Thermo-optical vacuum testing of IRNSS laser retroreflector array qualification model

We describe the activities performed by SCF_Lab (Satellite/lunar/GNSS laser ranging/altimetry and cube/microsat Characterization Facilities Laboratory) of INFN-LNF for the thermo-optical vacuum testing activity of a IRNSS (Indian Regional Navigation Satellite System) LRA (Laser Retroreflector Array), under contract for ISRO-LEOS. To our knowledge, this is the first publication on the characterization of the optical performance of an LRA operating at about 36,000 km altitude (typical of regional GNSS segments, namely QZSS, COMPASS-G) executed in fully representative, carefully lab-simulated space conditions. In particular, this is the only such publication concerning IRNSS. Since laser ranging to its altitude is more challenging than to GNSS altitudes (from about 19,100 km for GLONASS to about 23,200 km for Galileo), comparative measurements were long awaited by ILRS (International Laser Ranging Service) and we present measurements of the absolute laser return to ground stations of the ILRS in terms of lidar OCS (Optical Cross Section) at the IRNSS relevant value of velocity aberration, in turn derived from measurements of the full FFDP (Far Field Diffraction Pattern) over a very large range of velocity aberrations. These measurements were acquired: (i) on a full-size qualification model of a IRNSS CCR (Cube Corner Retroreflector) LRA that ISRO-LEOS provided to INFN-LNF; (ii) during the lab-simulation of a 1/4 orbit segment, in which the LRA CCRs are exposed to the perturbation of the sun heat at varying angles, from grazing incidence (90° with respect to the direction perpendicular to the plane of array), up to the perpendicular to the LRA, with a same time variation consistent with the actual space orbit. In this 1/4 orbit condition, the LRA experiences potentially large thermal degradations of the OCS, depending on the detailed thermal and mechanical design of the LRA. Since all GNSS constellations have different LRA designs or configurations, this is another reason of the interest in the laboratory optical performance of the IRNSS designs. In fact, we tested two different IRNSS LRA configurations, without and with an auxiliary cover. The latter of the configurations is designed to give a somewhat increased performance. We also measured FFDPs and OCS: (1) in-air isothermal conditions (thus obtaining the LRA nominal performance); (2) in space conditions at a low, stationary value of the LRA bulk temperature; (3) in space conditions at a high, stationary value of the LRA bulk temperature (Figs. S1-S18, available in the online supplementary material for this paper). In all these three cases, we measured FFDPs and OCS at the following laser incidence angles: (a) normal to the LRA plane; (b) +9° to the normal to the LRA plane; (c) −9° to the normal to the LRA plane. This is the first time, to our knowledge, that the measurements (not simulations) at ±9° (values requested by ISRO-LEOS) are published for GNSS LRAs. Finally, we point out that we measured full-FFDP and OCS of single CCRs, for maximum diagnostic value and performance validation at subelement level. For all these reasons, the articulate and variate set of tests of the IRNSS LRA reported in this paper is unique.