Laser Retroreflector Array Development for STSAT-2

STSAT-2(Science and Technology Satellite-2) is a microsatellite to experiment advanced satellite technology and research space science and now has been completed the flight model development. STSAT-2 will be launched by KSLV-1 at Naro space center of Korea in 2009 and injected an elliptical orbit having 300km at the perigee and 1500km at the apogee. It has three missions: as the first mission is to predict total precipitable water using microwave radiometer (DREAM); the secondary mission is to measure precise distance between STSAT2 and SLR ground station for precision orbit determination of STSAT-2; the last mission is to verify the space core technologies of bus. STSAT-2 is equipped with a laser retroreflector array having hemispherical configuration with one-nadir-looking corner cube in the center and surrounded by an angled ring of eight corner cubes like Shunzhou-IV, ERS-1&2, and Envisat. This paper introduces STSAT-2 mission and describes the development result of LRA. Introduction STSAT-2 is the first Korean satellite, which is equipped with laser retroreflector array for SLR. The objectives of STSAT-2 consist of three missions, which are the domestic development of a low earth orbit 100kg satellite which will be launched by KSLV-1(Korea Space Launch Vehicle-1) from the domestic space center (Naro Space Center), the development of advanced technology for small spacecraft, and the development and operation of world-class space science payloads (Jun Ho Lee, 2005). STSAT-2 has an elliptical orbit having 300km at the perigee and 1500km at the apogee. Some period among one year has partially sunlight period without eclipse. The minimum eclipse time has 0% and the maximum eclipse time about 35% (Kyunghee Kim, 2007). STSAT-2 has two payloads: the main payload, microwave radiometer (DREAM, Dualchannel Radiometer for Earth and Atmosphere Monitoring) and the secondary payload, laser retroreflector array. The DREAM mission objectives are to acquire brightness temperature of the liquid water from Earth atmosphere and surface and to predict the total precipitable water after post-processing. The mission objective of spacecraft technologies is to develop a thermally, mechanically, electrically stable and radial resistant spacecraft system having high precision attitude determination and control capability in a high eccentric ellipsoidal orbit. For spacecraft technology experiments, STSAT-2 has the following instrumentations onboard: (1) Pulsed Plasma Thruster (PPT) Proceedings of the 16th International Workshop on Laser Ranging 635 (2) Dual-Head Star Tracker (DHST) (3) Fine Digital Sun Sensor (FDSS) (4) Compact on-board computer (5) High-speed data transmission (10Mbps) Figure 1 shows the STSAT-2 configuration and table 1 summarizes the specifications of STSAT-2 (SaTReC, 2005). Figure 1. STSAT-2 configuration Table 1. STSAT-2 specifications Items Values Items Values Orbit 3001500km, Circular orbit Inclination : 80 Expected Life 2 years Launch Date July 31, 2009 (STSAT-2A) January 31, 2010 (STSAT-2B) Payloads DREAM : 10.66kg LRA : 0.82kg RF Link Margin S-band : 3dB @ 10deg X-band : 3 dB @ 10deg Attitude Control 3-axis stabilization Power 200W @ BOL Weight 100kg Eccentricity 0.082435 Period 102.998min Satellite Laser Ranging Mission The SLR mission objectives are to determine the more precise orbit of STSAT-2 than possible with S-band tracking data alone, to calibrate the main payload DREAM, and finally to support the science research such as earth science and geodynamics. Additionally, the precise orbit determination of STSAT-2 can be used to evaluate the performance of the first Korean launcher (KSLV-1). SLR data of STSAT-2 will provide the unique opportunity to investigate the variations of an ellipsoidal orbit. Laser Retroreflector Array LRA consists of nine corner cubes and symmetrically mounted on a hemispherical surface with one nadir-looking corner cube in the center and surrounded by an angled ring of eight Proceedings of the 16th International Workshop on Laser Ranging 636 corner cubes like Shunzhou-IV, ERS-1&2, Envisat, ALOS, GFO-1 and Jason-1. LRA configuration is shown in Figure 2. The diameter of corner cube is 31.5mm and the height 22.3mm. Corner cubes were made of fused silica. Corner cube assembly consists of corner cube housing, corner cube, PTFE ring, flexture, corner supporter, and cover. PTFE ring is a kind of Teflon and was inserted to absorb vibration and shock and isolate thermal conduction between corner cube and flexture. LRA is equipped on the satellite‘s earth panel and allows laser ranging in the field of view angles of 360 degree in azimuth and 60 degree elevation. The dimensions of laser retroreflector array have 200mm in diameter and 65mm in height. The beam divergence is fourteen arcsec in STSAT-2A and twelve arcsec in STSAT-2B. Table 1 summarizes the specifications of laser retroreflector array (SaTReC, 2005). Figure 2. Laser retroreflector array configuration of STSAT-2 Table 2. Specifications of laser retroreflector array Items Values Items Values Corner Cube Diameter 31.5mm Corner Cube Height 22.3mm LRA Size 20065(Height)mm Field of View 60 Dihedral Angle Offset 1.5 (STSAT-2A) 1.2 (STSAT-2B) Weight 815g Divergence 142arcsec (STSAT-2A) 122arcsec (STSAT-2B) Material Fused Silica Surface Flatness /10 Coating No Coating Phase Center of Laser Retroreflector Array Figure 3 shows the center of gravity of satellite and the phase center of laser retroreflector array. O is satellite-based origin and CG the satellite center of gravity. The vector C is from satellite-based origin to the center of gravity of satellite, C=(-7.6, 381.4, 1.9)mm in the STSAT-2A and (-0.18, 412.16, 0.03)mm in the STSAT-2B. The distances between the center of gravity of satellite and the optical center of LRA are 267mm in the STSAT-2A and 271.2mm in the STSAT-2B. Here, PC is the phase center. The vector L is from the satellitebased origin to the LRA mass center, (-0.34, 381.92, 310.98)mm. Proceedings of the 16th International Workshop on Laser Ranging 637 For the nadir-facing cube, the phase center is -hn in the z direction from the plane of the front faces of that cube. For the STSAT-2 cubes, h=22.3mm and n=1.464. So the phase center for the nadir cube is -32.65mm in z direction. The z-component of array phase center for that cube is +2.87mm from the mass center of laser retroreflector array, but will vary at the level of a few mm depending upon which of the other cubes are visible from the tracking station. However, ignoring that effect here, and defining vector L as the vector from the satellite-based origin to the phase center of the retroreflector array, we have L =(-0.34, 381.92, 313.85)mm. The plane of the front faces of the cubes is +35.52mm in the z direction from the LRA mass center. Finally, the vector PC from the satellite center of gravity to the phase center of the retroreflector array is PC=L -C. So, PC=(7.26, 0.52, 311.95)mm in the STSAT-2A and (-0.16, -30.24, 313.82)mm in the satellite fixed frame. Figure 3 shows the phase center of laser retroreflecture array. LRA Location