider are not just scientific and technological, but also regulatory and legislative. Your answers will be carefully examined and taken into account by CCTF in their analysis of Time and Frequency metrology current and future needs and SI second and UTC evolution. You can find the survey at https://www.surveymonkey.com/r/CCTF_Survey_2020 and in the attached letter from CCTF president Noël Dimarcq you will find additional information on other activities at CCTF for your reference. Best regards, The CCTF Task Force on the redefinition of the second, subgroup A Co-chairs: Marina Gertsvolf (marina.gertsvolf@nrc-cnrc.gc.ca<mailto:marina.gertsvolf@nrc-cnrc.gc.ca>) Gaetano Mileti (gaetano.mileti@unine.ch<mailto:gaetano.mileti@unine.ch>) Secretary: Frederic Meynadier(frederic.meynadier@bipm.org<mailto:frederic.meynadier@bipm.org>)