C. Lops

SCF-Test of a Galileo-IOV retroreflector and thermal-optical simulation of a novel GNSS retroreflector array on a critical orbit

Thorough laboratory measurements performed at INFN-LNF (Istituto Nazionale di Fisica Nucleare-Laboratori Nazionali di Frascati), in the framework of the ETRUSCO (Extra Terrestrial Ranging to Unified Constellations) experiment, proved fundamental to characterize retroreflectors for GNSS (Global Navigation Satellite System) satellites. The standard test developed, SCF-Test, was important to outline the weaknesses of past retroreflectors payloads (in use on GPS, GLONASS and GIOVE A/B satellites). For the upcoming deployment of the Galileo constellation ESA requested, in 2010, a full SCF-Test campaign to characterize a prototype retroreflector of the first IOV satellites. We report the results of a standard SCF-Test and the test of a simulated orbit, called GCO (Galileo Critical halfOrbit). The experience gathered with the ETRUSCO experiment was important for the subsequent project, ETRUSCO-2, whose aim is to develop and measure, in a newly built facility, a full size array of retroreflectors to be deployed on GNSS constellations. Here we report preliminary concurrent thermal and optical simulations of a simulated array. A simplified structure of the array was subject to a simulated space environment in a GCO; the resulting temperature distribution inside each retroreflector, was the input of the optical software to determine the variation of the intensity, throughout the orbit, coming back at a ranging station. The goal is to limit as much as possible signal fluctuations with respect to current deployed arrays.

Next-generation laser retroreflectors for GNSS, solar system exploration, geodesy, gravitational physics and earth observation

The SCF_Lab (Satellite/lunar/gnss laser ranging and altimetry Characterization Facility Laboratory) of INFNLNF is designed to cover virtually LRAs (Laser Retroreflector Arrays) of CCRs (Cube Corner Retroreflectors) for missions in the whole solar system, with a modular organization of its instrumentation, two redundant SCF (SCF_Lab Characterization Facilities), and an evolutionary measurement approach, including customization and potentially upgrade on-demand. See http://www.lnf.infn.it/esperimenti/etrusco/ for a general description.

Next-generation laser retroreflectors for precision tests of general relativity

Since 1969, Lunar Laser Ranging (LLR) to the Apollo Cube Corner Retroreflectors (CCRs) has supplied almost all significant tests of General Relativity (GR). When first installed in the 1970s, the Apollo CCRs geometry contributed only a negligible fraction of the ranging error budget. Today, because of lunar librations, this contribution dominates the error budget, limiting the precision of the experimental tests of gravitational theories. The new MoonLIGHT-2/LLRRA-21 (Moon Laser Instrumentation for General relativity High-accuracy Tests) apparatus is a new-generation LLR payload made of a single large CCR unaffected by librations. Thanks to this new design, MoonLIGHT-2/LLRRA-21 can increase the precision of the measurement of the lunar geodetic precession up to a factor 100, compared to the Apollo CCRs. To optimize the MoonLIGHT2/LLRRA-21 design and its lunar deployment, we performed both experimental tests of MoonLIGHT-2 thermal properties in simulated space condition and GR test simulations using the Planetary Ephemeris Program software, developed by the Center for Astrophysics (CfA). The experimental test shown the expected thermal properties and will provide useful to optimize the payload for the launch while the GR simulations suggest that MoonLIGHT-2 measurements are practically independent from Moon librations (providing a significant improvement in GR test) and that the absence of a sunshade does not have a relevant impact on the precision of GR tests.

Next generation Lunar Laser Ranging and its GNSS applications

Over the past forty years, Lunar Laser Ranging (LLR) to the Apollo Corner Cube Reflector (CCR) arrays has supplied almost all of the significant tests of General Relativity, and provided significant information on the composition and origin of the Moon. These arrays are the only experiment of the Apollo program still in operation. Initially the Apollo Lunar arrays contributed a negligible portion of the error budget used to achieve these results. However over the decades, the performance of the ground stations has been greatly upgraded so that the ranging accuracy has improved by more than two orders of magnitude. Now, after forty years, because of the lunar librations, the existing Apollo retroreflector arrays contribute a significant fraction of the limiting errors in the range measurements. University of Maryland (UMD) and INFN/LNF are now proposing a new approach to the Lunar Laser Ranging Array technology, the experiment MoonLIGHT12. The new arrays will support ranging observations that are a factor 100 more accurate, reaching the micron level. The new fundamental physics and lunar physics that this new Lunar Laser Ranging Retroreflector Array for the 21st century (LLRRA-21) can provide, will be briefly described. The new lunar CCR housing has been built at the INFN/LNF3. In the design of the new array there are three major challenges: 1) validate that the specifications of the CCR required for the new array, which are significantly beyond the properties of current CCRs, can indeed be achieved, 2) address the thermal and optical effects of the absorption of solar radiation within the CCR, reduce the transfer of heat from the hot housing to the CCR and 3) define a method of emplacing the CCR package on the lunar surface such that the relation between the optical center of the array and the center of mass of the Moon remains stable over the lunar day/night cycle. Its evolutionary design may be suitable for future GNSS constellations guaranteeing ranging accuracy improvement (the concept of a single reflector introduces no laser pulse spreading at all angles), weight and area saving (being its absolute optical cross section equal to a large number of the CCRs that will be used for the upcoming GNSS constellations)4.