Current achievements and future evolution of the ROSA radio-occultation receiver

Abstract

ROSA is a dual-frequency GPS receiver developed for space applications and tailored for GPS radio occultation studies. It is developed and manufactured by Thales Alenia Space Italia under a contract with the Italian Space Agency (ASI) and is designed to establish accurate global profiles of temperature and water vapour in the lower atmosphere. Moreover, ROSA antenna extended coverage allows the retrieval of total electron count (TEC) profiles in the ionosphere for space weather applications. ROSA first flight mission is on-board the Indian Space Research Organization (ISRO) Oceansat-2 satellite, launched on Sep 23, 2009 from SHAR, Sriharikota by Indian PSLV launcher. Oceansat-2 is a mission developed by ISRO for applications dedicated to ocean studies and meteorology. OCS-2 carries three payload instruments: Ocean Colour Monitor (OCM), Ku-band Scatterometer (SCAT) from ISRO and the Radio Occultation Sounder for Atmospheric studies (ROSA) from Italian Space Agency (ASI). The accommodation of the ROSA instrument on board the Oceansat-2 spacecraft has been driven by the satellite configuration that allows the possibility to install only one Radio Occultation antenna in the direction of the spacecraft velocity. In fact the anti-velocity side is used to interface the satellite with the launcher. This means that only rising occultation events can be detected by the ROSA instrument in this mission. ROSA will also be a payload on the forthcoming Argentinean CONAE SAC-D (Aquarius) mission. The ROSA instrument was delivered to CONAE in January 2010 and will be integrated on the SAC-D satellite in April 2010. The accommodation of ROSA on the SAC-D satellite has been driven by the SAC-D Aquarius accommodation requirements, particularly stringent in terms of interferences near the L2 band. A dedicated RF filter box was designed to cope with and mitigate this interference. The ROSA instrument on SAC-D will be a full velocity and anti-velocity antenna instrument, therefore allowing tracking both rising and setting events. Finally a further contract from ISRO has been awarded to Thales Alenia Space Italia to host a ROSA instrument on-board the next ISRO MeghaTropiques mission. The mission is devoted to study tropical meteorological and climatic processes associating three radiometric instruments allowing to observe three inter-related components of the atmospheric engine: water vapour, condensed water and radiative fluxes. Also MegaTropiques will host velocity and anti-velocity antennas, although with a reduced FOV. Initial results from the on-going commissioning phase that is running on the ROSA on OCS-2 mission show promising performance considering that they are obtained without any tuning from ground. This tuning task has been started since February 2010 and is planned to complete by June 2010. Initial scientific parameter retrieval, like refractivity profiles and electron density profiles are derived and compared to the best possible collocated COSMIC profiles. The agreement shown is good. Of course, a large validation campaign will be carried out after the commissioning and this will be the true test bench of ROSA instrument. Initial data analysis gives evidence that, despite the constraints experienced (only one antenna, 15 deg tilting with respect to velocity direction, multipath effects) results are considered satisfactory. An evolution of the ROSA instrument is, however, started in order to design an enhanced version of the instrument (ROSA 2G) with focus in particular on instrument (antennas and receiver box) mass, size, power consumption reduction as well as enhanced features. In particular the following main targets are considered in the ROSA 2G design: — Increased number of occultation events — Better quality of occultations — Modularity of design The possible ways in which these objectives can be achieved are detailed in this paper. Moreover, we will focus on the review of the current ROSA characteristics and on the first results obtained from the commissioning phase on OCS-2 like, for example, the analysis of GPS raw data, quantity and quality of signal acquisitions in both navigation and occultation channels, real-time navigation solution quality check and other instrumental parameter of interest like signal to noise ratio statistics, high-frequency raw sampling (open-loop) functionality, performance of the onboard atmospheric model, as well as first scientific data retrievals. Then we will focus on the on-going ROSA 2G design, starting from the main drivers and instrument requirements and describing the reasons and the approach adopted towards the enhanced instrument. In particular, evidence will be given to the receiver technological aspects and feasibility of the multi-functional instrument features.