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MODERN METHODS for the DETERMINATION of POLAR MOTION and UT1 William E. Carter National Geodetic Survey National Ocean Survey, N

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MODERN METHODS for the DETERMINATION of POLAR MOTION and UT1 William E. Carter National Geodetic Survey National Ocean Survey, N MODERN METHODS FOR THE DETERMINATION OF POLAR MOTION AND UT1 William E. Carter National Geodetic Survey National Ocean Survey, NOAA Rockvi 11e, Md. 20852 ABSTRACT This paper is organ~redinto two major divisions according to the topics: polar motion and UT1. Each division is introduced with a brief review to provide a minimal perspective for readers un- familiar with the subject area. The applications of Doppler sate1 1i te observations, laser ranging to artificial satellites and the Moon, and astro- nomic radio interferometry to monitoring polar motion and UT1 are discussed. Emphasis is placed on detailing how and what each method is capable of measuring, fundamental 1imitations are noted, and the present status of the development of each method is reviewed. The paper concludes with a summary of the author's evaluations of the various methods as candidates for the next generation international polar motion and UT1 monitoring service. INTRODUCTION The "classical" methods of monitoring polar motion and UT1 have been based on visual, photographic, and photoelectric observations. of optical stars. The temporal and spatial resol utio~sand accuracies of these methods have been limited by such factors as; an inability to fully correct the observations for the effects of the Earth's atmosphere, in- accuracies in the relative positions and proper motions of the stars, the 1imi ted number and poor distribution of observatories, and instru- mental imperfections. Further refinements of the classical methods, some of which involve the application of modern technology, are continu- igg and are e;<pected to yield significant improvements. However, pro- foundly differeqt methods, which have developed as outgrowths of space exploration activities, promi se an order of magnitude improvement in our ability to monitor polar motion and UT1. It is these space-age methods, i.e., Doppler satellite observations, laser ranging to artificial satel- lites and the Moon, and astronomical radio interferometry that are discussed in this paper. POLAR MOTION IL. ar motion is the motion of the Earth's instantaneous poie (axis of r!,:ation) with respect to a reference point fixed to the crust of the T:.rth. -i'Fle theoretical basis for the existence of polar motion was presented b.? Euler in 1765, but the motion was not detected observationally until the late 1800's. S. C. Chandler discovered that the observed motion was actually the result of two primary components: a revolution of the true pr le around the principal moment of inertia axis counterclockwise when \I. .wed from the north, with a period of 1.2 years; and an annual revolu- c>In, also counterclockwise (Chandler, 1891). The 1.2 year period of tibl?first component (now commonly referred to as Chandlerian motion) did nct agree well with the much shorter period predicted by Euler's work. The discrepancy was quickly explained by S. Newcomb as being due to the elasticity of the Earth (Newcomb, 1891 ). The annual term is produced by the continuous redistribution of mass in meteorological and geo- physical processes. The motion of the pole is not totally predictable from a simple two- component model. Unexpected changes in the magnitude and direction of the motion occur, that result in a requirement to monitor the motion on a continuing basis. Regular monitorin of polar motion was undertaken by the International ~atitude Service 9 ILS) in 1899, and has continued without interruption until today. The ILS system uses the differential zenith distance metiod (Hoskinson and Duerksen, 1947) of determining l3titude with visual zenith telescopes (VZT). The stations are all located very near the same parallel of latitude (39" 08' N) so that the same star pairs can be observed from a1 1 observatories. The mean pole position defined by the ILS observatories for the period 1900-1905 has been adopted as the Conventional International Origin (CIO) . In 196,(! the IL' +is reorganized, according to resolutions of the Inter- national Astronumi cal Union, and the International Polar Motion Service (IPMS) wat ~unded(Yumi, 1964). The IPMS continues to publish polar positions oased only on the ILS observatories, but it also pub1 ishes values derived from a combination of VZT, Photographic Zenith Tube (PZT), as trr abe, and transit circle observations from approximately 75 obr xvatories. In 1955, the Rapid Latitude Service (RLS) was established, by action of the IAU, under the direction of the Bureau International de 1 'Heure (BIH), to predict the coordinates of the pole and provide time correc- tions with very short delays. The individuality of the RLS has since been abandoned and the rapid service is now provided as a routine function of the BIH. In 1968, the BIH adjusted the positions of their contributing observa- tories, predomina:ely