GNSS RO ionospheric electron density retrieval through multi-antenna-based data filling

Abstract

Global Navigation Satellite System (GNSS) Radio Occultation (GNSS RO) is a remote sensing technique that observes the Earth's atmosphere, including the ionosphere, based on the degree of refraction and delay of GPS signals received from Low Earth Orbit (LEO) satellites. GNSS RO has the advantage of being able to providing high-resolution ionospheric information over the global range and altitude. Due to this advantage, the demand for GNSS RO data is increasing in various fields that require ionospheric monitoring. In Korea, the RO receiver is on board on Korea Multi-Purpose Satellite 5 (KOMPSAT-5) and is in operation. In order to obtain precise atmospheric profiles from the RO data and process the RO data not only from the domestically developed RO system, but also from other international RO missions, a new algorithm considering the characteristics of RO data from each mission and its own software are required.

In this dissertation, an algorithm for retrieving the ionospheric electron density through multi-antenna data filling was proposed. In the case of satellites such as Challenging Ministration Payload (CHAMP), Gravity Recovery and Climate Experiment (GRACE) and KOMPSAT-5, which use occultation antenna for monitoring the ionosphere, uncertainty may be included because the total electron content (TEC) outside the LEO orbit (auxiliary side TEC) is built through modeling during the TEC calibration process. Instead of using modeled auxiliary-side TEC, the proposed method uses the auxiliary-side TEC observed by a precision orbit determination (POD) and connects the data by modeling the gap between the POD antenna and the occultation antenna.

Second, this dissertation develops a software for retrieving the ionospheric electron density from GNSS RO data. The algorithm in the software was based on the existing COSMIC Data Analysis and Archive Center (CDAAC)’s algorithm. As a result of comparative analysis with CDAAC’s algorithm, it was confirmed that the 1st Constellation System for Meteorology Ionosphere and Climate (COSMIC-1) electron density retrieved by the developed software are within the tolerance range accuracy requirement. In addition, the electron density profiles from GRACE RO retrieved by the proposed algorithm have similar trends with the CDAAC’s output, and also have good agreement with the electron density profiles observed by the collocated incoherent scatter radar (ISR).

Third, the performance assessment of the ionospheric electron density retrieved from KOMPSAT-5 RO by applying the proposed algorithm has been done. To validate the output from KOMPSAT-5 RO with reliable truth observations, digital ionosonde (digisonde) data, the incoherent scatter radar and COSMIC-2 RO data were used as reference dataset. The comparisons between digisonde NmF2 and KOMSPAT-5 RO NmF2 reveal a high degree of correlation of 0.90. The comparisons with both observations from ISR and COSMIC-2 RO show that the profiles retrieved from KOMPSAT-5 RO data have good agreement with the electron density profiles from the both ISR and COSMIC-2 RO.

Lastly, this dissertation proposes the methodology for assessing the performance of RO data, and applies the methodology to assess the performance of GeoOptics’ Community Initiative for Cellular Earth Remote Observation (CICERO) CubeSat RO by comparing it with the COSMIC missions. The results from performance assessment demonstrate that the miniature version of the GNSS RO receiver could satisfy certain accuracy requirements of the GNSS RO measurements.