Laser Retro-reflector Arrays on the Compass Satellites (2008)

COMPASS‖ is the regional satellite navigation system in China, and the constellation will consist of 12 satellites, of which 5 GEO, 3 IGSO and 4 MEO. The effective reflective areas of LRAs for these satellites are 770cm 2 for GEO and IGSO, 360cm 2 for MEO respectively. The diameter of the corner cubes is 33mm. Each corner cube is uncoated both on front and back faces. The paper presents the characteristics of the LRAs of the COMPASS satellites. Introduction The COMPASS, the Chinese name is Beidou , is the regional satellite navigation system in China, which will cover the most areas of the East Asia region, and expect to be operational by 2011.The COMPASS constellation will consist of 12 satellites with different orbital altitudes. There will be 5 GEO, 3 IGSO and 4 MEO. The orbital altitudes for GEO and IGSO are 36000 km, while 21500 km for MEO. All of those satellites will be equipped with the laser retro-reflector array (LRA) for precise orbit determination. The experimental COMPASS satellite M1 with a MEO orbit was launched on April 13, 2007 (UTC). The LRA onboard COMPASS-M1 has been successfully tracked by the ILRS stations since December 2008. Fig. 1 is the drawing of COMPASS-M1. Figure 1. Drawing of COMPASS-M1 Navigation Satellite Design and Performance of the LRA The orbital altitude of the COMPASS-M1 is 21500 km, so the velocity aberration for the COMPASS-M1 is about 5.2 arc seconds. We choose the diameter of the corner cubes for the COMPASS-M1 and COMPASS satellites follow-on is 33 mm, equals to 1.3 inches. For compensation of the velocity aberration, the 0.6 arc seconds (±0.3′′) dihedral offsets were adopted for the COMPASS-M1‘s corner cubes. All the surfaces of the corner cubes are without coating. The fused silica material for the corner cubes was made by the Shanghai LengGuang Co. in China. Fig. 2 and Table 1 show the photos and parameters of the Proceedings of the 16th International Workshop on Laser Ranging 618 COMPASS-M1 (MEO) and the COMPASS GEO/IGSO respectively. We choose the hexagon array for the GEO/IGSO in order to reduce the returned pulse spread and to achieve better ranging precision. Figure 2. The LRAs on MEO and GEO/IGSO Table 1. Parameters of COMPASS-M1 (MEO) and COMPASS GEO/IGSO The optical performance of each corner cube must be carefully measured by the ZYGO interferometer in our laboratory. Fig. 3 shows the ZYGO and one of the measurements. The LRA must pass all the space environmental simulation testing. Fig. 4 shows one of the testing. Figure 3. Testing of LRA‘s optical performance with ZYGO interferometer M1(MEO) GEO/IGSO Size 31.628 cm 4943cm Diameter of corner cube 33mm 33mm Number 42 90 Reflective area 360cm 2 770cm 2 Dihedral offset 0.6′′ 0.5′′ Weight 2.45 kg 5.0 kg Proceedings of the 16th International Workshop on Laser Ranging 619 Figure 4. Environmental testing of LRA Table 2 and 3 list the cross section, range and signal strength and the parameters of the LRA on different satellites respectively from Dr. David Arnold (private communication). Table 2. Cross section, range and signal for different satellites (by David Arnold) Sat. Cross section Range (0 deg.) Range 4 Signal strength Range (45 deg.) Range 4 Signal strength Lageos 15 5.8 1 1.000 6.8 1 1.000 Etalon 55 19.0 115 .032 20.5 82 .044 GPS 19 20.0 141 .009 21.5 100 .012 GIOVE-A 45 23.9 288 .010 25.4 195 .015 Glonass 80 19.0 115 .046 20.5 82 .065 CompassM1 80 21.0 189 .028 23.0 131 .041 ETS-8 140 36.