The laser retro reflector array for COMPASS-M1 is a 31.6x28 cm hexagonal array weighing 2.5 kg. The array consists of 42 corner cubes, each 33 mm in diameter. The fused silica cubes are uncoated on both the front and back surfaces. The reflective area of the array is 360 cm**2.

COMPASS-M1 Retroreflector Information Form: (24-Sep-2008)

(from http://ilrs.gsfc.nasa.gov/satellite_missions/ilrssupretro.html)

Satellite name Name: COMPASS-M1

Contact for retroreflector information: Yang Fumin
Phone number: 86-21-64696290
E-mail address: yangfm @ shao.ac.cn

A prerequisite for accurate reduction of laser range observations is a complete set of pre-launch parameters that define the characteristics and location of the LRA on the satellite. The set of parameters should include a general description of the array, including references to any ground-tests that may have been carried out, array manufacturer and whether the array type has been used in previous satellite missions. So the following information is requested.

1. Array type (spherical, hexagonal, planar, etc.), to include a diagram or photograph: Planar
2. Array manufacturer: Shanghai Astronomical Observatory, Chinese Academy of Sciences
3. Link (URL or reference) to any ground-tests that were carried out on the array:
4. Other missions using this LRA design and/or type of cubes:

For accurate orbital analysis it is essential that full information is available in order that a model of the 3-dimensional position of the satellite centre of mass may be referred to the location in space at which the laser range measurements are made. To achieve this, the 3-D location of the LRA phase centre must be specified in a satellite fixed reference frame with respect to the satellite's mass centre. In practice this means that the following parameters must be available at mm accuracy or better.

5. 3-D location (possibly time-dependent) of the satellite's mass centre relative to a satellite-based origin: Satellite CoG location (in-orbit, wet, boom deployed) within mechancal build system:
(1082.0, -0.4, -0.5) mm

6. 3-D location of the phase centre of the LRA relative to a satellite-based origin: LRA optical reference point (intersection of the optical axes of all 4 prisms within the array) in body-fixed satellite system coordinates:
(649.9, -562.5, 1112.3) mm

However, in order to achieve (6) if it is not directly specified (the ideal case) by the satellite manufacturer, and as an independent check, the following information must be supplied prior to launch.

7. Position and orientation of the LRA reference point (LRA mass-centre or marker on LRA assembly) relative to a satellite-based origin: LRA mass center: (649.9,-562.5, 1133.3) mm
8. Position (xyz) of either the vertex or the centre of the front face of each corner cube within the LRA assembly, with respect to the LRA reference point and including information of amount of recession of front faces of cubes: The plane of the front faces of the cubes is +14.0 mm in the Z direction from the LRA mass center
9. Orientation of each cube within the LRA assembly (three angles for each cube): Random
10. Shape and size of each corner cube, especially the height: Circular, diameter of 33 mm, 24.0 mm in height
11. Material from which the cubes are manufactured (e.g. quartz): Quartz
12. Refractive index of the cube material, as a function of wavelength ? (micron): 1.46 at 532 nm
13. Dihedral angle offset(s) and manufacturing tolerance: 0.6+/-0.3 arcsec
14. Radius of curvature of front surfaces of cubes, if applicable:
15. Flatness of cubes' surfaces (as a fraction of wavelength): 0.1 wavelength
16. Whether or not the cubes are coated and with what material: Uncoated

COMPASS-M1 array diagram

Vector C is from the satellite coordinate origin to the satellite's center of mass (CoM).
Vector L is from the satellite coordinate origin to the mass center of the LRA containing 42 corner cubes.
C = (1082.0, -0.4, -0.5) mm
L = (649.9, -562.5, 1133.3) mm
The plane of the front faces of the cubes is +14.0 mm in the Z direction from the LRA mass center.
The cubes' phase centers are -h x n in the Z direction from the plane of the cubes.
For the Compass-M1 cubes, h = 24.0 mm, n = 1.46. So phase centers are -35.0 mm in Z.
So z-component of array phase center is (-35.0+14) = -21.0 mm from the LRA mass center.
Let L' as the vector from the satellite coordinate origin to the phase center of the LRA. We have
L'= (649.9, -562.5, 1133.3-21.0)mm, i.e. L'= (649.9, -562.5, 1112.3) mm
Finally, the vector CP from the satellite center of mass to the phase center of the LRA is
CP = L'-C
So CP= (649.9, -562.5, 1112.3)-(1082.0, -0.4, -0.5) = (-432.1, -562.1, 1112.8) in the satellite fixed frame.