WESTPAC (Western Pacific Satellite)
WESTPAC, formerly known as WPLTN (Western Pacific Laser Tracking Network) satellite, is a commercial Australian communications and geodetic research satellite owned by Electro Optic Systems Pty Limited (EOS), Queanbeyan, NSW (New South Wales), Australia. WESTPAC is a passive satellite, the objective is to enhance the contribution of satellite laser ranging (SLR), in particular to space geodesy in the Western Pacific, to serve in the development of free space optical communications, and also to study the Fizeau effect that occurs when laser light is reflected from a satellite travelling at orbital velocities. Its primary mission is to act as a target with intrinsically very stable range biases for calibrating ground station biases and satellite signatures. 1) 2) 3)
Background: At the Western Pacific Laser Tracking Network (WPLTN) Executive meeting in Moscow on Dec. 2, 1995, it was announced that Electro Optic Systems (EOS) had entered into a joint project with the Russian Space Agency (RSA) to construct and launch a new SLR satellite (WPLTN-1) designed to overcome the limitations of all present targets in relation to millimeter geodesy.
Armand Fizeau (1819 - 1896) is a French physicist noted for the first experimental determination of the speed of light in 1849. He used a beam of light reflected from a mirror 8 km away. The beam passed through the gaps between the teeth of a rapidly rotating wheel. The speed of the wheel was increased until the returning light passed through the next gap and could be seen. Then “c” was calculated to be 315,000 km/s. Leon Foucault improved on this a year later by using rotating mirrors and got the much more accurate answer of 298,000 km/s. Fizeau's technique was good enough to confirm that light travels slower in water than in air.
The spacecraft was designed and built by the RISDE (Russian Institute of Space Device Engineering) of Moscow. A wide range of research capabilities have been included into the S/C design, in addition to its communications function.
The WESTPAC satellite is spherical in shape (24.5 cm in diameter brass body, mass of 23.8 kg) covered with 60 recessed laser reflectors mounted and evenly spaced to its outer surface (each separated by 26º). The retroreflector design utilizes the Fizeau effect to correct for velocity aberration effects instead of normal dihedral angle spoiling techniques. The Fizeau corner-cubes have a refractive index of 1.62, they were built by RISDE. Each cube is recessed so that the average 0.7 cubes retroreflect on any given shot. The prism mirrors reflect any incident laser light back in the incoming direction, irrespective of the incident angle. The distinguishing features of the WESTPAC design include: 4)
• Only a single corner-cube reflects on any shot; in fact on average only 0.7 cubes are active
• Its response is optimized for 1.54 µm wavelength, to provide for fully eyesafe ranging at any power
• A new process was developed to obtain the center-of-mass correction within 0.5 µm accuracy (the retroreflectors are positioned 91.0 mm from the center of mass)
• The corner-cube design and material assumes the Fizeau Effect which, if real, will decrease return signal levels dramatically at 532 mm wavelength.
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.
Figure 1: Illustration of the WESTPAC-1 satellite (image credit: EOS)
Launch: WESTPAC was launched as a secondary payload to RESURS-O-4 on July 10, 1998 on a Zenit launcher from Baikonur, along with other secondary payloads [FASat-Bravo (Chile), TMSat (Thai , SAFIR-2 (OHB Bremen), and TechSat/Gurwin-II (Israel)].
Orbit: Sun-synchronous circular orbit (LEO), altitude = 835 km, inclination = 98.86º, period =101 minutes. The satellite has a high ballistic coefficient (504.2 kg/m2), maximizing the ratio of the mass to the cross-sectional area. The life expectancy of WESTPAC is on the order of several decades.
The WESTPAC satellite is being tracked by the ILRS (International Laser Ranging Service) network of ground stations. The SLR data is being archived and distributed for the space geodesy and geodynamics community. 5) 6)
Figure 2: Alternate illustration of WESTPAC (image credit: EOS)
Table 1: Westpac parameters (Ref. 2)
The passive spacecraft with its onboard retroreflector array is functional for SLR tracking in 2012 with an expected life of many decades.
1) Information provided by C. Smith of EOS Pty Limited, Canberra, Australia
4) “WESTPAC Satellite: The Scientific-Technical note for user,” of RISDE, 1997, URL: http://www.dgfi.badw-muenchen.de/docs/Westpac_final.pdf
5) M. Rutkowska, “Satellite WESTPAC influence of the gravity field on the accuracy of orbit estimation,” Artificial Satellites - Journal of Planetary Geodesy,” (ISSN 0208-841X), Vol. 38, No. 3, 2003, pp. 97 - 104
6) M. Rutkowska, R. Noomen, “SLR Systems Characteristics in WESTPAC, LAGEOS-1 , and LAGEOS-2 Observations,”12th International Workshop on Laser Ranging, Matera, Italy, Nov. 13-17, 2003, URL: http://cddis.gsfc.nasa.gov/lw12/docs/Rutkowska_et_Noomen_poster3g.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.