GNSS Radio Occultation for Ionospheric Monitoring – Impact and Mitigation of High Solar Activity Effects

The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km. It owes its existence primarily to ultraviolet radiation from the sun. The ionosphere is becoming more relevant to human society with its reliance on modern technology, since the accuracy of navigation and quality of telecommunication is influenced by ionospheric conditions. The free electrons in the ionosphere affect the propagation of radio waves. Below about 30 MHz the ionosphere acts like a mirror, bending the path traveled by a radio wave back toward the Earth. At higher frequencies, such as those used by GPS, radio waves pass right through the ionosphere. They are, nevertheless, affected by it. Disruption of communications and navigation systems can have severe societal consequences. Even though the ionospheric observational techniques and the ionospheric models have gone through considerable development sustained over many decades, accurate monitoring and forecasting of the ionosphere conditions still presents stubborn challenges. The global navigation satellite system (GNSS)-based radio occultation (RO) has been proven to be a powerful technique for remotely sensing the earth’s troposphere, stratosphere, and ionosphere in the past decade [8]. Radio occultation is a relatively new technique that can be used to study the ionosphere, offering potentially global and continuous measurements. MARINER IV first applied the RO observation technique to observe the Mars atmosphere and ionosphere in 1965 [48]. MicroLab-1 GPS/MET was launched in 1995 and applied to monitor the Earth’s atmosphere and ionosphere by using GPS RO technique [37,50]. The Global Positioning System (GPS) to Low Earth Orbit (LEO) satellite paths essentially make long, near-horizontal measurements of the integrated content of ionospheric electron density; namely total electron content (TEC). These measurements are not simple to interpret, since the satellite transmission paths map out a complicated and continuously changing measurement geometry. Nevertheless, a strong advantage of this system is that it provides measurements over the oceans and into remote polar caps, thus enabling the ionosphere to be studied on a truly global-scale. The FORMOSAT-3/COSMIC (the most recent and advanced RO mission in operation) was launched in April 2006, and has six micro satellites in different orbital planes. The GPS radio occultation experiment (GOX) is one of the satellite mission objectives, and observes the ionosphere and atmosphere vertical structure by using the RO observation technique. RO observations, particularly from FORMOSAT-3/COSMIC, have significantly improved our capability of monitoring the global ionosphere. In the ionosphere, the important scientific RO data product is the retrieved electron density profile (Ne(h)) along the tangent points during an occultation event. The Abel inverse transform is the conventional method to analyze the tropospheric occultations and it was natural to adopt this approach for the ionospheric studies. It allows the vertical profile of electron concentration to be obtained, nominally at a single location between the GPS and LEO (onboard RO receiver). The resulting profile is therefore some average of the ionosphere traversed by the occulting ray paths between the two satellites. However, the classical approach of the Abel inversion assumes spherical symmetry of the electron density field in the vicinity of an occultation. In practice, the footprint of an occultation generally covers wide regions and averages any spatial variations connected with variable declinations of the magnetic field from the horizontal direction along the occulted ray path. Indeed, inhomogeneous electron density in the horizontal direction for a given occultation is believed to be the main source of error when using the Abel inversion. Large amount of research has been done, in last couple of decades, to improve the Abel inversion by removing or reducing the effect of ionospheric asymmetry. One potential and frequently studied and revised method is the Abel inversion aided by other horizontal information such as the global ionospheric map (GIM) [9,30,32,36,42,49,72]. However, due to variability of available maps and the fact that not much attention was paid to the large-scale Abel retrieval error, as illustrated by [54,92,93], no standard procedure have been globally accepted and Abel inversion is still the most widely used technique to produce the ionospheric products using RO technique. In our research, we have thoroughly investigated the spherical symmetry problem of Abel inversion; qualitatively as well as quantitatively. This was done to first understand what actually is happening when we apply such algorithm for RO data inversion. In the process of this investigation, we were able to find an effective way of quantifying the impact of ionospheric asymmetry on the final product of RO data inversion, i.e., vertical electron density profile. The asymmetry index is based on the electron density variation along the occulted ray path. It efficiently incorporates electron density gradients along the RO ray path to find a number that sums-up the impact of prevailing ionospheric condition, on the final RO product, by giving it a number on a scale from 0 to 1. Our results, based on model simulations, show that the designed algorithm is proving to be an effective technique to find such information quickly and accurately. Using the knowledge gained during this thorough investigation, to mitigate the impact of spherical symmetry hypothesis from RO data inversion, we have also implemented a very effective technique based on the NeQuick2 (electron density model) adaptation to RO-derived TEC. It relies on the minimization of a cost function involving experimental and model-derived TEC data to determine the NeQuick2 input parameters (local ionization parameters) at the wanted locations and time. These parameters are then used to evaluate the electron density profile along the ray perigee positions associated to the relevant RO event. The results indicate that the technique significantly improved the RO inversion product and is able to avoid the presence of negative electron density values in the reconstructed profiles. Furthermore, no external data, such as GIM maps or other data, is required to apply the technique. The technique is currently under development and, when fully developed, will 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. This will be a significant contribution for RO scientific community, specifically, when a large increase in RO observations (from approximately 1,500 to 12,000 per day) is expected with the launch of COSMIC-2 RO mission in the next few years.