-------------------------------- Experimental EOP C02 time series -------------------------------- Last update: February 2024 conctact: christian.bizouard@obspm.fr For geophysical study, long term UT1/LOD data is of primary interest. By providing a long term combined series UT1 /LOD, updated monthly, according to the last geodetic determinations, we hope to fufil a gap let by IERS. N.B.: These combined series are not yet accepted as an offcial IERS product. EOP C02 series is obtained by gathering three individual time series: 1) 1830-1955 : Delta T = TE - UT = TE - TAI - (UT - TAI) from (Jordi et al. 1994), and obtained from the analysis of lunar occultations - mean sampling time of 120 days. 2) 1955-1962 : AICAS (Astronomical Institute of the Czeck Academy of Science, Prague) UT1 time series from optical astrometry (to be found on this WEB site as a long term EOP time series) 3) 1962-nowadays: Combined C04 time series free from short term zonal tides (< 2 years) N.B.: UT1 - TAI before 1958 : TAI does not exist before 1958, and it is replaced by the stable time scale derived from celestial Meachnics, namely the Ephemeris Time TE = TAI + 32.184 s. UT1 is not formally defined as a time scale before 1956, and it is replaced by the Universal Time UT. Then, before 1958 we can "extrapolate" TAI by TE, and reconstruct the difference UT(UT1) - TAI by UT(UT1) - TAI = UT(UT1)-TE + TE-TAI = UT(UT1)-TE + 32.184 s. Characteristic of the combination: - For optical astrometric data (Jordi et al, AICAS), prior to 1962, UT-TE / UT1-TAI series are smoothed by gaussian filter with 365 day interpolation step. - C04 series are smoothed by gaussian filter having a 200 day interpolation step. - After filtering the total series, composed of UT-TE (prior to 1958) and UT1 - TAI (after 1958), are interpolated by cubic spline at 100 days step. - Variation of the lenght of day LOD, given by LOD / D = -dUT1/dt where D = 86400 s is the duration of the day, is then obtained from a two point time derivative of the 100 day UT1 time series. Reference: Jordi, C., L. V. Morrison, R. D. Rosen, D. A. Salstein, and G. Rosello, Fluctuations in the earth's rotation since 1830 from high-resolution astronomical data, Geophys. J. Int., 117, 811-818, 1994. Acronyms: TE : Temps des Ephemerides UT : Universal Time UT1 : Universa lTime 1 TAI : Variation of the Lenght Of Day