*************************************************************************** Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette No 05, 1 August 1996 *************************************************************************** Title: VLBI Continuous Observations of the Rotation of the Earth Author: Chopo Ma, VLBI Coordinator for IERS Dear Colleagues, The appended announcement from the Goddard VLBI group should be of interest to researchers using Earth orientation parameter (EOP) time series. This new research effort is designed to provide continuous EOP from VLBI with accuracies better than what is now routinely available once per week. Another item is the NASA Research Announcement NRA-96-MTPE-05. This includes Earth rotation and angular momentum of Earth systems. The text of the NRA is available as of July 29 on the Mission to Planet Earth Internet home page "http://www.hq.nasa.gov/office/mtpe/" under "MTPE Research Announcements". This NRA may be a vehicle for supporting research using EOP, including the future continuous data. Proposals must be submitted before the end of September. Chopo Ma VLBI Coordinator for IERS *************************************************************************** Dear Colleagues, The purpose of this message is to inform you about a new project that will become our primary focus over the next few years. The goal of the project is to understand the Earth's integrated response through continuous VLBI measurements of Earth rotation. By capitalizing on the Mark IV technology development now nearing fruition, the project will provide a wealth of data for Earth science research. VLBI's historical accomplishments include the first detection of contemporary plate motion, initial measurements of broad crustal deformation zones, and accurate measurements of nutation, polar motion, and UT1, which provided insights into the shape of the Earth's core and effects seen in atmospheric angular momentum. The advent of GPS enabled densification of measurements in areas of high tectonic activity. GPS is now assuming the bulk of the terrestrial reference frame measurements. It is time for us to move on to another project which VLBI can uniquely address. My paper at the WEGENER meeting in Oporto in June presented these ideas to the international community (Attachment 1). We suggest that the new project be called CORE -- Continuous Observations of the Rotation of the Earth. A draft outline of the project is in Attachment 2. The project is very ambitious. It will require continued coordination and cooperation among all our U.S. and international partners. It will also mean a change in the way we observe and acquire data, process the data, and analyze and distribute results. A more detailed description of the project is being prepared now. We solicit your input about the new project. We welcome any comments or questions you have at any time. We will keep you informed about our progress and we will be asking for your assistance during the transition period. We are very excited about the new project. We feel it can give new life to a beleaguered VLBI effort, and it will provide an exceptional data set for Earth science research. Best regards, Tom Clark NASA/GSFC Attachments: 1. VLBI Programs for the Next Millennium. 2. Continuous Observations of the Rotation of the Earth (CORE). ========================================================================== ========================================================================== Attachment 1 Summary of paper presented at WEGENER meeting, Oporto, Portugal June 5, 1996: VLBI PROGRAMS FOR THE NEXT MILLENNIUM Dr. Thomas A. Clark NASA Goddard Space Flight Center In 1967 the first astronomical VLBI measurements were made. By the mid- 1970's, geophysical sciences began to recognize the capabilities of the technique. In NASA, the newly-formed Crustal Dynamics Project served as the focal point for the development of the Mark-3 VLBI system. At that time, many of the principal scientific questions to be solved were related to plate tectonics: - Can we observe plate tectonics in "real-time"? - Do the contemporary rates of plate motion agree with average motions over geological time scales? - Is plate motion a smooth or episodic process on decade time scales? - How are motions distributed along tectonic plate boundaries? - Are the plates rigid? By the 1980's VLBI had provided exquisite answers to these questions. Yes, we could see plate motions. The VLBI-determined velocity (for North America-to-Europe motions) was in good agreement with geological models but was ~10% smaller than the Minster-Jordan RM-2 model for the North America-to-Pacific velocity. The concurrent re-examination of the million-year geological data resulted in the NUVEL-1 plate motion model which was in substantial agreement with VLBI. Mobile VLBI observations in western North America (including Alaska and Canada) defined a broad zone of deformation that extended well to the east of the San Andreas Fault into the Basin and Range area. In Alaska, motions associated with subduction island arcs and the docking of terrains were measured. Elastic deformation resulting from two distinct earthquake events was measured. The interior regions of the North American and European plates were found to be stable at levels <1 mm/yr. During this same period VLBI became the technique of choice for measuring the connections between the inertial celestial reference frame and the rotating earth -- polar motion, UT1, precession and nutation. The rotation of the Earth seen by VLBI was found to be closely correlated with the integrated angular momentum of the earth's winds. The motions of the Earth provided significant new insight on the details of the ellipsoidal core in the Earth's deep interior. In the 1980's and 90's many organizations around the world began to develop VLBI capability. Now we see 30-35 different stations producing geodetic results each year and ~10 stations participating in more than 25 experiments during each year. Many of these stations were built specifically to support geodetic programs and many are radio astronomy observatories willing to make their telescopes available to the geophysical community. In the nearly 30 years since the first astronomical VLBI programs, the geodetic capabilities of VLBI have improved by a factor of ~10 each decade. Tectonic velocities for some sites are now known with uncertainties as small as a few hundred microns/yr. UT1 and the length of a day are determined to a few microseconds of time. Polar motion, nutation and