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

[1] 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.