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 inver