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N79 21462 Rotation of the Earth and Polar Motion," Services- Bernard Guinot Bureau International De 1'Heure 61, Avenue De 1'Observatoire, Paris 75014, France

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N79 21462 Rotation of the Earth and Polar Motion, r N79 21462 Rotation of the Earth and Polar Motion," Services- Bernard Guinot Bureau International de 1'Heure 61, Avenue de 1'Observatoire, Paris 75014, France Abstract. The need of a continuous monitoring solar attraction can be modeled, but others due of the polar motion appeared at the end of the to various local causes and to the plate motions 19th century, and was at the origin of one of the are not sufficiently known to correct the ob- oldest international projects : the establishment servations.] Figure 1 shows on an auxiliary of the International Latitude Service. This ser- sphere of unit radius the directions of the in- vice still operates, but other organizations now stantaneous North pole P, of a reference fixed determine the polar motion, using the astrometric pole P0 and_pf the zenith Z of a station. The measurements of latitude and time and also origin of the astronomical longitudes is a fixed Doppler observations of artificial satellites. On point 0 on the equator of Po. We can see that the the other hand, since the advent of atomic clocks astronomical latitude cp and longitude L vary with in 1955, the universal time has become a measure the coordinates of the pole x and y. The univer- of the rotation of the Earth, which is also cur- sal time UTl is simply linked to the angular rently required and which must be evaluated by a motion in space of the PO meridian around P ; it Service. The services providing polar motion and is therefore dependent on x and y. The fundamen- universal time data will be described, the preci- tal equations used in classical services are, for sion and accuracy of these data will be estimated. each station i, UTC being the worldwide time reference, Introduction x(t)cosL0j. (1) The full description of the rotation of the Earth in space is traditionally given by the mo- [-x(t)sinL y(t)cosL 0,1.Itarf P 0,1. +[UT1-UTC] t tion of the rotation axis with respect to the ± - UTC]t , (2) Earth (polar motion), and in space (luni-solar precession, nutation), and by the angular posi- where <P0 -^ is the initial fixed latitude. L0ji, tion around the rotation axis (universal time the initial longitude, does not explicitly UTl). Precession and nutation can be fairly well appear, but it is used in deriving UTO^, which modeled, and require only occasional improvements is therefore computed assuming x = y = 0. of their representation ; polar motion and univer- sal time are still unpredictable and require continuous monitoring. Except for the ILS, the computations of <Pt ^ , While for several decades, since 1900. the - UTC]t are made by the contributing International Latitude Service (ILS) was the only observatories themselves, using the values of source of the pole coordinates x and y, the de- astronomical constants recommended by the Inter- velopment of new astrometric instruments led to national Astronomical Union. The role of the the organization of a new service, the Interna- central services is thus to combine the equations tional Polar Motion Service (IPMS) in 1962. But (1) and (2). There is no standard procedure the advent of atomic clocks in 1955 made obsolete to accomplish that ; the main choices are related the division of the work between polar motion and to universal time. The Bureau International de - the weighting factors, according to the quality 1'Heure (BIH), in charge of universal time,began of the observations, in 1955 to determine its own set of coordinates - the choice of initial latitudes and longitudes, of the p.ole needed in the evaluation of UTl. On the other hand, the successful recovery of the pole coordinates using Doppler observations of Transit satellites led national organizations of the USA to determine routinely these coordinates. Thus the user has the choice among several sets of pole coordinates (but there is only one for UTl), which, of course, differ. This is often considered as a nuisance. But it has, at least, one important advantage : it is a warning against too much faith in the published results. We have too many examples of analyses and inter- pretations where the limitations of the ILS data where ignored. Methods The astrometric methods refer to the direc- tions of the plumb-lines of the observatories. We will assume that the Earth is rigid and that these directions are fixed within the Earth. [This is not true ; some motions due to the luni- Fig. 1. Variation of astronomical latitude and Proc. of the 9th GEOP Conference, An Ititennitiomtl SvmpiMiitni tni thr Applictiiion* of GiWi'.vv IH Gemkninim -s. Ocluher2-S, 1178. Depl. of Geodetic Science Rept. No. 280, The longitude with the coordinates of the pole. Ohio State Univ.. Columbus. Ohio 43210. 