the positions of extragalactic radio sources are measured to accuracies of better than 100 microseconds-of-arc. In the past few years, GPS has replaced VLBI for determination of regional-scale (<1-2000 km) plate deformation and is producing spectacular results in global-scale tectonics. Low-cost unmanned GPS stations (each costing 2 to 3 orders of magnitude less than a VLBI station) have been deployed in a full-time, permanent global network of more than 100 stations. Regional arrays in seismically active areas (southern California, Japan) with hundreds of GPS stations are under development. Now, and into the next millennium, the role of VLBI is changing. The extended bandwidth of the new Mark-4 VLBI system will allow for unprecedented measurement precision. The new Mark-4 correlator capabilities (up to 16 stations) will permit larger networks of stations to participate in measurements. A number of other evolutionary changes in VLBI technology have allowed some individual stations to routinely produce data with an order-of-magnitude better precision. Those stations of the world that routinely produce very high quality VLBI data will be used to provide very high accuracy Earth orientation data. We have been examining the possibility of having continuous VLBI data from ~20 stations. Each station would observe once or twice per week within a network of 4-6 stations. Each network would "hand over" the measurement chores to another network on a daily basis. These continuous data sets would then be used to monitor "high bandwidth" variability of the Earth due to atmospheric and oceanic circulation changes, ice loading, and transient events. Occasionally -- a few times per year -- the stations which are not able to commit to full-time operation or which have not yet been able to improve their performance will join the high-accuracy "core network" for a "snapshot" measurement of the Terrestrial Reference Frame to continue VLBI's tectonic legacy. ========================================================================== ========================================================================== Attachment 2 Continuous Observations of the Rotation of the Earth (CORE) with an International VLBI Network DRAFT OUTLINE July 9, 1996 Science Goals and Accuracies: Continuous recording of the integrated effect on the Earth's orientation from all causes. Multidisciplinary research projects will be possible. The data are sensitive to AAM (atmospheric angular momentum) to very short periods. Scientists can study continuous momentum exchange among the oceans, the atmosphere (wind), and the solid Earth. An early detection of El Nino may be possible. Detection of the predicted Earth libration is likely. It may be possible to capture an earthquake signal in Earth rotation data. The geometry and dynamics of the Earth's core will be seen in the nutation time series. The data set will enable a high accuracy (< 1 milli-arcsec) celestial reference frame (CRF). Expected EOP accuracies are: UT1 ~3 micro-sec, polar motion ~80 micro-arcsec (daily values). Hourly accuracies will be ~3 times lower. Even better accuracies (sub-microsecond UT1, 20 micro-arcsec polar motion) would be possible with more tape and/or more stations. Observing Scenario: Designate seven different Mark IV networks, one for each day of the week. One network would be the present NEOS. Each network would include 4 stations, more if resources permit. Each network observes for 24 hours on one day of the week, then "hands over" the job to the next network. Participating stations would normally dedicate one or more days per week to observing. We could accommodate stations that cannot dedicate the entire year to weekly observing but must reserve some time for other commitments such as astronomy observing. We will also try to coordinate station commitments for less regular observing on any schedule that works best for the station. Long baselines are required for high accuracy Earth rotation measurements. Global station coverage is required for terrestrial and celestial reference frames. We will obtain geodetic ties between the daily networks with simultaneous observing once per month. Recording mode will be Mark IV, bit rate 0.5 Gb/s, using 4-5 thin tapes/day at each station. The Mark IV correlator processes 24 hours of these observations in ~15 hours. International Collaboration: We are seeking international partners to join the CORE network. Commitment required of partners: Upgrade station data acquisition system to Mark IV. Demonstrate and maintain requisite standard of high technical performance. Upgrade station timing system to achieve absolute station timing at ~30 nsec level. Provide 24 hours of observing time on a regular basis, weekly if possible. Ship tapes to the Mark IV correlators in a timely, reliable manner. Purchase thin tapes as a contribution to the community tape pool. Participate in IGS, including acquisition of daily GPS data. Contribute some Mark IV correlator support. Each country contributes according to its resources and scientific/ technical interest. Required Resources and Performance Standards: The project requires Mark IV correlators and 15-20 Mark IV stations. Needs at least 28 station-days per week from the global VLBI network, for continuous data. Purchase of 500-1000 thin tapes. Tape shipping: 4-5 tapes/day/station. Stations participating in the network must maintain high technical performance standards at all times. Transition Plan: CONT96 (24 days spanning September-November, 1996): The longest span (11 weeks) of high resolution EOP data yet obtained. Sensitive to all tidal periods from 1 hour to 77 days. Simultaneous networks will assess underlying accuracy. We will hold a workshop in fall 1996 to refine the science goals and to obtain scientific community support. Obtain commitments from international partners during 1996-7. During 1997, observing emphasis will be on Earth rotation, using Mark III capabilities. We will evaluate the consistency and accuracy of measurements made using different networks. We will study network design using simulations to optimize the science goals. We will develop high technical performance standards for network stations. Upgrade stations to Mark IV capability and upgrade station timing systems during 1996-8. Finish construction of the Mark IV correlators, with first fringes in early 1997 and operational status ~1 year later. The project will evolve towards continuous Mark IV observing gradually, beginning with a few times per week and building up as resources permit.