Latitudinal, solar, and vertical variability of higher-order ionospheric effects on atmospheric parameter retrievals from radio occultation measurements

During Global Positioning System radio occultations (GPS/RO), the GPS signals always travel, at least partially, through the Earth's ionosphere. Hence, all GPS/RO measurements include ionospheric signatures that degrade the GPS signal quality, limiting the accuracy of GPS/RO measurements and of their derived products. Since the ionospheric effects are frequency‐dependent, they can partially be removed, to the first order, by linearly combining dual‐frequency GPS/RO measurements; small residual effects still remain and influence the accuracy of GPS/RO‐derived products. In our study, we show how the first‐order ionospheric residual effect, due to geometrical raypath splitting of the GPS signals, and the second‐order ionospheric residual effect impact the estimation of GPS/RO‐derived atmospheric parameters. Using 40 randomly selected GPS/RO events from the Challenging Minisatellite Payload (CHAMP) during periods of low (Rz12 = 2; 2008), moderate (Rz12 = 49; 2004), and high (Rz12 = 114; 2002) solar activity, we examine the variability of the ionospheric residual effects as function of altitude, latitude, and solar activity. We show that the first‐order ionospheric residual effect introduces a maximum negative temperature systematic bias of about 0.1K at 50 km, which decreases with decreasing altitude. The second‐order ionospheric residual effect attains values of about −2 mm, which leads to a minimum and maximum temperature systematic bias of about −0.5K at 5 km and +2.0K at 50 km, respectively. We also observe that the magnitude of the second‐order ionospheric residual effect increases with increasing latitude and solar activity, in both hemispheres, following a sinusoidal pattern. Heading to Solar Cycle 24—expected to peak in May 2013 with a Rz12 ∼90—we conclude that it is necessary that the first‐ and second‐order ionospheric residual effects be considered in GPS/RO data processing methodologies to enhance the current atmospheric models. Key Points Models higher‐order ionospheric effects in a GPS/RO geometry Quantifies, for the first time, the impact of those effects on RO temperatures Emphasizes the importance of accounting those effects on future RO missions