Determination of UT1 by VLBI

Very Long Baseline Interferometry (VLBI) is the only space geodetic technique which is capable of estimating the Earth's phase of rotation, expressed as Universal Time UT1, over time scales of a few days or longer. Satellite-observing techniques like the Global Navigation Satellite Systems (GNSS) are suffering from the fact that Earth rotation is indistinguishable from a rotation of the satellite orbit nodes, which requires the imposition of special procedures to extract UT1 or length of day information. Whereas 24 hour VLBI network sessions are carried out at about three days per week, the hour-long one-baseline intensive sessions (‘Intensives’) are observed from Monday to Friday (INT1) on the baseline Wettzell (Germany) to Kokee Park (Hawaii, U.S.A.), and from Saturday to Sunday on the baseline Tsukuba (Japan) to Wettzell (INT2). Additionally, INT3 sessions are carried out on Mondays between Wettzell, Tsukuba, and Ny-Alesund (Norway), and ultra-rapid e-Intensives between E! urope and Japan also include the baseline Metsähovi (Finland) to Kashima (Japan). The Intensives have been set up to determine daily estimates of UT1 and to be used for UT1 predictions. Because of the short duration and the limited number of stations the observations can nowadays be e-transferred to the correlators, or to a node close to the correlator, and the estimates of UT1 are available shortly after the last observation thus allowing the results to be used for prediction purposes.

Coseismic displacement of the 27th September 1997 Umbria - Marche (Italy) earthquakes detected by GPS: campaigns and data

On September 26,1997 two earthquakes of Mw 5.7 (00.33 GMT) and Mw 6.0 (9.40 GMT), occurred in the Umbria-Marche region (Central Apennines, Italy). The epicentres were located in an area of the Apenninic chain that experienced historical earthquakes up to X degrees of the MCS scale. During the time span 1992-1996, the Italian Istituto Geografico Militare (IGM) set up a new national geodetic network measured by Global Positioning System space geodetic technique, consisting of more than 1200 vertices uniformly distributed on the Italian peninsula and islands. From October 7 to 11, 1997, a short while after the main shocks of the Umbria-Marche seismic sequence, we reoccupied thirteen stations belonging to the IGM and TYRGEONET networks to measure coseismic displacement. The determinations of the post-seismic coordinates at 13 GPS monuments detected significant coseismic displacements. The comparison between the preseismic and postseismic data sets show maximum displacements of 14 cm and 25 cm in the horizontal and vertical components respectively. In this paper, the GPS network, the field work, the data processing procedures and the computed coseismic displacements measured at the geodetic monuments are discussed with the aim to provide a data set useful to the scientific