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IAA Transactions, No. 8, ``Celestial Mechanics'', 2002
Analysis of SLR observations of the Etalon geodetic
satellites
T. V. Ivanova, N. V. Shuygina
Institute of Applied Astronomy, St.Petersburg, Russia
This paper is devoted to the results of an analysis of satellite laser range ob­
servations of two Russian Etalon geodetic satellites. They were launched in 1989
(in January and May, respectively). They have near--circular orbits (eccentricities
are equal to 0.0007) with semi--major axis of about 25000 km and inclination of
65 ffi with respect to the Earth's equator, each orbit lying in one of the orbital
planes used by the Glonass Navigation System. Etalon1&2 laser range observa­
tions are available since June 1, 1992. All range measurements were taken from
the Crustal Dynamics Data Informational System (CDDIS) and European Da­
ta Center (EDS). Each measurement represents a normal point produced from
two--way ranges averaged over 2­minutes interval. A number of Etalon1&2 ob­
servations is much less than that of Lageos satellites because the height of the
Etalon orbit is three times more than the height of the Lageos orbit. A total
number of an approximately 10 years period of observation is about 46000 for
each satellite.
These observational data were analyzed by means of the problem--oriented
programming system for ephemeris astronomy ERA (Ephemeris Research in As­
tronomy) [1], which follows basically the IERS Conventions [2]. Initial site posi­
tions were taken from ITRF2000 solution. Transformation from the Terrestrial
Reference Frame to Celestial Reference Frame is carried out using IAU (1976)
precession, IAU (1980) nutation, celestial pole offsets and Earth rotation param­
eters taken from EOP (IERS) C04 series.
The a priori accuracy varies within considerably wide ranges: from a few
millimeters to about 1 meter. Moreover, there are many misprints in source da­
ta which cannot be corrected automatically. That is why the observations were
weighted anew according to the special procedure depending on the real quality
of station.
The data analysis is performed in several steps. The whole time span of about
10 years was divided into 21 days arcs. It turned out that 21--day interval is
the optimal period of time with sufficient number of normal points. At the first
90

step six coordinates and velocities and along--track accelerations, and reflection
coefficients were adjusted. The data fitting was produced using the least squares
method. Usually it was sufficient to perform two or three iterations to achieve
the convergence. The average value of the post--fit residuals is about 7 cm.
At the next step the global improvement was done over the whole time span.
At this stage the improved satellite orbits were used to obtain both corrections
to the station coordinates and Earth orientation parameters.
The next step of the investigation is to obtain the lowest order harmonics
of geopotential by means of Etalon1&2 SLR observations. Meanwhile there ap­
peared some difficulties in taking into account the accurate radius correction and
its dependence on the system of observing laser station. The corrections to the
station coordinates prove to be in agreement with that received from the process­
ing of Lageos measurements.
References
1. Krasinsky G. A., Vasilyev M. V. ERA: knowledge base for ephemeris and
dynamical astronomy. In: Proceedings of IAU Colloquium 165, Poland, 1996,
239.
2. Mc. Carthy, IERS Conventions (1996), IERS Technical Note 21, Obs. de
Paris, 1996.
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