Main data sources for the orbit determination of Low Earth Orbiting (LEO) satellites are advanced space-born GPS receivers providing full global coverage of orbit data. The alternative method of Satellite Laser Ranging (SLR) is used as a supporting and independent means for orbit determination. Its limitation is the rather poor global and temporal coverage, but the SLR data is free from ambiguities and directly related to the terrestrial reference frame.
Within the design and development activities for the CHAMP mission it was decided at GFZ Potsdam to equip this small satellite with a Laser Retro Reflector (LRR) of novel design for external calibration and validation of the onboard BlackJack GPS receiver with at least centimeter resolution. A basic requirement for such a laser reflector is to enable the worldwide SLR station network to track the satellite with high accuracy and with a sufficiently high link budget under both night and daytime ranging conditions. Recent improvements of the SLR technology suggested to make the design of the reflector suited for a measurement resolution even in the millimeter range.
The lower the number of individual reflecting prisms within such a laser reflector, the higher is the expected ranging accuracy. An ideal LRR target would thus be a single cube corner reflector, but because of its limited angular field it is not well suited for very low orbiting satellites like CHAMP and GRACE. A reasonable compromise was found in a way that the array is formed of only four cube corner prisms mounted in a compact frame with the outer dimensions of (100x100x48) mm only.
This design ensures that only one prism is contributing to the signal in general, except for some cases (near culmination of the satellite) where the signals of two prisms are interfering. However because of the small dimensions of the array this signature cannot be resolved by present SLR systems. This was validated by SLR stations with millimeter ranging accuracy during the CHAMP and GRACE missions.
Special care was taken to compensate for the effect of velocity aberration in order to deliver high signal strength for the SLR stations from the CHAMP LRR. This was achieved by making the diffraction far field of the LRR consisting of two spots which contain the reflected laser signal.
The main parameters of the GFZ LRR for LEO satellites are:
Clear aperture of the front face
Dihedral angle offset
-3.8" (smaller than 90 deg)
Radius of curvature of the front face
+500 m (convex)
Index of refraction @ 532nm
1.461 (fused quartz)
Nominal separation of the far field maxima
Nominal width of the far field peaks (20% intensity of max.)
A detailed description of the LRR can be found here .
The performance of the GFZ-made LRR array on the missions CHAMP and GRACE is excellent. For this reason it was delivered also for further space missions: the German radar satellites TerraSAR-X and TanDEM-X , the Korean KOMPSAT-5 , the Spanish PAZ and the ESA magnetic field mission SWARM which consists of 3 individual satellites.