13 •T.'T- Classical Astrometry Doppler Visual zenith telescope Visual zenith telescope Doppler receivers for Transit instrument TRANSIT satellites (international Astrolabe stations) Photographic Zenith Tube 5 instr. 75 instr. 20 iristr. X \ f International International Bureau interna- Defense ^Mapping Latitude Service Polar Motion Serv. tional de 1'Heure Agency (USA) ILS IPMS BIH *~ DMA y V •**• J / 9 y x, y, UT1 x, y Fig. 2. Data flow to the services. - the way of removing some systematic errors, noise by the pair variance (or Allan variance) - the averaging time, ? ( I2 v ( T,) = mean ofU i+l " V - the smoothing techniques on observational data a « • and evaluated results. The Doppler determinations of polar motion is an example of the satellite techniques for the This function is represented by stability curves study of Earth rotation, which are discussed by (Fig. 3), as it is customary for the characteri- Aardoom [1978] at this Conference. Therefore we zation of the stability of oscillators [Barnes will only point out that in these techniques, et al., 1971J . Thus instead of speaking loosely instead of referring the observations directly of the precision of the results, we-can speak of to the quasi-non-rotating directions of stars, their stability. one uses intermediary objects, the directions of which are computed in the non-rotating reference In general, for small values oft, aa(t) frame. Providing that the motion of the object follows a law in l/\ft, as in the case of white can be correctly modeled, UT1, x and y can be noise, but for larger values oft, O"a ( ~C) derived from the observations. In practice, a reaches a minimum which is called the flicker a spurious drift of UT1 cannot be avoided, but the floor. For still larger values oft, a (~0) coordinates of the pole can be obtained to a generally increases. In this latter domain it is large extent free from systematic errors and sometimes difficult to make the distinction drifts. Although astrometry only measures angles, we will use the meter as the unit for polar motion, assuming that the radius of the auxiliary sphere X and Y is the polar radius of the Earth. Figure 2 shows the general organization of the services in 1978. Precision and accuracy of the results A matter of importance is the degree of confi- dence the user may have in the results published by the Services. This problem is not easily sol- ved. In most cases, the data are given without any information on their precision. In some cases a standard deviation is given, computed from the internal consistency of the data contributing to the determination of a raw value over a given averaging time r. But even when this information is given, it is far from being sufficient : it is 10 20 SO 100 5 10 well known that the errors are not a white noise days years and that the standard deviation does not vary proportionally to 1/tfr . Fig. 3. Precision of the pole coordinates as a Let us suppose .that AQ, A^,...,^ are measured function of the averaging timer, ^(r) is the quantities obtained at instants t0, to + T,..., pair variance. AST stands for global astrometric to + nt , by averaging over intervals T. If a0, services : IPMS and BIH special solution for as- a^,..., ajj are the random errors of these quanti- trometry alone. The current BIH results includ- ties, it is possible -to characterize the random ing DMA have about the same precision as DMA. between random and systematic errors. For which requires well calibrated micrometer screws. instance, for the Earth rotation parameters, the The stations, located on the 39°8'N parallel increase of °a(^) witht can be the consequence are now : of changes in the network of observing stations, changes of programs and methods in the stations, Mizusawa, Japan, longitude 141° E and/or plate motions. Kitab, URSS, 67° E In the following, the stability curves will be Carloforte, Italy. 8° E given. One must be aware that they represent an Gaithersburg, USA, 77° W estimation. For values of E smaller than a month, Ukiah USA, 125° W we can assume that the observational noise is larger than the true noise of the observed quan- Initially, the declination errors were re- tities themselves, and the use of a high-pass moved by the so-called chain method. Two or more filter gives fairly reliable values of the ran- groups of stars are observed every night. Assum- dom errors a. But for larger values of t> , one ing that the latitude does not vary during the has to make less reasonable assumptions. The night, the difference between group results re- "three-corner-hat method" is not available presents the contribution of declination errors. because there are only two truly independent As only night observations are possible, a full series of data, the astrometric and the Doppler.
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