# Assessment of ZTD Derived from COSMIC Occultation Data with ECWMF, Radiosondes, and GNSS
###### Abstract
The convergence time of GPS-only and BDS-only solutions was reduced by 8.1% and 14.5%, respectively, compared to the hybrid neutral atmosphere delay model-3 of University of New Brunswick (UNBB3m) model, and the reductions were 6.9% and 11.2%, respectively, for the Global Pressure and Temperature-3(GPT3) model [14; 15]. Radio occultation (RO) data are suitable for tropospheric observations due to their global coverage, high vertical resolution, and generally uniform distribution from a climatological perspective [16]. The RO observations from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)-1 and those from the recently launched COSMIC-2 and other constellations have been applied to numerical weather forecast models [17]. The introduction of open-loop occultation signal tracking technology in COSMIC-1/-2 has improved the low-level atmospheric detection capability of radio occultation technology [18; 19]. This makes it possible to obtain more than 40% of events with a bottom detection height less than 1 km, and the atmospheric refractive index with higher accuracy in the lower atmosphere. Many scholars use the atmospheric occultation profile with moisture information from RO products to evaluate its total precipitable water based on multiple data sources: Ground-Based GNSS, Radiosonde, Microwave Satellite, and Numerical Weather Prediction (NWP) reanalysis data [20; 21]. When the atmosphere grid model is used as the background field, one-dimensional variational assimilation introduces the error from the limitation of the time and space resolution of that grid model into wetPrf which is the one-dimensional variational assimilation result between the refractive index from the occultation observation and the atmosphere background field. Now, more and more satellite constellations in low orbit of earth with the improved GNSS RO instruments are planned or carried out, such as COSMIC-2, GeoOpt, Korea Multi-Purpose Satellite-5(KOMPSAT-5), Metop-A/-B/-C, PAZ, SPIRE, TerraSAR-X (TSX), TanDEM-X (TDX), and so on, which would greatly improve the time and space resolution of GNSS-RO events. However, the atmospheric three-dimensional information provided by GNSS RO should be used in more ways, such as improving precision positioning. Although the Numerical Weather Model (NWM) analysis field data can be used as tropospheric data sources, there are deviations at the bottom, and the horizonal resolution of grid is only up to 25 km. The existing IGS stations or Continuously Operating Reference Stations (CORS) can only provide corrections in the continental area, and they are also unable to properly correct the position of aircraft and airships at a special height. However, the ZTD around the world at a specific height can be derived by integrating the retrieved refractivity index of GNSS-RO from the specific height to the top of the troposphere. The ZTD from GNSS-RO can continue to increase its distribution resolution as low-orbit satellites continue to increase to improve precision positioning independently or as supplements of the tropospheric data sources such as NWM data and GNSS-ZTDs. For facilitating the use of RO data to enhance precision positioning, it is necessary and meaningful to verify ZTDs from the dry atmosphere profiles of GNSS-RO with that from GNSS-ZTD, as well as analyze the influence on ZTDs' accuracy of the matching constraints such as the temporal, horizol, and vertical distance. Based on ZTDs from ECWMF (echZTD), radiosondes (sonZTD), and GNSS (gnsZTD), we carried out the assessment of ZTDs obtained by the occultation technique (occZTD). First, we present the four ZTD data sources and calculation methods, especially the ZTD correction method based on the barometric height formula for the height of the GNSS-ZTD which does not match the bottom tangent point height of the RO data.