Spherical symmetry of electron density distribution is a common assumption applied to invert Radio Occultation (RO) observations in the ionosphere. The technique works fine for low solar or geomagnetic activity periods but fails to provide reliable Ne profile in active periods due to large ionospheric gradients present along RO ray paths. After performing a thorough study on the qualitative and qualitative effects of ionospheric asymmetry on the final product, we have quantified the level of asymmetry in the ionosphere present along the trajectories followed by the GNSS signal during a RO event with respect to time, location and prevailing solar and geomagnetic activity, correlating such levels with retrieval errors . This concept is the base of processor 3C . Processor 3C (when fully developed) not only provides a solution to a long awaited problem (spherical symmetry hypothesis) to be solved related to RO data processing, but also defines some new concepts on which RO technique may be used in future research related to the ionosphere. A complete block functional diagram of this processor 3C with data inputs and outputs are shown in Fig 1. A list of case studies processed by the developed prototype is given in table 1. Based on functionality, we have divided processor 3C in three modules 3C.1, 3C.2 and 3C.3 (as shown in Fig 2).
|1||26/27 Sept, 2011
||18 - 24 / 01 - 04
|2||15/16 Jul, 2012
||08 - 24 / 01 - 16
|3||09 Mar, 2012
||05 - 24
|4||17/18 Mar, 2013
||11 - 24 / 01 - 04
|5||25 Oct, 2011
||01 - 12
|6||01 Oct, 2012
||01 - 12
|7||01 Jun, 2013
||04 - 14
Figure 1: Processor 3C block diagram. Based on functionality processor 3C is divided in three modules 3C.1, 3C.2 and 3C.3
Module 3C.1: Global maps of asymmetry index
This module will generate global asymmetry maps considering ideal RO geometries crossing two different background ionospheres based on the NeQuick2 and the IRI2012. Each map is generated for a particular observation direction. The asymmetry level will only be computed for one trajectory of the RO event (the one with a ray perigee placed at ?100 km). Users will not be able to see any of these asymmetry maps which will only be used in the background processing of output1 and output2 (see Fig 1).
Module 3C.2: Electron density profile retrieval (standard techniques) effectiveness
Based on asymmetry maps calculated in module 3C.1, this module will help to predict the expected quality of RO final product, when inversion algorithms based on spherical symmetry assumption are used to invert the RO data considered in the case studies. The following color code is used to provide the information:
Module 3C.3: Model-aided Electron density profile retrieval
Results of module 3C.3 (output2) are based on the research that is being performed to implement a different RO data inversion approach which tries to overcome the spherical symmetry assumption with the help of an ionospheric model. This has been achieved through an optimization procedure by forcing the ionization level of the background ionospheric model (F10.7 in the case of NeQuick), in order to generate a STEC (slant-TEC) value which matches the one available from a real RO observation. This optimization is applied considering all the RO ray paths, in order to generate a vertical profile of electron density considering the values estimated by the background model (optimized for each observation) close to the tangent point of each ray. The idea is shown on Figure 2. However, the technique is currently under development and it is not yet able to improve the results of standard inversion technique in all cases and being worked out for further improvements, even if for some cases a better agreement with collocated ionosondes is found .
Figure 2: Application of the concept of model-aided inversion. (a) geometry of a real RO event with ionization level parameter (Az_n) calculated for each ray, (b) resulting electron density profile (magenta curve) calculated using the same ionospheric model which is used to calculate Az_n parameters. This profile is a function of ‘Az_n’ and ray perigee positions (P) associated with each ray of the RO event. For comparison, COSMIC profile based on standard inversion (blue curve) and co-located model extraction (black curve) are also shown.
Further improvement in this technique (module 3C.3) may lead to significant contribution for the improvement of radio occultation data inversion techniques in future. This will be a significant contribution for RO scientific community, specifically, when a large increase in RO observations (from ~1,500 to ~12,000 per day) is expected with the launch of COSMIC-2 RO mission in the next few years.
 Shaikh, M.M., Notarpietro, R., Nava, B., The Impact of Spherical Symmetry Assumption on Radio Occultation Data Inversion in the Ionosphere: An Assessment Study, Advances in Space Research(2013), doi: dx.doi.org/10.1016/j.asr.2013.10.025.
 Shaikh M.M., Notarpietro R., Nava B., Implementation of Ionospheric Asymmetry Index in TRANSMIT Prototype, Mitigation of Ionospheric Threats to GNSS: An Appraisal of the scientific and technological outputs of the TRANSMIT project, ISBN: 978-953-51-4143-3 (http://www.intechopen.com/books/mitigation-of-ionospheric-threats-to-gnss-an-appraisal-of-the-scientific-and-technological-outputs-of-the-transmit-project/implementation-of-ionospheric-asymmetry-index-in-transmit-prototype).
 Shaikh M.M., Notarpietro R., Nava B. (under internal review), D2.1b - Radio Occultation Model for Ionospheric Profiling, Final deliverable based on the outcome of ESR4 work in TRANSMIT project, August, 2014.