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IVS Observing Program

Session Descriptions: 2001

This section has brief descriptions of each type of session coordinated by IVS. Refer to the master files for the details of each session such as start times, participating stations, correlator, and current status.


CORE-A:

The purpose of CORE-A is to validate the CORE concept of measuring EOP continuously using different networks. Comparisons of the EOP results from simultaneous sessions in 1997 and 1998 have shown fairly good agreement, but there are some puzzling systematic differences. It is hoped that additional data to be obtained during 1999 and part of 2000 will contribute to understanding this data set.

CORE-B:

The purpose of the CORE-B sessions is to obtain EOP data on days adjacent to the main NEOS/CORE-A and CORE-3 sessions and thus extend the span of continuous data. The secondary purpose of CORE-B is to provide observing sessions during which the stations can demonstrate their performance and their ability to participate in future regular CORE sessions.

APSG:

The primary purpose of the APSG sessions is to continue monitoring the relative motions of the plates in the Asia-Pacific region. These sessions involve sites on the four plates. Tsukuba and Fairbanks are on the North American Plate, Hobart is on the Australian Plate, Seshan and Urumqi are on the Eurasian Plate, and Kokee is on the Pacific Plate. Thus, these sessions will produce critical data for the investigation of the current crustal motions in the Asia-Pacific region combined with the data from the APSG SLR and GPS campaigns which will also be performed in the same month as the APSG experiments. Also, these experiments will improve the accuracy of the geocentric coordinates of the participating stations, especially Urumqi and Tuskuba.

EUROPE:

The purpose of this experiment is to determine the station coordinates and their evolution in the European geodetic VLBI network. This process will be done with the highest precision possible.

IRIS-S:

The purpose of the IRIS-S sessions is to continue the series of measurements for the determination of earth rotation parameters. IRIS-S experiments are set up to provide Earth orientation parameters (UT1, polar motion, and nutation), baseline components and positions of extragalactic radio sources. This network produced excellent EOP results with the addition of Fairbanks. Even with the inclusion of the O'Higgins antenna, this network with its long north-south and east-west baselines as well as with its wideband capability provides a good EOP determination.

CORE-OHIGGINS:

The purpose of the CORE-OHIG (Southern Terrestrial Reference Frame) sessions is to tie together optimally the sites in the southern hemisphere. The participating sites include all the sites in the south - O'Higgins, Fortaleza, Hartebeesthoek, and Hobart - as well as Kokee Park. The last two sites are the southern-most of the northern sites and have been included to make the geometry more robust and to increase the number of observations per scan. Because these sessions concentrate on the southern sites and do not use observing time to accurately tie these sites to far northern sites (as the Global Sessions do) they should yield a very accurate regional TRF around the South Pole. Syowa (Japan/Antartika) are willing to participate in this experiment. The characteristics of this network have changed slightly due to more weigh of the southern part.

Research and Development - VLBA:

There are six bi-monthly coordinated astrometric/geodetic experiments each year that use the full 10-station VLBA plus up to 10 geodetic stations currently capable of recording VLBA modes. These experiments are being coordinated by the geodetic VLBI programs of three agencies: 1. USNO will perform repeated imaging and correction for source structure.; 2. NASA will analyze this data to determine a high accuracy terrestrial reference frame.; and 3. NRAO will use these sessions to provide a service to users who require high quality positions for small numbers of sources. Sources for this series of experiments will be selected using the proposed approach. For each experiment we will select a set of 70-80 sources out of the pool of ~400 Northern Hemisphere ICRF sources. About 40-50 of these will remain the same from experiment to experiment and will be chosen to optimize the goals of the three groups.

BONN-R&D:

The primary purpose of this type of expeirment is to investigate the influence of polarization impurity on measurement groups and phase delays.

Syowa Anarctica:

These experiments were designed to strengthen the reference frame in the southern hemisphere, and to detect plate motion, in close collaboration with the JARE (Japanese Antarctic Research Expedition) project.

NEOS:

The National Earth Orientation Service (NEOS) sessions are conducted to determine the Earth orientation and to provide a robust link on a regular basis between the terrestrial reference frame (TRF) and the Celestial Reference Frame. Current NEOS operations consist of one 24-hour duration NEOS-A observing session, on Tuesday-Wednesday of each week, for Earth orientation, together with daily one hour duration ``intensives'' for UT1 determination. The operational NEOS-A network currently includes the VLBI stations at Gilmore Creek (Alaska), Kokee Park (Hawaii), Wettzell (Germany), Fortaleza (Brazil), Ny Alesund (Norway), Algonquin Park (Canada), Matera (Italy), Westford (Massachusetts), and Yellowknife (Canada), although at most 6 stations observe in any one observing session. All NEOS sessions are routinely processed in time for the next IERS Bulletin A on the following Thursday.

