Introduction

1. Service.
2. Science.
3. Research and development.

1. regional tectonic: EUROPE, APSG, SYOWA-ANT, CORE-OHIG, JADE, JAPAN-TIES;
2. celestial reference frame: RDV, SURVEY, CRF;
3. research and development: CONT-M;
4. additional EOP points: CORExx, IRIS-S.

How should we change our observing schedule in 2002?

Proposal

1. two mini-CORE: 16 days of continuous observations of EOP.
   Scientific goals:
   1. to detect anharmonic signal in UT1 and polar motion or at least to set the upper limit of such a signal.
   Analysis of residuals in EOP with high resolution after removal of harmonic sub-daily signal caused by ocean tides reveals some signal [1]. Visual inspection of plots of hourly estimates of UT1 and polar motion as well as their wavelet transform shows some systematic signals. Is that systematic signal real or not is unclear. Theory does not predict significant non-tidal contribution to UT1 and polar motion but also does not ban its existence. What is the level of non-tidal high frequency EOP signal, what is its nature is unresolved scientific problem.

   Solving this problem promise not only new scientific results but has important impact on EOP service. If there is a non-negligible anharmonic, non-tidal high-frequency constituent in UT1 and polar motion then continuous observations of EOP are necessary. If not, then moderate gaps in data do not deteriorate quality of EOP product.

   In order to detect anharmonic signal in EOP we have to observe it continuously. Even weekend gaps make interpretation of the obtained signal problematic.

   2. to confirm or refute the sequence of S₁, S₂, S₃ ... S_n peaks in the sub-daily band of EOP spectrum discovered in GPS analysis [1]. The "saw-tooth pattern" on the spectrum looks suspicious. Rothacher at el. wrote (p. 13736) that "the origin of these terms is unclear. We expect it to be an artifact in the GPS series". They continues (page 13735) "we have seen that the orbital effects (typically with a period of 12 hours) might be responsible for the signal seen in the immediate vicinity if the 12- and 24-hour periods". In a conclusion the authors noted that "further studies are needed to clarify this issue".

      Analysis of sparse VLBI data in 1996 by J. Gipson [2] and later in 2000 by L. Petrov [3] did not reveal signal at S₃ and S₄. However, analysis of VLBI and GPS data was done quite differently. Huge gaps, about 50% of the time interval, makes interpretation of the results more difficult. It is attractive to have simultaneous full data series, without gaps, and to analyze them by using exactly the same numerical procedures. Comparison of such analyzes will give us much stronger arguments in favor of the hypothesis that the S_n comb is an artifact of GPS. Or, who knows, maybe VLBI would confirm its existence.

      GPS results indicates that S₃ may have the amplitude 2 musec. Current accuracy of hourly estimates of UT1 is better than 10 musec. Analyzing 384 hours of observations would allow us to determine S₃ with precision better than 0.5 musec what is sufficient for detection of the signal under consideration.

   Mode: the same as it was originally proposed for CORE: each days has its own network.

2. Golden nutation sessions.
   Scientific goal: to provide monitoring of anharmonic component of nutation spectrum: FCN and, presumably, annual, atmosphere-driven component of nutation.

   20 years of VLBI observations allowed us to determine amplitudes of forced nutations with precision of 5-20 muas. Additional observations do not promise to improve the accuracy of determination of these harmonic terms significantly. At the same time, we can expect that there is anharmonic constituents of nutation spectrum around retrograde FCN frequency and observations confirm it. Therefore, monitoring free core nutation as well as variable annual and, probably, semi-annual constituents is necessary.

   Mode: monthly 12-14 station experiments. Since we already have 6 RDV experiments, we have to put 6 other experiments in the gaps between RDV.

3. Gravitation experiment.
   Scientific goal: to measure the speed of gravity propagation.
   Mode: main 9-station session around 2002.09.08 (Sunday) 16:00 UT and two auxiliary sessions. [4]

Resources. CORE-IRIS, CORE-1, CORE-2, CORE-3, CORE-4 in 2002 would take 648 station days, 326 tapes and 237 processing days.

Each 16-days long mini-CORE session, would take 256 tapes and 128 station/days, 48 processing days. After 16 days mini-Core sessions correlators need to work two months for processing only the backlog and incoming NEOS-A. Unfortunately, due to the tapes shortage we cannot make 32-days long session instead of two 16-days long.

6 14-stations golden nutation sessions require 84 station/days, 18 processing days, 54 tapes.

3 9-stations gravitation experiments require 27 station/days, 7.5 processing days, 18 tapes.

Total resources: 367 station/days, 122 processing days, 256 tapes -- two times less than in the CORE-IRIS, CORE-1, CORE-2, CORE-3, CORE-4 scenario. We see that we can achieve much more ambitious goals by concentrating resources in time (mini-CORE) and in space (golden nutation). In the future we can steadily increase the length of mini-CORE sessions and finally have them 365-days long. Retrospectively we can notice that 14 days long CONT94 sessions gave more science than 4 years of pre-CORE observations. I believe the strategy of concentration of resources is more advantageous than the strategy of their uniform distribution.

I propose to use remaining resources for other scientific programs and for research and development. Some portion of CORExx/IRIS sessions can be converted to regional tectonic sessions.

Research and development.

Objectives of research and development sessions are

1. Investigation of instrumental errors and the way of their reducing and/or calibration.
• nodding observations for calibration of phase delay observables and investigation of apparent group delay errors;
• special polarization experiments;
2. Validation of new models.
   For validation of new atmosphere model we can consider making observations in non-standard schedules, i.e.:
   1. observing at sub-networks with very fast antennas;
   2. observing at sub-networks with antennas with very low elevation limit;
   3. nodding observations.
3. Testing of new schedules. How can we exploit better sensitivity of Mark-4 system?
   • Higher SNR versus greater number of scans.
   • Parallel sessions at several subnetworks like CONT-M.
   • Investigation of phase referencing technology (one of the applications: to observe interplanetary spacecrafts).

References

1. M. Rothacher, G. Beutler, R. Weber, J. Hefty. "High-frequency variations in Earth rotation from Global Positioning System data", JGR, vol. 106, B7, p. 13711-13738, 2001.
2. J. Gipson, "Very long baseline interferometry determination of neglected tidal terms in high-frequency Earth orientation variations", JGR, vol. 101, p. 28051-28064, 1996
3. L. Petrov, "Estimation of EOP from VLBI: direct approach", in Proceedings of the IAU Colloquim 180, ed. by K. J. Jonston, Washington, D.C., 2000, p. 254-258.
4. L. Petrov "Gravitational VLBI experiment on 08 September 2002", Internal memo, 2001, URL: http://gemini.gsfc.nasa.gov/pet/discussion/grav/grav.html