Measurement of the LAGEOS-I Spin Axis

Measurement of the LAGEOS-I Spin Axis

Douglas G. Currie, Kenneth E. Kissell, Petras V. Avizonis, Dennis D. Wellnitz
Astro-Metrology Group
Deptartment of Physics
University of Maryland at College Park

The Astro-Metrology Group at the University of Maryland at College Park has developed methods of measurment of satellite spin dynamics based on experimental measurements made by the group on the LAGEOS-I geodedic satellite using the Goddard Space Flight Center's 48" telescope.

The Problem:

The LAGEOS series of satellites are essentially laser ranging satellites placed in high earth orbits, with no active electronics to 'broadcast' the current state of the satellite. Recent interest in using these satellites to measure the Lens-Thirring effect (General Relativistic Effect) has defined the need for knowledge of the current dynamics of the satellite.

Our Approach:

Through the use of both high speed photometry and simple video data which provide 30Hz photometry, it is possible to obtain sufficient information passively to determine both the rotation rate as well as the orientation of the satellite spin axis. The cartoon figure below shows the geometry nescessary to see a flash from fresnell reflection of the sun off of the front surface of a retro-reflector (CCR) in the satellite's skin. From this, and knowledge of the satellite design (lattitudes of concentric CCR bands, number of CCR's in a band) and photometry rate, it is possible to uniquely determine the axis orientation.


The figure below represents the results of six separate measurements of the satellite's spin axis. The actual solutions provide the RA and Dec of the axis, which are fit independantly as a functions of time. This generates a smooth curve (based on the assumption that the motion of the axis is smooth and continuous on a time scale of days) with appropriate time intervals between measurements. When combined, we get a polar representation of the precession/evolution of the spin axis orientation. .

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