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Australia's Satellites & Programs > WESTPAC
WESTPAC - the Western Pacific Laser Tracking Network (WPLTN) satellite, launched in July 1998 and owned by Canberra-based Electro Optic Systems Pty Limited (EOS).
The WESTPAC satellite (developed and owned by EOS) was launched from Baikonour Cosmodrome in Kazakhstan on 10 July 1998.
The satellite was constructed for EOS by the Russian Institute for Space Device Engineering (RISDE) and launched by the Russian Space Agency (RSA).
The satellite itself is passive, and is used for EOS's space research programs to calibrate the performance of satellite laser ranging systems (SLR), in the development of free space optical communications and also in the study of the Fizeau effect that occurs when laser light is reflected from a satellite traveling at orbital velocities.
WESTPAC is also used by the Western Pacific Laser Tracking Network (WPLTN) in studies of geodesy, crustal deformation and plate tectonics, geodynamics, atmospheric science and earthquake prediction. The key element of these studies is accurately determining the position of centre of mass of the satellite (for orbit prediction and comparison) from satellite laser ranging data. The satellite orbit too must be very stable to maximise the accuracy of orbit prediction and measurement.
To provide a stable orbit the satellite has a high ballistic coefficient, (504.2 kg/m2), that is ratio of mass to cross sectional area is maximised. To this end the body of the satellite is 23.8 kg with a diameter of 245mm. On the surface of the satellite are sixty, evenly spaced, retro-reflectors, each separated by 26 degrees. These prism mirrors reflect any incident laser light back in the incoming direction, irrespective of the incident angle.
Accuracy in determining the space craft center of mass from laser range measurements depends on accurately positioned retro-reflectors, and these are placed at a distance of 91.0mm from the center of mass, to less than 0.5mm tolerance and also precisely aligned along the radial axis of the sphere.
For ranging accuracy it is also important that laser returns from one and only one retro-reflector are obtained. To facilitate this, the fields of view of the retro-reflectors (28mm diameter) are restricted by shielding behind tubular diaphragms, with a 20.5mm opening, placed 31.5mm in front of the retro-reflector itself, providing a 26 degree field of view or around the spacecraft body, laser light incident on the satellite will always be reflected, but by one reflector only. The prism retro-reflectors are aluminum coated.
Due to diffraction effects, the retro-reflectors return the incident laser light with a divergence of around 5 arc seconds for light at a wavelength of 550nm, which is about the same as the incident laser beam.
To minimise size variations due to temperature changes as the satellite travels from sun to shade during its orbit, WESTPAC is covered in special thermal stabilising white coating. In operation the WESTPAC satellite travels in free, non-orientated flight, at an altitude of 835 km above the Earth's surface.