Creation of the new industry-standard space test of laser retroreflectors for the GNSS and LAGEOS

We built a new experimental apparatus (the "Satellite/lunar laser ranging Characterization Facility", SCF) and created a new test procedure (the SCF-Test) to characterize and model the detailed thermal behavior and the optical performance of cube corner laser retroreflectors in space for industrial and scientific applications. The primary goal of these innovative tools is to provide critical design and diagnostic capabilities for Satellites Laser Ranging (SLR) to Galileo and other GNSS (Global Navigation Satellite System) constellations. The capability will allow us to optimize the design of GNSS laser retroreflector payloads to maximize ranging efficiency, to improve signal-to-noise conditions in daylight and to provide pre-launch validation of retroreflector performance under laboratory-simulated space conditions. Implementation of new retroreflector designs being studied will help to improve GNSS orbits, which will then increase the accuracy, stability, and distribution of the International Terrestrial Reference Frame (ITRF), to provide better definition of the geocenter (origin) and the scale (length unit). Our key experimental innovation is the concurrent measurement and modeling of the optical Far Field Diffraction Pattern (FFDP) and the temperature distribution of the SLR retroreflector payload under thermal conditions produced with a close-match solar simulator. The apparatus includes infrared cameras for non-invasive thermometry, thermal control and real-time movement of the payload to experimentally simulate satellite orientation on orbit with respect to both solar illumination and laser interrogation beams. These unique capabilities provide experimental validation of the space segment for SLR and Lunar Laser Ranging (LLR). We used the SCF facility and the SCF-Test to perform a comprehensive, non-invasive space characterization of older generation, back-coated retroreflectors of the GIOVE-A and -B (Galileo In-Orbit Validation Elements) and the GPS-35 and -36 designs. First, using a full GPS flight model at laser wavelengths of 532 and 632 nm, we found its "effective optical cross section" in air, under isothermal conditions, to be six times lower than the Retrorefiector Standard for GNSS satellites (100 x 10(6) m(2) at 20,000 km altitude for GPS and 180 x 10(6) m(2) for Galileo at 23,200 km altitude), issued by the International Laser Ranging Service (ILRS). Under the simulated thermal and space conditions of the SCF, we also showed that in some space configurations the "effective optical cross section" is further reduced, by the thermal degradation of the FFDP. Using the same SCF-Test configuration on an individual GIOVE prototype cube, we measured severe thermal degradation in optical performance, which appears to be caused by the retroreflector metal coating and the non-optimized thermal conductance of the mounting. Uncoated retroreflectors with proper mounting can minimize thermal degradation and significantly increase the optical performance, and as such, are emerging as the recommended design for modern GNSS satellites. The COMPASS-MI, GLONASS-115 GNSS satellites use uncoated cubes. They provide better efficiency than those on GPS and GIOVE, including better daylight ranging performance. However, these retroreflectors were not characterized in the laboratory under space conditions prior to launch, so we have no basis to evaluate how well they were optimized for future GNSS satellites. SCF-Testing, under a non-disclosure agreement between INFN-LNF and the European Space Agency (ESA), of prototype uncoated cubes for the first four Galileo satellites to be launched (named "IOV", In-Orbit Validation satellites) is a major step forward. An SCF-Test performed on a LAGEOS (LAser GEOdynamics Satellite) engineering model retroreflector array provided by NASA, showed the good space performance on what is now a reference ILRS payload standard. The IOV and LAGEOS measurements demonstrated the effectiveness of the SCF-Test as an LRA diagnostic, optimization and validation tool in use by NASA, ESA and ASI. (C) 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.