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Determination of the relative orientation between radio and dynamical reference frames using VLBI observations of spacecraft

N. V. Shuygina

Institute of Applied Astronomy, Russian Academy of Sciences, 8 Zhdanovskaya st., St.Petersburg, 197042, RUSSIA


The experience of determination of the link between radio sources and dynamical reference frames is presented. To obtain this connection we used a series of very long baseline interferometry (VLBI) observations of the spacecraft Phobos-2 and angularly nearby quasars. Orbit determination of the spacecraft was carried out by numerical integration of relativistic equations of motion taking into account all major planet perturbations. The coordinates of perturbing planets were calculated by the DE200/LE200 ephemerides. The dynamical reference frame is determined by equator and equinox of these ephemerides. The resulting spacecraft orbit was fitted to the VLBI and tracing (range and Doppler frequency shift) data by means of weighted least squares procedure with the uncertainties at the level of a few kilometers. After precise orbit determination using VLBI measurements only we obtained three rotation angles between two systems. The radio and ephemeris dynamical frames were found to be coincident in all three axis to less than 100 milliarcseconds.


1. Introduction

In order to specify positions of astronomical objects it is necessary to have a reference frame. Celestial reference system may be defined kinematically (through the positions of extragalactic radio sources - EGRS), or dynamically (through the geocentric or heliocentric motions of artificial satellites, the Moon, and planets). Although we may have a conceptual ideal reference system, each realization of reference frame will be somewhat different, and there will be a need to determine the transformations from one reference frame to another (Moritz & Mueller (1987)). Table 1 shows several conventional quasi-inertial reference systems with examples of possible connection.

Table 1: Examples of linking different reference frames with their accuracies.

Nowadays the most accurate reference frame is constructed on the basis of EGRS positions defined by methods of radio interferometry with very long baselines (VLBI). The internal accuracy of such systems is of the order of . To obtain the connection between the frame mentioned and practically used dynamical frame, differential VLBI observations of spacecraft and angularly nearby quasars are used (Newhall et al. (1986)). We have analysed such measurements of the spacecraft Phobos-2 to link radio sources and dynamical reference frames.

2. Method

The relative orientation of the radio and dynamical reference frames can be described by a sequence of three rotations

where is barycentric unit position vector of quasar in the radio reference frame, represents the same vector in the dynamical frame, are well-known matrices of rotation about , , axis respectively.

The quasar coordinates are considered to know exactly from some extragalactic source catalogue Sovers et al. (1988). Thus calculated time delay of a quasar may be written as a function of the rotation angles (in parentheses we represent parameters to be obtained during differential correction process only)

The time delay for a spacecraft , described as a function of time and initial apparatus coordinates and , is

The calculated differential time delay as the difference between the quasar and the spacecraft delays is therefore given by

The observed interferometry time delay ,

is represented by a function of the parameters and their first-order corrections. After linearization process we obtain an equation of condition in the form


Using theoretical expressions for a quasar (Hellings (1986)) and a spacecraft time delays we get directly the condition equations. The system of equations 2.6 being written for all VLBI observations allows us to calculate orientational parameters and corrections to the position and velocity of the spacecraft. But in an orbit determination process these data are to be supplemented with another type of observations, for example, range measurements. Thus processing of the real observations divides into two parts: firstly from all radar and interferometry data we determined a precise orbit of the spacecraft, and then from the differential VLBI measurements only we obtained orientation parameters system of interest.

3. Observations

In order to obtain a mutual orientation of radio source and dynamical reference frames we used a complete set of all available VLBI observations of the spacecraft Phobos-2 received within the joint experiment ``Phobos'' in 1988-1989. During 150 days from ``Deep Space Network''stations 13 differential interferometry observations of apparatus Phobos-2 on the background of 4 quasars were received. Barycentric coordinates of this quasars are given in Table 2.

Table 2: Coordinates of the quasars.

These observations were supplemented with 175 radar data (time delay and Doppler frequency shift) received during the same time interval from 3 stations on the territory of the former SU. The information on all observations is presented in Table 3.

Table 3: Information on observations of the spacecraft Phobos-2.

4. Results

Orbit determination of the spacecraft was performed by numerical integration of the relativistic heliocentric equations of motion, taking into account perturbations of all major planets and Schwarzschild's terms due to the Sun. For the calculation of coordinates of perturbing planets and the Moon the DE200/LE200 ephemerides were used. So the dynamical reference frame under consideration is determined by equator and equinox of these ephemerides.

The orbit was computed using a linearized weighted least squares estimation algorithm. Root mean square (RMS) residuals given in Table 4 indicate how well the resulting orbit is fitted to various observations and the convergence of the iteration process. So on the first step we define a precise orbit of the spacecraft or in other words dynamical reference frame.

Table 4: The RMS residuals.

Table 5: Resulting () for VLBI observations.

After that, using differential VLBI observations only one can obtain three rotation angles of radio sources and dynamical reference frames. By means of least squares procedure we have got the following values of rotation angles

with correlation between them , , . The final individual residuals for all VLBI measurements are illustrated in Table 5.

5. Conclusion

In this paper we have analysed all available differential VLBI data of the spacecraft Phobos-2 to get a connection between the dynamical and radio sources reference frames. The calculations can be considered as preliminary ones but we have just confirmed that even so small a number of such measurements gives us the opportunity to obtain reasonable values of rotation angles. We hope that future projects and also the increase of observational accuracy permit us to achieve more precise and reasonable results.


The author expresses sincere thanks to Dr G.A.Krasinskii for the attention he paid to this work and to the Russian Foundation of Fundamental Investigations for the travel grant which enables the participation in the conference.


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Wed Feb 22 22:10:17 GMT 1995