Implementation of ROSA radio occultation data handling into EUMETSAT and GRAS SAF processing

Topic of the GRAS-SAF Visiting Scientist activity 16 has been the implementation of ROSA radio occultation data handling into EUMETSAT and GRAS SAF processing tools. ROSA data referred to observations taken on-board OCEANSAT-2 mission. Even if we are talking of standard Radio Occultation data, the format of raw binary data is peculiar, and processing chains already implemented for other Radio Occultation missions, should be adapted to this further format. Moreover, ROSA antennas, ROSA coverage, ROSA sampling rate, etc. are different from GRAS or COSMIC data. Main goals of this VS activity have therefore been the implementation of ROSA processing into the EUMETSAT YAROS prototype (in order to produce excess-phases, amplitudes and bending angles over impact parameter) and into the GRAS SAF ROPP processor (for the processing into bending angle, refractivity and higher level atmospheric profiles), dealing mainly with data interfacing issues. The first part of the activity was carried out in EUMETSAT, where the YAROS prototype has been adapted to handle ROSA data. Before doing that, an in-depth analysis has been done in order to identify all the ROSA data characteristics and issues. In particular, the Thales Alenia Space "decoding" software has been completely review, in order to understand how these data were formatted inside the native binary ROSA data stream coming from the OCEANSAT-2 telemetry, and inside the standard Level1 Engineered data file available for users. This preliminary analysis allows the definition of several ROSA data structures that could be easily implemented directly inside the YAROS prototype. At the end of this first part, two different releases of the EUMETSAT prototype were available: the first one able to manage only the scientific ROSA engineered data, containing the basic observables for Radio Occultation purposes (the Level 1a data, the one normally made available for users); the second one which implements ad-hoc data structures where all the data generated by the ROSA receiver (and contained in the raw binary format, the Level 0 data) are stored. About thirty hours of ROSA observations, for which orbit and clocks were already available, were used as input to the adapted YAROS/ROSA prototype and completely analyzed. Only the YAROS prototype adapted to ROSA Engineered Level 1 format was used. A quality check of L1 and L2 basic observables (mainly L1 and L2 Excess-Phase and Signal-to-Noise ratio profiles) contained in the Level 1a YAROS output file has been performed. Even if a complete ROSA dataset is available for one entire month of observations, the production of all the necessary POD products for the entire month was not undertaken. Its in-depth analysis will be carried out in the future, when the Memorandum of Understanding between Italian Space Agency and EUMETSAT for the ROSA data processing will be signed. During this activity we preferred to concentrate the work on the YAROS prototype adaptation to ROSA data, making the prototype ready to handle all the ROSA data available, and not only the one contained in the Level1 Engineered data normally available to the user. In order to let these NetCDF Level1a YAROS/ROSA output files to be further processed into higher Radio Occultation products, a converter from NetCDF 4.0 to NetCDF 3.0 file (with the required data structures correctly set for the GRAS-SAF ROPP processing). This was performed during the second part of the Visiting Scientist activity at DMI, where YAROS/ROSA Level 1a files were both processed to bending angles, to refractivity and higher level products. Analysis of the bending angle and refractivity data against ECMWF and collocated radio occultation data were also performed, in order to have a preliminary idea on the quality of the final atmospheric products that can be achieved analyzing ROSA data. Moreover, YAROS/ROSA Level 1a files were also processed into Level 1b products (bending angle profiles) using the L1a2L1b YAROS code and compared with the correspondent forward modelled ECMWF collocated data. As a general comment on the ROSA data quality it has to be noted that, even thought L1 data are in line with that observed by the other Radio Occultation instruments, L2 data shows some problems. Problems which are mainly related to OCEANSAT-2 issues. ROSA on-board OCEANSAT-2 was equipped only with the Velocity Radio Occultation antennas, therefore only rising events can be recorded. Moreover local multipath on-board OCEANSAT-2 is very strong because of solar panels and because of the scatterometer antenna which both are moving. In particular this local multipath was not modelled or measured on ground, since an in-orbit platform manoeuvre was made necessary and created a permanent and unexpected yaw bias on the platform. And these problems mainly impact L2 tracking, which basically starts too high in atmosphere and which is affected by long data gaps. Only for a small number of events L2 is available in troposphere. In these cases, the quality of atmospheric profiles retrieved using the GRAS-SAF ROPP processing chain is in line with that obtainable from other Radio Occultation missions. But, for all the other cases for which L2 is available higher in the atmosphere, modifications to the L2 extrapolation/interpolations and different Statistical Optimization setting are probably necessary to adapt the GRAS-SAF ROPP_PP Processor to such bad data. From the other point of view, YAROS software correctly handle such ROSA data, even if a high number of rejected profiles are seen with respect what normally happens in processing data from other Radio Occultation observation

Wet Refractivity tomographic reconstruction over small areas using an ad-hoc GPS receivers network

