A key objective for Westpac 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 must be very stable to maximize 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 maximized. To this end the body of the satellite is made from a dense material.
The total mass 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 centre of mass from laser range measurements depends on accurately positioned retro-reflectors, and these are placed at a distance of 91.0 mm from the centre of mass, to less than 0.5 mm tolerance and also precisely aligned along the radial axis of the sphere.
To achieve ranging accuracy laser returns can be achieved from one and only one retro- reflector at any time, and from any angle of acquisition. To achieve 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 acceptance angle for each retro-reflector. As the retro-reflectors are spaced 26 degrees around the spacecraft body, laser light incident on the satellite will always be reflected, but by one reflector only.
The prism retro-reflectors are aluminium coated. Due to diffraction effects the retro-reflectors return the incident laser light with a divergence of around 5 arcseconds for light at a wavelength of 550nm, which is about the same as the incident laser beam.
To minimize size variations due to temperature changes as the satellite travels from sun to shade during its orbit, WESTPAC is covered in a special thermal stabilizing white coating. In operation the WESTPAC satellite travels in free, non-oriented flight, at an altitude of 835 km above the Earth’s surface.
The satellite is passive with an expected useful lifetime of at least 500 years at its 820 km orbit. Westpac is used for EOS’s space research programs to calibrate the performance of optical tracking systems, in the development of free space optical communications and also advanced studies into optical phenomena in space, such as the Fizeau effect that occurs when laser light is reflected from an object travelling at orbital velocities.
The satellite is dual use, and can provide data to infill the database of precision orbital data for international studies of geodesy, crustal deformation, plate tectonics, geodynamics, atmospheric science and earthquake prediction.
Westpac is occasionally licensed by EOS for use by collaborating agencies for space research.