the same observatories that are included in the IPMS system, to insure the coincidence of the BIH pole with the CIO. Since that time, the BIH reference system has been maintained indepen- dently from the ILS system. In 1972, the BIH began to include pole position information obtained by Doppler satellite observations in their solutions. The Doppler values used are the two-day solutions of the Transit navigational system observations, presently pub1 ished by the Defense Mapping Agency. The methods used to combine the Doppler data with the optical data are detailed in the BIH Annual Report for 1976. Pole positions derived from Doppler observations of artificial sate1 - 1i tes are available from as early as 1967, but the earl iest data are of lower quality than the post 1972 data. The developmental work of the Doppler Polar Motion Service (DPMS) was accomplished at the Naval Weapons Laboratory (Anderle, 1973). The Defense Mapping Agency DMA) took over operational responsibility in April 1975 (Oesterwinter, 1978). The Doppler polar positions are available directly from DMA, and are also published in U. S. Naval Observatory Time Service Publication Series 7. To briefly summarize, polar motion values are determined and distributed today by the IPMS, BIH and DMA. The IPMS utilizes only optical data, the BIH utilizes a combination of optical and Doppler satellite data, and DMA utilizes only Doppler data. Monthly means are usually quoted as having uncertainties in the 20 to 40 cm range, but the positions pub- lished by the different services often differ by 1 to 2 meters. Many questions still remain unanswered even after almost 80 years of continually monitoring polar motion. Some of these questions cannot be answered unless significant improvements are made in the spatial and temporal resolutions of the observations. An improved monitoring system, based on more modern methods, is badly needed. Candidate methods are: Doppler satellite observations, laser ranging to the Moon and artificial sate11 i tes, and astronomic radio interferometry. Polar Motion Determinations by Doppler Satellite Observations The material presented in this section has been extracted primarily from papers by Anderle (1973) and Oesterwinter (1978). Radio signals suitable for Doppler observations are transmitted by U. S. Navy Navigation System sate1 lites. The satellites are in nearly circu- lar polar orbits at heights of about 1,000 km. They continuously trans- mit at two carrier frequencies, 399.968 MHz and 149.988 MHz (nominal values). The oscillators typically drift a few parts in 1011 per day. Both frequencies are generated from the same osci 11ator to faci 1i tate the determination of ionospheric refraction effects. Pole positions are obtained as part of the bi-daily updating of the or- bit of each satellite. The gravity field model and the positions of the base stations are held fixed in a least squares solution which esti- mates the x and y coordinates of the pole, six constants of orbital integration, one drag scaling factor, a frequency and tropospheric scaling factor for each satellite pass, and the coordinates of any new points being surveyed. The bi-daily solutions from as many as five different satellites are combined to derive 5-day mean positions of the pole. The Doppler pole positions are determined in the "Doppler network" coordinate system. In 1970, an attempt was made to make the origin of the Doppler coordinate system close to the CIO by estimating the coordi- nates of the base stations in a solution in which the gravity field co- efficients and the BIH pole positions were held fixed. The network does vary with time due to station failures, modifications and upgrades, and the augmentation of the 17 to 20 base stations by one tu ten, or more, temporary stations during various operational campaigns. The standard error of the pole positions vary considerably depending upon the distribution and number of observations combined in each solu- tion. Oesterwinter (1978) concludes that the standard deviation of a two-day polar coordinate solution is now better than 40 cm, and for a five-day mean, under 20 cm. The dominant source of error is believed to be residual errors in the gravi ty fie1d model . Polar Motion Determinations by Satellite Laser Ranging The material presented in this section has been extracted primarily from papers by Kolenkiewicz et a1 . (1977) and Smith et a1 . (1978). Many artificial Earth satellites have been equipped with retroreflectors to facilitate tracking by laser ranging systems. Polar motion can be determined from satellite laser ranging from a single station, if an accurate satellite ephemeris is available, or a network of stations. In the case when only a single tracking station is in operation, only one component of polar motion, i.e., the component along the station meridian, can be monitored. The procedure is to establish a precise reference orbit by tracking the satellite for a reasonable period of time, say a month or so, and then compare subsequent observations made over periods of perhaps 6 to 12 hours, to this reference orbit.
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