One of the most attractive scientific issues in the use of GNSS (Global Navigation Satellite System) signal from a meteorological point of view, is the retrieval of high resolution tropospheric water vapour maps. The real-time (or quasi real-time) knowledge of such distributions could be very useful for several applications, from operative meteorology to atmospheric modeling. Moreover, the exploitation of wet refractivity field reconstruction techniques can be used for atmospheric delay compensation purposes and, as a very promising activity, it could be applied for example to calibrate SAR or Interferometric-SAR (In-SAR) observations for land remote sensing. This is in fact one of the objectives of the European Space Agency project METAWAVE (Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water vapour Effects), in which several techniques were investigated and results were compared to identify a strategy to remove the contribution of water vapour induced propagation delays in In-SAR products. Within this project, the tomographic reconstruction of three dimensional wet refractivity fields on a small atmospheric volume (16km x 20 km x 10 km height, from 2 km to 4 km horizontal resolution and 1 km vertical resolution), was performed considering real tropospheric delays observations acquired by a GNSS network (9 dual frequency GPS receivers) deployed over Como area (Italy), during 12-18 October, 2008. Acquired L1 and L2 carrier phase observations have been processed in terms of hourly averaged Zenith Wet Delays. These vertical informations have been mapped along the correspondent line of sights (by up-sampling at 30 second sample times the 15 minutes GPS satellites positions obtained from IGS files) and inverted using a tomographic procedure. The used algorithm performs a first reconstruction (namely, the tomographic pre-processing) based on generalized inversion mechanisms, in order to define a low resolution first guess for the following step. This second step inverts GPS observables using a more refined algebraic tomographic reconstruction algorithm, in order to improve both vertical and horizontal resolution. Despite limitations due to the network design, internal consistency tests prove the efficiency of the adopted tomographic approach: the rms of the difference between reconstructed and GNSS observed Zenith Wet Delays (ZWD) are in the order of 4 mm. A good agreement is also observed between our ZWDs and corresponding delays obtained by vertically integrating independent wet refractivity fields, taken by co-located meteorological analysis. Finally, during the observing period, reconstructed vertical wet refractivity profiles evolution reveals water vapour variations induced by simple cloud covering. Even if our main goal was to demonstrate the effectiveness in adopting tomographic reconstruction procedures for the evaluation of propagation delays inside water vapour fields, the actual water vapour vertical variability and its evolution with time is well reproduced, demonstrating also the effectiveness of the inferred 3D wet refractivity fields. Even if results obtained were satisfactory, limitations due to the observation geometry, to the GNSS propagation delay information extraction form observables and to the applied tomographic technique will be highlighted, in order to trace the road-map toward future improvements in this challenging fiel

Effects of Ionospheric Asymmetry on Electron Density Standard Inversion Algorithm Applicable to Radio Occultation (RO) Data Using Best-suited Ionospheric Model

The "Onion-peeling" algorithm is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density (Ne) distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density vertical profile Ne(h). In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods. Using simulated RO Total Electron Content (TEC) data computed by means of the best-suited ionospheric model and ideal RO geometries, we evaluated the asymmetry level index for quasi-horizontal TEC observations. This asymmetry index is based on the Ne variations that a signal may experience along its ray-path (satellite to satellite link) during a RO event. The index is strictly dependent on RO geometry and azimuth of the occultation plane and is able to provide us indication of the errors (in particular those concerning the peak electron density NmF2 and the vertical TEC) expected in the retrieval of Ne(h) using standard Onion-peeling algorithm. On the basis of the outcomes of our work, and using best-suited ionospheric model, we will try to investigate the possibility to predict the ionospheric asymmetry expected for the particular RO geometry considered. We could also try to evaluate, in advance, its impact on the inverted electron density profile, providing an indication of the product qualit

IMPACT OF IONOSPHERIC HORIZONTAL ASYMMETRY ON ELECTRON DENSITY PROFILES DERIVED BY GNSS RADIO OCCULTATION

The ‘Onion-peeling' algorithm is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density profile. In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods. In this work, using simulated RO TEC data computed by means of the NeQuick2 ionospheric electron density model and ideal RO geometries, we tried to formulate and evaluate an asymmetry level indicator for quasi-horizontal radio occultation observations. This asymmetry index is based on the electron density variation that a ray may experience along its propagation path (satellite to satellite link) in a RO event. Our previous qualitative assessment based on ideal simulations of RO events shows very high correlation between our asymmetry index and Onion-peeling retrieval errors (Shaikh M.M. et al 2013): errors produced by Onion-peeling in the retrieval of NmF2 and VTEC are larger at the geographical locations where our asymmetry index indicates high asymmetry in the ionosphere. In this contribution, an analysis of the asymmetry index has been carried out for the first time using real radio occultation geometries taken from COSMIC mission. This has been done for COSMIC events for which, considering the same RO geometry, simulated Limb-TEC (LTEC) under NeQuick2 background were quite close to the real LTEC observations (providing ‘quasi' co-located vertical profiles of electron density after inversion). On the basis of the outcomes of our work, for a given geometry of a real RO event and using a suitable ionospheric model, we will try to investigate the possibility to predict ionospheric asymmetry expected for the particular RO geometry considered. We could also try to evaluate, in advance, its impact on the inverted electron density profile, providing an indication of the expected product quality, if standard Onion-peelingalgorithm will be adopted to invert the observables. Results presented in this paper are initial outcomes based on our asymmetry evaluation algorith

Storm Identification, Tracking and Forecasting Using High-Resolution Images of Short-Range X-Band Radar

second, the isolation of false merger (loosely-connected storms