Abstract
Abstract. A modification to the standard bending-angle correction used in GPS radio occultation (GPS-RO) is proposed. The modified approach should reduce systematic residual ionospheric errors in GPS radio occultation climatologies. A new second-order term is introduced in order to account for a known source of systematic error, which is generally neglected. The new term has the form κ(a) × (αL1(a)-αL2(a))2, where a is the impact parameter and (αL1, αL2) are the L1 and L2 bending angles, respectively. The variable κ is a weak function of the impact parameter, a, but it does depend on a priori ionospheric information. The theoretical basis of the new term is examined. The sensitivity of κ to the assumed ionospheric parameters is investigated in one-dimensional simulations, and it is shown that κ ≃ 10–20 rad−1. We note that the current implicit assumption is κ=0, and this is probably adequate for numerical weather prediction applications. However, the uncertainty in κ should be included in the uncertainty estimates for the geophysical climatologies produced from GPS-RO measurements. The limitations of the new ionospheric correction when applied to CHAMP (Challenging Minisatellite Payload) measurements are noted. These arise because of the assumption that the refractive index is unity at the satellite, made when deriving bending angles from the Doppler shift values.
Highlights
GPS radio occultation (GPS-RO) measurements are routinely assimilated into operational numerical weather prediction (NWP) systems (Healy and Thépaut, 2006; Cucurull et al, 2007; Aparicio and Deblonde, 2008; Poli et al, 2009; Rennie, 2010)
This study has focused on a source of systematic residual ionospheric error, originally noted in VK94
The results suggest it may be beneficial to introduce an additional term in the standard bending-angle ionospheric correction scheme (Eq 6)
Summary
GPS radio occultation (GPS-RO) measurements are routinely assimilated into operational numerical weather prediction (NWP) systems (Healy and Thépaut, 2006; Cucurull et al, 2007; Aparicio and Deblonde, 2008; Poli et al, 2009; Rennie, 2010). This work has possible relevance to the generation of accurate monthly mean geophysical climatologies of the stratosphere, retrieved from GPS-RO measurements It may be useful when producing GPS-RO climatologies with the new average-bending-angle method, which has been discussed recently (Ao et al, 2012; Gleisner and Healy, 2013; Danzer et al, 2014). The new method does not correct the residual errors arising from horizontal gradients in the ionosphere, or those caused by the earth’s geomagnetic field We expect that these effects are likely to produce random noise on individual profiles, but to first order they should average out, and not affect monthly and seasonal climatologies.
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