GNSS and SAR Signal Delay in Perturbed Ionospheric D-Region During Solar X-Ray Flares

We investigate the influence of the perturbed (by a solar X-ray flare) ionospheric D-region on the GNSS and SAR signals. We calculate a signal delay in the D-region based on the low ionospheric monitoring by very low frequency (VLF) radio waves. Results show that the ionospheric delay in the perturbed D-region can be important and, therefore, should be taken into account in modeling the ionospheric influence on the GNSS and SAR signal propagation and in calculations relevant for space geodesy. This conclusion is significant because numerous existing models ignore the impact of this ionospheric part on the GNSS and SAR signals due to its small electron density which is true only in quiet conditions and can result in significant errors in space geodesy during intensive ionospheric disturbances

Behaviour of electron content in the ionospheric D-region during solar X-ray flares

One of the most important parameters in ionospheric plasma research, also having a wide practical application in wireless satellite telecommunications, is the total electron content (TEC) representing the columnal electron number density. The F-region with high electron density provides the biggest contribution to TEC while the relatively weakly ionized plasma of the D-region (60 km { 90 km above Earth's surface) is often considered as a negligible cause of satellite signal disturbances. However, sudden intensive ionization processes, like those induced by solar X-ray flares, can cause relative increases of electron density that are significantly larger in the D-region than in regions at higher altitudes. Therefore, one cannot exclude a priori the D-region from investigations of ionospheric influences on propagation of electromagnetic signals emitted by satellites. We discuss here this problem which has not been sufficiently treated in literature so far. The obtained results are based on data collected from the D-region monitoring by very low frequency radio waves and on vertical TEC calculations from the Global Navigation Satellite System (GNSS) signal analyses, and they show noticeable variations in the D-region's electron content (TECD) during activity of a solar X-ray °are (it rises by a factor of 136 in the considered case) when TECD contribution to TEC can reach several percent and which cannot be neglected in practical applications like global positioning procedures by satellites. [Projekat Ministarstva nauke Republike Srbije, br. III-44002, 176001, 176002, 176004 and TR36020