| Abstrakt: |
A new one‐dimensional variational (1D‐Var) retrieval method for ionospheric GNSS radio occultation (GNSS‐RO) measurements is described. The forward model implicit in the retrieval calculates the bending angles produced by a one‐dimensional ionospheric electron density profile, modeled with multiple "Vary‐Chap" layers. It is demonstrated that gradient based minimization techniques can be applied to this retrieval problem. The use of ionospheric bending angles is discussed. This approach circumvents the need for Differential Code Bias (DCB) estimates when using the measurements. This new, general retrieval method is applicable to both standard GNSS‐RO retrieval problems, and the truncated geometry of EUMETSAT's Metop Second Generation (Metop‐SG), which will provide GNSS‐RO measurements up to about 600 km above the surface. The climatological a priori information used in the 1D‐Var is effectively a starting point for the 1D‐Var minimization, rather than a strong constraint on the final solution. In this paper the approach has been tested with 143 COSMIC‐1 measurements. We find that the method converges in 135 of the cases, but around 25 of those have high "cost at convergence" values. In the companion paper (Elvidge et al., 2023), a full statistical analysis of the method, using over 10,000 COSMIC‐2 measurements, has been made. Plain Language Summary: This paper presents a new way of estimating the density of electrons in the ionosphere—the part of the Earth's atmosphere in which atoms are ionized by radiation from the Sun. Radio signals sent between GNSS navigational satellites and receivers in a low orbit around the Earth are delayed, and their paths are bent, by the presence of electrons in the ionosphere. Previous attempts to use these observations to estimate the electron density have been based on the delay of the signals. The new approach outlined in this paper uses the bending incurred by the radio waves instead. Such "bending angles" have been used to infer some properties of the lower atmosphere for many years, and are widely available. We demonstrate that by extending these measurements to greater heights, they can provide useful information about the ionosphere as well. Key Points: A new method of deriving ionospheric electron densities, using the difference between bending angles at two different frequenciesIt is based on a 1D variational retrieval, the solution of which is the best fit to the a priori background and the observationsThe forward model assumes the ionosphere to consist of several idealized "Vary‐Chap" electron density layers [ABSTRACT FROM AUTHOR] |
