First estimates of the second‐order ionospheric effect on radio occultation observations

Abstract

[1] This study examines the impact of the second-order ionospheric effect on radio occultation (RO) data products. We propose a new linear combination between dual frequency GPS observables, which retrieves slant total electron content free from the second-order ionospheric effect. Our STEC values differ from those obtained by independent techniques by a maximum of 3 total electron content units (TECU), depending on the geographic location and geomagnetic activity. Additionally, we suggest an alternative method of computing the second-order ionospheric delay in RO experiments, which does not require the use of geomagnetic and ionospheric models. First estimates show that the second-order ionospheric delay for the RO experiments falls within the range [−10, −8] mm, which is of the same order of magnitude with second-order ionospheric delay estimates from ground-based experiments. Finally, as a by-product of our model, we retrieve weighted mean geomagnetic field values, which we compare with theoretical estimates computed by the International Geomagnetic Reference Field-10 (IGRF-10) model. Our estimations agree with the IGRF-10 model between 0.23% and 7.0%.Citation: Vergados, P., and S. D. Pagiatakis (2010), First estimates of the second-order ionospheric effect on radio occultation observations,

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

“…When including the second‐order ionospheric effect, the observation equations for the L 1 and L 2 (in m) carrier‐phase GPS signals at f 1 and f 2 frequencies (in Hz) are expressed as follows [ Vergados and Pagiatakis , 2010]: where ρ is the observed GPS–LEO line‐of‐sight (LoS) length (in m), c is the speed of light in vacuum (in m/s), dT and dt are the LEO and GPS clock offsets (in seconds), respectively, d trop is the tropospheric delay (in m), STEC = ∫ GPS LEO n e ds is the slant total electron content (defined as the integrated free electron number density along the signal path, between a GPS and a LEO satellite in the RO geometry; in TECU), and 〈 B || 〉 = B cos θ · n e ds is the geomagnetic field (in T/m 2 ) weighted by the free electron number density n e along the GPS–LEO propagation path, and θ is the angle between the GPS signal propagation vector and the geomagnetic field ; λ 1 , λ 2 (in m) and N 1 , N 2 are the wavelengths and the unknown integer carrier‐phase ambiguities at L 1 and L 2 channels, respectively, and constant K = 1.1283 · 10 12 (in m 3 /Ts 3 ). The GPS and LEO Inter Frequency Biases (IFBs), multipath and phase thermal noise on the L 1 and L 2 channels are given by and respectively.…”

“…The second‐order ionospheric effect is proportional to both, the TEC and the geomagnetic field along the GPS‐LEO signal path and inversely proportional to the third power of frequency. The cause of the second‐order ionospheric effect is the Faraday effect [e.g., Kedar et al , 2003; De Roo et al , 2004; Vergados and Pagiatakis , 2010], which arises from the interaction of the GPS signals with the Earth's ionosphere under the influence of the Earth's magnetic field. The ionosphere‐free linear combination only removes a fraction of the second‐order ionospheric effect from the GPS/RO measurements and thus, an additional ionospheric residual effect will always remain, which we will refer to as the second‐order ionospheric residual effect.…”

“… Hoque and Jakowski [2010] found that the magnitude of the second‐order ionospheric residual effect during RO events can reach −14.6 cm (in L 1 ) and −6.9 cm (in L 2 ) under high solar activity, whereas is decreases to −13.1 cm (in L 1 ) and −6.2 cm (in L 2 ) under moderate solar activity. Vergados and Pagiatakis [2010] derived an analytical expression for the RO TEC between a GPS and a LEO satellite including the second‐order ionospheric effect in the GPS observation equations. Utilizing Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ionospheric RO measurements, they estimated that a systematic bias of ∼1–3 TECU is observed in the RO TEC values whenever the second‐order ionospheric residual effect is not considered.…”

The impact of large scale ionospheric structure on radio occultation retrievals

“…Second, the refractive index gradient at each frequency depends on the magnetic field along the raypath, which is not accounted for in standard "first-order" calculations of ionospheric dispersion (so-called "higher-order" ionospheric effects). See Syndergaard (2000), Vergados and Pagiatakis (2010) and Bassiri and Hajj (1993).…”

Electron number density profiles derived from radio occultation on the CASSIOPE spacecraft

“…Neglecting the small higher‐order terms in the Taylor series expansion of the Appelton‐Hartree equation, the differential delay between the L1 and L2 frequencies is proportional to the TEC. The effects of higher‐order terms on ionospheric and atmospheric retrievals have been investigated by Vergados and Pagiatakis [, ], for example. Using GPS RO TEC observations T ec ( s ), as input equation is employed to recover electron density profiles.…”