Jinling Li

VLBI ecliptic plane survey: VEPS-1

We present here the results of the first part of the VLBI Ecliptic Plane Survey (VEPS) program. The goal of the program is to find all compact sources within 7 degrees.5 of the ecliptic plane that are suitable as calibrators for anticipated phase referencing observations of spacecraft, and determine their positions with accuracy at the 1.5. nrad level. We run the program in two modes: search and refine. In the search mode, a complete sample of all sources brighter than 50 mJy at 5 GHz listed in the Parkes-MIT-NRAO and Green Bank 6. cm (GB6) catalogs, except those previously detected with VLBI, is observed. In the refining mode, the positions of all ecliptic plane sources, including those found in the search mode, are improved. By 2016 October, thirteen 24 hr sessions that targeted all sources brighter than 100. mJy have been observed and analyzed. Among 3320 observed target sources, 555 objects have been detected. We also conducted a number of follow-up VLBI experiments in the refining mode and improved the positions of 249 ecliptic plane sources.

Measurements of the VLBI experiments during the first campaign of the Asian-Pacific space geodynamics (APSG) program

During the first campaign of the Asian-Pacific space geodynamics (APSG) program in October 1997, two VLBl experiments were successfully organized and coordinated by the Astrometry and Geodesy VLBl Group of Shanghai Astronomical Observatory, the Chinese Academy of Sciences, cooperated with geodetic VLBl group, GSFC, NASA, USA. Six VLBl stations participated in the experiments, including Seshan and Urumqi station of China, Gilcreek station in Alaska and Kokee station in Hawaii of USA, Kashima station of Japan and Hobart station at Tasmania of Australia. Baseline lengths are from 1 900 to 11 000 km and the mean relative uncertainty of the baseline length measurements is 1.0 x 10-9. In addition, the rates of the baseline lengths among the six stations and their three-dimensional velocities are solved out via global analysis of the two APSG sessions and the historical observations of the six stations as well as other VLBl observations from the global observation network. These results are appreciable to the studies of the modern crustal movement in the Asian-Pacific region. Especially,an 8 mm/a eastward motion and a 14 mm/a north by northeast motion are detected respectively for Seshan and Urumqi stations relative to the stable part of the Eurasian plate. The motions directly illustrate the effect of the northward movement of Indian plate on the modern crustal motions of the northwestern and the eastern part of China, which is of important significance to the study of the modern crustal motion of China.

An analysis of the polar motion series from VLBI observations

From the reduction of 2893 globally distributed astrometric and geodetic VLBI sessions from August 1979 to the end of 1998, coordinates of 722 radio sources at J2000.0, coordinates and velocities of 128 stations at J1997.0 and about 20 years Earth Orientation Parameters were estimated. From the analysis of the resultant polar motion series, the following are demonstrated: (i) During the VLBI data span the Markowitz wobble is not exhibited. (ii) The amplitudes of both annual and Chandler wobble show temporal variations, with the former being more obvious than the latter. (iii) Wavelet analysis shows that all the signals in the polar motion series are characterized by temporal variation in amplitudes. If we take any signal as strictly periodic, it is impossible to remove it completely from the polar motion series by least-squares fit because the hypothesis of a constant amplitude conflicts with VLBI measurements. (iv) By applying a filter, the secular polar motion was found to be (2.74 ± 0.01) masla towards (83.9 ± 0.3) ° W longitude, which is smaller in rate and more westward in direction compared with those determined from optical observations or the combination of optical and space geodetic observations.