CRF:

The Celestial Reference Frame (CRF) sessions are intended purely to provide astrometric observations useful in improving the current CRF, and in extending and densifying the CRF by observing "new" sources. CRF sessions are scheduled roughly 4 times per year.

NAVEX:

The (NAVnet-EXperimental (NAVEX) sessions are irregular sessions organized by the USNO for various reasons. Goals of previous NAVEX sessions have included technique improvement, investigation of various sources of systematic error, observations angularly close to the Sun as a test of General Relativity, and observations to extend the celestial reference frame. They use a variable VLBI network depending on the antennas available and the goal of the session.

ASTROMETRY:

Our earlier S/X band Southern Hemisphere VLBI astrometry program (Russell et al. 1994, AJ,107,379; Reynolds et al. 1994, AJ,108,725; Johnston et al. 1995, AJ,110,880) proved most successful in providing the fundamental Southern Hemisphere reference frame for the International Celestial Reference Frame (ICRF) (Ma et al. 1998, AJ,116,516), as well as the basis for the southern component of the ATCA reference frame currently in use. We plan to increase the number of reference frame sources significantly, both to better define the reference frame and to provide more sources for phase referencing observations. We note that much of the present VLBA observations are now undertaken in phase referencing mode. The astrometric observations will all be made in MkIIIA/MkIV mode and correlated in Washington. These will use Parkes, Hobart, Tidbinbilla, Hartebeesthoek, Kokee Park, Kashima and Fortaleza.

IMAGING:

Source structure becomes an important limit to the precision of VLBI reference frame measurements at sub-milliarcsecond levels, where the point-source approximation commonly made in astrometric analysis breaks down badly for many sources. The removal of these systematic effects requires imaging the individual sources, in order to model the structure and compute corrected observables (delays and rates) in the astrometric reduction. The structure in these compact sources often varies with time and, ideally, imaging at several epochs is required to define a time-dependent source model. A program of multi-epoch imaging of all the Northern sources of the Radio Optical Reference Frame (RORF) is currently underway using the VLBA, with promising results. We propose to extend this important work to the south by imaging at 8.4GHz, with milliarcsecond resolution, all flat-spectrum sources with catalogued flux densities S(5 GHz) > 1 Jy south of declination -20. These observations will be make using the S2 VLBI recording system and correlated in Sydney, Australia.

CORE-3:

The CORE-3 sessions marked the start of the regular CORE oberving program. These sessions will provide a bi-weekly 48-hour continuous data set from January 2001 through June 2001 (NEOS + CORE-3) and a weekly 48-hour continuous data set from July 2001 through December 2001. The CORE-3 sessions will be recorded in a Mark 4 mode which requires that the participating stations have Mark 4 capability.

CORE-1:

The CORE-1 sessions provide a monthly 72 hour continuous data set (CORE-1 + NEOS + CORE-3). These sessions will help reach CORE's goal of continuous data. The CORE-1 sessions will be recorded in a Mark 4 mode which requires that the participating stations have Mark 4 capability.

CONT-M:

The CONT-Mini1 & 2 sessions along with the NEOS-A413 session provides a 3 day continuous data set. This network runs simultaneously with the CONT-Mini 3, 4, and 5 network. The data set investigates the possible causes for the differences in Earth orientation parameters (EOP) measured with different VLBI networks. EOP results from previous measurements obtained simultaneously on two independent networks have shown larger than expected differences between the networks. The previous 24-hour sessions with simultaneous networks (mostly CORE-A/NEOS) have typically been spaced two weeks apart. Depending on the cause of the EOP differences, the EOP values from the two networks might be expected to track each other better when the sessions are run back-to-back, without interruption. By observing continously over 3 days with the CONT-mini networks, we therefore expect to obtain useful information about the origin of the EOP differences.

CORE-C:

CORE-C provides additional observing sessions for stations (Algonquin, Yellowknife, and Fortaleza) that have demostrated by their performance and their ability that they can participate in regular CORE sessions but do not have Mark IV capabilities.