The 20m radio telescope consists of a parabolical dish and a counterweight. Depending upon whether the gravimeter "sees" this arrangement head-on or from the side, the gravity attraction is maximum or minimum in these two respective extremes.

The mass distribution in the telescope can be approximated by a 2×45 ton dipole that rotates according to the pointing commands. These are frequent and may imply large slews especially during Geo-VLBI experiments.

The gravity attraction of the mass dipole is given by

where H, H

From Geo-VLBI schedules

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Exp. name Year Day UT start Year Day UT stop

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cl09sx1 2009 174 08:57:12 2009 216 08:48:43

eg040fon 2009 166 08:28:40 2009 166 22:30:27

ey008aon 2009 217 06:44:31 2009 218 08:06:25

f09m1on 2009 303 07:23:26 2009 303 10:50:46

r1385on 2009 180 16:30:09 2009 181 17:04:10

r1386on 2009 187 12:11:00 2009 188 17:06:23

rd0908on 2009 266 14:13:04 2009 267 17:57:06

rg001con 2009 184 11:36:36 2009 185 17:01:26

tl09c6 2009 230 12:04:50 2009 295 11:25:37

tl09x1on 2009 216 09:32:57 2009 217 07:46:34

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we have extracted time series of pointing to radio sources, right ascension and declination, and transformed the angles to the horizontal system.

The gravimeter record was reduced from tides and atmospheric effects, was band-pass filtered (Fig.1), downsampled to 30 s intervals, and chopped to retain sections synchronous with the VLBI schedules (Fig.2)

A least-squares fit of the variable factors in Eq. 1 gave the following results:

RESULTS: ADMITTANCES, LOCAL COPHASES

SITE/FILE: gdrc.ts lon/lat: 11.9260 57.3964

Normalized Chi^2 of fit : 4.57D-04, R= 2.14D-02 , X_1 X_2 = 1.00 2.30, Nev= 4

The error information below is compatible with a unit normalized Chi^2.

<rslist>d> rmsu= 2.138407516121582E-002 weff= 1.00000000000000 rchi= 1.00064262960750

SYMB #b Regression signal Admittance parameter +- 68.3% cfd

PSIN 1 <gdrc4pointing.mc> -0.000396 +-0.000343

PCOS 2 <gdrc4pointing.mc> 0.001146 +-0.000357

/ 3 Linear [uGal]/(12717*8.33333D-03[h]) 0.000058 +-0.000671

/ 3 Linear [uGal]/[year] 0.004822 +-0.055471

-W 4 Const 1.000D+00 -0.000003 +-0.000191

Remarkably, 1/2 arctan( PSIN/PCOS coefficients ), 1/2 arctan(-396/1146) = -9

Fig. 1 - Band-pass filter for suppressing drift and microseisms. The filter was applied on both the pointing and the

gravity time-series before the least squares fit. The sample frequency, 1/30 Hz, is denoted by fs (1/30 Hz in the experiment).

The band-pass filter is of window-design type. The pass-band is Fourier-integrated in the frequency domain, and the time series is windowed using a Kaiser-Bessel window with design parameter 2.1 (Harris, 1982) and a taper length of ± 60.

Fig 2.a -The whole range of pointing and gravity time series. The ordinate units are really uGal - sorry. The best-fitting amplitude of the pointing time-series has been increased with a factor 300. Figure 2b shows a short sub-interval

Fig.2b - suggesting that there are more important noises than antenna pointing effects. This serie was filtered more narrow than specified in the text. The actual filter had an upper corner frequency of 0.2 f

Fig 3 - The bearing angle of the gravity station as seen from the 20m radio telescope. This aerial photo (GoogleEarth) is too old to show the building of the gravity lab.