Bulletin G6od6sique (1992) 66:346-354 Bulletin G od sique © Springer-Verlag 1992 Radiointerferometric polar motion observations with high temporal resolution A. Nothnagel 1, 2, G.D. Nicolson ~, J. ChampbeU 2 and H. Schuh 2, 3 1 Hartebeesthoek Radio Astronomy Observatory/FRD, PO Box 443, Krugersdorp 1740, South Africa 2 Geodetic Institute of the University of Bonn, Nussallee 17, W-5300 Bonn, Federal Republic of Germany 3 New affiliation: Geodetic Institute of the University of Bonn 4 New affiliation: Deutsche Forsehungsanstalt ffir Luft- und Raumfahrt (DLR), K61n Received April 15, 1992; Accepted July 6, 1992 Summary. For several years polar motion has been routinely monitored with the technique of geodetic Very Long Baseline Interferometry (VLBI) at five- day intervals. Here we present the results of the first two series of VLBI measurements for polar motion monitoring observed on a quasi daily basis employing only a single baseline. The high sensiti- vity of the long north-south baseline between radio telescopes at Wettzell in Germany and Hartebeest- hock in South Africa for both components of polar motion permitted relatively short and inexpensive measurements of only two hours duration per ses- sion. The results of this series agree very well with the pole path determined with the IRIS network using 4 observatories in each observing session of 24 hour duration. With the polar motion results of the two series spanning about 35 days each, spectral analyses were performed which have shown a fort- nightly period with a high degree of probability. These measurements demonstrate the potential of long north-south VLBI baselines for monitoring polar motion with very high temporal resolution for studies of short period fluctuations. Introduction For a period of more than 10 years the technique of geodetic Very Long Baseline Interferometry (VLBI) has been used to measure the rotation of the Earth. Networks of radio telescopes separated by more than one earth radius regularly observe compact ~ extragalactic radio sources, such as quasars, which form an ideal reference frame for studies of the Earth's rotation. The difference in arrival times of signals from such radio sources at the stations yields a time delay which is the principal observable in geodetic VLBI. The geometric time delay r of signals arriving at two radio telescopes is to first order a function of the baseline vector b between the two telescopes and the unit vector in the direction of the radio source k: -!/, = k (1) c The negative sign reflects the conventions used in defining r and b. c is the velocity of light. For practical reasons the baseline vector b is nor- mally expressed in a three dimensional Cartesian coordinate system defined by a number of VLBI radio telescopes while the radio source positions form a quasi inertial reference frame in space at a given epoch (J2000.0). In order to express b and k in the same coordinate system a number of transformations are necessary (e.g. Ma 1978). These can be applied to either vec- tor. In a unified system eq. (1) is written as: _% r = WSNP k (2) c where W is the rotation matrix for polar motion (wobble), S is the diurnal spin matrix, N is the nu- tation matrix and P is the precession matrix. Any parameters in equation (2) or combination of para- meters can be determined from VLBI experiments depending on the aims and configuration of the network (Shapiro and Knight 1970). Considerable efforts are undertaken with networks of permanent radio observatories in the northern hemisphere to determine the orientation of the Earth'S spin axis in projects called IRIS (Internatio- nal Radio Interferometric Surveying) (Carter et al. 1985) or NAVNET (Eubanks 1991 ). These networks observe up to 25 radio sources several times in a Correspondence to: A. Nothnagel